Bromination of Alkynes in Ionic Liquids - A Kinetic Investigation

The kinetic behaviour and the product distributions of brominations of several arylalkynes with Br2 in the room-temperature ionic liquids (ILs) 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6] and 1-butyl-3-methylimidazolium bromide [bmim][Br], have been investigated at different temperatures. In [bmim][Br], alkynes stereospecifically gave the anty addition products, the reactions following a second-order rate law.In [bmim][PF6], mixtures of syn and anti addition products were obtained, and the reaction followed a second- or third-order rate law, depending on the structure of the alkyne and the concentration of Br2. The data obtained for the reactions in [bmim][Br] are interpreted on the basis of a mechanism involving a product- and a rate-determining nucleophilic attack by bromide on the alkyne-Br2 Π complex. The data relating to the electrophilic addition in [bmim][PF6] are explained in terms of the initial 1:1 alkyne-Br2 Π complex, the ionisation of which, probably catalysed either by a second halogen molecule or by the imidazolium cation, gives a bromirenium (or β-bromovynil cation) Br- or [Br3]- intermediate. Such intermediates then collapse to give the corresponding dibromo adduct(s). The kinetic constants and the activation parameters for the reaction in [bmim][Br] are compared with those relating to the second-order reaction of the same alkynes with tetrabutylammonium tribromide ([NBu4][Br3]) in 1,2-dichloroethane (DCE). On the basis of the kinetic data, a significant role for solvent viscosity in determining the reaction rate may be envisaged.