Chapter 8 — Aldehydes, Ketones and Carboxylic Acids

NCERT Class 12 Chemistry Chapter 8 covers aldehydes, ketones, and carboxylic acids — polar carbonyl-group compounds central to organic synthesis, biochemistry, and industrial applications such as solvents, perfumes, plastics, and food preservatives.

Aldehydes, ketones, and carboxylic acids are important classes of organic compounds containing the carbonyl group (C=O). Aldehydes have the carbonyl carbon bonded to hydrogen and carbon, ketones to two carbon atoms, and carboxylic acids carry a –COOH group. They are prepared from primary and secondary alcohols, alkynes, nitriles, Grignard reagents, and acyl halides. Key reactions include nucleophilic addition, aldol condensation, Cannizzaro reaction, Tollens' and Fehling's tests, Clemmensen and Wolff-Kishner reductions, esterification, and Hell-Volhard-Zelinsky halogenation. These compounds occur widely in nature and industry — in flavours, perfumes, solvents, plastics, and food preservatives.

1. 01Aldehydes are more reactive than ketones in nucleophilic addition reactions due to both steric (one vs. two bulky substituents) and electronic (greater electrophilicity of carbonyl carbon) factors.
2. 02Aldehydes are distinguished from ketones using mild oxidising agents: Tollens' reagent gives a silver mirror with aldehydes; Fehling's reagent gives a reddish-brown precipitate (aromatic aldehydes do not respond to Fehling's test).
3. 03Aldehydes and ketones with at least one α-hydrogen undergo aldol condensation in dilute alkali to give β-hydroxy aldehydes or ketones; aldehydes without α-hydrogen undergo Cannizzaro reaction (disproportionation) with concentrated alkali.
4. 04Carboxylic acids are significantly more acidic than alcohols and phenols; electron-withdrawing substituents increase acidity by stabilising the carboxylate anion, while electron-donating groups decrease it.
5. 05Carboxylic acids are prepared by oxidation of primary alcohols and aldehydes, hydrolysis of nitriles and esters, treatment of Grignard reagents with CO2, and side-chain oxidation of alkylbenzenes.
6. 06Industrial uses span solvents (acetone, ethyl methyl ketone), food preservation (sodium benzoate), perfumery (benzaldehyde, vanillin), manufacture of nylon-6,6 (hexanedioic acid), and biological specimen preservation (formaldehyde/formalin).