Radical Chain Carriers in N-Bromosuccinimide Brominations

Article abstract of DOI:10.1021/ja00353a044

Relative reactivities per hydrogen (r's) for neopentane-CH2Cl2 have been redetermined for photobromination by Br2, r = 0.03-0.4, NBS + Br2, r = 1-2.5, and NBS + 1,1-dichloroethylene (DCE) as a Br^. trap, r > 20.In the last case β-bromopropionyl isocyanate (BPI) is the major product.In Br2 reactions r's vary with experimental conditions and are increased significantly by a number of carbonyl and succinimide derivatives.BPI is also a major product in the brominaton of CHCl3 in the presence of DCE and in the UV photolysis of NBS in CD2Cl2 even in the presence of Br2.For CH2Cl2-CD2Cl2 mixtures, k_H/k_D ca. 10 for both NBS-Br2 and NBS-DCE systems.The kinetics of decomposition of NBS in CH2Cl2 in the presence of Br2, initiated by UV light, azobis(isobutyronitrile) and benzoyl peroxide are zero order in NBS, with k for attack of the chain-carrying radical on CH2Cl2 estimated as 55 at 50 deg C.In the presence of DCE, benzoyl peroxide initiated reactions in CH2Cl2 are also zero order, with rates corresponding to a rate constant for ring opening k_d ca. 1660.Addition of cyclohexane produces an unexpected increase in rate corresponding to k_d ca. 1.5*10⁴, and k for reacton of the succinimide radical with cyclohexane ca. 10⁴.If these rate constants are valid, they require a radical lifetime of ca. 10^-4 s, and this and their low value appear inconsistent with a high-energy excited state of the succinimide radical.It is proposed that the chain carrier in the NBS-Br2 system is a reaction product of NBS and a bromine atom, for which some possible structures are suggested.