α- and β-Carbon Substituent Effect on S_N2 Reactivity. A Valence-Bond Approach

The α- and β-carbon substituent effect, on S_N2 reactivity and reactivity-selectivity, is discussed by using a (previously described) correlation diagram model of S_N2. The reaction barrier (E) is a fraction (f) of the energy gap (I_N:-A_RX) between the two curves which intersect to yield the reaction profile. I_N: is the ionization potential of the nucleophile (N:) and H_RX is the electron affinity of the substrate (RX). The fraction (f) of I_N:-A_RX which enters the activation barrier depends inter alia on the degree of delocalization of the three-electron bonds, e.g., (R^…X)^-. The more delocalized the three-electron bond, the larger the / Thus, reactivity trends arise from the interplay between the electron surge aspect (I_N:-A_RX) and the bond-interchange aspect (e.g., the degree of delocalization of the three-electron bonds) of the S_N2 transformation. It is shown that -halo substitution (on R) delocalizes the three-electron bond and effects a small improvement in the acceptor ability of the substrate, and therefore it slows down S_N2 reactivity. The largest delocalization is effected when the α-substituent(s) is (are) identical with the leaving group. In these cases, one observes the strongest rate retardation, Φ-acceptor α-substituents improve the substrate acceptor ability markedly without greatly delocalizing the three-electron bonds. Therefore, these substituents will enhance reactivity but mainly toward powerful nucleophiles. The effects of other α-substituents (e.g., CH₃O, Ph, SiR₃, etc.) and β-substituents (e.g., F, Cl, Br, RO, etc.) are also discussed in this light. The reactivity reversals often reported in the literature are suggested to be manifestations of the gap-slope interplay.