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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: xshadow on June 01, 2021, 07:21:32 AM

Title: Allylic bromination : NBS
Post by: xshadow on June 01, 2021, 07:21:32 AM
Have a doubt:
If I have a ciclopentadiene and use 2eq NBS I'll get two bromination at the same allylic position or I get two bromination each  one in  a  different allylic site

Thanks!!
Title: Re: Allylic bromination : NBS
Post by: Babcock_Hall on June 01, 2021, 09:28:41 AM
Would you care to make a prediction and explain your reasoning?
Title: Re: Allylic bromination : NBS
Post by: xshadow on June 01, 2021, 02:42:57 PM
Would you care to make a prediction and explain your reasoning?

After first   cyclopentadiene allylic bromination  I have:

-  one allylic position with an Hydrogen and a bromine
- the other allylic position with two hydrogen

Now the second NBS equivalent could react with the allylic position already brominated or with the pther one

Now looking at the mechanism I get a radical when the C-H bond is cleaved by Br·

Now I don't know  if I can say that a radical carbon is an electrophilic  carbon and an halogen near it will destabilized that.

So the bromination will occour on the other allylic position where I don't have any halide

Another explanation  is the steric hindrance
Br has a big steric hindrance and is hard for the second Br· to attack the allylic postion where is already bonded a Br


But I don't know if these explanations are good

Thanks....
Title: Re: Allylic bromination : NBS
Post by: Babcock_Hall on June 01, 2021, 06:47:06 PM
I don't know the answer.  However, I would argue that in some instances (bromination of alkanes), if there are steric effects, they are swamped out by relative stabilities of the radicals.
Title: Re: Allylic bromination : NBS
Post by: rolnor on June 02, 2021, 04:17:48 AM
I dont understand, there is only one allylic site?
https://en.wikipedia.org/wiki/Cyclopentadiene
Title: Re: Allylic bromination : NBS
Post by: xshadow on June 02, 2021, 08:20:12 AM
I dont understand, there is only one allylic site?
https://en.wikipedia.org/wiki/Cyclopentadiene

Sorry!!.my mistake
The molecule  is cyclopentene
Title: Re: Allylic bromination : NBS
Post by: rolnor on June 02, 2021, 12:42:21 PM
I think you get mainly one bromine in each site first then the tri and tetra bromo. But its really not easy to say, you have to do the experiment.
Title: Re: Allylic bromination : NBS
Post by: xshadow on June 02, 2021, 03:39:50 PM
I think you get mainly one bromine in each site first then the tri and tetra bromo. But its really not easy to say, you have to do the experiment.

I thought that too..not so easy to predict

Thanks :)
Title: Re: Allylic bromination : NBS
Post by: Babcock_Hall on June 02, 2021, 05:04:39 PM
I would look for the more stable radical.