JEE Chemistry Aldehydes Ketones and Carboxylic Acids Notes Set 08

Structure and Nomenclature

The carbonyl group, >C=O, has an sp2 carbon with three sigma bonds in one plane (bond angle about 120°) and a pi bond to oxygen. Oxygen is more electronegative, so the carbon is partly positive and the group is strongly polar.
In an aldehyde the carbonyl carbon holds at least one hydrogen (\( \ce{RCHO} \)); in a ketone it is bonded to two carbon groups (\( \ce{RCOR'} \)). Simple ketones have identical groups and mixed ketones have different ones.
IUPAC: aldehydes take -al and the CHO carbon is C1; ketones take -one with the lowest locant. Common names: methanal (formaldehyde), ethanal (acetaldehyde), propanone (acetone), phenylmethanal (benzaldehyde), methyl phenyl ketone (acetophenone), diphenyl ketone (benzophenone), ethanedial (glyoxal), trichloroethanal (chloral).
Isomerism: aldehydes and ketones with the same carbon count are functional isomers (propanal and propanone); ketones also show chain and position isomerism, and aldehydes show chain isomerism.

Preparation of Aldehydes and Ketones

Nucleophilic Addition and Reactivity

The typical reaction is nucleophilic addition: the nucleophile attacks the planar carbonyl carbon and the oxygen takes a proton. Reactivity falls in the order \( \ce{HCHO > RCHO > ArCHO > RCOR > ArCOAr} \). Alkyl groups donate electrons and add steric hindrance; the aryl ring conjugates with C=O.
Addition of HCN gives a cyanohydrin, which hydrolyses to a hydroxy acid (ethanal gives lactic acid, racemic). Sodium bisulphite gives a crystalline adduct with aldehydes and methyl ketones. Alcohols give hemiacetals and, with dry HCl, acetals. Grignard reagents give alcohols.
Ammonia derivatives condense with loss of water: NH2OH gives oximes, NH2NH2 hydrazones, PhNHNH2 phenylhydrazones, semicarbazide semicarbazones and 2,4-DNP orange-red crystals. Ammonia itself gives aldehyde ammonia with ethanal, urotropine with methanal and hydrobenzamide with benzaldehyde.
Electron-withdrawing groups raise reactivity, so chloral forms a stable hydrate and acid chlorides are more reactive than aldehydes. The ketone PCl5 reaction gives gem-dichlorides.

Oxidation, Reduction and Distinguishing Tests

Aldehydes oxidise easily to acids; ketones need strong oxidants and break C-C bonds. Reduction: \( \ce{LiAlH4} \), \( \ce{NaBH4} \) or \( \ce{H2/Ni} \) give alcohols; Clemmensen (Zn-Hg/HCl) and Wolff-Kishner (\( \ce{N2H4} \)/KOH, glycol) give the alkane. Magnesium amalgam reduces acetone to pinacol.

Reactions of the Alpha-Hydrogen and Name Reactions

Alpha-hydrogens are acidic because the conjugate base is a resonance-stabilised enolate. Aldol addition: two molecules of ethanal in dilute alkali give 3-hydroxybutanal, which on heating loses water to give crotonaldehyde. Crossed aldol (Claisen-Schmidt) of benzaldehyde with ethanal gives cinnamaldehyde, and with acetophenone gives chalcone.
Cannizzaro reaction: aldehydes without alpha-hydrogen (methanal, benzaldehyde, 2,2-dimethylpropanal) give alcohol plus carboxylate; in crossed Cannizzaro, methanal is oxidised to formate and the other aldehyde is reduced. Glyoxal gives an intramolecular Cannizzaro reaction to glycollate.
Perkin reaction gives cinnamic acid from benzaldehyde, acetic anhydride and sodium acetate. Benzoin condensation: cyanide catalyses two benzaldehyde molecules to benzoin, an alpha-hydroxy ketone. Haloform reaction: methyl ketones with halogen and alkali give a haloform and a carboxylate.
Wittig reaction converts C=O to C=C with a phosphorus ylide. Acetone with conc. \( \ce{H2SO4} \) forms mesitylene; with HCl it forms mesityl oxide and phorone; with dilute alkali it forms diacetone alcohol. Enol content is largest in pentane-2,4-dione.

Important Compounds and Uses

Methanal: gas at room temperature, 40 percent aqueous solution is formalin (preservative); with phenol gives Bakelite; with ammonia gives urotropine; polymerises to paraformaldehyde and trioxane. Ethanal: boils at 294 K; trimer paraldehyde (hypnotic) and tetramer metaldehyde (slug killer); with excess chlorine gives chloral.
Acetone: boils at 329 K, miscible with water, a good solvent; ketene forms on pyrolysis near 1000 K. Benzaldehyde: almond smell, used in perfumery and dyes; oxidises in air to benzoic acid. Silver mirrors are made by reducing Tollens' reagent with methanal.