A study of thermodynamic acidities of enols with the semiempirical computational method AM1

Abstract

Using the semiempirical computational method AM1 we have computed the enthalpies of ionization of enols 1–21 and correlated these values with solution and estimated pKa's. From this initial study it appears that, in general, it is possible to predict the pK's of acyclic enols within ± 1 pKa unit. On the basis of this correlation we predict the pKa's of 1-hydroxy-1,3-butadiene (3-butenal enol) (9.7) and 1,1-difluoroacetone enol (9.6) and suggest that the previously estimated pKa of 1,1-dichloroacetone enol (6.2) is low by approximately 3 pK units. We find computationally (this has been established experimentally) that the Z-enol of phenylacetaldehyde is less acidic than the E-enol. Keywords: enols, acidities, computational studies, AM1.