December 01, 2021, 01:26:48 AM
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Topic: Allylic bromination : NBS  (Read 541 times)

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Offline xshadow

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Allylic bromination : NBS
« 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!!

Offline Babcock_Hall

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Re: Allylic bromination : NBS
« Reply #1 on: June 01, 2021, 09:28:41 AM »
Would you care to make a prediction and explain your reasoning?

Offline xshadow

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Re: Allylic bromination : NBS
« Reply #2 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....

Offline Babcock_Hall

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Re: Allylic bromination : NBS
« Reply #3 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.

Offline rolnor

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Re: Allylic bromination : NBS
« Reply #4 on: June 02, 2021, 04:17:48 AM »
I dont understand, there is only one allylic site?
https://en.wikipedia.org/wiki/Cyclopentadiene

Offline xshadow

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Re: Allylic bromination : NBS
« Reply #5 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

Offline rolnor

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Re: Allylic bromination : NBS
« Reply #6 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.

Offline xshadow

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Re: Allylic bromination : NBS
« Reply #7 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 :)

Offline Babcock_Hall

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Re: Allylic bromination : NBS
« Reply #8 on: June 02, 2021, 05:04:39 PM »
I would look for the more stable radical.

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