A Theoretical Study of Substituent Effects on Allylic Ion and Ion Pair SN2 Reactions
- Resource Type
- Authors
- S. S.-H. Leung; Elambalassery G. Jayasree; Andrew Streitwieser; Godwin Sik Cheung Choy
- Source
- ChemInform. 37
- Subject
- chemistry.chemical_classification
Allylic rearrangement
Double bond
organic chemicals
Substituent
Ab initio
Ionic bonding
General Medicine
Transition state
Ion
chemistry.chemical_compound
chemistry
Computational chemistry
SN2 reaction
- Language
- ISSN
- 1522-2667
0931-7597
An ab initio study of ionic and ion pair displacement reactions involving allylic systems has been carried out at the RHF/6-31+G* level. The geometries and natural charges show the absence of conjugative stabilization in the ionic transition states, thus differing from traditional explanations. The high reactivity of allyl halides is explained by electrostatic polarization of the double bond. Substituent effects were also studied; in general, electron-withdrawing groups lower the barriers of the ionic SN2 reactions but increase the barriers of the ion pair reactions. The allylic reactions are compared with related benzylic systems. Hammett correlations give ρ of opposite sign for the ionic and ion pair displacement reactions, in agreement with some experimental results.