[maccha]

I'm confused about propagation in free radical chain reactions. If you have propene, for example, how do you know if a Bromine free radical would add to the pi bond or abstract an allylic hydrogen? Thanks

[Schrödinger]

Under what conditions and from what initial compound is the bromine free radical being generated? And is there any specific reaction(s) you have in mind?

[orgopete]

This is a good question. I reasoned that this reaction must be a kinetic versus thermodynamic process. A radical addition of HBr and radical bromination generates a Br•. But one adds and the other abstracts. In HBr, the addition radical is quenched with HBr. If HBr is low or absent, then it must reverse to an alkene. Slowly, abstraction must take place. This is propagated or quenched with bromine.

I don't know the data on this, but allylic brominations are most frequently done with NBS. Two things, NBS absorbs HBr keeping its concentrations low. Two, Br₂ concentrations are very low to prevent a radical addition or electrophilic addition to compete.  Obviously, one could drip in bromine. I don't know if anyone has done this reaction and whether dibromides are encountered in their products.