Substituent effects in substrates on activation parameters in the bimolecular nucleophilic reactions in solution

Abstract

Changes of the activation parameters, ΔH^≠ and ΔS^≠, in the S_N2, S_NAr and acyl-transfer reactions with charged and neutral nucleophiles in various solvents were correlated with σ constants of the substituents in the aromatic ring of the substrates. The resultant δΔH^≠ and δΔS^≠ reaction constants are linearly related for variations of substituents at the substrate. Correlation of δΔH^≠vs. δΔS^≠ allows one to estimate the contribution of changes of the internal enthalpy, δΔH^≠_int, to the enthalpy reaction constant, δΔH^≠, which gives a single linear dependence on the Hammett ρ reaction constants for all bimolecular nucleophilic reactions. The deviations from dependence of δΔH^≠_intvs. ρ can be interpreted in terms of changes of the transition state structure or reaction mechanism. The results obtained show that the substituent effects in the substrates and nucleophiles on the charge development in the transition state are governed by the magnitude of δΔH^≠_int.