Gas-phase SN2 reactions of chloride ion with alkyl bromides at atmospheric pressure. Temperature dependence of the rate constants and energies of the transition states

Article abstract of DOI:10.1021/ja00078a053

Rate constants for the gas-phase reactions Cl^- + RBr → RCl + Br^-, where R = Me, Et, and n-Bu, have been determined over the temperature range 35-150°C by a kinetic ion mobility mass spectrometer (KIMMS) at 640 Torr of buffer gas pressure and by a pulsed e-beam high-pressure mass spectrometer (PHPMS) at 3 Torr of buffer gas pressure. For the case of R = Me, different rate constants and different temperature dependencies of the rate constants are observed at 640 versus 3 Torr of buffer gas pressure. This effect of pressure is thought to be due to increased collisional stabilization of the entrance-channel intermediate, (Cl^-·MeBr), with increased pressure. Less significant pressure effects observed for the cases of R = Et and n-Bu are thought to reflect efficient stabilization of the entrance-channel intermediates of these reaction systems at both buffer gas pressures. If it is assumed that the rate measurement at 640 Torr reflect the high-pressure limit of kinetic behavior, the energies (δE₀) of the S_N2 transition states (TS) of these reactions can be determined from simple transition-state theory (TST); δE₀ = -2.2, 0.0, and -1.3 kcal/mol for R = Me, Et, and n-Bu, respectively. The determination of δE₀ described here for the case of R = Me is particularly significant because δE₀ for this system could probably not be accurately deduced from kinetic measurements made at 3 Torr or lower pressure, where the excited intermediate, (Cl^-·MeBr)*, has been shown to behave in a nonstatistical manner.