Unit 8: Aldehydes, Ketones and Carboxylic Acids

Carbonyl and carboxyl compounds — central to organic synthesis and biochemistry

8.1 Nomenclature and Structure

Aldehydes, Ketones and Carboxylic Acids Overview

        Key structural features: Carbonyl carbon is sp² hybridised, trigonal planar (120°). C=O bond is polarised — carbon is electrophilic (δ⁺), oxygen is nucleophilic (δ⁻).
    

8.2 Preparation

        Oxidation chain: 1° Alcohol → Aldehyde (PCC) → Carboxylic acid | 2° Alcohol → Ketone | 3° Alcohol → No oxidation

        Tollens' test distinguishes aldehydes from ketones (silver mirror test)

8.3 Nucleophilic Addition Reactions

        Aldehydes > Ketones in nucleophilic addition — less steric hindrance + carbonyl carbon more electrophilic.
    

Important Derivatives

Reagent	Product	Name
H₂N-OH	>C=N-OH	Oxime
H₂N-NH₂	>C=N-NH₂	Hydrazone
H₂N-NHC₆H₅	>C=N-NHC₆H₅	Phenylhydrazone
H₂N-NH-CO-C₆H₃(NO₂)₂	>C=N-NH-C₆H₃(NO₂)₂	2,4-DNP (orange ppt — carbonyl test)

8.4 Named Reactions

Cannizzaro Reaction

Aldehydes without α-hydrogen (HCHO, PhCHO) undergo disproportionation in conc. NaOH → alcohol + carboxylate salt.

Aldol Condensation

Aldehydes/ketones with α-H + dilute NaOH → β-hydroxy aldehyde → α,β-unsaturated aldehyde (heating).

Haloform Reaction

Methyl ketones (CH₃CO-) give iodoform test (yellow CHI₃ ppt) with I₂/NaOH.

8.5 Carboxylic Acids

Acidity

Most acidic organic compounds (pKa ~4-5)
Carboxylate ion is stabilised by resonance (two equivalent structures)
Electron-withdrawing groups increase acidity: Cl₃CCOOH > HCOOH > CH₃COOH
Turns blue litmus red, reacts with NaOH and NaHCO₃

Key Reactions

Esterification: RCOOH + R'OH ⇌ RCOOR' + H₂O (H₂SO₄ catalyst)
With SOCl₂/PCl₅: RCOOH → RCOCl (acyl chloride preparation)
With NaHCO₃: RCOOH + NaHCO₃ → RCOONa + CO₂↑ + H₂O
HVZ reaction: α-halogenation of carboxylic acids (RCH₂COOH → RCH(Br)COOH)
Decarboxylation: RCOOH + NaOH/CaO → RH + Na₂CO₃

Carboxylic acids are more acidic than phenols and alcohols because the carboxylate ion is stabilised by two equivalent resonance structures (charge equally distributed over two oxygens).

Summary

Aldehydes, ketones, and carboxylic acids contain the carbonyl (>C=O) and carboxyl (-COOH) groups respectively. Aldehydes are more reactive than ketones in nucleophilic addition due to steric and electronic effects. Important named reactions include Cannizzaro (disproportionation of non-enolisable aldehydes), Aldol condensation (α-hydrogen containing aldehydes/ketones), and haloform reaction (methyl ketones). Carboxylic acids are the most acidic organic compounds due to resonance stabilisation of carboxylate ion. Tollens' and Fehling's tests distinguish aldehydes from ketones.