New insight into the formation mechanism of imidazolium-based halide salts.

By performing density functional theory calculations, the Menshutkin reaction between the N-methyl imidazole with chloroethane is reexamined to rationalize the experimental discovery. The calculated results show that the reaction proceeds via a S(N)2 mechanism with a barrier of 119.1 kJ mol(-1) which is much lower than that reported in previous literature according to a five-membered transition state mechanism. Moreover, it is found that the barrier is further reduced to 98.1 kJ mol(-1) in toluene solution. The present result validates the experimental finding that the Menshutkin reaction for synthesizing N-alkyl imidazolium halide salts proceed smoothly at lower heating temperature.