Q.Give five methods of preparation of carboxylic acids. OR Explain the following :

Carboxylic acids can be prepared by oxidising alcohols/aldehydes or alkylbenzene side chains, by hydrolysing nitriles or amides, by carbonating Grignard reagents, or by hydrolysing acid derivatives (acyl chlorides, anhydrides, esters) — each route builds or exposes the –COOH group from a precursor already carrying (or capable of carrying) that carbon.

1. Oxidation of primary alcohols and aldehydes: primary alcohols and aldehydes are oxidised to carboxylic acids by strong oxidising agents such as acidified KMnO₄ or K₂Cr₂O₇: RCH₂OH → RCHO → RCOOH.

2. Oxidation of alkylbenzenes: any alkyl side chain on an aromatic ring, however long, is oxidised entirely down to a single –COOH group directly attached to the ring by hot alkaline KMnO₄ (followed by acidification): C₆H₅CH₃ (toluene) → C₆H₅COOH (benzoic acid).

3. Hydrolysis of nitriles/amides: nitriles (RCN) are hydrolysed by aqueous acid or alkali, usually via the amide, to give the carboxylic acid (or its salt, which is then acidified): RCN + 2H₂O/H⁺ → RCOOH + NH₄⁺.

4. From Grignard reagents: a Grignard reagent reacts with dry CO₂ (carbonation) to give a magnesium carboxylate, which on acid hydrolysis gives the carboxylic acid with one more carbon than the original alkyl/aryl halide: RMgX + CO₂ → RCOOMgX → (H₃O⁺) → RCOOH.

5. Hydrolysis of acid derivatives (acyl chlorides, anhydrides and esters): the common acid derivatives all give back the parent carboxylic acid on hydrolysis. Acyl chlorides and anhydrides are hydrolysed readily by water (faster with aqueous base, then acidified), while esters need acid- or base-catalysed hydrolysis: RCOCl + H₂O → RCOOH + HCl; (RCO)₂O + H₂O → 2RCOOH; RCOOR' + H₂O/H⁺ ⇌ RCOOH + R'OH (or RCOOR' + NaOH → RCOONa + R'OH, then acidify).

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