SN2 Transition State. 5. Effect of a-Substituents on SN2 Reactivity and the SN2–SN1 Borderline Problem. A Molecular Orbital Approach

Daniel Kost, Kalman Aviram

Research output: Contribution to journalArticlepeer-review

32 Scopus citations

Abstract

Molecular orbital (MO) theory is applied to the analysis of the SN2 identity reaction, with various a-substituents (H, planar NH2, planar BH2, OH, CHO, F, and CH3) and nucleophile leaving group pairs (H-, NH3, and BH4-). Ground states, transition states, and encounter complexes have been optimized at the 4-31G level. We find that π-interactions between an a-substituent and a π-type occupied orbital associated with the reaction coordinate axis are of major importance in governing transition-state (TS) energy and reaction rate. π-Donors in the maximum overlap (“on”) conformation slow the reaction, whereas π-acceptors accelerate it. π-Donors which lack cylindrical symmetry (e.g., methoxy and hydroxy) can avoid the repulsive interaction by a 90° rotation relative to the reaction coordinate (“turning off” the interaction). Inductive electron withdrawal, when neutralized from π-effects, seems to have an accelerating influence. Four-electron repulsion in the “on” geometry of a TS with a π-donor is minimized through “Loosening” of the TS and thereby making it more “SN1-like”. Weak nucleophiles (and powerful leaving groups) also generate loose transition states with carbocationic character. The destabilizing effect of π-donors (in the on geometry) diminishes gradually with extension of the TS, until it turns to a stabilizing interaction with the developing carbocation. Thus, in terms of geometry, a distinct crossover between a tight SN2 TS with a preferred off conformation to an SN1-like TS with a stable on conformation is found. The diminishing rate enhancement of a-halocarbonyl compounds with decreasing nucleophilic power of the nucleophile, as well as the rate enhancement observed for p-nitrobenzyl chloride with powerful nucleophiles and rate retardation with neutral nucleophiles, is rationalized in terms of this analysis and the effect of the nucleophile on SN2 TS tightness. The dependence of rate retardation by a-F on the leaving group is treated in a similar manner.

Original languageEnglish
Pages (from-to)2006-2013
Number of pages8
JournalJournal of the American Chemical Society
Volume108
Issue number8
DOIs
StatePublished - 1 Jan 1986

ASJC Scopus subject areas

  • Catalysis
  • General Chemistry
  • Biochemistry
  • Colloid and Surface Chemistry

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