General acid catalysis in the enolization of acetone

We have used iodometry to study the enolization of acetone in water catalyzed by a series of general acids, comprised of hydrochloric acid, methanesulfonic acid, 24 aliphatic monocarboxylic acids, nine aromatic monocarboxylic acids, eight aliphatic dicarboxylic acids, and 20 monoanions of dicarboxylic acids. The log k–pK profile for unbuffered solutions of strong and moderately strong acids shows a maximum near pk ≈ 0. The Brønsted α value for a set of eight aliphatic monocarboxylic acids in which effects of bulk, charge, and polarizability are at a minimum is 0.56. Steric effects, probably augmented by polarizability effects in some cases, cause positive deviations from the Brønsted line drawn with respect to these standard acids. Anionic carboxylic acids are also more reactive than would be predicted from their equilibrium acid strengths, whereas cationic acids tend to be less reactive. Using D2O as solvent has only a small effect on the rate of carboxylic acid catalysis. Using acetone-d6 gives values of kH/kD in the range of 7.0–8.0 at 25 °C, values consistent with proton or deuteron being transferred between two bases of comparable strength, the carboxylate anion and the enol form of acetone. Keywords: general acid catalysis, enolization, Brønsted relation, steric effects, deuterium isotope effects.