Org Unit 6-7.pdf

UNIT 6 CARBONYL REACTIONS Carbonyl Reaction  Addition  Addition – Elimination  Enolization-ketolization 1  6.1. Carbonyl addition Oxygen is more electronegative than carbon. Therefore the electrons in the C=O bo...

UNIT 6 CARBONYL REACTIONS Carbonyl Reaction  Addition  Addition – Elimination  Enolization-ketolization 1  6.1. Carbonyl addition Oxygen is more electronegative than carbon. Therefore the electrons in the C=O bond are attracted to oxygen , producing a highly polarized bond. Oxygen is more electronegative than carbon. Therefore the electrons in the C=O bond are attracted to oxygen, producing a highly polarized bond.  Nucleophiles can add to carbonyl group under either acidic or basic condition. 2 1. Hydration – Addition of H2O 3   Hydrates are unstable species. Backward reaction may occur. 2. Acetal Formation  Aldehydes and ketones react with alcohols to form acetals. 4  E.g  Hemiacetals and ketals are not stable, can change to acetals.  In the formation of acetals two molecules of alcohols adds to the carbonyl. 5  Diols that bear two hydroxyl groups in a 1,2 or 1,3 relationship to each other yield cyclic acetals on reaction with either aldehydes or ketones. 6  When the carbonyl and the –OH groups found in the same molecule, cyclic hemiacetals formed.  Glucose exist as a cyclic hemiacetal. Glucose Glucose (Hemiacetal form) 7 3. Addition of Hydrogen cyanide: Cyanohydrins If HCN is added to carbonyl, a cyanohydrin results. O OH + C H + HCN R C R R R CN Cyanohydrin O OH CH3CH2C H + HCN CH3CH2C H CN O HO CN + HCN 8 3. Addition of Grignard Reagents Grignard reagent-organometallic compound Grignard reagent adds to carbonyl to form alcohols 9 4. Addition of Hydrogen Catalysed by Pt, Pd, or Ni 4. Reduction of Carbonyl The most common metal hydride used to reduce carbonyl compounds are Lithium aluminium hydride and sodium borohydride 10 Reduce only the carbonyl not the C-C double bond 11 6.2. Addition – Elimination reaction If the added Nu contain unshared electrons, it may donate them to form a double bond, there by eliminating the original carbonyl oxygen atom. 12 1. Imines and related compounds 1 amine 2 amine 13 14 Oxime formation Hydrazone formation 15 2. Ester Formation 16  6.3 Enolization-ketolization reactions Aldehydes and ketones may exist as an equilibrium mixture of two forms, called the keto form and the enol form. This type of structural isomerism is called tautomerism. For simple ketones and aldehydes, the keto form predominates. 17 The α-hydrogen in a carbonyl compound is more acidic than a normal hydrogen bonded to a carbon atom. This α-hydrogen can easily removed by a base to form enolate ion. 18 Enols are related to an aldehyde or a ketone by a proton-transfer equilibrium known as keto-enol tautomerism. Base catalysed enolization 19 Application of enolization-Ketonization reactions 1. Halogenation Aldehydes and ketones react with halogens by substitution of one of the α-hydrogens: 20 2. Alkylation of Enolate anions Base used for alkylation is LDA (Lithium diisiopropyl amide) -bulky base -avoided side reaction 21  3. Aldol condensation Is nucleophilic addition of an enolate ion in to another carbonyl. 22 23 The β-hydroxy aldehyde products of aldol addition undergo dehydration on heating, to yield α, β-unsaturated aldehydes: 24  Mixed Aldol condensation When different carbonyl reacts, mixture of aldol addition products possible. The mixed aldol condensations that are the most synthetically useful are those in which: 1. Only one of the reactants can form an enolate; or 2. One of the reactants is more reactive toward nucleophilic addition than the other. Formaldehyde cannot form an enolate 25  Aldol cyclization  Intramolecular aldol reaction of diketones are useful for making 5 or 6-membered rings.  Rings > 6 and < 5 are not common due to ring strain. 26  UNIT 7 OXIDATION-REDUCTION REACTION Oxidation reaction involves either gaining of oxygen atoms or loss of H-atoms. Reduction reactions involves gaining of H and losing of heteroatoms such as O, N, X. 27  7.1 Oxidation Reaction Oxidation of Alcohol 28  Pyridinium chlorochromate (PCC) stop oxidation at aldehyde Biological Oxidation of Alcohols 29 Oxidation of Aldehydes Mild oxidizing reagent like Ag2O can selectively oxidize aldehyde in to the presence of other functional group. 30 Tollen’s Test Test for aldehyde functional group If a silver mirror is deposited on the wall of the container oxidation has occurred. Oxidation of multiple bonds 31  7.2 Reduction Reaction Catalytic hydrogenation Hydride reduction (Chemical reduction) O R C H NaBH4 O R C R LiAlH4 O R C O R O 32 R C OH  O R C H NaBH4 O R C R LiAlH 4 O R C O R O R C OH 33