Peroxides and Radicals!

[jonathankimmm]

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Hey guys had a question from watching Chad's Ochem Videos

In the videos: ALKENES

It shows a hydroboration-oxidation reaction where you have 1.BH3.THF, 2.H2O2, OH-
and it adds an H and OH , SYN addition.

however later he adds 1.HBR, 2.ROOR
and it has no stereochemistry with 4 total products.

he says the peroixde makes a radical which has sp2 so there is no stereochemistry.

But in the first reaction, it has peroixde yet it is syn addition. Does the addition of BH3 THF just overide the peroxide?

 

[jmh12345]

What you need to remember about reactions with peroxides is not about the syn addition. What the peroxides do is make the product be anit-markovnikov meaning the Hydrogen bonds to the more substituted carbon.

The syn addition is due to the mechanics of the reaction not the peroxides.

HBr reacting with an alkene without peroxides, the Hydrogen will go to the least substituted carbon. With peroxides it will go to the more substituted. As Chad says, "The peroxide (ROOR) roars and scares the H into the position it wouldn't normally want.

Hope this helps! Good luck studying!

AA: 21 TS: 21 PAT: 21