Chemistry for Health Sciences II Lectures 15-16 PDF

Chemistry for Health Sciences II Two credits Term 3, 2024 King Saud bin Abdulaziz University for Health Sciences Lectures 15 – 16 At the end of these lectures, students should be able to: Write IUPAC names for aldehydes and ketones Explain the physical properties (boiling point & solubility) of the...

Chemistry for Health Sciences II Two credits Term 3, 2024 King Saud bin Abdulaziz University for Health Sciences Lectures 15 – 16 At the end of these lectures, students should be able to: Write IUPAC names for aldehydes and ketones Explain the physical properties (boiling point & solubility) of the above functional groups. Recognize and write the reactions to prepare aldehydes or ketones (i.e. oxidation of primary or secondary alcohols) Recognize and write the reactions of aldehydes or ketones (i.e. oxidation & reduction reactions, addition reactions hemiacetals and acetals). Differentiate between Tollen's test and Benedict's test. Structures and Physical Properties Structure and Physical Properties Aldehydes and ketones are polar compounds The carbonyl group is polar – The oxygen end is electronegative Aldehydes and ketones do not form Hydrogen bonding with each other. They have weak intermolecular dipole-dipole forces working between molecules. Hydrogen Bonding in Carbonyls Aldehydes and ketones cannot form intermolecular hydrogen bonds However, water can form hydrogen bonds to them Physical Properties Carbonyls boil at Higher temperatures than: – Hydrocarbons – Ethers Lower temperatures than: – Alcohols Nomenclature and Common Names Naming Aldehydes Locate the parent compound – Longest continuous carbon chain – Must contain the carbonyl group Replace the final –e of the parent with –al Number the chain with the carbonyl carbon as 1 Number and name all substituents Naming Aldehydes For substituted aldehydes, we number the chain starting with the aldehyde carbon. What is the name of this molecule? 1. 2. 3. 4. Parent chain – 5 carbons = pentane Change suffix – pentanal Number from carbonyl end – L to R Number / name substituents – 2-methyl 5 4 3 2 is 2-methylpentanal To S 1 Nomenclature of Aldehydes Aldehyde group has priority over the multiple bond hydroxyl, alkyl, and halogens group. For cyclic aldehydes, the suffix –carbaldehye is used. Aromatic aldehydes often have common names: IUPAC and Common Names With Formulas for Several Aldehydes 1 Ketones Simplest ketone MUST have 3 carbon atoms so that the carbonyl group is interior Base name: longest chain with the C=O Replace the –e of alkane name with –one Indicate position of C=O by number on chain so that C=O has lowest possible number Ketone group has priority over the multiple bond, hydroxyl, alkyl, and halogens group. 4-methyl-2-heptanone Nomenclature of Cycloketones Common names of ketones are formed by adding the word ketone to the names of the alkyl or aryl groups 12 Problems CHCHCHCH CHCHCH H É CH 2 butanone none but 2 13 Examples: Write the correct names for the following: a. 3-Methylbutanal (no number is necessary for the aldehyde function) b. 2-Butenal (the number locates the double bond between C-2 and C-3) c. Cyclobutanone d. 3-Heptanone 14 Important Aldehydes and Ketones Methanal (formaldehyde) - gas used in aqueous solutions as formalin to preserve tissue Ethanal (acetaldehyde) - produced from ethanol in the liver causing hangover symptoms Propanone (Acetone) - simplest possible ketone – Miscible with water – Flammable – Both acetone and methyl ethyl ketone (butanone) are very versatile solvents Preparation of Aldehydes and Ketones Preparation of aldehydes and ketones Principal means of preparation is oxidation of the corresponding alcohol – Primary alcohol produces an aldehyde – Secondary alcohol produces a ketone – Tertiary alcohol does not oxidize This oxidation process removes two hydrogens 1° Alcohol Oxidation An aldehyde is formed 0 on 2° Alcohol Oxidation A ketone is formed KMnOn Oli Heron The oxidizing agents: (KMnO4/OH- or H2CrO4 or PCC or CrO3 /H2SO4 {Jone’s reagent}) PCC Cro H So 3° Alcohol: No oxidation 3o alcohols cannot undergo oxidation: Examples OH CrO3 H+, acetone (Jones' reagent) cyclopentanol O cyclopentanone CH3 CH3 O PCC CH3 OH 5-methyl-1-hexanol {C5H5NH+ ClCrO3–} CH3 H 5-methylhexanal c Reactions of Aldehydes and Ketones 1. Oxidation and Reduction a. Aldehydes: oxidized to carboxylic acids b. Ketones: no further oxidation c. Aldehydes and ketones are reduced to alcohols: aldehydes to primary alcohols and ketones to secondary alcohols 2. Addition a. Alcohols to give hemiacetals and acetals b. Reaction with nitrogen compounds Oxidation of Aldehydes Aldehydes are easily oxidized to carboxylic acids by almost any oxidizing agent – So easily oxidized that it is often difficult to prepare them as they continue on to carboxylic acids Visually Distinguishing Aldehydes From Ketones Visual tests for the aldehyde functional group based on its easy oxidation are: Tollen’s Test – Silver ion is reduced to silver metal – Use a basic solution of Ag(NH3)2+ – The silver metal precipitates and coats the container producing a smooth silver mirror c 19 Tollen’s Test Benedict’s Test Benedict’s Test – Reagent is a buffered aqueous solution of Cu(OH)2 and sodium citrate – Reacts with aldehydes, but not with ketones – Cu2+ is reduced to Cu+ Solution of Cu2+ is a distinctive blue color Color fades during the reaction as Cu+ precipitates as the red solid, copper(I) oxide, Cu2O - No reaction with ketone Cu2+ solution 𝐶𝑢 (aq) + Cu2O precipitate Aldehyde → 𝐶𝑢 𝑂 𝑠 + 𝐶𝑎𝑟𝑏𝑜𝑥𝑦𝑙𝑎𝑡𝑒 𝑎𝑛𝑖𝑜𝑛(aq) Benedict’s Test on Glucose Benedict’s Test Reduction of Aldehydes & Ketones Both aldehydes and ketones are readily reduced to alcohols – Reduction occurs with hydrogen as the reducing agent Classical reaction is hydrogenation – React with hydrogen gas – Requires a catalyst – Ni, Pt, Pd H IH revisit 0 Niored Reduction of Aldehydes and Ketones  Aldehydes and ketones are reduced to alcohols.  The most commonly used reducing NaBHa reagents are sodium borohydride (NaBH4), lithium aluminum hydride (LiAlH4)., and their derivatives. H 0080 Na H B H H no Sodium borohydride H 0080 o Li H Al H H Lithium aluminum hydride no Reduction of Aldehydes and Ketones Examples: µ Aqueous acid (H+, H2O) is added afterwards to protonate the alkoxide ion. 2 A Reduction of Aldehydes and Ketones c co c 4h my a 30 Addition Reactions Principal reaction is the addition reaction across the polar C=O double bond – Very similar to the addition hydrogenation of alkenes – Requires catalytic acid in the solution Product of the reaction is a hemiacetal – Hemiacetals are quite reactive – Undergo a substitution reaction with the –OH group of the hemiacetal is exchanged for another –OR group from the alcohol – Reaction product is an acetal – This reaction is reversible Hemiacetal and Acetal Formation or HemiKetal Generalized structure of a hemiacetal/ketal or Ketal Generalized structure of an acetal/ketal Hemiacetal and Acetal Formation Product of the addition reaction is a hemiacetal followed by formation of an acetal OH Aldehyde Alcohol catast R o Acid Acetal R R OR OR R R HemiKetal and Ketal Formation Retone OH R OR R HemiKetal OR OR R R HemiKetal Ketal Keto-Enol Tautomerism Aldehydes and ketones may exist as an equilibrium mixture of 2 forms (known as tautomers), called Keto form and the enol form. These two forms are isomers and differ in the location of: – – – – A hydrogen atom A double bond This type of isomerism is called Tautomerism. The keto form has a C=O while the less stable enol form has a C=C. The keto form is usually the most stable -H -C Examples Write formulas for the keto and enol forms of acetone. Problem. Draw the structural formulas for the enol form of A. Acetaldehyde B. Cyclohexanone A. -H -C 400 O H H C H µ C H Acetaldehyde O H C C H H enol form H H O O H B. H Qµ H Cyclohexanone enol form Most simple aldehydes and ketones exist mainly in the keto form. Alkyl hydrogen atoms bonded to a carbon atom in a (alpha) position relative to Keto or enol group. Because of the acidity of α hydrogens carbonyls undergo keto-enol tautomerism. H O O H H enol form of phenol wie eius keto form of phenol co ** Aromaticity can effect stability because of the resonance, When considering the below molecules, the aromatic molecule phenol (enol from) is more stable and favors than keto form. Carbonyl compounds that do not have an -H cannot form enols and exist only in the keto form. O H C O C O C H benzaldehyde benzophenone H formaldehyde Summary of Reaction Equations or HemiKetal or Ketal