Reactions and uses of carboxylic acids

Heating with mineral acids such as H₂SO₄, or with P₂O₅, gives the anhydride.

Esterification: an acid and an alcohol or phenol give an ester with concentrated H₂SO₄ or HCl gas as catalyst. The mechanism is nucleophilic acyl substitution: protonated C=O is attacked by the alcohol, a proton shift turns –OH into –OH₂⁺, water leaves, and the protonated ester loses H⁺. Removing water or ester as it forms drives the reversible reaction.

PCl₅, PCl₃ or SOCl₂ replace –OH by Cl to give the acyl chloride. Thionyl chloride is preferred because its by-products, SO₂ and HCl, are gases that escape, leaving the product easy to purify.

Ammonia gives the ammonium salt, which on strong heating loses water to give the amide.

LiAlH₄, or better diborane, reduces acids to primary alcohols. Diborane leaves ester, nitro and halo groups largely alone; NaBH₄ does not reduce –COOH.

Decarboxylation: the sodium salt heated with sodalime (NaOH and CaO in the ratio 3 : 1) loses CO₂ to give a hydrocarbon. Kolbe electrolysis of an aqueous alkali-metal salt gives a hydrocarbon with twice the carbons of the acid's alkyl group.

Hell-Volhard-Zelinsky reaction: an acid with an α-hydrogen, treated with chlorine or bromine and a little red phosphorus, gives the α-halocarboxylic acid.

In aromatic acids –COOH deactivates the ring and directs to meta. They do not undergo Friedel-Crafts reactions: the ring is deactivated and the AlCl₃ catalyst binds to the carboxyl group.

Uses: methanoic acid in rubber, textile, dyeing, leather and electroplating; ethanoic acid as a solvent and as vinegar; hexanedioic acid for nylon-6,6; benzoate esters in perfumes; sodium benzoate as a food preservative; higher fatty acids for soaps and detergents.