Using incoming nucleophile primary hydrogen-deuterium kinetic isotope effects to model the SN2 transition state

The primary hydrogen-deuterium incoming nucleophile KIEs for the S(N)2 reactions between parasubstituted benzyl chlorides and borohydride ion in DMSO at 30.000 +/- 0.002 degrees C are small (less than or equal to 1.14) and insensitive to a change in substituent at the alpha-carbon. The small Hammett rho (0.51) and rho(Gamma) (-0.52) values found when the para substituent on the benzene ring of the substrate is altered indicate there is very little charge on the alpha-carbon in the transition state. The large, constant secondary alpha-deuterium KIEs of 1.089 +/- 0.002 and the large chlorine leaving group KIEs of 1.0076, 1.0074, and 1.0078 found for the p-methyl-, the p-hydrogen-, and the p-chlorobenzyl chloride reactions suggest that the transition states for these reactions are unsymmetric with short H-C-alpha and long B-H and C-alpha-Cl bonds. The decrease in the chlorine leaving group KIE from 1.0076 +/- 0.0003 for the p-methylbenzyl chloride reaction to 1.0036 +/- 0.0003 for the p-nitrobenzyl chloride reaction indicates the C-alpha-Cl bond shortens markedly when a strongly electron-withdrawing substituent is on the ex-carbon. Unfortunately, the bond strength hypothesis is the only theory that predicts the changes observed in transition-state structure and it only indicates the bond that changes but not how the transition-state structure is altered.