Why Does Bromine Sometimes Add to an Alkene and at Other Times Abstract an Allylic Hydrogen Atom? A Computational Investigation of Hydrogen Bromide Addition and Allylic Bromination.

The mechanisms of radical-mediated addition of hydrogen bromide to an alkene and of allylic bromination rely on a balance between competing addition and hydrogen abstraction by the bromine radical. In the past, addition has been postulated to be rapid and reversible, while abstraction is much slower but thermodynamically more favorable. G4 calculations are used to investigate this scenario in the case of cyclohexene. While generally supportive of the standard interpretation of the reaction, the calculations suggest that the kinetic barrier to hydrogen abstraction is often more entropic than enthalpic. A variety of other alkenes are also studied to test the limits of this framework. While the barriers to abstraction and the stability of the abstraction and addition radicals vary widely, only in the most extreme cases, such as cyclobutene, cyclopropene, and α-methylstyrene, do these variations compromise the balance between the competing reactions to an extent that interferes with effective allylic bromination. It was also observed that some bromine addition radicals are bridged in structure, especially the more highly substituted ones, while chlorine addition radicals are only rarely bridged.