Nucleophilic Addition of Olefins. 12. Solvent-Induced Change in the Rate-Limiting Step of the Hydrolysis of Benzylidenemalononitrile in Acidic Me2SO-Water Solution

Article abstract of DOI:10.1021/ja00298a035

A kinetic study of the reversible, four-step hydrolysis of benzylidenemalononitrile (BMN) to form benzaldehyde and malononitrile in water and Me2SO-water mixtures (50percent, 60percent, and 70percent Me2SO (v/v)) is reported.At pHo_OH, Scheme I, k₃₄^H2O), is rate limiting in all solvents.Above pH 2 the breakdown of the anionic intermediate, PhCH(O^-)CH(CN)2 (T^-_O, Scheme I, k₄), is rate limiting in water, while in the presence of Me2SO oxygen deprotonation of T^o_OH (k₃^B) becomes partially rate limiting at low buffer concentrations.A detailed kinetic analysis, coupled with a few assumptions about rates of diffusion-controlled proton transfers, allows one to estimate the rate and equilibrium constants of all kinetically important steps.This analysis shows that the solvent-induced change in the rate-limiting step is caused by an enhanced rate of breakdown of T^-_O(k₄) in the Me2SO-containing solvents.This rate enhancement can be understood in terms of an increased "push" by the less solvated anionic oxygen of T^-_O.This increased "push" is somewhat attenuated by a reduced "pull" which arises from the slight destabilization of CH(CN)2(1-) in the Me2SO-containing solvents.On a more quantitative level the equilibrium constant, K₄, for the breakdown of T^-_O into benzaldehyde and CH(CN)2(1-) correlates remarkably well with the estimated solvent activity coefficients of T^-_O <*> and CH(CN)2(1-) <*>.A correlation of the solvent effect on the rate constant, k₄, of the breakdown of T^-_O with the same solvent activity coefficients suggests that the sensitivity of k₄ to <*> is greater than its sensitivity to <*>.This "imbalace" is attributed to a rate-retarding effect which arises from early desolvation of the oxyanion in the transition state , an effect which is magnified in the less aqueous solvents.The direct breakdown of T^o_OH into benzaldehyde and malononitrile (k₃₄^H2O) probably occurs by a mechanism in which C-C bond cleavage is concerted with the removal of the OH proton by the solvent.Suggestive, though inconclusive, evidence is presented in favor of a transition state (2) in which oxygen deprotonation is coupled with protonation of the departing carbanion.