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 β-substituted alkyl bromides (X-CH 2 CH 2 Br, 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 β-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 (ρ F = 1.83). In contrast, analysis of the available condensed phase data for these substrates indicates that halogens strongly retard the reaction (ρ 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.