Give four methods of preparing carboxylic acids, with an equation for each. Why do carboxylic acids have higher boiling points than alcohols of similar molar mass?

1. 1 · Oxidation of primary alcohols. RCH₂OH → (i) alkaline KMnO₄ (ii) H₃O⁺ → RCOOH.
2. 2 · Oxidation of alkylbenzenes. C₆H₅CH₃ → (KMnO₄/KOH, heat) C₆H₅COOK → (H₃O⁺) C₆H₅COOH. The whole side chain becomes -COOH.
3. 3 · Hydrolysis of nitriles. RCN → (H₂O, H⁺) RCONH₂ → RCOOH.
4. 4 · Grignard reagent and CO₂. RMgX + CO₂ (dry ice) → RCOOMgX → (H₃O⁺) RCOOH; one carbon added.
5. 5 · Explain the boiling point. Acid molecules pair up as dimers held by two hydrogen bonds, which are not fully broken even in the vapour.
6. 6 · Draw the dimer. Two RCOOH molecules facing each other, each O-H joined to the other's C=O by a dotted line. Label 'hydrogen bond' and 'dimer'.
7. 7 · Close in one line. So acids are made by oxidation, hydrolysis or carboxylation, and they boil high because of dimers.

Scoring tip: Write the hydrolysis step (H₃O⁺) for the Grignard and alkylbenzene routes; examiners deduct for stopping at the salt.