33.1 Carboxylic acids

An alkylbenzene with at least one benzylic hydrogen can be oxidised by hot alkaline KMnO₄. After acidification, the side chain is converted to a carboxylic acid attached directly to the ring.

The carbon skeleton beyond the benzylic carbon is removed during vigorous oxidation, so different alkylbenzenes can give the same benzoic-acid ring product.

Methylbenzene + hot alkaline KMnO₄, then dilute acid → benzoic acid. The methyl carbon becomes the carboxyl carbon.

Do not oxidise the aromatic ring itself or stop at an alcohol; this is a strong side-chain oxidation followed by acidification.

Carboxylic acids react with chlorinating agents such as PCl₃ with heat, PCl₅ or SOCl₂ to replace the hydroxyl group by chlorine, forming an acyl chloride.

The acyl chloride is more reactive because chloride is a better leaving group than hydroxide. It can then be used to make esters, amides or other acyl derivatives.

Ethanoic acid + PCl₅ → ethanoyl chloride plus phosphorus-containing products and HCl. The exact by-products depend on the reagent.

Do not treat the reaction as simple substitution at a saturated carbon; the carbonyl carbon is the electrophilic centre.

Carboxylic acids are already highly oxidised at the carbonyl carbon, but some can undergo further oxidation under strong conditions, depending on the structure.

Do not assume every carboxylic acid has an ordinary oxidation product. Identify the carbon framework and the oxidant before predicting cleavage or further oxidation.

Methanoic acid is readily oxidised to carbon dioxide and water because it still has a hydrogen attached to the carbonyl carbon; ethanoic acid is much more resistant under typical syllabus conditions.

“Carboxylic acid cannot oxidise” is too absolute, while “all acids oxidise to CO₂” ignores structural and condition boundaries.

The relative acidity is carboxylic acid > phenol > alcohol. Carboxylate ions delocalise negative charge over two oxygens; phenoxide delocalises into the ring; alkoxides localise charge on one oxygen.

Compare conjugate-base stability and electron-donating or withdrawing substituents, not just the O–H bond itself.

A carboxylic acid reacts with aqueous carbonate to release CO₂; phenol is too weak for this under ordinary conditions, and ethanol is weaker still.

All three can donate H⁺, but “weak acid” is relative: a weak acid can still be substantially stronger than another weak acid.

Chlorine withdraws electron density through sigma bonds. This stabilises the negative charge of a carboxylate ion, so a chlorine-substituted carboxylic acid is more acidic than the unsubstituted acid.

The effect depends on distance: chlorine closer to the carboxyl group exerts a stronger inductive effect, and multiple chlorines reinforce it.

2-chloropropanoic acid is more acidic than propanoic acid; 3-chloropropanoic acid is also stronger than propanoic acid but less strongly affected by the greater separation.

Do not use chlorine’s aromatic directing effects here; acidity is controlled by its inductive withdrawal through the acid’s carbon chain.