Gas-Phase Identity SN2 Reactions of Halide Anions and Methyl Halides with Retention of Configuration
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- 作者：Mikhail N. Glukhovtsev ; Addy Pross ; H. Bernhard Schlegel ; Robert D. Bach ; and Leo Radom
- 刊名：Journal of the American Chemical Society
- 出版年：1996
- 出版时间：November 13, 1996
- 年：1996
- 卷：118
- 期：45
- 页码：11258 - 11264
- 全文大小：234K
- 年卷期：v.118,no.45(November 13, 1996)
- ISSN：1520-5126
文摘
High-level ab initio molecular orbital calculations atthe G2(+) level of theory have been carried out onthe identity front-side nucleophilic substitution reactions withretention of configuration, X- + CH3X, forX = F,Cl, Br, and I. Overall gas-phase barrier heights do not show astrong variation with halogen atom and are calculatedto be 184.5 (X = F), 193.8 (X = Cl), 178.9 (X = Br), and 171.4 kJmol-1 (X = I), substantially higher thanthecorresponding barriers for back-side attack (-8.0 for X = F, 11.5for X = Cl, 5.8 for X = Br, and 6.5 kJmol-1 forX = I). The difference between the overall barrier for back-sideattack and front-side attack is smallest for X = I(164.9 kJ mol-1). Central barrierheights for front-side attack decrease in the following order: 241.0(X = F), 237.8(X = Cl), 220.0 (X = Br), and 207.4 kJmol-1 (X = I). The minimum energypathways for both back-side andfront-side SN2 reactions are found to involve the sameion-molecule complex(X-···H3CX), with thefront-sidepathway becoming feasible at higher energies. Indeed, our resultssuggest that the chloride exchange inCH3Cl,which has been found in gas-phase experiments at high energies, may bethe first example of a front-side SN2reaction with retention of configuration at saturated carbon.Analysis of our computational data in terms offrontierorbital theory suggests that elongation of the bond between the centralatom and X could be a significant factor indecreasing the unfavorable nature of the front-side SN2reaction with retention of configuration in going from X=F to X = I. Ion-molecule complexesCH3-X···X-, which may be pre-reactioncomplexes in direct collinear halophilicattack, were found for X = Br and I but not for X = F and Cl.The calculated complexation energies(
Hcomp) forhalophilic complexes are considerably smaller (7.3 and 19.4 kJmol-1 for X = Br and I, respectively) thanthose forthe corresponding pre-reaction complexes for SN2 attack atcarbon (41.1 and 36.0 kJ mol-1 for X = Brand I,respectively). Nucleophilic substitution reactions at the halogenatom in CH3X (X = F-I) (halophilic reactions)arehighly endothermic and appear to represent an unlikely mechanisticpathway for identity halide exchange.
Hcomp) forhalophilic complexes are considerably smaller (7.3 and 19.4 kJmol-1 for X = Br and I, respectively) thanthose forthe corresponding pre-reaction complexes for SN2 attack atcarbon (41.1 and 36.0 kJ mol-1 for X = Brand I,respectively). Nucleophilic substitution reactions at the halogenatom in CH3X (X = F-I) (halophilic reactions)arehighly endothermic and appear to represent an unlikely mechanisticpathway for identity halide exchange.