Steric and solvation effects in ionic S(N)2 reactions

Chen, Xin, Regan, Colleen K., Craig, Stephen L., Krenske, Elizabeth H., Houk, K. N., Jorgensen, William L. and Brauman, John I. (2009) Steric and solvation effects in ionic S(N)2 reactions. Journal of the American Chemical Society, 131 44: 16162-16170. doi:10.1021/ja9053459


Author Chen, Xin
Regan, Colleen K.
Craig, Stephen L.
Krenske, Elizabeth H.
Houk, K. N.
Jorgensen, William L.
Brauman, John I.
Title Steric and solvation effects in ionic S(N)2 reactions
Formatted title
Steric and solvation effects in ionic SN2 reactions
Journal name Journal of the American Chemical Society   Check publisher's open access policy
ISSN 0002-7863
1520-5126
Publication date 2009-11
Sub-type Article (original research)
DOI 10.1021/ja9053459
Volume 131
Issue 44
Start page 16162
End page 16170
Total pages 9
Place of publication Washington, United States
Publisher American Chemical Society
Language eng
Formatted abstract
This paper explores the contribution of solvation to the overall steric effects of SN2 reactions observed in solution. The reactions of chloride ion with a series of alkyl chloronitriles, RCH(CN)Cl (R = methyl, ethyl, isopropyl, tert-butyl) were investigated both experimentally and theoretically. These reactions serve as a model system for the parent reactions, Cl− + RCH2Cl, which are too slow to measure. Each additional substitution at the β-carbon lowers the reactivity, clearly demonstrating a steric hindrance effect. The magnitude of the steric effect, however, is not significantly different in the gas phase and in solution. We conclude that the solvation energies of the corresponding SN2 transition states must be similar regardless of size of the substituent. This lack of size dependence in the current system is in sharp contrast with many other ionic systems such as ionization of simple alkyl alcohols, where solvation depends strongly on size. We propose that the weak size dependence results from the compensation between a direct shielding effect of the substituent and an indirect ionic solvation effect, which arises from the geometric perturbations introduced by the substitution. The conclusion is further supported by calculations using polarizable continuum models and QM/MM simulations.
Q-Index Code C1
Q-Index Status Provisional Code
Institutional Status Non-UQ

Document type: Journal Article
Sub-type: Article (original research)
Collection: School of Chemistry and Molecular Biosciences
 
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