Mechanism of substitution at a boron atom. Part III. The borderline between bimolecular and unimolecular substitution on tetrahedral boron halide adducts

Abstract

Kinetic parameters for the reaction of dinitroaniline and dinitronaphthylamine with the acetonitrile adducts of boron trichloride and boron tribromide in acetonitrile have been obtained and compared with thermodynamic data on the dissociation energies of N–B and B–X bonds in the adducts. The bimolecular pattern of substitution is observed for chloride, but for bromide, the reaction is near to the borderline with unimolecular substitution, although still exhibiting a rate dependence on nucleophile. The thermodynamic data show that boron trichloride and boron tribromide are approximately equal in Lewis acid strength in acetonitrile as solvent and reference base, contrary to expectation.