62168-25-6

In the photobromination of alkyl bromides, hydrogen bromide is shown to act as a catalyst and a kinetic study implies that two molecules of the acid are involved in catalysis.The catalysis is specific in two ways: only HBr is effective and it operates only with alkyl bromides having a β-hydrogen available for substitution.HBr favours the formation of 1,2-dibromides.This catalytic pathway is superimposed on the classical, uncatalysed mechanism.Isotopic labelling experiments show that an elimination-readdition pathway may also account for part of the reaction (maximum 20percent) but cannot explain the migration of bromine which is observed in the formation of β-dibromides.

A new method for studies of pi -succinimidyl (S// pi ) radicals is described, one that makes possible the study of reactions of this radical with a variety of substrates not accessible by the use of Br//2-NBS. NBS systems containing BrCCl//3 at mole fractions greater than 0. 3 show all the characteristics associated with S// pi behavior, and they function in the presence of olefins which serve as Br//2 scavengers. If CCl//4 is substituted for BrCCl//3, the system is clearly S// sigma . The S// pi behavior is contrasted with S// sigma and Br multiplied by (times) reactivities for H abstractions from a variety of substrates and for additions to tert-butylethylene, isobutylene, and 1,3-butadiene. In early-transition-state systems, for H transfer, the strength of the bond being broken and the strength of the bond being made are not the major factors in determining reactivities. The behavior in late-transition-state systems is influenced by both bond strengths. The S// pi radical shows intermediate behavior. These conclusions are supported by primary deuterium isotope effects for methylene chloride and chlororoform. The Appendix addresses a number of questions raised by the preliminary study of NBS reactions by Walling et al.