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-phenylac etonitrile, 2-X (X = NO2, SO2CH3, CN, CF3, Br, and Cl) by piperidine a nd morpholine and for the reverse reaction (k(-1)(BH)) have been deter mined 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) + lo g(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-nitrop henyl moiety lags behind proton transfer. A consequence of this imbala nce is that the intrinsic rate constant decreases with increasing elec tron withdrawing strength of X, For pi-acceptor substituents (NO2, SO2 CH3, CN) there is a further decrease in k(0) due to a lag in the deloc alization of the charge into X. The intrinsic rate constants depend ve ry little on the Me(2)SO content of the solvent which is shown to be t he result of compensation of mainly two competing factors, One is the stabilization of the polarizable transition state by the polarizable M e(2)SO which increases k(0); the other is attributed to a lag in the s olvation of the developing carbanion behind proton transfer at the tra nsition state which leads to a decrease in k(0).