Overview — Carbonyl Compounds and Carboxylic Acids

Aldehydes (RCHO), ketones (R₂C=O) and carboxylic acids (RCOOH) contain the carbon–oxygen double bond. They flavour foods, serve as solvents and reagents, and are central to organic synthesis. This lesson covers nomenclature, preparation, polar C=O chemistry, nucleophilic addition, distinguishing tests, α-hydrogen reactions (aldol, haloform, Cannizzaro), and the preparation, acidity and reactions of carboxylic acids.

Section 1: Aldehydes and Ketones — Structure & Preparation (27.1)

Carbonyl C and O are sp² hybridised; molecule is planar about C=O. Oxygen is more electronegative → Cδ⁺=Oδ⁻. Carbon is electrophilic (Nu attack); oxygen is nucleophilic/basic (E⁺/H⁺). Dipole interactions raise b.p. vs hydrocarbons; H-bonding with water gives lower members good solubility.

Aldehydes more reactive than ketones: (i) only one +I alkyl group → more positive carbonyl C; (ii) less steric crowding.

Naming: alkanals (-al), alkanones (-one); common names acetone, acetophenone, benzaldehyde, formaldehyde. Carbonyl carbon gets lowest number; for ketones the -one suffix is numbered (propan-2-one). Aromatic aldehydes often keep common names (benzaldehyde).

Preparation (overview): oxidation of 1° alcohols → aldehydes (controlled, e.g. PCC or distillation of product); 2° alcohols → ketones (K₂Cr₂O₇/H⁺ or PCC); ozonolysis of alkenes (reductive work-up); hydration of alkynes (Hg²⁺/H⁺ → ethanal from ethyne, methyl ketones from terminal alkynes); Friedel–Crafts acylation (ArH + RCOCl/AlCl₃) → aromatic ketones; Rosenmund (RCOCl + H₂/Pd-BaSO₄ → RCHO — poisoned catalyst stops at aldehyde); Stephen reduction of nitriles (SnCl₂/HCl then H₃O⁺ → RCHO); dry distillation of calcium carboxylates for ketones (higher level).

Physical properties: lower aldehydes/ketones are liquids with characteristic odours. Boiling points higher than hydrocarbons of similar mass (dipole–dipole) but lower than alcohols (no O–H H-bonding between pure carbonyl molecules). Methanal, ethanal and propanone mix well with water; solubility falls as the hydrophobic R grows.

Section 2: Reactions of Aldehydes and Ketones (27.1.4)

A. Nucleophilic addition: HCN → cyanohydrins (useful C–C bond; hydrolyse to α-hydroxy acids); NaHSO₃ adducts (crystalline, reverse on acid/base); alcohols → hemiacetals then acetals (acid; protect carbonyls); water → gem-diols (hydrate, often reversible); Grignard → alcohols after acid work-up (HCHO → 1°, RCHO → 2°, R₂C=O → 3°); ammonia derivatives → imines, oximes, hydrazones, 2,4-DNP hydrazones (orange/yellow crystalline solids for m.p. identification of both aldehydes and ketones).

B. Reduction: to alcohols (NaBH₄ mild, LiAlH₄ stronger, catalytic H₂/Ni). Deoxygenation to alkanes: Wolff–Kishner (NH₂NH₂, KOH, high-boiling alcohol — base-compatible) vs Clemmensen (Zn-Hg/HCl — acid-compatible). Choose the method that does not destroy other acid- or base-sensitive groups.

C. Oxidation: aldehydes → carboxylic acids easily (even air, or KMnO₄/K₂Cr₂O₇). Tollen’s: ammoniacal AgNO₃, [Ag(NH₃)₂]⁺ → silver mirror. Fehling’s: Cu²⁺/tartrate alkaline → brick-red Cu₂O (aliphatic aldehydes; aromatic aldehydes often fail Fehling but give Tollen’s). Ketones resist these mild oxidants — classic distinction. Strong oxidants can still cleave ketones under forcing conditions (exam: mild tests are the focus).

D. α-Carbon chemistry: α-H is acidic (enolate resonance) → keto–enol tautomerism. Halogenation at α-C (acid or base); excess base + I₂ → haloform (CHI₃ yellow ppt for methyl ketones CH₃COR and compounds oxidisable to them, e.g. ethanol, ethanal). Aldol: dilute NaOH on carbonyls with α-H → β-hydroxy aldehyde/ketone; heat drives dehydration to α,β-unsaturated carbonyl (conjugated). Crossed aldol needs care (one component without α-H preferred). Cannizzaro: aldehydes without α-H (HCHO, PhCHO) + concentrated NaOH → disproportionation: one molecule oxidised to acid salt, one reduced to alcohol. Never confuse: aldol needs α-H + dilute base; Cannizzaro needs no α-H + concentrated base.

Section 3: Carboxylic Acids (27.2)

RCOOH — formic, acetic, propanoic… alkanedioic acids (oxalic, malonic), benzoic acid and substituted aromatics. Carboxyl carbon is sp²; the –COOH group is planar. In pure liquids/solids, acids form strong H-bonded dimers → high melting and boiling points relative to alcohols of similar mass. Lower members are water-soluble; long-chain fatty acids are not.

Preparation: oxidation of alkenes (hot alkaline KMnO₄, cleaves double bond), 1° alcohols and aldehydes; side-chain oxidation of Ar–CH₃ (or longer alkyl) → ArCOOH (KMnO₄/H⁺ or CrO₃ — ring must have benzylic H); Grignard + CO₂ then acid work-up (adds one carbon); hydrolysis of nitriles and cyanohydrins (acid or base, then acidify). Industrial acetic acid: methanol carbonylation and related routes (context only).

Acidity: pKₐ typically ~4–5 in water. Dissociation gives resonance-stabilised carboxylate (negative charge shared by both oxygens) — much stronger acids than alcohols (pKₐ ~16) or phenols (pKₐ ~10). −I / electron-withdrawing substituents increase acidity (ClCH₂COOH > CH₃COOH; Cl₃CCOOH is very strong); alkyl groups slightly decrease acidity. Benzoic acid is a solid aromatic acid; o-substituents can affect strength via steric and electronic effects (exam: know the inductive trend first).

Reactions: form salts with bases (NaOH, Na₂CO₃); liberate CO₂ from NaHCO₃/Na₂CO₃ (distinction from most phenols — phenols need stronger base); Fischer esterification with ROH/H⁺ (reversible; remove water to drive); form acid chlorides (SOCl₂ preferred — gaseous by-products; also PCl₅/PCl₃); amides via ammonium salt then heat dehydration; anhydrides by dehydrating agents or from RCOCl + RCOONa; reduction with LiAlH₄ → 1° alcohols (NaBH₄ usually does not reduce free RCOOH); α-halogenation (Hell–Volhard–Zelinsky) with X₂ and red phosphorus → α-halo acids for further substitution.

Derivatives: reactivity order roughly acid chloride > anhydride > ester > amide toward nucleophiles. Interconversions (e.g. RCOCl → ester or amide) are central to multi-step synthesis. Esters hydrolyse to acids (acid or base; base saponification is irreversible).

Exam Connections and Chapter Summary

High-yield checklist: (1) polar C=O and why RCHO > R₂C=O (electronic + steric); (2) Nu addition products — cyanohydrin, acetal, oxime, 2,4-DNP; (3) Tollen’s/Fehling vs 2,4-DNP (oxidation vs carbonyl detection); (4) aldol needs α-H + dilute base, Cannizzaro needs no α-H + concentrated base; (5) iodoform for CH₃COR; (6) Wolff–Kishner (base) vs Clemmensen (acid) for C=O → CH₂; (7) acid preparation routes and pKₐ / −I trends; (8) NaHCO₃ CO₂ test vs phenols; (9) esterification and acid chlorides.

Builds on L26 (oxidation of alcohols to carbonyls and acids) and L25 (haloform chemistry). Next in the module: nitrogen compounds (L28) and biomolecules (L29). Practice writing the mechanism of nucleophilic addition and the aldol sequence — they appear repeatedly in board and competitive exams.