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 para-substituted benzyl chlorides and borohydride ion in DMSO at 30.000 ± 0.002 °C are small (≤1.14) and insensitive to a change in substituent at the α-carbon. The small Hammett ρ (0.51) and ρ r (−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 α-carbon in the transition state. The large, constant secondary α-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 α and long B−Η and C α −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 α −Cl bond shortens markedly when a strongly electron-withdrawing substituent is on the α-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.