Equilibria and UV-spectral characteristics of BrCl, BrCl2- and Br2Cl- species in 1,2-dichloroethane - Stereoselectivity and kinetics of the electrophilic addition of these species to alkenes

UV/Vis spectrophotometric measurements have been used to determine the equilibrium constants and the absorption spectra of BU4N+Br2Cl- and BU4N+BrCl2- species in 1,2-dichloroethane (DCE) at 25 degreesC. The formation constants and the absorption spectra of BrCl from Br-2 and Cl-2 and of BU4N+Cl3- species from Bu4N+Cl- and Cl-2 were also determined in this solvent at 25 degreesC. The kinetics and the products of bromochlorination of several aryl-substituted olefins with BrCl, with Br-2 in the presence of an excess of Bu4N+Cl- (Br-2/ Cl-), and with BrCl in the presence of an excess of BU4N+Cl- (BrCl/Cl-) were investigated in DCE at different temperatures. The kinetics and product distribution data for the reactions with Br-2/Cl- or BrCl/Cl- are interpreted on the basis of a mechanism involving a product- and rate-determining nucleophilic attack by chloride on the olefin-halogen (Br-2 or BrCl) pi complex. The data relating to the electrophilic addition of BrCl are interpreted in terms of the initial formation of a 1:1 olefin-BrCl pi complex, the ionization of which, catalyzed by a second halogen molecule, gives a bromonium. (or beta -bromocarbenium) BrCl2- ion pair able to collapse to the corresponding bromochloro adducts. A significant proportion of the ion pair intermediates may, however, actually be present as bromonium (or beta -bromocarbenium) Br2Cl- and Br-3-ion pairs, and the collapse of these should be responsible for the formation of the dibromides in BrCl addition