Keto-Enol Tautomerism. II. The Calorimetrical Determination of the Equilibrium Constants for Keto-Enol Tautomerism for Cyclohexanone and Acetone

Abstract The vapor phase enol contents of cyclohexanone and acetone were determined by calorimetric measurements of the enthalpies of hydrolysis of their corresponding enol methyl ethers to ketones and methanol in 91.67% THF–H2O (0.15 M H2SO4). The relative order of enol content, together with cyclopentanone in our previous study, was cyclohexanone>cyclopentanone≥ acetone, which was the same as that reported by Gero and by Schwarzenbach, but different from Bell’s observed order (cyclopentanone>cyclohexanone>acetone). The enthalpies calculated from Benson’s correlation of thermodynamic properties for the enolization reactions of these ketones and for the hydrolysis reactions of enol methyl ethers agreed with our relative ordering. Furthermore, data on reactions of compounds with 5- and 6-membered rings lead to the prediction that cyclohexanone contains more enol than cyclopentanone does. The kinetics of hydrolysis of enol methyl ethers was also discussed.