Substituent effects on the activation parameters of SNar reactions

The reaction constants δΔH≠ and δΔS≠ derived from the dependences of the ΔH≠ and ΔS≠ activation parameters on substituent constants σ in SNAr reactions of substituted benzenes with anionic and neutral nucleophiles in various solvents were found to be linearly related upon variation of substituents in the substrate and nucleophile. The δΔH≠—δΔS≠ dependences make it possible to estimate the contribution of the internal enthalpy δΔH≠t into the enthalpy reaction constant δΔH≠. The δΔH≠int values are linearly related to the Hammett p values. Deviations from the linear dependence δΔH≠int—n were interpreted in terms of change of the reaction mechanism. Substituent effect on the development of charge in the transition state is determined by the δΔH≠int value.