An extended model of the SN2 reaction between a substituted alkane and nucleophile

This paper is devoted to the development of qualitative quantum-chemical models of bimolecular nucleophilic substitution (S(N)2 reaction) of substituted alkanes. To this end, we invoke the algebraic expressions for occupation numbers of basis orbitals of heteroatom-containing bonds obtained previously in the form of power series (J. Mel. Struct. (Theochem), 434 (1998) 43). The third-order terms of this series absent in the previous contribution but necessary for investigation of the SN2 reaction, are additionally derived. The well-known two-orbital model of the reaction (based on taking into account only the direct interaction between the electron-donating orbital of nucleophile (phi ((+)d)) and the electron-accepting orbital of the heteroatom-containing bond of the substituted alkane (phi ((-)a))) is shown to follow from second-order terms of the above-mentioned power series. Taking into account the third-order terms of the same series yields an extended model of the reaction, wherein the indirect interactions between orbitals phi ((+)d) and phi ((-)a) also are explicitly included, and orbitals of the nearest (C-alpha-C-beta and C-alpha-H) bonds with respect to the heteroatom-containing (Z-C-alpha) bond play the role of the principal mediators of this new interaction. The indirect interactions between orbitals phi ((+)d) and phi ((-)a) are shown to contribute significantly to a greater efficiency of the back attack of the nucleophile vs, the frontal one. (C) 2001 Elsevier Science B.V. All rights reserved.