HBr + alkene

[hausee]

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if no other condition given, which will be the favoriable reaction intermediates for reaction R-CH=CH-R + HBr?

1. carbon cation
2. radicals

This is from ADA full test. I think it can go either way, right? if no condition given? (means with light and regular atmosphere)

Thanks😴

 

[akeurogh]

wouldn't it be HBr added to the alkene with a carbocation intermediate?

 

[hausee]

it can also go through radicals

 

[PrivateJoker]

Pretty sure it will always say HV or light if they want you to recognize that it is going to go through a radical intermediate rather than a carbocation intermediate. Regardless, the product should be the same.

 

[americanpierg]

Pretty sure it will always say HV or light if they want you to recognize that it is going to go through a radical intermediate rather than a carbocation intermediate. Regardless, the product should be the same.

products aren't the same. if the mechanism is carried out through a radical intermediate, the br will attach first, and then the H will attach to the more substituted carbon, leading to anti markovnikov. if it is through a carbocation, the h will attach first, and then the Br will attach to the more substituted carbon, IE markovnikov. the radical mechanism will not lead to a change in sterochemistry, while the carbon cation mechanism will lead to a mixture.

if a question like that does come up, i would pick the carbocation method just because the other one would require an input of energy, IE light and wouldnt be favored in the overall picture. if u just threw hcl and an alkene together, u get a reaction through a carbocation intermediate. you would have to add light if you want a radical intermediate.

 

[zellyen]

i thought hbr only goes anti markonikov in the presence of peroxides (ROOR)

 

[americanpierg]

i thought hbr only goes anti markonikov in the presence of peroxides (ROOR)

peroxides and radicals are anti tho peroxides with clorine addition are NOT anti

 

[zellyen]

yea i somehow missed the fact that you said radical vs carbocation intermediates on the previous post

a question tho: how do I know when HBR + alkene will go with the radical or the carbocation intermediate route? do i assume radical when i see heat/light and carbocation if i see nothing?

 

[hausee]

I guess if there is no reaction condition given, it will go through carbon cation; If light/peroxide is included, it go through radicals.