Towards mechanisms of bimolecular nucleophilic reactions in solution—probing the variation of the activation parameters in the reactions of aromatic compounds

Variation of the activation parameters in the S_N2, acyl‐transfer, S_NAr, S_NV, and Ad_N reactions offers a uniquely useful probe for the mechanistic features of these reactions in solution. New approach uses the substituent effects on the aromatic ring to the variation of the activation parameters, ΔH^≠ and ΔS^≠, in the above reactions in the frameworks of the Hammett‐like equations in order to evaluate the resultant δΔH^≠ and δΔS^≠ reaction constants. Compensation relationships of δΔH^≠ versus δΔS^≠ allow one to estimate the contribution of changes of the internal enthalpy, δΔH^≠_int, to the enthalpy reaction constant, δΔH^≠, that is inherent to bimolecular nucleophilic reactions and gives a single linear dependence on the Hammett ρ reaction constants for these reactions. The deviations from dependence of δΔH^≠_int versus ρ serve as useful points of interpretation of changes of the transition state structure or reaction mechanism. The results obtained show that the substituent effects in the substrates, nucleophiles, and leaving groups on the mechanistic features in bimolecular nucleophilic reactions are governed by the magnitude of δΔH^≠_int. Copyright © 2011 John Wiley & Sons, Ltd.