A new method for studies of T-succinimidyl (S,) 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 Br2-NBS. NBS systems containing BrCC1, at mole fractions greater than 0.3 show all the characteristics we have associated with S, behavior, and they function in the presence of olefins which serve as Br2 scavengers. If CC14 is substituted for BrCCI,, the system is clearly S,. The S, behavior is contrasted with So and BP 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, radical shows intermediate behavior. These conclusions are supported by primary deuterium isotope effects for methylene chloride and chloroform: Br., 11-13; S,, 6; S,, 1.4-1.5. The Appendix addresses a number of questions raised by the preliminary study of NBS reactions by Walling et aL3] The failure to adequately deoxygenate their reaction mixtures accounts for discrepancies in products. Their major objection to the attribution of our succinimidyl studies to S, and S, is based on two points: they report rates of reaction that are stated to be too low for accommodation to our hypothesis, and they find that brominations with Br2 in the presence of trifluoroacetic anhydride, benzoic anhydride, succinimide, or N-phenylsuccinimide show lower selectivity than Br.. Nonetheless, on this basis an alternate to S, is proposed to be a bromine atom adduct to NBS or the other carboxylic acid derivatives. This substitute proposal is not consistent with either the facts or the theoretical treatments. The arguments they base on absolute rates employ highly variable rate determinations and some assumptions in the treatment of the data that are essentially the conclusions sought.
