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

Changes of the activation parameters, Delta H-not equal and Delta S-not equal, in the S(N)2, SNAr and acyl-transfer reactions with charged and neutral nucleophiles in various solvents were correlated with sigma constants of the substituents in the aromatic ring of the substrates. The resultant delta Delta H-not equal and delta Delta S-not equal reaction constants are linearly related for variations of substituents at the substrate. Correlation of delta Delta H-not equal vs. delta Delta S-not equal allows one to estimate the contribution of changes of the internal enthalpy, delta Delta H-int(not equal), to the enthalpy reaction constant, delta Delta H-not equal, which gives a single linear dependence on the Hammett rho reaction constants for all bimolecular nucleophilic reactions. The deviations from dependence of delta Delta H-int(not equal) vs. rho 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 delta Delta H-int(not equal).