Hey all, i'm learning about alkene mechanisms and had a general question regarding AMA. Can something that usually only does MA have an exception and do AMA? If so, how, thanks.

When adding water across a double bond, the hydroboration-oxidation reaction will give you anti markovnikov addition.

 

http://en.wikipedia.org/wiki/Hydroboration-oxidation_reaction

Ok, however, on a test I just took there was a reaction with an alkene and HBR where the HBR did AMA which ALWAYs is MA. You had to explain why the AMA of HBR was more stable, how's this possible?

What was the alkene?

 

And was there a peroxide present?

I forgot the alkene, but yah I believe peroxide was present.

With HBr + Alkene, the reaction usually follows markovnikov's rule. This is due because MA favors the more stable carbocation.

 

Now when a peroxide is added, the mechanism changes. The peroxide acts as the initiation step for a radical chain reaction:

 

http://en.wikipedia.org/wiki/Free_radical_addition

So other than in the presence of peroxide, is this the only time HBR would bond via AMA to an alkene in terms of stability? Thanks btw.

Yeah, its the only one I can think of off the top of my head (read: I haven't looked too hard). Its one of those textbook example things that you just memorize.