SN2 Reactions in the Gas Phase. Temperature Dependence of the Rate Constants and Energies of the Transition States. Comparison with Solution

Article abstract of DOI:10.1021/ja00316a023

The rate constants for the gas-phase reactions Cl^- + RBr = ClR + Br^-, where R = Me, Et, n-Bu, i-Pr, and i-Bu, were determined for temperatures between 25 and 390 deg C with a pulsed electron beam high ion source pressure mass spectrometer.The rate constants for Me decreased with an increase of temperature (negative temperature dependence).Et and n-Bu had almost no temperature dependence while i-Pr and i-Bu had positive temperature dependence.An analysis of the data on the basis of theory provides approximate values for ΔE₀, the energy of the transition state <*> relative to the energy of the reactants.These ΔE₀ values are as follows: Me, -2.5; Et, 0.8; n-Bu, -0.5; i-Pr, +5.1; i-Bu, +5.7 kcal/mol.The δΔE₀ are compared with relative activation energies: δE_a in solution (C.K.Ingold and A.J.Parker) and calculated strain energies δΔE_strain due to steric repulsions in the transition state (C.K.Ingold and D.F.DeTar).An approximate agreement between the three sets of data is found.This finding supports the assumption of Ingold that steric effects in the transition state dominate the relative rates of this reaction series.The temperature dependence of the rate constants in the gas phase is of interest to ion-molecule reaction theory.It provides a graphic demonstration for the effect of the central barrier in the double-well reaction coordinate.When ΔE₀ is negative, negative temperature dependence is observed.When -ΔE₀ is small (Me, n-Bu) the reaction proceeds with chemical activation at the very low pressures used in ion cyclotron resonance but with near Boltzmann transition-state distribution at the higher pressures used in high-pressure mass spectrometry.When ΔE₀ is positive, the reaction proceeds with positive temperature dependence and boltzmann transition-state distribution.