Aldehydes, Ketones and Carboxylic Acids

2.1 Core concepts

• In aldehydes the carbonyl carbon is bonded to one H (and one C/H), while in ketones it is bonded to two carbons; carboxylic acids have the carboxyl group –COOH, a carbonyl fused with hydroxyl (NCERT §8.0, p. 227).
• Common names of aldehydes are derived from common names of the parent carboxylic acid by replacing "–ic acid" with "aldehyde"; positions are marked with Greek letters α, β, γ (NCERT §8.1.1, p. 228).
• IUPAC: aliphatic aldehydes end in "–al" (chain numbered from CHO), ketones in "–one" (numbering begins from end nearer the carbonyl); ring-attached CHO uses suffix "carbaldehyde" (NCERT §8.1.1, p. 229).
• The carbonyl carbon is sp²-hybridised, trigonal planar with ~120° bond angles; oxygen carries two lone pairs and the C=O bond is polar due to oxygen's higher electronegativity, making C electrophilic and O nucleophilic (NCERT §8.1.2, p. 231).
• Aldehydes & ketones are prepared by (i) oxidation of 1°/2° alcohols, (ii) dehydrogenation of alcohols over Ag/Cu, (iii) ozonolysis of alkenes, and (iv) acid-catalysed hydration of alkynes (ethyne → ethanal; higher alkynes → methyl ketones) (NCERT §8.2.1, p. 231-232).
• Rosenmund reduction: acyl chloride + H₂ on Pd/BaSO₄ → aldehyde; Stephen reaction (RCN + SnCl₂/HCl → imine → RCHO) and DIBAL-H reduce nitriles and esters to aldehydes (NCERT §8.2.2, p. 232).
• Aromatic aldehyde preparations: Etard reaction (toluene + CrO₂Cl₂ → benzaldehyde via chromium complex); CrO₃ in acetic anhydride (via benzylidene diacetate); side-chain chlorination then hydrolysis; Gattermann–Koch (C₆H₆ + CO + HCl with anhydrous AlCl₃/CuCl) (NCERT §8.2.2, p. 232-233).
• Ketones: acyl chloride + R₂Cd (R₂Cd from CdCl₂ + RMgX) → ketone; nitrile + RMgX then hydrolysis → ketone; Friedel–Crafts acylation (arene + acyl chloride / anhydrous AlCl₃) → aryl ketone (NCERT §8.2.3, p. 233-234).
• Physical properties: methanal is gas, ethanal a volatile liquid; bp of aldehydes/ketones is higher than hydrocarbons & ethers (dipole–dipole) but lower than alcohols (no intermolecular H-bonding); lower members are miscible with water through H-bonding (NCERT §8.3, p. 235).
• Nucleophilic addition mechanism: Nu attacks the electrophilic carbonyl C perpendicular to the sp² plane; hybridisation changes sp² → sp³, giving a tetrahedral alkoxide intermediate that picks up H⁺ — net addition of Nu⁻ and H⁺ across C=O (NCERT §8.4(1), p. 236).
• Reactivity in nucleophilic addition: aldehyde > ketone because (a) ketones have two bulky groups (steric hindrance) and (b) the two alkyl groups donate electrons and reduce electrophilicity of the carbonyl carbon (NCERT §8.4(1)(ii), p. 236).
• Important nucleophilic additions: HCN (base-catalysed) → cyanohydrin; NaHSO₃ → bisulphite addition product (equilibrium favours aldehydes; useful for purification); alcohols/HCl-dry → hemiacetal → acetal (gem-dialkoxy); ethylene glycol with ketone → cyclic ketal (NCERT §8.4(1)(iii), p. 237-238).
• Addition–elimination with H₂N–Z gives >C=N–Z: ammonia → imine, RNH₂ → Schiff's base, NH₂OH → oxime, NH₂NH₂ → hydrazone, PhNHNH₂ → phenylhydrazone, 2,4-DNP → 2,4-dinitrophenylhydrazone (yellow/orange/red solids — useful for characterisation), H₂N-NHCONH₂ → semicarbazone (NCERT Table 8.2, p. 238).
• Reduction: NaBH₄ / LiAlH₄ / catalytic H₂ reduce C=O to alcohol; Clemmensen (Zn-Hg/conc. HCl) and Wolff-Kishner (NH₂NH₂ then NaOH/KOH in ethylene glycol) reduce C=O all the way to CH₂ (NCERT §8.4(2), p. 238-239).
• Oxidation: aldehydes → carboxylic acids easily (HNO₃, KMnO₄, K₂Cr₂O₇, and even mild Tollens' & Fehling's); ketones resist oxidation and only break C–C bonds under harsh conditions; Tollens' (ammoniacal AgNO₃) gives silver mirror; Fehling's (Cu²⁺ in alkaline Rochelle salt) gives reddish-brown Cu₂O ppt (aromatic aldehydes don't respond to Fehling's) (NCERT §8.4(3), p. 239).
• Haloform: methyl ketones (CH₃CO–) and CH₃CH(OH)– compounds + NaOX → sodium carboxylate (one C less) + CHX₃ (iodoform test); the reaction does not affect C=C double bonds (NCERT §8.4(3)(iii), p. 240).
• α-Hydrogen acidity → Aldol condensation: with dilute alkali, aldehydes/ketones with α-H give β-hydroxy carbonyl (aldol/ketol) that loses water to give α,β-unsaturated carbonyl; cross-aldol with two different α-H carbonyls gives four products (NCERT §8.4(4), p. 241-242).
• Cannizzaro reaction: aldehydes with NO α-H (HCHO, PhCHO) undergo disproportionation with concentrated alkali — one molecule is oxidised to the carboxylate salt, the other is reduced to alcohol (NCERT §8.4(5)(i), p. 242).
• Aromatic aldehydes/ketones undergo electrophilic substitution at the ring with the C=O group acting as a deactivating, meta-directing group (NCERT §8.4(5)(ii), p. 243).
• Carboxylic acids: common names end in "–ic acid" (formic from ants, acetic from vinegar, butyric from rancid butter); IUPAC names use "–oic acid" with COOH carbon numbered one; –COOH on a ring uses "carboxylic acid" suffix (NCERT §8.6.1, p. 244-245).
• Carboxyl carbon is less electrophilic than carbonyl carbon because of resonance donation from the –OH oxygen (NCERT §8.6.2, p. 245).
• Preparations of carboxylic acids: (1) KMnO₄/K₂Cr₂O₇/CrO₃ (Jones) oxidation of 1° alcohol or aldehyde; (2) vigorous oxidation of alkyl benzenes (any 1°/2° alkyl side chain → COOH, but 3° unaffected); (3) hydrolysis of nitriles → amide → acid (mild conditions stop at amide); (4) RMgX + dry CO₂ then H⁺ → RCOOH (one C extra); (5) hydrolysis of acid chlorides, anhydrides and esters (NCERT §8.7, p. 245-247).
• Acidity: RCOOH + active metal → H₂; RCOOH + NaHCO₃ / Na₂CO₃ → CO₂ (this CO₂ test distinguishes –COOH from phenols, which do not react with hydrogencarbonate); carboxylate anion is stabilised by two equivalent resonance structures placing –ve charge on two O atoms — hence RCOOH > phenol > alcohol in acidity (NCERT §8.9.1, p. 249-250).
• pKa values (NCERT lists): HCl pKa = –7.0; CF₃COOH (strongest carboxylic acid here) 0.23; benzoic acid 4.19; acetic acid 4.76; smaller pKa = stronger acid (NCERT §8.9.1, p. 250).
• Substituent effect on acidity — EWG (–NO₂, –CN, halogens, –CF₃) stabilise carboxylate and increase acidity; EDG (–OCH₃, –CH₃) decrease it. Order: Ph < I < Br < Cl < F < CN < NO₂ < CF₃. On aromatic ring, p-OMe-C₆H₄-COOH (pKa 4.46) < benzoic (4.19) < p-NO₂-C₆H₄-COOH (3.41) (NCERT §8.9.1, p. 250-251).
• C–OH cleavage reactions: (1) heating with H₂SO₄ or P₂O₅ → anhydride; (2) Fischer esterification with alcohol + conc. H₂SO₄/HCl gas (nucleophilic acyl substitution: protonation → addition of alcohol → loss of H₂O); (3) PCl₅/PCl₃/SOCl₂ → acid chloride (SOCl₂ preferred — by-products SO₂ + HCl are gases); (4) NH₃ → ammonium salt → on heating → amide (NCERT §8.9.2, p. 251-252).
• –COOH group reactions: LiAlH₄ or B₂H₆ reduce –COOH to 1° alcohol (NaBH₄ does NOT reduce –COOH; diborane is selective and does not touch ester/nitro/halo); decarboxylation of sodium salt with sodalime (NaOH + CaO, 3:1) → R–H + CO₂; Kolbe electrolysis of carboxylate salts gives hydrocarbons with double the C atoms (NCERT §8.9.3, p. 252-253).
• Hell–Volhard–Zelinsky (HVZ): RCH₂COOH with Cl₂ or Br₂ + small amount of red P → α-halocarboxylic acid; aromatic acids — ring electrophilic substitution gives meta product because –COOH is deactivating and meta-directing; benzoic acid does NOT undergo Friedel–Crafts (AlCl₃ binds to –COOH) (NCERT §8.9.4, p. 253-254).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Fig. 8.1 — Orbital diagram for the carbonyl group: sp² carbon, three σ bonds, π bond above and below the plane (p. 231).
• Resonance structures of C=O — neutral (A) and dipolar (B) explaining polarity and high dipole moment (p. 231).
• Fig. 8.2 — Nucleophilic attack on the carbonyl carbon perpendicular to the sp² plane, producing a tetrahedral alkoxide that captures H⁺ (p. 236).
• Table 8.1 — Common vs IUPAC names of aldehydes & ketones (p. 230).
• Table 8.2 — N-substituted derivatives (>C=N–Z): imine, Schiff's base, oxime, hydrazone, phenylhydrazone, 2,4-DNP-hydrazone, semicarbazone (p. 238).
• Table 8.3 — Common vs IUPAC names of carboxylic acids (formic/methanoic ... benzoic/benzenecarboxylic) (p. 244).
• Resonance pair stabilising carboxylate anion (two equivalent O bearing –ve charge) versus phenoxide (–ve charge on C atoms) — explains why RCOOH > PhOH in acidity (p. 250).
• Ranked bp comparison at MW ≈ 58–60: n-butane (273 K) < methoxyethane (281) < propanal (322) < acetone (329) < propan-1-ol (370) (p. 235).

2.4 Common confusions / NTA trap points

• Tollens' vs Fehling's: BOTH oxidise aldehydes, but Fehling's does NOT respond to AROMATIC aldehydes (benzaldehyde) — a favourite trap (p. 239).
• HVZ uses red phosphorus + Cl₂/Br₂ (not just Cl₂/Br₂ alone); also it acts at the α-C, not the ring (p. 253).
• Reactivity in nucleophilic addition is aldehyde > ketone — students sometimes invert this; remember both steric and electronic factors (p. 236).
• Cannizzaro requires NO α-H and conc. alkali (HCHO, PhCHO, (CH₃)₃CCHO, 2,2-dimethylpropanal qualify); aldehydes with α-H instead do aldol (p. 242).
• Iodoform test is given by methyl ketones AND CH₃CH(OH)–R (ethanol gives iodoform, methanol does not); iodoform does not affect C=C (p. 240).
• NaBH₄ does NOT reduce –COOH; use LiAlH₄ or diborane (B₂H₆). Diborane is selective — leaves nitro/halo/ester untouched (p. 252).
• Benzoic acid is deactivating and meta-directing despite the lone pairs on O — and it does NOT undergo Friedel–Crafts (AlCl₃ chelates with –COOH) (p. 253-254).
• Phenyl/vinyl directly attached to –COOH INCREASES acidity (against expectation from resonance) because sp²-C is more electronegative than sp³-C (p. 251).
• For ketone preparation from acyl chloride, use R₂Cd (dialkylcadmium), NOT RMgX directly (Grignard goes through to tertiary alcohol) (p. 233).
• Aromatic ring of toluene side-chain oxidation: KMnO₄/CrO₃ takes 1°/2° alkyl → –COOH; tertiary alkyl is NOT oxidised (p. 246).
• Rosenmund vs Stephen — Rosenmund reduces acyl chloride to aldehyde using H₂/Pd-BaSO₄; Stephen reduces nitrile via SnCl₂/HCl then hydrolysis (p. 230).
• Etard reaction uses CrO₂Cl₂ (chromyl chloride) on toluene to give benzaldehyde after hydrolysis (p. 231) — distractor names the wrong oxidant.

2.5 Quick reaction map — aldehydes, ketones and carboxylic acids