Types of Reactions in Organic Chemistry PDF

**[Types of Reactions in Organic Chemistry]** **[Substitution Reactions]** [Halogenation of Alkanes] \*\*Definition: A substitution reaction is a chemical reaction in which an atom or a group of atoms in a molecule is replaced by another atom or group at atoms.\*\* Alkanes undergo substitutions...

**[Types of Reactions in Organic Chemistry]** **[Substitution Reactions]** [Halogenation of Alkanes] \*\*Definition: A substitution reaction is a chemical reaction in which an atom or a group of atoms in a molecule is replaced by another atom or group at atoms.\*\* Alkanes undergo substitutions in their reactions with halogens in the presence of ultraviolet light. CH~4~ + Cl~2~ + CH~3~Cl + HCl\ - One of the hydrogen atoms in the methane has been replaced by a chlorine atom to form the new compound chloromethane.\ - If excess chlorine is used, further substitutions take place to form dichloromethane(CH~2~Cl~2~), trichloromethane (CHCl~3~), and eventually tetrachloromethane (CCl~4~) [Mechanism of Monochlorination of Methane] \*\*Definition: The mechanism of a reaction is the detailed step-by-step description of how the overall reaction occurs.\*\* This mechanism is known as free-radical substation. **Initiation\ **A chlorine molecule is broken down into two chlorine atoms in the presence of ultraviolet light **Propagation\ **A chlorine atom attacks a methane molecule to form hydrogen chloride and a species called methyl free radical **Propagation\ **A methyl free radical attacks a chlorine molecule to form chloromethane and a chlorine atom The chlorine atom produced in step 2 can then react with another molecule of methane. This sets up a chain reaction. \*\*Definition: A chain reaction is a reaction that continues on and on because a product from one step of the reaction is a reactant for another step of the reaction.\*\* **Termination\ **When most of the reactants have been used up during Steps 2 and 3, there are only a small number of Chlorine radicals and methyl radicals left over. These combine to form Cl~2~, chloromethane and ethane **Evidence** **Explanation** ------------------------------------------------------------------------------------- ----------------------------------------------------------------------------------- Reaction takes place when exposed to UV light Suggests a free-radical mechanism whereby Cl~2~ is broken down into Cl radicals. For every photon of light absorbed, thousands of chloromethane molecules are formed Chain reaction is taking place Ethane is found in the products Two methyl free radicals must be combining to form ethane Free radicals speed up the reaction Only a reaction which uses free radicals would be sped up by adding free radicals Inhibitors such as oxygen slow down the reaction Shows a chain reaction is taking place [Esterification\ ]When a carboxylic acid is reacted with an alcohol, with H~2~SO~4~ as a catalyst, an ester is formed. This reaction is referred to as saponification and is used in the manufacture of soap. **[Mandatory Experiment: To prepare a sample of soap]** Diagram of a chemistry experiment Description automatically generated**[(1)]** **[(2)]** 1. Add 2.5 g of lard, 2.5 g of potassium hydroxide and 20 cm^3^ ethanol, along with a few anti-bumping granules, to the flask. Swirl to allow proper mixing. 2. Set up the reflux apparatus, making sure to grease all joints 3. Reflux the mixture for 20 minutes, using a water bath 4. Remove the ethanol by distillation. 5. Dissolve the residue in a minimum of hot water (approximately 15 cm^3^). 6. Add this solution to the brine. The soap should precipitate out. 7. Filter the soap. 8. Test the soap for its lathering qualities by shaking a small sample of it with water. It is important to wash your hands afterwards to remove any potassium hydroxide still present. **[Addition Reactions]** \*\*Definition: An addition reaction is one in which two or more molecules react together to form a single molecule.\*\* \- Always will occur on a double or triple bond\ - Geometry will change from planar to tetrahedral bonds\ - Used in the hydrogenation of vegetable oils\ - Use to form plastics [Hydrogenation of Vegetable oils] **Polarisation\ **The C=C double bond in ethane has a high concentration of negative charge. As the Br~2~ approaches the ethene, the electrons are repelled away from the ethene, polarising the Br~2~ **Heterolytic fission\ **The Br~2~ molecule splits into Br^+^ and Br^-^ ions **Carbonium ion formation\ **The Br^+^ attacks the electron rich C=C double bond. This forms a carbonium ion **Ionic Addition\ **The carbonium ion is then attacked by the Br^-^ ion. This results in the formation of 1,2-dibromethane [\ Evidence for this mechanism] If this reaction is carried out in bromine water with Chlorine ions added, the normal 1,2-dibromoethane product is formed, but two other products form also; 1-bromo-2-chloroethane and 2-bromoethanol. These form because of the Cl^-^ or the OH^-^ [Polymerisation Reactions] \*\*Definition: Polymers are long chain molecules made by joining together many small molecules.\*\* Polymers are repeating structures consisting of thousands of monomers linked together.\ - (Poly)chloroetheane, PVC, is a common plastic \*\*Definition: The repeating unit of a polymer is that part of the polymer whose repetition produces the complete polymer chain except for the end groups.\*\* Polyvinyl chloride can be synthesised from ethene **[Elimination Reactions]** \*\*An Elimination reaction is one in which a small molecule is removed from a larger molecule to leave a double bond in the larger molecule.\*\* Ethene can be produced from ethanol by means of an elimination reaction. It involves the removal of a molecule of water from a molecule of ethanol by passing the ethanol through aluminium oxide. **[Redox Reactions]** When a primary alcohol reacts with an oxidising agent, the primary alcohol is converted to an aldehyde. Two hydrogen atoms are removed from the primary alcohol If a secondary alcohol reacts with acidified sodium dichromate, a ketone is formed Aldehydes and ketones can be reduced back to alcohols in the presence of hydrogen and a nickel catalyst **[Mandatory Experiment: To study the reactions of ethanal with (i) acidified potassium permanganate solution, (ii) Fehling's reagent and (iii) ammoniacal silver titrate]** [ **Oxidation by acidified potassium manganate(VII)**] 1. **Using graduated disposable pipettes, place in a test-tube 2 cm^3^ of ethanal, 1 cm^3^of potassium manganate(VII) solution and 4 cm^3^ of dilute sulfuric acid.** 2. **Warm the test-tube in a water-bath and shake gently. Observe and record any colour change in a table of data copied into your practical report book** **[Oxidation by Fehling\'s solution ]** 1. **Using a graduated disposable pipette, place in a test-tube 1 cm^3^ of Fehling\'s solution no. 1.** 2. **Using a separate pipette, add 1 cm^3^ of Fehling\'s solution no. 2 -- swirl the contents so that the blue precipitate initially formed will dissolve.** 3. **Add 1 cm^3^ of ethanal, heat gently and shake.** 4. **Observe any change and record your observations.** **No red precipitate is observed. Ketones are not oxidised by Fehling's solution** **[Oxidation by Tollen\'s reagent ]** 1. **Using a graduated disposable pipette, place in a clean test-tube 3 cm^3^ of silver nitrate solution and 1 cm^3^ of sodium hydroxide solution.** 2. **Add aqueous ammonia solution drop by drop, with shaking, until the precipitate formed in stage 1 is just dissolved.** 3. **Add two or three drops of ethanal, mix by shaking and warm in a water bath.** 4. **Observe any change and record your observations,** 5. **Rinse out the test-tube with dilute nitric acid and then water.** 1. Test with magnesium: Coil the 5 cm clean strip of magnesium loosely, drop it into one of the test tubes, and swirl. Record your observations.\ Fizzing is observed in the test tube, a mild pop is heard. 2. Test with sodium carbonate Add 1 g of anhydrous sodium carbonate powder into the second test tube, and swirl. Record your observations.\ The lighted taper is extinguished and limewater turns a milky white colour 3. Esterification (i) With care add 2 drops of concentrated sulfuric acid to the third test tube. [Synthesis of PVC from Ethene\ ]Step 1: [ ] **Ethene and chlorine react to form 1,2-dichloroethane.\ ** **Step 2: Heat is used to thermally crack the 1,2-dichloroethane into chloroethene and HCl.\ ** **Step 3: The chloroethene undergoes a polymerisation reaction to form polychloroethene (PVC)** [ ] **[Chromatography and Instrumentation in Organic Chemistry]** [Mass Spectrometry ] \*\* Definition - The Principle of Mass Spectrometry is that charged particles moving in a magnetic field are deflected by different amounts due to their masses. This separates the particles according to their masses.\*\* There are five stages in Mass Spectrometry A diagram of a magnetic field Description automatically generated with low confidence(3) 1\. Vaporisation: The sample material is vaporised into a gas. 2\. Ionisation: An electron gun fires high-energy electrons at the gaseous sample. This knocks electrons off the sample particles, leaving the sample as a group of positively charged ions. 3\. Acceleration: A negatively charged plate attracts the positive ions. This accelerates the particles so that they travel at high speed through the spectrometer. 4\. Separation: A magnetic field of a particular strength is used to deflect the particles. Particles that are too light are deflected too much and hit the side of the spectrometer. Particles that are too heavy are not deflected enough and hit the side of the spectrometer. Only particles that have a certain mass are deflected by just the right amount and make it through the spectrometer to the detector. 5\. Detection: A detector senses the number of positive ions hitting it and displays the result on a mass spectrum. [Mass Spectrum Output ] The peaks shown in the mass spectrum have units of mass on the horizontal axis. The height of each peak represents the relative abundance of particles of that mass. We can see that the peak at 57 is roughly four times higher than the peak at 86. This means that there are 4 times as many particles of mass 57 as there are of mass 86. Uses of Mass Spectrometry -Drug testing and discovery\ -Food contamination detection\ -Pesticide residue analysis\ -Isotope ratio determination\ -Protein identification\ -Carbon dating. **[Chromatography]** \*\* Definition - Chromatography is a separation technique in which a mobile phase carrying a mixture moves in contact with a selectively adsorbent stationary phase.\*\* Types of Chromatography Paper Chromatography -- Stationary Phase -- Chromatography Paper Mobile Phase -- Water or Solvent A diagram of a paper chromatography Description automatically generated(4) 1. Spot of sample mixture is placed approx. 1 cm above the water line. 2. The mobile phase soaks up through the stationary phase and dissolves the materials in the mixture. 3. Each material dissolves to a different extent. 4. Less soluble materials will appear as a spot closer to the bottom of the paper. 5. More soluble materials will appear as a spot higher up the paper. 6. This separates the components in the mixture. Uses of Paper Chromatography Analysis of food colourings, dyes and indicators. **[Other Types of Chromatography ]** Column Chromatography Stationary Phase -- Silica Gel in Glass Tube Mobile Phase -- Water, Ethanol, or other solvent Passing a solvent through a column is called elution. The solvent is known as the eluent. Uses: Separation of dyes in food colouring **[Gas Chromatography]** Definition - The Principle of Gas Chromatography is that a mixture of components is carried by a gaseous mobile phase is separated based on their different interactions with a solid stationary phase and the gaseous mobile phase. (5) Stationary Phase -- Coiled column filled with coated silica gel Mobile Phase -- Inert Carrier Gas Uses -Measuring Alcohol levels in Urine Samples\ -Drug Testing in Athletes **[GC-MS]** Gas Chromatography is often used with Mass Spectroscopy. After separation by GC, each component is put through a Mass Spectrometer to identify each component. High Performance Liquid Chromatography (HPLC) The pump supplies the high pressure needed for the mobile phase. The reason the mobile phase needs high pressure is that the column is packed very tightly, and the elution would take too long if the pump wasn't used.\ (6) Stationary Phase: Coated Silica Gel Mobile Phase: Suitable liquid solvent (ethanol) under high pressure Uses -Testing for Growth Hormones in food\ -Measuring quantity of Caffeine in drinks **[Spectroscopy]** [Infra-red Spectroscopy (IR)] Organic compounds absorb infrared radiation. Different frequencies of IR light are absorbed by different types of bonds. So a C=O would absorb a different frequency of IR then an O-H or a C=C bond. By looking at how an organic compound absorbs IR light, we get an IR spectrum, which is used as a fingerprint for that compound.  Uses -Identifying Plastics\ -Identifying Illegal Drugs\ -Used to analyse breathalyser results [Ultraviolet Spectroscopy (UV)] This instrument measures how a compound absorbs UV light of different wavelengths/frequencies. Like IR spectroscopy, this also results in a fingerprint for each compound. However, unlike IR, UV spectroscopy can also measure the concentration of the compound, making this a quantitative analysis. UV spectroscopy is commonly used with HPLC in order to identify each compound in a mixture, and their concentrations. **[Mandatory Experiment: To separate rhe components of ink using paper chromatography]** 1. Add solvent to the bottom of the tank to a depth of about 10 mm. Cover the tank, and allow to stand for a few hours, to allow the tank to become saturated with solvent vapour. 2. Make a line with a pencil near the top of a rectangular sheet of chromatography paper, and another line about 3 cm from the bottom. 3. Place a small spot of each indicator and of the mixture of indicators at different points on the line near the bottom of the paper, using a capillary tube. Dry using a hair drier, and repeat. 4. Place the chromatogram in the tank, ensuring that the solvent level in the tank is below the line on which the indicator samples are spotted. Run the chromatogram, until the solvent reaches the line near the top of the paper. 5. Remove and dry. 6. Calculate and record the R*~f~* values of each indicator. (Ammonia vapour is used to locate phenolphthalein -- the paper is held in vapour coming from a boiling solution of dilute ammonia.) 7. Identify the indicators present in the mixture. **[Exam Questions]** [2014 -- HL -- Section A -- Question 2] A student prepared a sample of soap in the school laboratory. The experiment was carried out in the four stages illustrated on the previous page. At Stage 1, using a water bath, the student refluxed for approximately 20 minutes 4.45 g of glyceryl tristearate (an animal fat) together with an excess of sodium hydroxide pellets, anti-bumping material and about 30 cm^3^ of ethanol. The reaction shown in the following balanced equation took place.\ The apparatus was then allowed to cool and rearranged for Stage 2, distillation, again using a water bath. After distillation, the contents of the distillation flask were decanted or washed into a beaker containing brine -- Stage 3. Filtration was used in Stage 4 to isolate the soap which was then thoroughly washed.\ (a) What is the purpose of refluxing in Stage 1 of the preparation? To allow time for reaction without losing volatile material\ Name the type of reaction that occurred during this stage. Saponification\ (b) What substance was removed by distillation in Stage 2? Ethanol\ (c) Explain the function of the brine in Stage 3. To precipitate the soap\ (d) Why was it necessary to wash the soap thoroughly in Stage 4? To remove sodium hydroxide\ How should the student have washed the soap? Brine\ (e) Draw the structure or give the name of the co-product of the reaction. Propane-1,2,3-triol\ Where was the co-product located at the end of the process? In the brine\ (f) Given that the sodium hydroxide was in excess, calculate the maximum yield in grams of soap that could have been obtained in this preparation. 4.45/890 = 0.005 mol of fat\ 0.005 x 3 = 0.015 mol of soap\ 0.015 x 306 = 4.59g\ (g) Suggest, with reference to its structure, how a soap like sodium stearate can dissolve both the non-polar oils and the ionic salts in sweat from the skin\ - C~17~H~35~ is non-polar and dissolves oils.\ --COO^--^Na^+^ attracted to salts in sweat. [2013 -- HL -- Section A -- Question 2].In a practical examination, chemistry students were required to perform a number of tasks in a laboratory. They had access to all the necessary reagents and glassware and also to the required safety equipment and clothing.\ (a) How could a student have carried out a simple chemical test to confirm that a colourless liquid sample was ethanoic acid and not ethanol? Ethanoic Acid: Addition of acidified potassium permanganate will not give a colour change\ (b) A sample of ethene gas was supplied in a stoppered test tube. Describe fully how the gas could have been shown to be unsaturated.\ - Add a bromine solution to a test tube of ethene.\ - Colour change: Brown to colourless\ (c) Describe with the aid of a labelled diagram how a student could have used chromatography to separate a mixture of indicators.\ - Paper chromatography.\ - Apply mixture using dropper spot on paper slightly above eluent in beaker\ - Solvent moves up separating components\ (d) One of the tasks in the practical examination was to measure the melting points of two benzoic acid samples (A and B) and to use the results to determine which was the purer sample. The melting points obtained by one of the students were as follows: sample A = 117 -- 120 °C; sample B = 120 -- 121 °C.\ Which was the purer sample? A\ Justify your answer. Has a wider range\ The students were required to recrystallize the impure benzoic acid. What solvent should they have used for the recrystallization? Water\ Explain why this solvent is suitable. Very soluble in hot but slightly soluble in cold\ (e) The diagram shows a steam distillation apparatus assembled incorrectly by one of the students. Identify the flaw in the assembly and state how it should have been rectified.\ Have end of safety tube under water **[References]** 1. Instituteforeducation.com 2. Scoilnet.ie 3. Chemguide.com 4. BYJU's.com 5. Researchgate.net 6. Chemguide.com