benzoic acid

Meaning: Benzoic acid, C₆H₅COOH, is the simplest aromatic carboxylic acid, a carboxyl group attached to a benzene ring. It is a weak acid (pKa about 4.2). Losing H⁺ gives benzoate, whose negative charge is delocalized over both oxygens by resonance. Ring substituents change acidity through their effect on benzoate: electron-withdrawing groups such as NO₂ stabilize it and strengthen the acid, while electron-donating groups such as CH₃ or OCH₃ generally weaken it.

Equation: Ka = [C₆H₅COO⁻][H₃O⁺] / [C₆H₅COOH], with concentrations in mol/L at equilibrium and Ka taken as unitless; pKa = −log Ka.

On the MCAT: A common passage separates a mixture dissolved in ether. Aqueous NaHCO₃ or NaOH converts benzoic acid to sodium benzoate, an ion that dissolves in the water layer; adding HCl to that layer reprotonates it and benzoic acid precipitates. Benzoic acid is also the product when toluene, or another alkylbenzene with a benzylic hydrogen, is oxidized by KMnO₄. In electrophilic aromatic substitution, the carboxyl group is a deactivating, meta-directing substituent.

Common confusion: Benzoic acid (pKa about 4) is far more acidic than phenol (pKa about 10), so bicarbonate deprotonates benzoic acid but not phenol. Also, the neutral acid is poorly soluble in water; it is the benzoate salt that dissolves.