THE EFFECT OF ALKYL-GROUPS ON CARBANION STABILITY .2. ANALYSIS OF ALPHA-ALKYL AND BETA-ALKYL SUBSTITUTION ON EXPERIMENTAL AND THEORETICAL ACIDITIES

Multivariational analyses were performed to separate the contribution of electronic and steric effects of alkyl groups on the experimental h ydrocarbon acidities of four different sets of compounds, consisting o f hydrocarbons (SET-I), acetophenone derivatives (SET-II), and acetone CH(3)COCHR(1)R(2) derivatives, where the acidity of the protons on th e alkyl substituted site, (SET-III), and the acidity of the methyl pro tons are both investigated (SET-IV). Significant regression was obtain ed upon backward elimination for the first and last set. Due to the co linearity of the parameters in SETs-II and -III the significance of th e correlation is not so high. As expected, the gas phase acidity of th e hydrocarbons in SET-I is very sensitive to the polarizability and el ectronegativity of the alkyl substituents. The substituent have simila r effects on the solution acidities of the ketone derivatives in SETs- II, -III, though suppressed due to solvation, unlike the cases in SET- II and SET-III, the effects of the substituents in SET-IV, where the s ubstituents are relatively far away from the carbanion center generate d are strongly reduced and reversed. This different behavior may be at tributed to the domination of the solvent effects. Theoretical aciditi es of these compounds were calculated using the semiempirical methods such as MNDO, AM1 and PM3. The results are, quantitatively and qualita tively, unsatisfactory for the gas phase, as well as for the solution acidities. Multivariational analysis of substituent effects shows the insufficiency of the present parametrization to reflect the substituen t effect properly. It seems that all these semiempirical methods are u seless in prediction of carbon acidities, without reparametrization us ing carefully selected sets of compounds.