Double bond isomerization of butene catalyzed by 1-ethyl-3-methyl-imidazolium chloride: Concerted or stepwise mechanism?

The double bond isomerization of butene catalyzed by 1-ethyl-3-methyl-imidazolium chloride ionic liquid has been examined by performing density functional theory calculations at the B3LYP/6-31G (d, p) level. The stepwise H-migration pathway is compared with the concerted H-migration pathway proposed in the early literature. The calculated elementary step barriers in the stepwise mechanism are 35.1 and 24.7 kcal/mol, in contrast to 55.1 kcal/mol found in the concerted mechanism. This fact implies that the isomerization reaction may proceed according to the stepwise rather than concerted mechanism. The present results provide a general mechanism picture for the double bond isomerization of olefin in imidazolium-based ionic liquids.