Hydroboration Selectivity

[aDentite]

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Markovnikov or Anti-Markovnikov? Achiever Test 1, Question 82 says that that alkene + HBr/hv --> goes anti-markov. I've seen in other sources (Chad, Kaplan, Topscore) that this reaction goes Markovnikov.

I know Hbr/ROOR is always anti-Markovnikov, but what dictates the selectivity of Hbr/hv?

 

[UCB05]

Markovnikov or Anti-Markovnikov? Achiever Test 1, Question 82 says that that alkene + HBr/hv --> goes anti-markov. I've seen in other sources (Chad, Kaplan, Topscore) that this reaction goes Markovnikov.

I know Hbr/ROOR is always anti-Markovnikov, but what dictates the selectivity of Hbr/hv?

In the presence of ROOR or hv, the reaction proceeds by radical anti-Markovnikov addition. The reason for this is that ROOR and hv initiate radical formation, leaving the alkene to attack the Br* radical to form the most stable radical intermediate. Next, this intermediate grabs a H* to result in an anti-Markovnikov product.

This is different from the more familiar alkene addition in which the double bond attacks the H in HBr to form the most stable carbocation intermediate, which is then attacked by Br- to form the Markovnikov product.

ROOR and hv are radical initiators, which hints at anti-markovnikov radical addition.

 

[redchesus]

ROOR and hv are radical initiators, which hints at anti-markovnikov radical addition.

I think it only works for HBr though, because I know the peroxide effect doesn't apply to other hydrogen halides

To OP: this is bromination btw, not hydroboration (there's no boron 🙂)

 

[aDentite]

To OP: this is bromination btw, not hydroboration (there's no boron 🙂)[/QUOTE]


Ah, true... I've been studying way too long 👎


Thank you both.

 

[UCB05]

I think it only works for HBr though, because I know the peroxide effect doesn't apply to other hydrogen halides

To OP: this is bromination btw, not hydroboration (there's no boron 🙂)

That's correct. Different steps in initiation or propagation turn out to be unfavorable or slow when using different hydrogen halides, so these rules should only be applied to HBr.