Can One Predict Changes from SN1 to SN2 Mechanisms?

The reactions of substituted benzhydryl bromides Ar2CHBr with primary and secondary amines in DMSO yield benzhydryl amines Ar2CHNRR', benzophenones Ar2C=O, and benzhydrols Ar2CHOH. Kinetic investigations at 20 degrees C revealed the rate law -d[Ar2CHBr]/dt = (k(1) + k(2)[HNRR')[Ar2CHBr], where the amine independent term k(1) gave rise to the formation of Ar2C=O and Ar2CHOH and the amine-dependent term k(2)[HNRR'] was responsible for the formation of Ar2CHNRR'. Clear evidence for concomitant S(N)1 and S(N)2 processes was obtained. While the rate constants of the S(N)1 reactions correlate with Hammett's sigma(+) constants (rho = -3.22), the second-order rate constants k(2) for the S(N)2 reactions are not correlated with the electron releasing abilities of the substituents, indicating that the transition states of the S(N)2 reactions do not merge with the transition states of the S(N)1 reactions. The correlation equation log k(20 degrees C) = s(E + N), where nucleophiles are characterized by N and s and electrophiles are characterized by E (J. Am. Chem. Soc. 2001, 123, 9500-9512), was used to calculate the lifetimes of benzhydrylium ions in the presence of amines and DMSO. The change from S(N)1 to S(N)2 mechanism occurred close to the point where the calculated rate constant for the collapse of the benzhydrylium ions with the amines just reaches the vibrational limit; that is, the concerted S(N)2 mechanism was only followed when it was enforced by the lifetime of the intermediate. The nucleophile-specific parameters N and s needed for this analysis were determined by studying the kinetics of the reactions of a variety of amines with amino-substituted benzhydrylium tetrafluoroborates (Ar2CH+BF4-) of known electrophilicity E in DMSO. Analogously, the rates of the reactions of laser flash photolytically generated benzhydrylium ions Ar2CH+ with DMSO in acetonitrile were employed to determine the nucleophile-specific parameters Nand s of DMSO, and it is reported that DMSO is a significantly stronger O-nucleophile than water and ordinary alcohols.