The kinetic acidity of halomethyltrimethylammonium salts

Abstract

The kinetic acidities of members of a series of halomethyltrimethylammonium salts (Me₃N⁺CH₂X, BF₄⁻) have been measured under conditions of ⁻OD/D₂O at 50 °C, with the rate constants being found to increase X = Cl < Br < I. The observed trend is rationalized on the basis of the polarizabilities and electron repulsion factors of the respective halogen substituents. The observed rate constants indicate a lowering of the ΔG^‡ for deuterium/hydrogen exchange of 12–13 kcal mol⁻¹ due to the presence of the α-halo substituents. The kinetic acidity of chloromethyldimethylsulfonium BF₄⁻ was ≈10⁶ that of the analogous chloromethylammonium salt.