The enol content of simple carbonyl compounds: a kinetic approach

Abstract

Following a suggestion of Lienhard and Wang, we demonstrate that the enol content of simple carbonyl compounds can be estimated as the ratio of the rate constants for acid-catalyzed enolization of the carbonyl compound and acid-catalyzed hydrolysis of the corresponding methyl enol ether. Values so estimated are in excellent agreement with those determined by our thermochemical method. Sufficient data are available for the following; compound, pKEnol: acetaldehyde, 4.7; propionaldehyde, 4.2; isobutyraldehyde, 2.7; acetone, 7.0; cyclopentanone, 6.7; cyclohexanone, 5.3; cycloheptanone, 7.2; cyclooctanone, 6.6; p-nitroacetophenone, 4.9; p-bromoacetophenone, 6.2; acetophenone, 6.6; p-methylacetophenone, 7.0; p-methoxyacetophenone, 7.3; and by estimating the rate constants for enolization, the following; indanone, 7; 2-phenyl-2-propanone, 4; cyclopropyl methyl ketone, 8; chloroacetaldehyde, 2 (corrected for hydration); phenylacetaldehyde, 2.