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.
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.
Continue Revising — NCERT Solutions for Class 12 Chemistry:
Chapter 1: Solutions | Chapter 2: Electrochemistry | Chapter 3: Chemical Kinetics | Chapter 4: The d- and f-Block Elements | Chapter 5: Coordination Compounds | Chapter 6: Haloalkanes and Haloarenes | Chapter 7: Alcohols, Phenols and Ethers | Chapter 8: Aldehydes, Ketones and Carboxylic Acids
