The Walling, El-Taliawi, and Zhao "Carnonyl Effect" in Radical Brominations Is an Example of HBr Reversal. It Is Not Relevant to ? and ? Radical Chemistry.

Article abstract of DOI:10.1021/ja00318a050

Walling, El-Taliawi, and Zhao claim that our "S_? chemistry" is reproduced in photobrominations employing Br2 in the presence of carbonyl compounds (no NBS).Photobromination of alkanes by N-bromosuccinimide (NBS) shows selectivities that vary, but which show limiting values for significant ranges of rection conditions.We have attributed these different limiting values to three reaction paths which differ in involving either Br^., S_?, or S_? as the intermediate hydrogen-abstracting radical.Walling, El-Taliawi, and Zhao (WEZ) have reported selectivities they belive are attributable to a carbonyl-bromine intermediate rather than our S_?.Evidence is now presented to show that the WEZ results are due to HBr in varying amounts with or without carbonyl compounds present.This represents a fourth type of selectivity and one that is sensitive to HBr concentrations and probably arises in their systems from the following reaction: There is no carbonyl effect.This fourth type had been deliberately excluded from our work, so that reactions involving HBr reversal are not relevant to our published work on succinimidyl chemistry.Thus, the selectivities (per H basis) for the neopentane/methylene chloride competitions stand without modification, as published earlier: 0.067, 1.0 and 17 for B^., S_?, and S_?, respectively.