Elementary SN2 reaction revisited. Effects of solvent and alkyl chain length on kinetics of halogen exchange in haloalkanes elucidated by Empirical Valence Bond simulation

Abstract

We studied halogen exchange reactions between chloride ion and three bromoalkanes, namely methyl bromide, propyl bromide and hexyl bromide, by quantum chemistry methods and multiscale Empirical Valence Bond (EVB) simulations. These reactions proceed by the mechanism of bimolecular nucleophilic substitution (SN2). The gas phase reaction which was used as reference was characterized by quantum calculations at the M06-2X/6-31+G(d,p) level; the computed reaction energy and activation barrier were used for the calibration of tunable EVB parameters. Effects of solvent were simulated by the EVB methodology with explicit inclusion of water molecules. The corresponding gas phase barriers are 10.30, 13.25 and 15.61kcal/mol for methyl, propyl and hexyl bromide, respectively. The calculated activation free energies in aqueous solution are 21.15, 23.28 and 26.50kcal/mol, respectively. Aqueous environment dramatically impedes all three reactions relative to the gas phase, which is consistent with previous experimental and computational studies. In agreement with elementary features of SN2 reactions, elongation of the alkyl group increases the barrier and decreases the rate constant both in the gas phase and in the solution.