Photodecomposition of N-Bromosuccinimide: Radical Chain Carriers and Their Interrelations

Article abstract of DOI:10.1021/jo00386a008

The direct photolysis of N-bromosuccinimide (NBS) in the absence and presence of bromine and its bromine atom initiated decomposition were investigated by selective irradiation of NBS or Br2 and by critically examining the β-bromopropionyl isocyanate (BPI) yields and the relative selectivity of intermolecular H abstraction from cyclohexane and CH2Cl2.Experimental results showed (i) that direct photolysis of NBS gave extra amounts of BPI, (ii) that the bromine atom initiated NBS decomposition also generated BPI, and (iii) that the interaction of bromine atoms with NBS or the succinimidyl radical with bromine set up a rapid equilibrium of the radicals.In the NBS + Br2 system the relative selectivities of intermolecular H abstraction, k_C6H12/k_CH2Cl2, were 16-22, practically the same as those for a bromine atom (15-18), and the BPI percent yields decreased systematically as both initial and observed bromine concentrations increased in the range of 1-100 mM.Therefore, the photodecomposition of the NBS + Br2 system involves the succinimidyl radical and bromine atom in a fast equilibrium and as the radical-propagating species, without the need to invoke another radical.The succinimidyl radical generated from direct photolysis of NBS is proposed to be a vibrationally excited hot species that undergoes much faster ring opening than the ground-state species.These results provide a satisfactory explanation to resolve the most recent controversy over the number of radical intermediates involved in the NBS photodecomposition.