Carboxylic Acids

Structure and acidity

Carboxylic acids R-COOH have an sp² carboxyl carbon bearing both a carbonyl (C=O) and a hydroxyl (-OH). They are acidic (pK_a ~4-5) because the carboxylate anion R-COO⁻ is resonance-stabilised: the negative charge is shared equally over two oxygens. By contrast, alcohols have pK_a ~16-18 because their alkoxide has no resonance stabilisation. Electron-withdrawing groups increase acidity: trichloroacetic acid (pK_a 0.7) > dichloroacetic (1.3) > chloroacetic (2.85) > acetic (4.76). The effect attenuates with distance — γ-chlorobutyric acid has a much smaller effect than α. The FSc Punjab Textbook Chemistry XII Chapter 14 lists the trends; Clayden Chapter 12 gives the rigorous resonance treatment.

Preparation routes

Five reliable routes: (1) oxidation of 1° alcohols or aldehydes with KMnO₄ or K₂Cr₂O₇/H₂SO₄; (2) oxidation of alkylbenzenes — any alkyl side chain on benzene oxidises all the way down to -COOH on the ring carbon (toluene → benzoic acid); (3) Grignard + CO₂ followed by H₃O⁺ gives a carboxylic acid one carbon longer than the halide; (4) hydrolysis of nitriles RCN + H₂O/H⁺ or OH⁻ → RCOOH (the nitrile itself comes from RX + KCN); (5) hydrolysis of esters, amides, or acyl halides.

Reactions of -COOH

Acid-base: with NaOH → sodium carboxylate + water; with NaHCO₃ → carboxylate + CO₂ + H₂O (a key distinguishing test from phenol, which does not liberate CO₂ from bicarbonate). Esterification (Fischer): RCOOH + R'OH ⇌ RCOOR' + H₂O catalysed by H₂SO₄; mechanism is PADPED — protonate, add, deprotonate, protonate, eliminate, deprotonate (Clayden mnemonic). Reduction with LiAlH₄ gives a 1° alcohol; NaBH₄ is too weak to reduce -COOH. Conversion to acyl chloride uses SOCl₂, PCl₃, or PCl₅; SOCl₂ is preferred because by-products SO₂ and HCl are gaseous.

Decarboxylation and the Hell-Volhard-Zelinsky reaction

Decarboxylation: heating sodium carboxylate with soda lime (NaOH/CaO) gives RH + Na₂CO₃. β-keto acids and β-diacids decarboxylate readily on warming because the transition state has a six-membered cyclic arrangement (Clayden). Kolbe's electrolysis: anodic oxidation of carboxylate gives R• + CO₂ + e⁻; two R• combine to R-R. HVZ reaction: RCH₂COOH + Br₂/PBr₃ → RCHBrCOOH (α-bromination); the α-bromoacid is a useful synthon for α-amino acids (Strecker / Gabriel-style amination).

Acid derivatives — the reactivity ladder

Reactivity towards nucleophiles: acyl chloride RCOCl > anhydride (RCO)₂O > ester RCOOR' > amide RCONR₂ > carboxylate RCOO⁻. A more reactive derivative can be converted to a less reactive one but not vice versa under mild conditions (so RCOCl + R'OH → RCOOR' is easy; ester to acyl chloride needs harsh conditions). All derivatives hydrolyse to the parent acid: amides resist hydrolysis most stubbornly (used as protecting groups; require strong acid or base and heat). Saponification (ester + NaOH → carboxylate + alcohol) is the basis of soap-making from triglycerides. Morrison & Boyd Chapters 20-21 are the canonical references.