Class 12 Chemistry Chapter 8 Aldehydes, Ketones and Carboxylic Acids – Revision Notes

Quick revision notes for Class 12 Chemistry Chapter 8 – Aldehydes, Ketones and Carboxylic Acids, covering nomenclature, preparation, name reactions, distinguishing tests, and the acidity of carboxylic acids. Ideal for last-minute board exam revision.

Last Updated: September 23, 2026

Nomenclature and Structure

Aldehydes (–CHO) and ketones (>C=O between two carbons) share the polar carbonyl group; carboxylic acids (–COOH) combine a carbonyl with a hydroxyl. Aldehydes are generally more reactive than ketones towards nucleophilic addition (less steric hindrance, only one electron-donating group on the carbonyl carbon).

Preparation

Aldehydes/ketones are made by oxidation of alcohols (PCC for aldehydes, stopping short of the acid), Rosenmund reduction of acid chlorides (H₂/Pd–BaSO₄, poisoned catalyst) for aldehydes, and Friedel–Crafts acylation for aryl ketones. Carboxylic acids are made by oxidation of primary alcohols/aldehydes, hydrolysis of nitriles/esters, and via Grignard reagents+CO₂.

Name Reactions

Aldol condensation: a carbonyl compound with α-hydrogen self- or cross-condenses (dilute base) to a β-hydroxy carbonyl compound, which dehydrates on heating to an α,β-unsaturated carbonyl compound. Cannizzaro reaction: an aldehyde with no α-hydrogen disproportionates with concentrated alkali into the alcohol+the carboxylate salt. Haloform (iodoform) reaction: positive only for methyl ketones (CH₃CO–) and compounds oxidisable to them (including ethanol and acetaldehyde), giving a yellow CHI₃ precipitate. Clemmensen reduction: Zn(Hg)/conc. HCl reduces C=O all the way to CH₂ under acidic conditions. Wolff–Kishner reduction: NH₂NH₂/KOH, heat, does the same reduction under basic conditions (for acid-sensitive substrates). Hell–Volhard–Zelinsky (HVZ) reaction: a carboxylic acid with an α-hydrogen+Cl₂/Br₂ with catalytic red phosphorus gives the α-halo acid.

Distinguishing Tests

Tollens’ test: a silver mirror forms with any aldehyde (aliphatic or aromatic); ketones give no reaction. Fehling’s test: positive only for aliphatic aldehydes — aromatic aldehydes like benzaldehyde do not respond. Iodoform test: positive for methyl ketones and CH₃CH(OH)–-type alcohols (and acetaldehyde). NaHCO₃ test: carboxylic acids effervesce (release CO₂); phenols and esters do not.

Acidity of Carboxylic Acids

Carboxylic acids are acidic because the carboxylate ion delocalises its negative charge equally over two electronegative oxygen atoms, a highly effective form of resonance stabilisation — more effective than phenoxide’s delocalisation partly onto less electronegative ring carbons, which is why carboxylic acids are stronger acids than phenols despite phenoxide having more resonance structures. Electron-withdrawing substituents (halogens, –NO₂) increase acid strength and this effect weakens rapidly with distance from –COOH; electron-donating substituents (alkyl, –OCH₃) decrease it.

Written by Satish

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