Kinetics of the halogen-exchange reaction between alkyl halides (RY) and boron trihalides (BX3). Evidence for formation of adducts RY,BX3

Abstract

A kinetic study has been made of the halogen-exchange reactions: RY + BX₃→ RX + BX₂Y (R = Prⁿ, Prⁱ, Buⁿ, Bu^s, or Me₃Si; Y = Cl or Br; X = Br or l), where X is heavier than Y, in cyclohexane or CCl₄ at 306·5 K by use of ¹H and ¹¹B n.m.r. spectroscopy. For all these reactions, the initial rate (r₀) depends on the initial concentrations of the reactants according to the relationship: r₀=k₃[RY]₀²[BX₃]₀+k₂[RY]₀[BX₃]₀. In most of the systems studied, the reactions are simply third-order (k₂ negligible), but cases showing second-order (k₃ negligible) or mixed-(second- plus third-)order kinetics are also found. Reactions of Bu^tCl with BBr₃ or Bl₃ are immeasurably fast, whereas those of PrⁿCl or BuⁿCl with BBr₃ are immeasurably slow under the conditions used. Only for reactions of isobutyl halides, which showed more complicated kinetic behaviour, was any isomerization of the alkyl groups detected (in these cases to t-butyl halide). The temperature-dependence of the rate of reaction of PrⁱCl with BBr₃(in CCl₄) has also been studied. Possible mechanisms for the reactions are suggested, requiring initial adduct formation of type RY,BX₃.