The Alkyl Inductive Effect, II Theoretical Calculation of Inductive Parameters

Abstract

Three models of alkyl groups, “delocalization”, “through-the-bond”, and “electric field” models, are presented, all of which enable the calculation of σI(R) from first principles, and excellent agreement is demonstrated for the calculated and experimental values of σI(R). For the “delocalization” model it is found that —σi(R) = 0.0455 + 0.0232(1—1/n), where n is the number of C atoms in an w-alkyl group, and for an infinite C-chain R group, σI(R∞) is —0.0687, identical to the value found by a different method in Part I of this series. The “through-the-bond” model gives —σI(R) = —0.0559 + 0.1015 [formula ommited] Ci/(2i—1)2, where Ci is the number of C-atoms in the ith position from X in RX; and for the “electric field” model, we obtain —σI(R) = 0.0463 + 0.0102 [formula ommited] Cidi-2, where di is the calculated distance from C1 to Cn in the most probable conformation of the R-group. It is concluded that Taft's [formula ommited]) values have a real significance whether or not the physical and chemical effects of alkyl substitution reside ultimately in an internal induction mechanism, or in alkyl group polarization by charged centers in the molecule, or a combination of the two. © 1979, Walter de Gruyter. All rights reserved.