Alkene radical reactivity

[jwest88]

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So sometimes when an alkene is reacted with NBS. The allylic proton is removed to produced a resonance stabilized radical. However, I've also seen a situation where the NBS reacts directly with the alkene and the radical goes to the most substituted carbon.

Does anyone know when one is preferred over the other?

 

[paymaant]

i believe allylic radicals are more stable, therefore more favorable. whenever you react a molecule with NBS, look for allylic carbons!

 

[kl352]

when you have alkene with NBS with light, then there will be allylic bromination with the bromide substituting a hydrogen. when you have alkene with NBS and water, then a halodydrin will form with bromide and a hydroxyl group adding anti to the double bond. this forms a bromonium ion where water attacks through markinovkov so the hydroxyl group will be on the more substituted carbon