Kinetics of proton transfer from 2-nitro-4-X-phenylacetonitriles to piperidine and morpholine in aqueous Me(2)SO. Solvent and substituent effects on intrinsic rate constants. Transition state imbalances

Rate constants (k(1)(B)) for the deprotonation of 2-nitro-4-X-phenylacetonitrile, 2-X (X = NO2, SO2CH3, CN, CF3, Br, and Cl) by piperidine and morpholine and for the reverse reaction (k(-1)(BH)) have been determined in 90% Me(2)SO-10% water, 50% Me(2)SO-50% water, and water (X = NO2, SO2CH3, CN only). Bronsted beta(B) values (dlog K-1(B)/dpK(a)(BH)), Bronsted alpha(CH) values (dlog K-1(B)/dlog K-a(Ch)), and intrinsic rate constants (log k(0) = log(k(1)/q) for pK(a)(BH) - p K-a(CH) + log(p/q) = 0) were calculated from these data. alpha(CH) is smaller than beta(B), implying an imbalance which is consistent with a transition state in which delocalization of the negative charge into the 2-nitrophenyl moiety lags behind proton transfer. A consequence of this imbalance is that the intrinsic rate constant decreases with increasing electron withdrawing strength of X, For pi-acceptor substituents (NO2, SO2CH3, CN) there is a further decrease in k(0) due to a lag in the delocalization of the charge into X. The intrinsic rate constants depend very little on the Me(2)SO content of the solvent which is shown to be the result of compensation of mainly two competing factors, One is the stabilization of the polarizable transition state by the polarizable Me(2)SO which increases k(0); the other is attributed to a lag in the solvation of the developing carbanion behind proton transfer at the transition state which leads to a decrease in k(0).