carboxylic acid

Meaning: A carboxylic acid has the formula R–COOH: a carbonyl carbon bonded to an –OH group and to R (or H). Names end in -oic acid. Losing H⁺ gives a carboxylate, RCOO⁻, whose negative charge is delocalized over two oxygens by resonance. That stable conjugate base makes the acid much stronger than an alcohol (pKa about 4–5 versus about 16), though still weak.

Equation: Ka = [RCOO⁻][H₃O⁺] / [RCOOH], and pKa = −log Ka, where brackets are equilibrium concentrations in mol/L.

On the MCAT: Expect acidity rankings. Electron-withdrawing groups such as halogens near the carboxyl stabilize the carboxylate and lower the pKa, so trichloroacetic acid is stronger than chloroacetic acid, which is stronger than acetic acid. Electron-donating alkyl groups weaken the acid slightly. In spectroscopy, look for a very broad O–H stretch near 2500–3300 cm⁻¹, a C=O stretch near 1710 cm⁻¹, and a ¹H NMR signal near 10–12 ppm. In extractions, aqueous NaHCO₃ converts the acid to a water-soluble carboxylate, and adding strong acid regenerates it. Alcohols give esters; LiAlH₄ reduces the acid to a primary alcohol.

Common confusion: Students call carboxylic acids strong acids, or credit the O–H bond alone for the acidity. They are weak acids, only partly dissociated in water, and the acidity comes mainly from resonance stabilization of the carboxylate.