Temperature Coefficients of the Rates of Acid-Catalyzed Enolization of Acetone and Ketonization of Its Enol in Aqueous and Acetonitrile Solutions. Comparison of Thermodynamic Parameters for the Keto-Enol Equilibrium in Solution with Those in the Gas Phase

Article abstract of DOI:10.1021/ja00193a033

Rates of hydrogen ion catalyzed enolization of acetone and ketonization of acetone enol were measured over a range of temperatures in water and in acetonitrile solution.The data give ΔH(excit.) = 20.0 +/- 0.1 kcal mol^-1, ΔS(excit.) = -12.1 +/- 0.3 cal K^-1 mol^-1, and ΔH(excit.) = 20.1 +/- 0.5 kcal mol^-1, ΔS(excit.) = -5.8 +/- 1.7 cal K^-1 mol^-1, for enolization in water and acetonitrile, respectively, and ΔH(excit.) = 9.7 +/- 0.4 kcal mol^-1, ΔS(excit.) = -8.6 +/- 1.4 cal K^-1 mol^-1, and ΔH(excit.) = 11.4 +/- 0.2 kcal mol^-1, Σ(excit.) = 1.6 +/-0.8 cal K^-1 mol^-1, for ketonization in water and acetonitrile, respectively.These values lead to ΔH⁰ = 10.3 +/- 0.4 kcal mol^-1, ΔS⁰ = -3.5 +/- 1.5 cal K^-1 mol^-1, for the keto-enol equilibrium in water, and ΔH⁰ = 8.7 +/- 0.6 kcal mol^-1, ΔS⁰ = -7.4 +/- 1.9 cal K^-1 mol^-1, for the equilibrium in acetonitrile.This is the first determination of thermodynamic parameters for a simple ketone-enol equilibrium in solution; the results are remarkably similar to the thermodynamic parameters for this reaction in the gas phase.A mechanism involving acid catalysis of the bromination of acetone enol by N-bromosuccinimide, the process used to monitor enolization in acetonitrile solution, is ruled out.