Effects of substituents on activation parameters in SN2 reactions at aliphatic carbon in solution

Changes of the activation parameters in aliphatic S(N)2 reactions with anionic and neutral nucleophiles in various solvents, Delta H-not equal and Delta S-not equal, were correlated with a constants of the substituents. The resultant SAW and S.A.S reaction constants are linearly related for variations of substituents at the substrate, leaving group and nucleophile. Correlations of delta Delta H-not equal versus delta Delta S-not equal allow the estimation of 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. The deviations from the dependence of delta Delta H-int(not equal), versus rho can be interpreted in terms of changes in the transition state structure in S(N)2 reactions. The results obtained show that the substituent effects on the charge development in the transition state are governed by the magnitude of delta Delta H-int(not equal). Copyright (C) 2009 John Wiley & Sons, Ltd.