CHEM 223 Lab Manual S25 PDF

Spring 2025 IMPORTANT: STUDENTS REGISTERED FOR THIS LABORATORY COURSE MUST ALSO BE REGISTERED FOR THE ACCOMPANYING RECITATION Recitation attendance is mandatory, and a significant portion of the CHEM 223 grade comes from exams & quizzes given during recitation. Labs for this course will alternate between in-person (in-person) and virtual (on-line) labs. The in-person and virtual days will be switched for odd and even sections. Check the table in the syllabus for your schedule. Table of Contents Day 1/2 Intro. Orientation: Lab Procedures and Safety Rules 2 Day 1/2 Exp. Melting Points 9 Day 2/1 Virtual: Chromatography 11 Day 3/4 Crystallization 20 Day 4/3 Virtual: Distillation 22 Day 5/6 Molecular Modelling 25 Day 6/5 Virtual: Acid-Base Extraction Theory 32 Day 7/8 Acid-Base Extraction Practice 36 Day 8/7 Virtual: Le Châtelier's Principle 39 Day 9/10 Nucleophilic Substitution 41 Day 10/9 Virtual: IR Spectroscopy 43 Day 11/12 Oxidation of Cyclohexanol 50 Day 12/11 Virtual: SN1 and SN2 Preparative Reactions 52 Day 13/14 Synthesis and Reactions of Alkenes 57 Day 14/13 Virtual: Hydrogenation Reactions 61 - Appendix I – Properties of Selected Hazardous Chemicals 63 - Appendix II – Microscale Extraction 66 - Appendix III – Microscale Recrystallization 67 January 19, 2024 Revision PRH Day 1/2 (in-person), Orientation: Questions on Routine Lab Procedures and Safety Rules To be read for understanding before the first in-person lab period. There will not be enough time to cover everything during lab time. Read the SYLLABUS as well. Arrive with any questions you have. Required Safety Video for each to watch before the first day of lab https://www.youtube.com/watch?v=OmLyP5vZw4Q Lab instructors will ask questions about this video on the lab quiz. COURSE DESCRIPTION In this course, you will first learn how to separate, purify, and characterize organic compounds. You will synthesize a number of organic compounds and employ the techniques that you have learned to separate, purify, and/or characterize the products of your own reaction products. GRADING Your grade will be calculated as described in the syllabus, and will be based on pre-lab quizzes, lab performance, written lab reports for in-person labs, tests for the virtual labs, and the recitation assessments. Keep in mind that good execution of laboratory techniques, adherence to safe laboratory practices (including cleanliness and proper waste disposal), the quality and quantity of the products you make, the organization of your work (including how well you have planned your work before hand), and how well you understand the chemical processes that occur are all factored into your grade, as well as the quality of your written reports. REQUIRED TEXT Pavia, Kriz, Lampman, and Engel: A Small Scale Approach to Organic Laboratory Techniques, Fourth Edition. (This lab manual will refer to this laboratory textbook as “Pavia”) PLANNING AND EXECUTION In the organic chemistry laboratory, you will plan and execute your work more independently than in previous laboratory courses. In order to finish each day's assignment in the time allowed, you must arrive at the lab prepared to do your experiment by reading and understanding the assigned sections of Pavia and this lab manual. For the virtual labs, don't just look through the material - you need to study and learn from it. Your learning from these virtual labs will support your in-person labs for CHEM 223, as well as your future lab experiences. A very important part of your learning is the lab recitations (information online on your CHEM 223 recitation blackboard site). Take good notes during the recitations and review them carefully when planning each experiment. The prelab assignments for each in-person experiment must be completed BEFORE you arrive at the lab session. Many of the experiments that you will complete this semester are not taken directly from the Pavia textbook, though you can find many similarities with these labs there. Part Six of the Pavia textbook (starting at page 548) should be especially helpful to you, as it contains descriptions of the techniques you will use throughout the two semesters of Orgo lab. The importance of studying the recitation material and applying what you have learned cannot be exaggerated. The key to success is not just planning your work carefully before you enter the laboratory, but also understanding important factors involved in the chemistry. It is essential that you start working promptly as soon as you arrive at the lab, rather than socializing with other students. You will be so busy in some experiments that you likely won't have time to talk much. Sometimes during Orgo 1 and 2 labs, you will need to work on different parts of an experiment, or even on different experiments, simultaneously in order to finish on time. This is a real-world experience, as researchers multi-task in the lab whenever they can safely do so. It is very important that you finish all work within the scheduled class time and within the total time allotted for the experiment. No work will be allowed outside the scheduled lab time, including washing glassware and taking melting points. Everybody must physically leave the laboratory by the scheduled end of class. No additional time will be given to any student who falls behind on their work. You will work individually on some procedures, but there are also some procedures that you will perform in pairs or as a small group. This information is given with each in-person experiment in this manual. One drawer of equipment will be issued to a pair of students – you and your assigned partner will be responsible for keeping everything in good condition throughout the semester, even if the two of you don’t necessarily work together on any experiment. January 19, 2024 Revision PRH Page 2 When you hold a flask in the hood for your instructor to see and ask a question about the contents and what is happening, you must be able to describe what you put in the flask, and the sequence of operations you have carried out in arriving at that point. (Indicating that you aren't sure if you did a step is a bad sign, but your lab instructor cannot help you if you are not honest.) Your lab instructor will not simply provide answers, but will try to help you figure out the solution to your problem. You should always be prepared to discuss what you are doing and try to arrive at a solution to your own problem rather than to rely on your instructor to solve your problem. LABORATORY SAFETY ALL STUDENTS must wait outside the laboratory until their instructor has entered and has given permission to enter the room. Participation on all fourteen scheduled lab days, both virtual and in-person, is required, especially the first in-person meeting of your lab section for the semester, when safety and procedures with be discussed. There is also a lab experiment to be performed on the first day. During that session, all students will be acquainted with safe laboratory practices, the safety features of the laboratory, and the procedures to be followed in the case of an emergency. Students will also be provided with a copy of laboratory rules. Appendix I of this lab manual contains additional information on the hazardous properties of some chemicals used in CHEM 223. You will not be allowed to proceed with the lab course until you are familiar with the rules and safety procedures. This focus on safety might seem a little silly since you have been in a lab before, but serious lab accidents can happen, and virtually every lab accident involves the violation of a lab safety rule, sometimes what would appear to be a minor rule. I have experienced first or second hand far too many mishaps, some trivial, and some quite serious. The first in-person lab day also has an experiment for you to do. If you miss the first meeting due to registration delays, you should contact the lab coordinator and your lab instructor immediately to schedule a make-up time to perform the check-in procedure and go through the safety protocols. *** SAFETY GOGGLES MUST BE WORN AT ALL TIMES IN THE LABORATORY. *** Failure to comply with any lab regulation, will result in the deduction of performance points and/or ejection from the laboratory for that lab period, or for the entire lab course. Your safety in the lab is of utmost importance to us. Careless violations of safety rules WILL carry penalties; flagrant violations of safety rules will carry sever penalties, up to and including permanent expulsion from the course. Following is a summary of the most important rules; any additional rules specific to particular labs will be covered during the lab instructor's prelab presentation. And it is worth noting that cuts and burns are the most common injuries in student labs, but stiches and longer-term eye care have been required in rare cases. Also note that aqueous solutions of strong bases (e.g., NaOH) are at least as dangerous as solutions of most strong acids (e.g., HCl), especially in the eyes. Students are not allowed in the lab unsupervised. Safety glass must be worn by EVERYONE in the lab if ANYONE is using lab equipment and/or chemicals. You must buy your own safety goggles and have them at the lab. No unapproved procedures are to be performed. If you are curious, great, but ask your instructor first. We want you to have fun in the lab, but it is a quiet fun: no loud or rowdy behavior, and don't be in a hurry. AND ABSOLUTELY NO PRANKS. No eating or drinking in the lab, including chewing gum and water. Smart phones, computers, etc. can only be used in support of the lab in progress, e.g., as a calculator, timer, or camera. Checking your email is grounds for immediately expulsion from that day's lab and a grade of zero. Although we avoid as many chemical risks as possible, you will use chemicals capable of harming you if you are careless. We will inform you of particular hazards during the prelab presentations, so pay attention; however, all substances have risks. When in doubt, ASK. Under no circumstances are you allowed to take any equipment or chemicals out of the lab, including anything that would have been thrown out. January 19, 2024 Revision PRH Page 3 If you find a chemical in the fume hood, you are to dispense it in the fume hood. All spills of any significance, broken anything, problems with equipment, etc. must be reported to the instructor. Some spills and accidents, hopefully minor, will happen. The penalty for hiding a lab accident is SEVERE. Check and double-check the identity and concentration of all chemicals before using them. There is a huge difference between 0.1 M NaOH and 1.0 M NaOH. And 10 M NaOH is especially hazardous. Never return chemicals to the original container and risk contaminating a bottle-take out only what you need. Longer hair and anything else that dangles and can't be removed must be tied back at all times. Do not show up at lab with clothing that exposes your skin to unnecessary risks-no bare midriffs (e.g., crop tops), no short skirts, or no short shorts, etc. AND under no circumstances will scandals or other open-toed shoes be allowed. If you show up for lab dressed inappropriately for any day's activity, you will be asked to leave and may not be able to make up the lab. LAB CLEANLINESS (also a safety issue) 1. Make sure that the area around your workspace is clean while you’re working on your experiment and before you leave the laboratory. If someone has to clean up after you, there will be a penalty. 2. If you spill something or otherwise make a mess during a procedure, you must clean it up. Unless it is a trivial spill (water, brine, etc.) ask the lab instructor how to clean up the material. (Perhaps they will think it wiser that they do it for you.) 3. There are designated disposal containers for broken glass, chemicals (solid and liquid, chlorinated and non- chlorinated), lab gloves (legally classified as medical waste), etc. located throughout the room. If you are not sure where to dispose of something, ask your lab instructor. Improper disposal of lab waste could result in a very significant fine being levied on Hunter College/CUNY. All lab waste must go into the correct container, as mandated by Federal Law MAKING UP AN IN-PERSON LAB You might be able to make-up a missed in-person lab session, but only with the permission of the lab coordinator. The absence must be due to an emergency or a valid reason that the lab coordinator deems legitimate. Email your lab instructor and the lab coordinator well in advance of an anticipated absence to request approval to make up the lab. Procedure Send an email to your lab instructor and the lab coordinator as soon as possible after your absence (or before your absence, if it is anticipated in advance). Please include your lab section (2 digits, not the 5-digit number), when it meets, and the name of your lab instructor. Briefly explain the reason for your absence (documentation might be requested). The lab coordinator will help select a lab section for you to join for the make- up, if one is available. You may be asked to withdraw from the course if you miss too many in-person lab sessions. Every make-up lab will be during a time when that same experiment is being done by another section. Please make every attempt to attend all your regular lab sessions and reserve make-ups for emergencies only. Permission for make-ups will not be granted in cases of student misconduct (e.g., thrown out of lab for violating rules) or negligence (e.g., slept in, forgot about class, etc.). If it proves to be impossible to have you make up the absence in the laboratory, you will be assigned a make-up written report that will likely take more time than the lab would have to complete. This is only allowed once a semester, and only if you were proactive about scheduling an in-person makeup lab. MAKING UP A VIRTUAL LAB If you are unable to do the on-line lab in the time prescribed, contact your lab instructor and he/she will allow you to do the virtual lab late. However, without a valid excuse, there will be a late penalty. If you have a computer glitch while taking a virtual lab exam, do whatever you can to document it and send an email to you lab instructor while copying the lab coordinator to request a reschedule. January 19, 2024 Revision PRH Page 4 THE LABORATORY NOTEBOOK You need a bound lab notebook dedicated solely to this lab course. A loose-leaf or spiral notebook is NOT acceptable. (A spiral lab notebook with pre-numbered pages is acceptable.) The cheapest and easiest to find would be the classic "composition notebook" (example shown), which often has a marble design on the front and can cost as little as a dollar or two. Notebooks where pages can be easily removed without being able to tell are not acceptable. If you wish to keep your recitation and study notes for this lab course in the same notebook, please put them in a separate section at the end. In many settings a lab notebook is considered a legal record, and you must learn to keep records in your notebook accordingly. The notebook is to be sufficiently complete and well organized so that anyone who reads it (e.g., a fellow student) can know what was done in each experiment and can do the procedure from what is written in it, including reproducing any mistakes that were made, if they wish to. This laboratory notebook has essentially the same requirements as a notebook used to record data in a research laboratory. All data are to be recorded at the time they are observed or obtained. This includes weights, boiling and melting points, observations of physical changes, results, and conclusions. Separate pieces of copy/loose-leaf paper are NOT to be used for recording data to be transcribed later. Your laboratory instructor will periodically check your notebook at random or at the end of a laboratory session to ensure that your data is being properly recorded at the time you conducted the experiment. Each experiment must have a title and brief description, with structures and chemical names and reaction(s) given as appropriate. The notebook should be neat, but you will need to occasionally make corrections by drawing a line through a wrong entry and noting the correct information. Make all entries in ink, and NEVER erase or obscure an entry. It must be a complete, original record. The record made at the time of the observation is the official record. You should have a Table of Contents at the beginning of your notebook, and all of the pages should be numbered consecutively. Start every new experiment on a new page. Instead of copying details of a procedure verbatim, refer to the page in the lab manual or other sources where the procedures are given, but there should be a sufficient but brief summary of the procedures, enough to allow a reader to follow what was done. The notebook is a log of your work and entries and should be dated daily. As you conduct the experiment, you must write a description of the actual procedure that you followed including all observations. The preliminary write-up, as indicated in each experiment, must be in your notebook before you begin the experiment. All preliminary write-ups must include a list of less common hazards and toxicities of the compounds involved. We must emphasize that your notebook should be up-to-date at all times during the laboratory period, and your instructor will periodically examine it to ensure this. To repeat: You must only use indelible ink and you are NOT permitted to use corrective fluid (“white-out”) or tape, or to otherwise obscure any entry. Our legal system considers this a possible indication of fraud. In Summary, the rules for notebooks are as follows. It is to be used for information relating to this lab course only Use only indelible ink, never pencil Never use whiteout or the like, and never scribble out an entry Put a single line through any mistake in an entry Never remove a page-put a single large X across a page if necessary Every new lab experiment should start on a fresh page with the date and an informative title A brief purpose for the lab, in your words, should be below the title The notes you take preparing for the experiment should be clearly labeled as such A summary experimental procedure should come after your notes; this is what you intend to do The procedure you used must be entered in detail as you do the procedure, what you actually did If you make a mistake during the procedure and realize it, you should clearly indicate so in your notes and how you addressed the mistake Your observations should be recorded in your procedure as you make them All entries should be well organized and labeled as to meaning All data should be labeled and units must be included, and honor significant figures (2 and 2.00 are not the same) January 19, 2024 Revision PRH Page 5 All data should be entered directly into the notebook, not on a slip of paper Always enter everything you actually did and everything you observed A short, simple summary should conclude the entry for each experiment and should include whether the purpose of the experiment was accomplished and what you learned Remember, a research notebook is a record that needs to be easily read by other individuals, and you must keep yours that way. Sloppy notebook entries have lost patents on significant medicines and even killed patients. Lab Reports You need to write a lab report of some sort for each in-person lab day. These are due at the beginning of the following lab day, unless your lab instructor indicates otherwise. The requirements for lab reports are given below in the context of an example. Not all labs are identical in nature, so variations will be appropriate, but please stay as close to the intent of the suggested format as practical to help keep the grading easier. If your lab instructor has trouble reading your report or finding key data that should be easy to find, there will be a deduction. This example of a minimal lab report is based on an older melting point lab. Don't make your report long for the sake of being long, but, when in doubt, err towards inclusion. Excessive wordy (padded) reports or the mis-use of unnecessarily fancy terms may incur a penalty. Keeping you language simple is always recommended. Note that lab reports, like scientific publications, are written in the third person passive voice. ("The sample was heated for 15 minutes.") Please ask if you don't know what this is. This applies to all sections of the lab report. This usage reflects the intent to relay an objective report of what was done. If you want to add a personal comment ("I...." or "We...."), it should be in a footnote. And be consistent, not just in tense, but in formatting style etc. SAMPLE LAB REPORT [I have imbedded some general comments in this format.] Joe Student CHEM 223 Melting Point Determination Lab Partners: Sam Testube and Marie Flaske Purpose [Note: Don't overuse colons. They make no sense after most headings.] The purpose of this lab was to become familiar with the procedures for measuring melting points and their use in the characterization and identification of organic compounds. This was done by measuring the melting points of two known compounds, urea and cinnamic acid, and their mixtures, and by the use of mixed melting points to identity an unknown as benzoic acid, acetanilide, and vanillin. [Note the use of the passive voice (shows objectivity) and the past tense (reporting what you did).] [For a lab with chemical reaction, the reactions would also be shown here, either with specific structures if the lab is a preparation, or generic structures when the reactivity of various structures is being compared.] Chemicals and Equipment Melting point apparatus (Mel-Temp) and capillary tubes Digital thermometer Urea, cinnamic acid, and 75:25, 50:50, and 25:75 mixtures of them Unknown sample, and benzoic acid, acetanilide, and vanillin as reference samples [Note that chemical names are not capitalized differently than normal words.] [In a formal lab report there would be a lot more detail here, but what is here is fine-routine glassware and chemicals are never listed in formal lab reports.] Summary Procedure Each compound was put into a capillary melting point tube with a small sample well packed into the bottom. The January 19, 2024 Revision PRH Page 6 melting point apparatus was set to heat somewhat quickly until close to the anticipated melting point, and then reset to give a slow temperature rise well before melting was anticipated. Each sample was watched carefully to observe any changes, and the temperatures corresponding to the initiation of melting and completion of melting were recorded. If the heating rate during melting was felt to be too rapid resulting in an inappropriately wide melting point range, the melting point was redetermined. For samples with unknown melting points, a more rapid determination was made, followed by an appropriately slow melting point determination. [Never split tables (unless longer than a page) or leave any type of heading dangling on a page by itself.] Results Table 1, Melting Point of Urea, Cinnamic Acid, and their Mixtures. Sample Initial Melting Melting Complete Reported Range o o o Urea (U) 132.3 C 135.4 C 132-5 C 3.2 oC 75:25 U:CA 97.2 oC 101.2 oC 97-101 oC 4.0 oC 50:50 U:CA 97.2 oC 98.3 oC 97-98 oC 1.1 oC 25:75 U:CA 97.4 oC 98.8 oC 97-99 oC 1.4 oC Cinnamic acid (CA) 133.7 oC 136.4 oC 134-136 oC 2.7 oC [In Table 1, I have rounded off the reported values-tenths don't mean much here, but didn't for the range, where the tenths do mean something. Temperature changes are much easier to determine than absolute temperatures.] Table 2, Melting Point of an Unknown and Its Mixtures with Reference Compounds. Sample Initial Melting Melting Complete Reported Range Unknown 2 114.9 oC 115.4 oC 115 oC 0.5 oC Mixed w/Vanillin 63.1 oC 70.1 oC 63-70 oC 7.0 oC Mixed w/Acetanilide 114.4 oC 115.5 oC 114-6 oC 1.1 oC [In Table 2, 7.0 oC included the ".0", as the zero resulted from the calculation-it has meaning. Also note the range of 0.5 with the leading zero. NEVER use.5 in a report or even write it a lab notebook or anywhere else. It is far too easy to miss the ".", making "0.5" much safer.] Discussion The results with urea and cinnamic acid (Table 1) clearly showed the impact mixing two compounds has on the melting point, even when those two compounds have very similar melting points. It was curious that all three mixtures had similar melting point behavior. This may or may not be general, and more compounds would have to be examined to know. Certainly a very small amount of impurity would have less of an effect. The melting point ranges for the urea and cinnamic acid were unusually wide, as pure compounds usually have melting point ranges of less than 0.5-1.0 oC. It is possible that the samples supplied were not as pure as they could be, but it is also possible that a slower heating rate would have given a narrower melting point range. Still, the drop in the melting point seen clearly demonstrated the drop in the melting point of mixtures, even though a wider range was not observed. The results from the unknown shown in Table 2 more clearly illustrated the expected change in melting point range. The unknown had a good range of 0.5 oC, indicating a relatively pure sample. Its mixture with vanillin (reported melting point 82 oC) showed both a drop in melting point and an increase in the melting point range to 7.0 oC. This indicated that the unknown was not vanillin. The unknown's identity was indicated to be acetanilide based on the second mixed melting point determination in the table. January 19, 2024 Revision PRH Page 7 The most important lessons from this lab are as follows. Melting points are characteristic of a given crystalline compound. Instead of a single temperature, a melting point range is usually seen due to the presence of impurities and the changing temperature during melting, the heating rate. To minimize the impact of heating rate, melting points should be determined patiently with a slow change in temperature during melting. Intentional mixtures of samples will either give a melting point that is significantly lower and with a wider range if the samples are different, or will give a comparable melting point and range if the samples are the same. Therefore, determining mixed melting points is an important method for confirming the identity of two crystalline compounds. [Your lab report should always end with the most important lessons you learned doing this lab. Even though they should be focused and brief, these final comments should reflect your understanding of each lab and should answer the questions, "Why did we do this lab and why is it significant in organic chemistry?"] Assigned Post-Lab Questions [Some, but not all, of your labs have questions to be answered at the end of your report.] END SAMPLE LAB REPORT RECITATION YOU MUST BE REGISTERED FOR A RECITATION SECTION You should consider that the recitation is the equivalent of a one-credit course. Don't let yourself become one of the many students who receive a lower grade for the entire lab course due to lower scores on their recitation, especially the exams. It is essential from a viewpoint of safety alone to attend all the recitations, and attendance and participation will be monitored for that reason. However, the recitation is also critical from the standpoint of your grade since it will account for about 20% of the total possible points for the course. We would like to stress again the importance of studying and planning your work before you start experiments. Students who are well prepared and really understand what they are doing in the lab will enjoy the work and might even look back on their organic chemistry lab as a fun learning experience. Those who do not understand the experiments will experience frustration and, likely, failure, in addition to exposing themselves and others to the risk of a potentially serious accident. We will do our best to help you enjoy this lab course and achieve good results, but if you don't do your preparation and planning, no one will be able to help you enough for you to do a good job. If an instructor determines that a student has not adequately prepared for an experiment resulting in safety issues, the student may be sent away from the laboratory and will not be allowed to do make-up work in another section. January 19, 2024 Revision PRH Page 8 Day 1/2 (live) cont., Melting Point Experiment READ: Pavia – Technique 9 – pages 645-654 THIS LAB WILL BE DONE INDIVIDUALLY INTRODUCTION Each compound has a specific melting point, and it is a very important criterion for the purity of solids, since pure compounds melt within very precise temperature ranges. Most impurities will lower the melting point from its normal temperature and cause it to melt over a range of temperatures instead of quickly. Your lab instructor will explain to you the use of the melting point apparatus, as well as show you how to fill the melting point capillaries. Please note that you need only a small amount of compound for melting point determination, but having too little, as well as too much, causes problems. EXPERIMENTAL PROCEDURE Part A: Determine the melting point behavior of the following compounds and mixture, and observe how the mixture is different than the pure compounds trans-cinnamic acid, urea, and a 1:1 mixture of trans-cinnamic acid and urea Record your observations, including anything thing you observe during the melting point determination. If you go too fast or too slow, you will have to repeat the melting points. This will happen, but the lab instructor is there to help you get it right. HINT: There are three slots for inserting melting point capillary tubes inside the melting point apparatus. You should use up all available slots in the apparatus at the same time whenever possible instead of melting one sample at a time. You may also need to repeat one or more samples until you get good results. Running melting points takes patience and focus. If you get distracted at the wrong time, you could lose 30 minutes of precious lab time. Part B: Determine the possible identity of an unknown compound through it melting point behavior. The identity of your unknown compound will be one of the following. Determine the possibilities by running it's melting point. Compound Lit. MP (°C) Compound Lit. MP (°C) Naphthalene 80-82 4-Methoxybenzoic acid 182-185 Anthracene 216-217 1-Naphthol 95-96 Benzoic acid 122-123 3-Nitroaniline 112-114 Benzophenone 49-51 4-Nitrophenol 112-114 p-Bromoacetanilide 165-169 3-Nitrobenzoic acid 140-143 Cholesterol 147-148 Salicylic acid 159-160 4-Chlorobenzoic acid 239-242 Sorbic Acid 135 (sh) trans-Cinnamic acid 133-135 o-Toluic acid 103-105 p-Toluic acid 180 (sh) *Lit. = literature, MP = melting point, (sh) = sharp, no discernable range. Part C: Positively identity the unknown compound from Part B by recording a mixed melting point measurement with a known reference sample. This is a routine part of melting point procedures and should always be performed as long as a pure reference sample is available. As an example, if you think you have 3-nitroaniline or 4-nitrophenol (same melting point), you'll January 19, 2024 Revision PRH Page 9 run a mixed melting point with your unknown on both, and the one that keeps it's pure melting point is almost certainly your compound. For any melting point this semester, a reference value from the literature ("lit. value") should be included with an appropriate reference. LAB REPORT There are NO ASSIGNED POST-LAB QUESTIONS for this experiment. January 19, 2024 Revision PRH Page 10 Day 2/1 (virtual), Chromatography READ: Pavia - Techniques 19 and 20 - pages 780 - 815. This assignment consists of reading the background information given, then watching three videos and answering a series of questions on those videos and your understanding and application of what you learned. You would be wise to read through the questions before you watch the videos. When you are done, you will take an on- line exam via Blackboard. Do NOT underestimate the amount of learning needed to do well on the exam. Background Material Chromatography is, in general, an analytical or preparative purification technique where a compound spends part of its time moving through a system in a mobile phase, and part of the time not moving because of its association with a stationary phase. This is illustrated below. stationary phase A A A B mobile phase A A B B B B A B B stationary phase A A A B B This illustration shows a mobile phase going through a channel, and a stationary phase that lines the walls of that channel. Both Compound A and Compound B spend part of the time moving with the mobile phase, and part of the time associated with the stationary phase. For either compound, the equilibrium between the mobile and stationary phases is rapid, so that every single molecule, on average, spends the same percent of the time moving as other like molecules. Because Compound B, as illustrated, spends a smaller percent of its time associated with the stationary phase than Compound A, it moves faster, and Compounds A and B are separated. This is the basis for all chromatography. NOTE, when two compounds behave identically in chromatography, it is consistent with them being the same compound, but the conclusion is certainly not justified on that basis alone that they are identical. GLC, Gas-Liquid Chromatography A mixture of organic compounds in the gas phase is pushed by an inert gas through a very small-bore column containing a porous support that has a very thin layer of a waxy compound attached to it. At the normal temperatures GLC is used, the wax makes an essentially liquid layer, hence the name, and compounds dissolve temporarily in it, then return to the gas phase. Usually, the compound that is lower boiling will pass through the column the fastest, but interactions with the wax can also be important. Liquid-Solid Chromatography Solid or liquid organic compounds can be dissolved in a solvent and then passed through many different solid stationary phases. The most common is silica gel, essentially finely ground quartz (common sand is quartz, SiO2), except that it is amorphous and has been activated by driving off water. Other substances are used, but silica gel is the usually the cheapest and easiest to use on a routine basis. Silica gel is used with organic solvents, commonly mixtures, rarely containing water. If we want to purify anywhere from trace amounts to significant amounts of a compound, we can use columns containing silica gel with diameters of less than an inch to more than a foot. However, if we just want to analyze purity, the silica gel can be adhered to a glass plate (or a piece of plastic or aluminum) using a "binder". A small amount of a mixture of compounds to be analyzed can then be put on the plate close to the bottom, and the plate is dipped into a solvent mixture that is wicked up the plate, essentially by capillary action, just like a piece of tissue paper dipped into a little water will draw the water up into the tissue paper. This analytical method is call tlc (thin-layer chromatography). Note that there is something called reversed-phase chromatography where the column material is non-polar and the solvent is polar, but we won't focus on that valuable method. Figure 1 shows an example of how a tlc plate might look with two compounds. The first plate on the left has three sample spots; they have been placed on a line made lightly with a pencil and a small cross-hash for each spot. (Ink can't be used because it contains colored organic compounds that won't stay put like the graphite from the pencil will.) One sample has been spotted on the left, a different sample on the right, and both on the middle mark, referred to as January 19, 2024 Revision PRH Page 11 a co-spot. As the solvent is drawn up the plate, indicated by the upper hashed line, the spots move as well. However, they don't move with the "solvent front" (the dotted line, which can be seen), but lag behind it, as then spend only some of the time in solution, while they spend the rest of the time interacting with the silica gel on the plate. Note the left spot moves farther than the right spot, indicating they are different compounds, the slower moving compound usually being more polar. The faster moving spot spends a higher percentage of its time in solution relative to the slower spot. Figure 1 TLC Example On the second of the six images, you can barely see the separation in the middle column; you can't be sure it is even real. On the third image, there is now a figure eight in the middle, and after that there are two distinct spots. By the end there is high confidence that they are, indeed, different compounds. The co-spot in the middle gives us much higher confidence in our results, as the solvent sometimes doesn't climb evenly. It should be noted that only colored compounds can be watched as the tlc elutes, that is, as the solvent rises up the plate. Usually, the compounds have to be visualized after the plate has been eluted by a variety of techniques (mentioned later). The ratio of the time a compound spends in solution to the time it spends adhered to the silica gel can be easily measure from the plate, as shown in Figure 2. This value is referred to as the Rf (retention factor) and is characteristic of a compound with a given solvent mixture. The calculation of the Rf values for the example given is shown, and is simply the ratio of the distance traveled by a given compound divided by the distance traveled by the solvent front, and can vary from 0 to 1. A value of 0 (zero) means a compound was completely bound to the silica gel and never moved with the solvent, while a value of 1 means the compound moved with the solvent the whole time and was never bound to the solid phase. When the spots are nice and round, the distance measurements are made to the middle of the spot. Sometimes the spots can end up somewhat tear shaped, in which case the measurements are made to the middle of the head of the tear, which is the most concentrated part. Such tear-shaped spots can be due to too much compound on the plate, poor solubility of the compound in the solvent used, slow equilibrium between the solution and stationary phases, or other problems. Separations are usually the best with Rf values between 0.25 and 0.5, but each separation is different and sometimes the best separation is at higher or lower Rf than this range. For silica gel, the dominant interaction with the silica is polar in nature, with hydrogen bonding being very important, and H-bond donors being more significant than H-bond acceptors, so more polar compounds, especially compounds that can hydrogen bond, run at lower Rf. If an initial attempt to run a tlc with new compounds has the spots too low on the plate, a more polar solvent mixture is used to move spots farther. If the spots move too far, a less polar solvent is used. Solvents with a single component sometimes work, but mixtures of two, or rarely three or four, solvents are much more common. In some cases, two different compounds may have almost the same Rf value, even though they move the ideal one-third of the way up the plate. In that case, using a totally different solvent mixture may give a good separation of the compounds. Because of this, two compounds that separate well on tlc must be different, but compounds that have the same Rf may be the same or may be different. (Proving two samples are different compounds is usually pretty easy, but proving they are the same and not just closely related compounds compound requires more data.) January 19, 2024 Revision PRH Page 12 Figure 2 Rf Determination For the top spot, Rf = 1.2/2.5 = 0.48 2.5” For the bottom spot, Rf = 1.0/2.5 = 0.40 Rf = 0.48 Rf = 0.40 1.2” 1.0” TLC Plate (SiO2) of the Separation of 3 Isomers (The lane on the right has all three samples mixed) By Izmaelt - Own work, CC BY-SA 3.0 https://commons.wikimedia.org/w/index.php?curid=23344911 Although Rf values are the standard way to report tlc data, they do suffer from several problems. 1) Solvent can evaporate from the solvent front at the top of the silica gel plate to various degrees, causing the spots to move farther and, therefore, have higher calculated Rf values than they should. 2) The silica gel can vary in activity depending on the amount of residual moisture it contains. Plates with more bound water (less activated) tend to give higher Rf values than very dry plates. 3) Some Rf values can be very sensitive to the exact ratio of the solvents used, and tlc solvents are often mixed quickly and rarely with high accuracy. For these reasons and others, Rf values are most useful when comparing the relative Rf values of two or more compounds run at the same time. Although Rf values are reported for practical reasons, nothing is better than what you actually see in the lab. A comment on visualizing the spots. Some compounds are colored, but hardly all of them, and even colored compounds can contain uncolored impurities. UV (ultra-violet) light can often be used to see where many uncolored compounds are, either due to a special additive in the silica gel, or the compounds ability to fluoresce like the colors on a black-light poster. (See Pavia for the technical details.) Compounds can also be "stained" in various ways. E.g., iodine (I2) associates with many compounds and can visualize their presence, and applying a little sulfuric acid and heating can scorch spots and make them visible. There are also many specialized stains that are used with various classes of compounds. Paper Chromatography Everything that applies to the more common silica gel chromatography applies as well to paper chromatography. It has the advantage of being much cheaper, and is also faster with very polar solvents (but hardly fast, as you will see). It works well for us in separating the various colors used to make food coloring. In the extra credit experiment you should note that the more polar the solvent mixture, the further the compounds move. Column Chromatography (See picture on next page.) This technique is performed by packing a glass tube or column with an adsorbent as shown on the next page. There are many different types of adsorbents (solid phases) that are used in column chromatography, and the choice of adsorbent depends on the types of compounds to be separated. The most common adsorbent is silica gel, but alumina and reversed-phase adsorbents can also be used (see Pavia for details). Silica gel is used to separate a wide variety of functional groups such as hydrocarbons, alcohols, ketones, esters, acids, azo compounds, and amines. Alumina is also used, and comes in three forms: acidic, basic, and neutral. Acidic alumina is used for separating acidic materials such as carboxylic acids and amino acids. Basic alumina is used to separate amines, while neutral alumina can be used to separate non-acidic and non- basic compounds. Likewise, cellulose, starch, and other complex sugars can be used as the stationary phase to separate special classes of natural products, and magnesium silicate is used for certain separations. There are even stationary phases that can separate larger molecules (e.g., proteins) based primarily on their size (SEC, size- exclusion chromatography). January 19, 2024 Revision PRH Page 13 A column may be packed 'wet' by mixing together a slurry of the solvent and adsorbent and pouring it into the tube, and then letting the solid stationary phase settle to the bottom. Alternatively, it can be filled with the dry adsorbent that is allowed to settle before solvent is be forced down the column to expel air. In both cases, the quality and uniformity of the solid phase packing is crucial for good separations. The mixture to be purified is then dissolved in a small amount of an appropriate solvent and added carefully to the top of the solid adsorbent, so to ensure that the packing is not disturbed. You run (elute or "develop") the column by adding more of the solvent to the top, and then collecting the fractions of eluent that come out at the bottom. For 'flash' column chromatography, moderate air pressure is used to push the solvent through the column quickly. A typical column early in the process of elution is shown. The contents of the fractions and the success of the separation can be determined by running TLC plates or other method. A column can be developed with a single solvent, a set mixture of solvents, or a solvent gradient (a solvent system which gradually increases in polarity). For example, a column may be developed first with a low-polarity solvent, such as hexane, and as fractions are collected in many test tubes the developing solvent is changed to 10:1, 5:1, and 1:1 hexane-methylene chloride. A polarity gradient must be used for mixtures of compounds with very different polarities. Non-polar compounds adsorb less readily to the polar stationary phase, and consequently will travel more along with the mobile phase. It is important to keep in mind that both overall polarity and the polarity of any functional groups play a role in interaction with silica gel. Since polar compounds are better adsorbed onto the polar stationary phase, they tend to travel more slowly. A polar solvent can best compete with the stationary phase to attract more polar compound, thus carrying it along with the mobile phase. So, the best mobile-phase (solvent system) will be sufficiently polar to compete with the stationary phase so that the compounds of interest are carried far enough down the column, but is still sufficiently attracted to the stationary phase so that they will not travel all the way down the column at once (along with the "solvent front"). Hydrogen bonding also plays a very significant role in how compounds interact with silica gel. Alcohols somewhat resist elution and may give wider bands, while carboxylic acids are challenging to move through silica gel and often elute in a very broad band that trails badly, i.e., that elute for a long time give getting slowly more and more dilute. Amines, even tertiary amines, also are difficult to elute because they are the strongest hydrogen-bond acceptors. Silica gel is both an acceptor and a donor. For larger molecules, these factors don't always present problems, but for more medium-sized molecules, the silica gel can be pretreated with a very dilute solution of a simple alcohol/acetic acid/simple amine before adding the silica get to the column. There are alternatives to silica gel that can be used that are more tolerant of acidic and basic functional groups, such as the previously mention alumina. A common non-polar solvent for chromatography is hexane. It can be used with a variety of polar solvents. The following solvents are listed in approximate order of increasing eluting power (related to but not the same as molecular polarity): cyclohexane, petroleum ether (actually a hydrocarbon mixture, not an ether), pentane, carbon tetrachloride, benzene, toluene, chloroform, ethyl ether, ethyl acetate, ethanol, acetone, acetic acid, methanol, and water. This order can vary for different types of compounds. You will learn to understand this order as your knowledge of the physical properties of organic compounds increases. The order of elution for common compounds from fastest (moves with a non-polar solvent) to the slowest (a more polar solvent is necessary) is roughly as follows: hydrocarbons, olefins, ethers, halocarbons, simple aromatics, ketones, aldehydes, esters, alcohols, amines, and carboxylic acids. Again, you will learn theses groups with time. January 19, 2024 Revision PRH Page 14 Videos you are required to watch and learn from All video credits are on YouTube Hyperlink Address (cut and paste if hyperlink fails) Total Length Khan Video https://www.youtube.com/watch?v=e3lRt9XdV0s 5:06 S3-Malta tlc https://www.youtube.com/watch?v=Xo97dvZKcOc 10:13 NUS Column Chrom. https://www.youtube.com/watch?v=ci2uu9Cuf5s 18:41 NOTES ON VIDEOS Khan Video Simple explanation of tlc. S3 Malta Video on Thin-Layer Chromatography (tlc) This video is not as focused as I'd like, but it has a lot of good things in it that other videos didn't have. Minute (0:25-0:50) - Use of tlc. Minute (0:50-1:40) - How to make spotting tubes. Everyone I know of buys the spotting tubes, but if you want to see some simple glass work, it is fun. I did have to do this once upon a time. Minute (1:40-3:50) - How to spot compounds on the tlc plate and elute the tlc plate. Elute means to move compounds up through the surface layer of silica gel (or down through a column) with a solvent, and the solvent mixture you use is call an eluent. Minute (3:50-5:15) - Visualization of compounds on the tlc plate. Minute (5:15-6:30) - Using tlc to identify compounds. Minute (6:30-8:40) - Using tlc to monitor a reaction. Note at the end of this segment, and the red and blue spots are the starting materials in a reaction, and the blue spot is the product. Minute (8:40-end) - Precautions NUS Video on Column Chromatography You can ignore the term "flash" chromatography. It is a very trendy term that has lost some of its original meaning. Using air pressure to make the column run faster is why the term applies here to some extent. Minute (1:36) - This is the first time I have every heard a ring stand called a retort stand. A retort is an ancient type of distillation flask. Minute (2:17) - "Two spoons full" is a rather non-scientific measure. Beginners can calculate the right amount of silica gel to use for the amount of compound they have and then weigh it out. With experience it becomes more like when I make chili-my eyes know how much to use of each ingredient. Minute (3:10) - He says to make a homogeneous slurry, with is impossible. Homogeneous in chemistry means a true solution. What he should say is that it is a uniform suspension (heterogeneous). Minute (3:42) - Silicosis is the deposition of any form of silica in your lungs. It takes forever for your body to get it out of your lungs. A buildup over time will cause serious trouble breathing and other significant medical problems. Minute (8:40) - I've put the structures of beta-carotene and chlorophyll below. Even though it is easy to see that chlorophyll is the more polar of the two, it is not as polar as you might have guessed. The magnesium is tightly "coordinated" by the 4 nitrogens of the chlorin ring system, and a lot of the molecule is pretty greasy. Minute (10:00) - Look closely at the yellow band as it starts to go down the column. It should be narrow and flat at the top and bottom of the yellow band. His technique isn't that good, but in his defense, he is distracted by making January 19, 2024 Revision PRH Page 15 the demo recording. For an easy separation like this one, it doesn't matter that much. Minute (12:30) - Feeling warm, as he referred to, is not reliable. You often have to hunt through dozens of fractions by tlc to find your compound. Minute (14:49) - The white areas are caused by the sudden increase in the concentration of the ethyl acetate. When it interacts with the silica gel, it actually gets hot enough to boil some of the solvent. Minute (16:30) - End - the rest is just cleanup, which you don't need to watch. Chlorophyll A (B is similar) beta-Carotene Questions and problems to do on your own for understanding (not to be turned in for grading) before you take the Blackboard Exam To check your learning before taking the Blackboard virtual lab exam, answer the following questions with clear explanations. 1) If you run a tlc with two random organic compounds, and the two spots have the same Rf value, can you conclude that they are the same compound? If not, what can you conclude? (Consider that many millions of organic compounds have been made and characterized.) 2) There are many chemicals that can be used to visualize spots on a tlc plate that have been run and dried. The permanganate used in the video to make the yellow spots on the purplish background relies on the oxidizing power of basic permanganate (MnO4-) solutions, which is a stronger oxidizing agent than chromic acid. You could also use chromic acid to visualize spots, and the yellow background would turn green wherever something is being oxidized. Many alcohols can be oxidized to ketones, but ketones are not easily oxidized. So, could you use easily use chromic acid to visualize spots when you are monitoring a reaction to make a ketone out of an alcohol? What would you likely observe if you tried? Also, if you used a little alcohol in a solvent mixture to elute your tlc plate, why is it important to completely dry the plate before you visualize your spots using an oxidizing agent such a permanganate or chromic acid? 3) Calculate the Rf values of the spots on the tlc plate shown on the right. A ruler and calculator are required, and you should measure to the middle of the spots. Show your calculations. (You can print this page, or just measure the distances on the screen.) 4) You have a solvent, say pure methylene chloride, that gives a compound you made an Rf of 0.20 on tlc plate, and you want to purify this compound using column chromatography. You know that the column you are going to use holds 35 mL of solvent in the volume occupied by the silica gel. Assume that the column behaves exactly like the tlc plate, how much solvent will you need to elute your compound? (In practice it will usually take more because the band for a compound gets wider as it goes down the column.) Hint: If the Rf were 1.0, it would take 35 mL, and if the Rf were 0.0, the compound would not even move on the silica gel and would never elute. January 19, 2024 Revision PRH Page 16 5) You try three different solvents to separate a mixture of two compounds you made. The results are shown to the right. Which one of these three would you use to purify the compound on a column and why? Would you want to try to adjust any of these solvent systems to improve the separation? How? Hint: You might want to assume that the column holds, say, 10 mL of solvent, and that the volume the compounds will elute in is 4 mL. Just as you did in Question 4, estimate the amount of solvent required to elute each of the compounds for all three solvents. 6) (Challenging!) Chemists often talk about the polarity of compounds and the polarity of the solvent that is in competition with the silica gel to interact with the compounds. (Remember the compounds are in equilibrium between being in solution and being bound to the silica gel's surface.) Consider the data in the two lists on the next page. On the left is the usual order of elution of functional groups for tlc or 10% ether 10% ethyl acetate 30% methylene in hexanes in hexanes chloride in hexanes column chromatography. Note that amines and carboxylic acids are so hard to elute from silica gel, that ammonia and acetic acid, respectively, are usually add as modifiers to the solvent at up to ~1%. On the right is the usual power of solvents to elute compounds. Considering this information, what is the relative importance of the two important polar interactions, dipole-dipole interactions and hydrogen bonding, in silica gel chromatography. Test Go to the Blackboard page for your lab section and take the exam to demonstrate your learning. WARNING, the virtual lab exams are intended to be challenging! (And they are open book.) 7) Extra Credit (5 to 15% of a single lab grade, depending on the quality of your work and report) Get some white paper coffee filters, food coloring, rubbing alcohol (91%, if they have it, 70% at least, not 50%), and whatever containers you have at home, and do your own tlc chromatography experiment to see what you can learn. You can also add colored and/or black markers or pens to see the colors the inks are made of as well. (Many black markers are not just made of black ink!) Doing this kitchen experiment will take some ingenuity on your part, but it can be done, because I did it in my kitchen. The in-person lab procedure we usually use is given below to give your ideas of how to proceed. If you actually do this activity and want credit for it, write up your experiment, take pictures, and turn it in as a separate report. This will be accepted for extra credit any time this semester before our reading day. Paper Chromatography Procedure (From a previous in-person lab) Preparing the filter paper (if necessary) 1) Take a piece of filter paper and using a sharp pencil put a small hole as close to the middle as you can. Then take a very small strip from another piece of filter paper and make a short, stubby wick about 2 cm long and 3 mm wide. You should be able to insert it into the hole in the filter paper with most of the wick on one side. This wick will be dipped into the solvent you use and draw it up into the filter paper. Check your filter paper and wick by placing it January 19, 2024 Revision PRH Page 17 on top of a Petri dish, and invert a second Petri dish on top of it. See Figure 3, as well as the sample setup you'll find in the lab. Figure 3 Paper Chromatography Setup Possible Alternate Setup Filter paper with hole Final setup With wick in place Solvent level Part 1: Picking a Chromatography Solvent System 2) Make the solvents you will start with by mixing 50 mL each of 25%, 50%, and 75% water in n-propanol. Each of these solids goes in the bottom of a Petri dish as your solvent reservoir. If there are enough Petri dishes, you'll be able to do all three solvents at once. 3) Draw a circle using a pencil around the hole, about 0.5 inches in radius. Put a hash mark on this circle where you intend to put your spots. (You should be able to fit 4 or more spots on one piece of filter paper.) Use the glass capillaries supplied to put a very small spot of each of the food colors on the hash marks you made. 4) Put your prepared filter paper on the Petri dish with the first solvent with the wick dipping into the solvent. Cover with the second Petri dish to minimize evaporation. (The better the seal, the better the results you will get.) Watch as the paper chromatography progresses, and stop when you think you have optimum results, which should be when the solvent is at least 2/3rds of the way to the edge. 5) With a pencil in hand, remove the filter paper and quickly trace a line around the paper where the solvent line is. Use this line and the center of the darkest part of the colored bands to calculate an Rf value for each colored band in each sample. (You can do this calculation while the next chromatography runs.) 6) REPEAT this procedure until you have done all three solvent mixtures. 8) After evaluating your results visually, you should eliminate the solvent with the worst results. For the other two, pick a solvent mixture in between them that might give you a better separation. For example, if you liked the results of the 25% and the 50% best, you might consider trying 30%, 35%, 40%, or 45%. You can choose one of the two mixtures in the middle, or one closer to the initial solvent that you think did a better job. 7) Run one final chromatography using your chosen solvent. While doing this follow-up experiment, you can finish calculating all the Rf values for your first three solvents. For each color in each solvent you should have 1, 2, or 3 values, depending on how many different colored bands a given food coloring separated into. As a reality check, review your Rf values, comparing them to the chromatograms. 8) Assess all four solvent systems you have tried visually, and pick your best to do the follow-up experiment. (You can calculate the Rf values for you last run with the experiment in Part 2 is running.) Part 2: Using Your Solvent System The best way to test if two compounds are the same or not, is to mix them together and see if 2 spots resolve using a good chromatography system. One reason for that is that Rf values depend on many variables, so making a comparison in a single run is always the best. You'll do that now to see if the green is composed of the same yellow January 19, 2024 Revision PRH Page 18 and blue dye that are used for those colors. To make this test better you will make initial spots that are as weak as you can meaningfully see, but not so weak you won't be able to see the spots after you elute with the solvent mixture you picked. 1) In the corner of two small containers tilted so drops will stay where you put them, put two drops of green food color. 2) To one add a single drop of blue coloring, and to the other add a single drop of yellow. Mix them with a spotting tube. 3) On a single piece of chromatography (filter) paper, put two light spots of each of the mixtures and two spots of just green coloring. Try for very light spots, but ones where you'll be able to see the eluted spots easily. One of each pair of spots will likely be darker than the other one, which is okay. 4) Elute the chromatography paper as before using your optimized solvent system. January 19, 2024 Revision PRH Page 19 Day 3/4 (in-person), Crystallization READ: Pavia - Techniques 8, 9, 10 and 11 - pages 632-682. Also read pages 548-587. THIS LAB WILL BE DONE IN PAIRS INTRODUCTION The technique of crystallization is one of the most valuable available for the purification of solids, and is by far the fastest and cheapest for larger-scale work. The basic ideas for this purification technique are easily understood and the manipulations are straightforward. In spite of this, crystallization remains as much an art as it is a science, as subtle observations and experience are important. Another problem can arise because a good crystallization frequently requires patience. In extreme cases, it can take weeks or even months for good crystal growth. A more serious frustration at times is that the best solvent to use cannot be chosen by any convenient rule, but must be found by trial and error. However, an understanding of the interactions between the solvent and the compound being crystallized is quite useful. The most fundamental property of a good solvent for crystallization of a solid is that the hot solvent dissolves the substance readily, while the cold solvent dissolve it only slightly. This means that you can start your hunt for a good solvent by looking for one that give borderline solubility at normal temperatures. From here you would have to adjust the temperature range, the ratio of solid to solvent, or even try combinations of solvents to find a mixture with just the right solvent properties. A single solvent is always best for reproducibility, but it is often not possible to find a single solvent with the right properties, so using a mixture of two solvents is often necessary. In crystallization of a mixture of solids, one is always faced with the practical problem of knowing how much solvent to use. If one of the impurities in you compound is poorly soluble in the chosen solvent, one could go on adding the hot solvent for a long time before the mixture is dissolved completely. If you use too much solvent the desired compound will not come out of solution when you cool it. In practice, you will probably make mistakes at times, and the only way to recover your compound is to concentrate the solution to see if the desired solid precipitates. Therefore, for best results, you must be able to judge whether any undissolved solid is the compound that you are trying to recrystallize, or some poorly soluble impurity. This usually requires keen observation. Crystallization using a pair of solvents is very useful for the purification of certain compounds. In this technique, one often dissolves the compound in a warm volume of the solvent in which the compound is more soluble, and then the second solvent in which the compound is less soluble is added slowly until a slight turbidity (cloudiness) appears. The solution is reheated and/or a drop of the first solvent is added to obtain a clear solution. The solution is then slowly cooled to obtain the crystallized compound. Note that for high quality crystals, slow cooling is always better, but rapid cooling and crystallization often gives acceptable results in terms of purity. For this experiment, you will recrystallize an unknown compound using water. Your unknown compound will be benzoic acid, salicylic acid, or sorbic acid. You need to look up their structures to include in your lab report EXPERIMENTAL PROCEDURE First, obtain a sample of your unknown compound from the lab instructor. Set up everything that you will need for recrystallization in the fume hood. Boil approximately 150 mL of water (get this started first thing, as it will take some time). Pour 5-10 mL of this hot solvent into a flask. Place on a hot plate to heat back up while you go on with your other preparations. Weigh out close to 2 g of your impure unknown and place it in a 125 mL Erlenmeyer flask. You do not need to weigh out an exact amount, but you should record as precisely as possible how much you did weigh out. Carefully add (in small amounts) the hot solvent with mixing. Remember the importance of using the minimum amount of hot solvent. You may have to decide whether the last traces of undissolved solid are part of your sample or an insoluble impurity, so look carefully for changes in the amount of solid present. Swirl the flask between additions, and, when almost completely dissolved, add a boiling chip and return to boiling on the hot plate. Remove the solution from the hotplate, gradually add a small amount of decolorizing carbon (Norite®, caution-frothing possible), and swirl the solution gently. Heat the solution to a slow boil for January 19, 2024 Revision PRH Page 20 approximately 5 minutes. Filter the hot solution using a heated funnel and fluted filter paper, or a Buechner funnel. It is recommended that you place the funnel in a heated oven at the start of the lab in order to ensure that it is hot for the filtration process. If the sample cools too quickly, crystals might form that clog the filter. NOTE It is important to keep the solution and the filtering apparatus hot during the filtration. Since solubility decreases as the temperature decreases, sometimes rapidly, the crystals may begin to crash out of solution while you are filtering, and this could result in considerable loss of time and product. To avoid this, add slightly more water than is needed to completely dissolve the impure solid. The excess water can then be evaporated after the Norite has been filtered off. The purpose of Norite is to remove small amounts of colored and other types of impurities. It is usually not necessary to use Norite in situations where you do not seem to have any colored impurities. While we are using Norite in this experiment for practice, you should only use Norite in the future if it seems necessary to do so. Cover the mouth of the flask containing the hot filtrate with a watch glass and allow it to cool, first to room temperature, then undisturbed in an ice bath. The more slowly a solution is allowed to cool, the better the quality and, often, the purity of the crystals you will obtain might be. When the product has crystallized completely, collect the crystals in a Büchner funnel on a sheet of filter paper. Rinse the Erlenmeyer flask with part of the filtrate to ensure complete transfer. Discontinue the suction when the crystals are still slightly moist. Wash the crystals with cold water. Apply suction again and press the crystals firmly with a clean glass stopper. Allow the crystals to dry in the air for an extended period of time, or gently heat them in the oven. HINT: While air drying the wet crystals is often the safest way to remove the solvent, it can also be very time consuming. It is faster to gently heat your wet crystals on a watch glass in the oven to evaporate the solvent faster. You must be careful to check the oven temperature and avoid heating the crystals close to their possible melting point. Determine the weight, yield, and the melting point of the dried recrystallized material. When you run the melting point, include a sample of the original sample for comparison. Based on the melting point range and the expected melting points, identify your unknown. To be certain of the identity of your compound, you must also take a mixed melting point with a known reference sample of the suspected material. Show your purified sample to your instructor and indicate the weight and melting point. You may then dispose of them as directed. Remember, your sample will be graded both on the appearance of the crystals and the yield obtained. The yield in this case will be based on the mass of purified unknown solid that you recover, and should be expressed as percent recovery, which is the mass of purified solid obtained is divided by the mass of impure solid that you started with times 100%. After the experiment is completed, you should hand in a sample of your recrystallized compound to your instructor. Ask your instructor how he wants it. LAB REPORT Make sure that you clearly indicate the number of your unknown sample. Assigned Questions: Answer Questions 1, 2 and 6 from Pavia, page 681-682, as well as the following question: The solubility of benzoic acid in water is 6.8 g/100 mL at 95 oC and 0.3 g/100 mL at 25 oC. You are given 10 g of benzoic acid. Calculate the amount of water that you would use to recrystallize the sample. If the crystals are collected at 25 oC what is the maximum possible recovery? January 19, 2024 Revision PRH Page 21 Day 4/3 (virtual), Distillation READ: Pavia - Techniques 13, 14, 15 and 16, pp ~700-760 (BP, Simple Distillation, Vacuum Distillation, and Steam Distillation) Importance There are two long-standing methods to purify both large and relatively small amounts of organic compounds. For compounds that are liquids, distillation is often the easiest method, while for compounds that are solids, recrystallization is often the best choice for cost and convenience. Background Knowledge The distillation of wine goes back at least to the ancient Greeks, who also used distillation to purify salt water to make it drinkable. Earlier possible references to distillation go back another millennium. Distillation is the heating of a liquid to induce a phase change to the gaseous state, and then condensing the vapor in a separate container in a purer state. If you have watched steam from a boiling pot of water condense on a colder surface, you have watched a crude distillation. Alembics, such as the one shown above, are among the oldest distillation flasks known https://commons.wikimedia.org/w/index.php?curid=475445 Any liquid that is capable of existing as a gas can establish an equilibrium with a certain amount of vapor associated with it, measured as its vapor pressure. The relative humidity that is referred to on the weather report is an indication of how close the water vapor in the air outside is to an equilibrium state with liquid water in the environment. At 80% you really notice that humid air, but it is not until you reach 100% humidity that equilibrium exists between the air and water in the environment. However, knowing the humidity is at 100% does not tell you the absolute amount of water in a certain volume of air without knowing the temperature. The hotter it is, the more water vapor the air can hold, i.e., the higher the vapor pressure at equilibrium. This is shown in the graph of the equilibrium vapor pressure of water versus temperature. Vapor Pressure of Water in Equilibrium with Liquid Water http://ummalqura-phy.com/HYPER1/watvap.html As the temperature of a container of water or other liquid increases towards the boiling point, the vapor pressure increases to that of atmospheric pressure. Under those ultimate conditions, distillation will occur if the vapor has a colder surface to condense on. Above the boiling point, a liquid can't exceed the boiling point by much. If you put a thermometer into a pot of pure boiling water, the temperature doesn't rise significantly above 100 oC, and the entire pot of water will stay at that temperature until it boils away. If this is done at near equilibrium, the vapor (the water in the gaseous state) above the boiling water will also be at 100 oC. In practice, a thermometer over an open pot of water won't read that temperature because it is mixing with the air in the room and is not in equilibrium, but in the lab, we can show this to be true at equilibrium in the apparatus shown below. (This is a generic picture, and any setup in the lab will likely differ in some details.) Most simple liquids can be distilled just like described for water, but invariably in an apparatus like the one shown below. In a common laboratory distillation, there is a heat source, here a heating bath (14) on a hotplate (13), a January 19, 2024 Revision PRH Page 22 distillation flask (2) holding the liquid being distilled, a thermometer1 (4), a condenser (5) that connects (via 6 & 7) to a stream of cold water to condense the vapor back into the liquid state, and a flask (8) to collect the distillate. For a simple distillation, the distillation column (3) is very short and serves only to connect the other pieces of the distillation apparatus. Under these conditions, most of the vapor that is formed reaches the condenser and is quickly made into a liquid again. If a pure liquid is being distilled under ideal conditions, the thermometer would read the same temperature for the entire active phase of the distillation. However, if a mixture is being distilled that is difficult to separate, the temperature would change gradually as the distillation proceeds. The reason a simple distillation can't separate liquids with similar boiling points is that each liquid has a significant vapor pressure at every temperature, and the resulting mixture of vapors is what condenses. Early in the distillation, the lower-boiling compound makes up a higher percentage of the distillate relative to what is being distilled. Consequently, the percent of that compound decreases during the distillation, and, as it does, the temperature of the vapor increases as the distillation continues. Common Laboratory Distillation Setup https://en.wikipedia.org/wiki/Distillation To separate compounds with boiling points that differ by >25 oC a fractional distillation is used, but only if a good apparatus and sufficient care are taken. The apparatus is the same, except that the distillation column (3) is much longer and is filled with glass or ceramic beads or other shapes that slow down the vapor as it rises towards the thermometer. The vapor condenses frequently as it tries to move up the column and, as it tries to run back down into the distillation flask, the hot vapor that is trying to rise through the column converts part of it back into vapor again. This results in multiple distillation cycles as the vapor tries to rise up the column. Each time a condensation- revaporization happens, the lower boiling liquid makes up a higher and higher percent of the vapor. Done efficiently through a good column, the vapor that makes it to the thermometer early in the distillation and on to the condenser is virtually pure with only traces of the higher-boiling compound. When the amount of the lower-boiling compound gets very low in a fractional distillation, a transition occurs, and the temperature rises to the boiling point of the higher-boiling compound. During this transition, the material being distilled is a mixture of the two compounds. The better the distillation, the smaller this mixed fraction is. After the last traces of the lower-boiling compound have been removed, the higher-boiling compound distills out at a constant temperature and is relatively pure. Example of a Common Distillation Lab In this lab you would do both a simple and a fractional distillation of a mixture of toluene (boiling point 111 oC) and hexane (BP 68 oC). How well you would do depends on a compromise between heating fast enough to get enough vapor to the thermometer to register a good temperature, and heating slow enough to see a reasonable separation. INTRODUCTION (Example Lab) Distillation can be used as a method for purifying what is mostly a single liquid, as a means of separating a liquid from dissolved solids, to recover those solids, or to separate a mixture of liquids. The starting liquid is heated and, when it boils, the vapors are condensed into a separate receiver. The resultant liquid (distillate) is collected in one or more fractions. Purification of a Single Liquid A single liquid will begin to boil when its vapor pressure is equal to the vapor pressure within the distillation flask, most commonly atmospheric pressure. For a pure liquid, the boiling temperature 1 The thermometer is placed too high in this picture. January 19, 2024 Revision PRH Page 23 should remain constant (within 2 oC) for the duration of the distillation, as long as a steady, minimal rate of distillation is achieved. Separation of a Liquid from a Non-volatile Component When circumstances allow a liquid to be removed quickly, either because it is a solvent being removed from a desired, non-volatile product, or it is the desired component being removed from non-volatile, undesired material, then a rapid distillation may be done. This can be done with a simple distillation setup, or faster with a rotary evaporator, which you will see in the lab and will eventually see in use. Fractional Distillation When a liquid mixture of two volatile components is distilled, boiling will begin when the sum of the two vapor pressures of the solution is equal to the atmospheric pressure. This is assuming "ideal behavior", which is not always the case. These partial pressures are dependent upon the vapor pressure of the pure liquid and its mole fraction in the solution. If the boiling points of two liquids differ significantly (by around 100 oC or more), good separation can be obtained by simple distillation, as the partial pressures of the two liquids will be very different. However, if the boiling points are closer to each other, one cannot obtain good separation by simple distillation. A different method must be employed to increase the efficiency of the distillation. This is accomplished by the use of a fractionating column in order to understand the behavior of two liquids as they are distilled using a fractionating column, you must understand the liquid/vapor composition curves in Pavia on pages 723, 730, 732, 742 and 743. You need to read and understand Techniques 14 and 15 to understand distillations. Sections 14.2, 14.3, and 15.1-15.6 should be particularly helpful. The column packing, often put into the same piece of glassware that is used as a large-tube condenser (must be wide enough for the packing material), provides a surface for multiple vaporizations and condensations to occur. In order for the column to function successfully, a temperature gradient must be maintained in steady state along its length (i.e.: the bottom of the column must be hotter than the top). This can be accomplished by heating the boiling flask slowly, and also by insulating the column with glass wool or cotton secured with aluminum foil. The liquid must move up the column slowly, thus ensuring a near equilibrium of vapor and liquid all along the column. As the difference in the BP of the two components increase, or as a better fractioning column is used, the efficiency of the separation increases. For very small differences in boiling point (on the order of 1 oC!), there is high- efficient (expensive) equipment. If one performs a fractional distillation at good efficiency, a plot of temperature vs. volume of the distillate would resemble the figure. Steam Distillation Sometimes relatively a small amount of a liquid that is mostly insoluble in water needs to be distilled, but the modest vapor pressure and amount of material make regular distillations problematic. In that case, a steam distillation might be done, where a stream of steam is generated and used to both heat a desired liquid and to carry the vapor over into a collection flask, where the two form separate phases. An example would be the steam distillation of 1-octanol, an alcohol with a boiling point of 195 oC. Smaller amounts of this compound can be steam distilled at the boiling point of water, 100 oC. The vapor of the water carries over the small amount of 1-octanol vapor, so a much larger amount of water needs to be distilled than the amount of 1-octanol. Another example of steam distillation is in the isolation of the terpene limonene from orange peel. The limonene is only isolated in limited amounts and has a BP of 176 oC, making steam distillation a good method to purify it from the non-volatile material components of the orange. See Pavia for more technical details on distillation. ADDITIONAL EXPERIMENTAL EXAMPLE 1) Instructions for a Simple Distillation To set up your apparatus, follow Figure 14.1 in Pavia on page 723 with the following modifications. Use a ring stand to secure both a 100 mL round-bottomed flask and a heating mantle, which is attached to a voltage regulator. January 19, 2024 Revision PRH Page 24 Start with the regulator set at ~50 and lower the setting to ~30 immediately when you observe the liquid start to boil. Use a 10 mL graduated cylinder to collect your distillate so you can easily determine the amount collected. Have a beaker handy to empty the graduated cylinder as it fills up, as you will be collecting more than 10 mL of distillate. Describe your apparatus in your notebook. When you have finished your setup, obtain approximately 60 mL of the methanol-water mixture assigned to you and pour it into the round-bottomed flask. Add one or two carborundum "boiling" stones. Make sure that all of the ground glass joints are securely fitted and that the water is flowing slowly through your condenser properly (in the bottom, out the top). Have your instructor check your apparatus before you begin the distillation. The mantle regulator should be set to maintain boiling in the flask at a steady rate. The distillate should come out at a rate of 1 drop every one to two seconds. Record the boiling point for each mL of distillate in your notebook. 2) Instructions for a Fractional Distillation The same basic set-up is used, except that a fractionating column (filled with the material supplied: glass beads, porcelain saddles, etc.) should be placed between the boiling flask and the distillation head (see Pavia p. 735). Do not run water through your fractionating column, just through the condenser. Use glass or cotton wool to insulate the column and secure the insulation with aluminum foil. Again, describe your apparatus in your notebook. Carry out the distillation on a fresh 60 mL sample of the methanol-water mixture using the fractional distillation set- up. As soon as boiling starts, turn down the regulator to keep the liquid boiling slowly. As you heat, a ring of condensate will rise slowly in the column. (You may peak briefly behind the insulation every now and then to follow it.) This rise of the front edge of the refluxing liquid should be gradual to allow for equilibration within the column. The ring of condensate should take at least several minutes to reach the top of the column. Once again, distill at a rate of 1 drop of distillate every 1-2 seconds. Once distillation actually begins, maintain a constant rate by slowly increasing the heat by adjusting the regulator as required. Record the distillation temperature for each mL of distillate in your notebook. If possible, make more frequent readings when the temperature starts to rise quickly. This rise may be preceded by a drop in temperature if your distillation rate is relatively slow. You may stop distilling after a second steady temperature is reached for 4-5 consecutive mL of distillate. Questions and problems to do on your own for understanding (not to be turned in for grading) before you take the Blackboard Exam 1. Discuss the results that might be obtained for the 2 distillations. Include a discussion of possible observations during the distillations and also a discussion of the theoretical aspects of simple vs. fractional distillation. 2. If a fractional distillation shows no better separation than a simple distillation, give possible reasons for this result. 3. What are possible sources of error and loss of material in distillations? 4. How would you use a distillation graph like the one given above to estimate the volumes of methanol and water in the original mixture. Comment on the accuracy of such a determination. Assigned Questions: Answer Questions 1, 2, 6 and 9 in Pavia on pp 750-751, as well as the following two questions. 1. Why is the ability to separate two liquids by fractional distillation drastically reduced if heat is applied too rapidly to the distillation flask? 2. The entire bulb of the thermometer placed at the head of a distillation apparatus should be just below the entrance to the condenser. Explain the effects on the temperature recorded if the thermometer were placed (a) well below the exit to the condenser and (b) above the exit. Test During the scheduled period for your section, go to the Blackboard page for your lab section and take the exam to demonstrate your learning. January 19, 2024 Revision PRH Page 25 Day 5/6 (in-person), Molecular Modelling Reference: Please consult the relevant sections in your lecture textbook. Pay close attention to the definitions of torsional strain, steric strain, angle strain, and 1,3-diaxial interactions. THIS LAB WILL BE DONE INDIVIDUALLY INTRODUCTION The three-dimensional shapes of molecules can be visualized best with the aid of molecular models. This is usually done with physical model kits with solid pieces that represent different types of atoms or molecules, or but can also be done virtually with computer software that represents the 3D shapes of molecules in a way that allow them to be rotated on the screen. In this exercise, molecular model kits will be used to build the 3D structures of molecules and to illustrate the conformations of acyclic compounds (no rings) as well as various derivatives of cyclohexane (6-member ring). Proteus® molecular modeling kits will be used for this exercise. These model kits have specific pieces that represent sp3, sp2, or sp hybridized atoms. They are designed to approximate the bond angles within molecules and to give some indication of spatial relationships between different groups on a molecular structure. Black colored pieces by convention represent carbon atoms, red pieces represent oxygen, and the blue pieces represent nitrogen. In addition, there are other colored extensions that are used as framework pieces representing atoms such as hydrogen (usually white), fluorine, chlorine, and iodine. A unique feature of these Proteus® model kits is that each of the bonding arms has a locking mechanism that allows you to manipulate and change the conformations of the models that you build without them coming apart. This is especially important when working with the chair conformations of cyclohexane derivatives, allowing you to see the impact that a simple conformational change (ring flip) can have on the relationship between substituents on the cyclohexane ring. DEFINITIONS It will be helpful here to define a few terms related to symmetry since you will have to examine the symmetry properties of the molecular models that you construct. A Plane of Symmetry divides a geometric figure or molecule into two equal halves so that one half of the shape is the mirror image of the other. A plane of symmetry is also referred to as a mirror plane, and examples of planes of symmetry of a variety of geometric shapes and molecules are illustrated below. (Do not confuse a mirror plane that bisects a molecule from the mirror image of a molecule.) Note that a molecule may have one or many planes of symmetry. January 19, 2024 Revision PRH Page 26 A Center of Symmetry is a point through which each atom in a three- dimensional structure can be reflected so that all atoms of that structure have identical counterparts equidistant from the point of symmetry on a line through the initial point and extending through the center of symmetry. For example, each point on the surface of a sphere has a point on the opposite side equidistant from its center, which is the center of symmetry. See the images of SF6 and CF4 for chemistry examples. Although both compounds have a lot of symmetry elements, only SF6 has a center of symmetry. An Axis of Symmetry is a line around which a three-dimensional structure can be rotated so the molecule will appear identical at a rotation of less than 360o. A 2-fold axis would require a rotation of 180o, 3-fold axis a rotation of 120o (see image of NH3), a 4-fold axis 90o, etc. For example, an equilateral triangle and a trigonal planar molecule such as boron trifluoride both have a threefold axis of symmetry. Alternatively, what this means is that rotation of a molecule with an x-fold axis of symmetry through an angle equal to 360/x degrees will result in what appears to be an identical structure. For example, a pentagon with a five-fold axis of symmetry will appear to be exactly the same whenever it is rotated through 360/5 = 72°. A hexagonal molecule like benzene has a six-fold axis of symmetry and appears to be identical when rotated through 60°. Note that geometric shapes and molecules can have several different types of symmetry at the same time. For example, an equilateral triangle has three two-fold axes of symmetry that are the lines that bisect each of the three angles, plus a three-fold axis of symmetry that goes through the middle perpendicular to the plane of the triangle, three mirror planes that are perpendicular to the plane of the triangle and bisect each of the three angles, and a trivia

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