Substituent effects in gas-phase substitutions and eliminations: beta-Halosubstituents. Solvation reverses S(N)2 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) hav e been studied in a quadrupole ion trap mass spectrometer. Branching ratios between S(N)2 and E2 products were measured and rate constants were determ ined. The beta -halogens increase both the S(N)2 and E2 rates, but the effe ct 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 relation ship and the results indicate that field-effects (i.e., electron-withdrawin g groups) strongly favor the reaction (rho (F) = 1.83). In contrast, analys is of the available condensed phase data for these substrates indicates tha t halogens strongly retard the reaction (rho (F) = -2.04). The dramatic rev ersal in substituent effects can be explained by a simple electrostatic mod el which suggests that solvation causes the system to shift to a more highl y ionized S(N)2 transition state.