Evidence for merging of mechanism in competing SN2 and E2 processes

Abstract

Dissected ratios k_OTs/k_Br(S_N2) and k_OTs/k_Br(E2) for competing substitution and elimination, respectively, have been determined for alkoxide ion (EtO^–/EtOH and Bu^tO^–/Bu^tOH) promoted reactions of simple alkyl derivatives RX (R = Me, Et, Prⁿ, Buⁱ, or Neopen; X = OTs, or Br). The ratios are shown to be severely distorted when compared with those for pure substitutions and pure eliminations. In general, a rise in k_OTs/k_Br(S_N2) is coupled with a fall in k_OTs/k_Br(E2). The observed distortion is consistent with the hypothesis that alkoxide ion promoted S_N2 and E2 reactions are not completely independent processes. A merged type mechanism is suggested, in which alkoxide ion interacts with both C_α and H_β before partitioning to S_N2 and E2 transition states occurs.