JEE Chemistry Aldehydes Ketones and Carboxylic Acids Notes Set 09

Carboxylic Acids: Preparation

1. Grignard reagent + CO₂ (dry ether), then H₃O⁺: R–MgX gives R–COOH.
2. Oxidation of primary alcohols or aldehydes with K₂Cr₂O₇/H⁺ or CrO₃/H⁺. 3. Hydrolysis of nitriles, amides, esters and acid derivatives with water and acid.

Carboxylic Acids: Reactions

1. Hell-Volhard-Zelinsky: red P + Br₂ halogenates the alpha carbon.
2. With SOCl₂ (or PCl₅) gives the acid chloride. With NH₃ and heat gives the amide.
3. Hunsdiecker reaction: silver salt + Br₂ gives R–Br with one carbon fewer. Soda lime (NaOH + CaO) decarboxylation gives the alkane with one carbon fewer.
4. Sulphonic acids are stronger than carbonic acid and liberate CO₂ from NaHCO₃.

Aldehydes and Ketones: Preparation

1. Rosenmund reduction: acid chloride + H₂/Pd-BaSO₄ gives an aldehyde. 2. Stephen reaction: nitrile + SnCl₂/HCl then H₂O gives an aldehyde.
3. PCC oxidises primary alcohols to aldehydes and secondary alcohols to ketones without over-oxidation.
4. Dry distillation of calcium salts: two identical salts give a ketone; a formate with another salt gives an aldehyde.

Aldehydes and Ketones: Reactions and Tests

1. Cannizzaro reaction: aldehydes without alpha hydrogen (HCHO, benzaldehyde, 2,2-dimethylpropanal) disproportionate in concentrated alkali to alcohol and carboxylate; in a crossed reaction HCHO is oxidised.
2. Aldol condensation: compounds with alpha hydrogen (ethanal, propanone, cyclohexanone) condense in dilute alkali and on heating give alpha,beta-unsaturated carbonyl compounds.
3. Condensations with NH₂OH (oxime), NH₂NH₂ (hydrazone), semicarbazide and 2,4-DNP (orange precipitate with any aldehyde or ketone).
4. Tests: Tollens' (silver mirror) only for aldehydes; Fehling's only for aliphatic aldehydes; iodoform test for CH₃CO– and CH₃CH(OH)– compounds.