Substituent effects in gas-phase substitutions and eliminations:: β-Halo substituents.: Solvation reverses SN2 substituent effects

The gas-phase reactions of benzoate and phenolate containing dianions with a series of P-substituted alkyl bromides (X-CH2CH2Br, X = H, F, Cl, Br) have been studied in a quadrupole ion trap mass spectrometer. Branching ratios between S(N)2 and E2 products were measured and rate constants were determined. The beta -halogens increase both the S(N)2 and E2 rates, but the effect is greater for the latter process and therefore these substituents lead to an increase in the amount of elimination. The kinetic data for the S(N)2 reactions can be analyzed via a two-parameter, linear free-energy relationship and the results indicate that field-effects (i.e., electron-withdrawing groups) strongly favor the reaction (rho (F) = 1.83). In contrast, analysis of the available condensed phase data for these substrates indicates that halogens strongly retard the reaction (rho (F) = -2.04). The dramatic reversal in substituent effects can be explained by a simple electrostatic model which suggests that solvation causes the system to shift to a more highly ionized S(N)2 transition state.