Effects of substituents on activation parameters in S N 2 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, Δ H ≠ and Δ S ≠ , were correlated with σ constants of the substituents. The resultant δ Δ H ≠ and δ Δ S ≠ reaction constants are linearly related for variations of substituents at the substrate, leaving group and nucleophile. Correlations of δ Δ H ≠ versus δ Δ S ≠ allow the estimation of the contribution of changes of the internal enthalpy, δ Δ H   ≠ int , to the enthalpy reaction constant, δ Δ H ≠ , which gives a single linear dependence on the Hammett ρ reaction constants. The deviations from the dependence of δ Δ H   ≠ intversus ρ 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 δ Δ H   ≠ int . Copyright © 2009 John Wiley & Sons, Ltd.