Competing Mechanistic Pathways in the Bromination of 1,3-Butadiene with Molecular Bromine, Pyridine-Bromine Complex, and Tribromide Ion

Article abstract of DOI:10.1021/jo00324a021

The kinetics and product distribution of the reactions of 1,3-butadiene with molecular bromine, with the pyridine-bromine charge-transfer complex (PyBr2), and with tetra-n-butylammonium tribromide in 1,2-dichloroethane and dichloromethane have been investigated.Whereas the reaction with Br2 is second order in halogen, those with the other two reagents are first order in the halogenating species, and evidence is presented that both act as independent electrophiles, rather than as sources of molecular bromine.In the reaction with PyBr2 (and to a lesser extent in that with Br3(1-) in the presence of pyridine) substantial amounts of N-(4-bromo-1-buten-3-yl)pyridinium bromide are formed besides the expected 1,2- and 1,4-dibromo adducts.This is converted into the corresponding tribromide as long as free Br2 or PyBr2 is present in the medium.The resulting tribromide ion remains as the only brominating species during the later stages of the reaction.The change from 1,2-dichloroethane to the slightly less polar dichloromethane as solvent produces a threefold decrease in rate of reaction with Br2 and a fourfold increase in that with Br3(1-).Possible mechanisms for the reactions of butadiene with Br2, PyBr2, and Br3(1-) are discussed in terms of the present and previous results on these reactions.These have revealed very significant differences in the ratios of 1,2- to 1,4-adducts on changing from molecular Br2 to complexed bromine as the reagents, which are interpreted in the light of these mechanisms.