Kinetic Study of the Addition of Trihalides to Unsaturated Compounds in Ionic Liquids. Evidence of a Remarkable Solvent Effect in the Reaction of ICl₂^-

The kinetic constants and activation parameters for the reactions of Br₃^- and ICl₂^- with some alkenes and alkynes have been determined in the ionic liquids [bmim][PF₆], [emim][Tf₂N], [bmim][Tf₂N], [hmim][TF₂N], [bm₂im][Tf₂N], and [bpy][TF₂N] (where emim = 1-ethyl-3-methylimidazolium, bmim = 1-butyl-3-methylimidazolium, hmim = 1-hexyl-3-methylimidazolium, bm₂im = 1-butyl-2,3-dimethylimidazolium, bpy = butylpyridinium, PF₆ = hexafluorophosphate, and Tf₂N = bis(trifluoromethylsulfonyl)imide) and in 1,2-dichloroethane. The rates of both reactions increase on going from 1,2-dichloroethane to ILs. Evidence suggests that, while the hydrogen bonding ability of the imidazolium cation is probably the main factor able to increase the rate of the addition of ICl₂^- to double and triple bonds, this property has no effect on the electrophilic addition of Br₃^- to alkenes and alkynes. Furthermore, in the case of the ICl₂^- reaction, the hydrogen bonding ability of ILs can be exploited to suppress the unwanted nucleophilic substitution reaction on the products by the Cl^- anion.