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Topic: Can NBS happen more than once?  (Read 568 times)

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Offline sharbeldam

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Can NBS happen more than once?
« on: February 01, 2024, 08:29:11 AM »
If a question has cyclohexene with NBS, we get one product which is Br on the allylic position.

But why does it not happen again? what if we had excess NBS or a question like that in the picture below? what would be the product? do we have selectivity for the second NBS? would the second Br attach to the allylic carbon on top or bottom? or the reaction wont occur at all?

Thanks.
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Offline Babcock_Hall

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Re: Can NBS happen more than once?
« Reply #1 on: February 02, 2024, 05:40:43 PM »
What particular piece of information should you look for, in order to make a prediction about regioselectivity?  I am not sure whether or not the piece of information I am thinking of is available, but let's suppose that it is.

Offline Babcock_Hall

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Re: Can NBS happen more than once?
« Reply #2 on: February 06, 2024, 10:30:01 AM »
My suggestion is to find the bond dissociation energies of the relevant C-H bonds.

Offline sharbeldam

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Re: Can NBS happen more than once?
« Reply #3 on: February 06, 2024, 05:52:56 PM »
any simpler method :(?
like without going that deep.
does the reaction happen at all? does the radical bromination happen only once then its stabilized .
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Offline Babcock_Hall

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Re: Can NBS happen more than once?
« Reply #4 on: February 06, 2024, 10:15:32 PM »
Suppose we go back to unsubstituted cyclohexene for a moment.  The C-H with the lowest bond dissociation energy (BDE) will be abstracted by the bromine radical.  There are three kinds of C-H bonds, sp2-CH (BDE ~ 464 kJ/mol), ordinary sp3 (BDE ~ 421 kJ/mol) and allylic C-H (BDE ~369 kJ/mol).  That is why the first substitution happens at the allylic position, and it is a pretty unsophisticated analysis.  The problem is that we to know the BDE for BrCH in the allylic position.

Offline sharbeldam

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Re: Can NBS happen more than once?
« Reply #5 on: February 07, 2024, 04:09:24 AM »
First of all , that makes sense thank you.
just to be clear, im not looking for the lazy way, im looking for an explanation that doesnt necessarily need numerical values that are hard to get (but i will try to get it from my college).

I was thinking along the line, that it happens in the allylic carbon first time because of stabilization by resonance (hence the lower dissociation).

now second time we still have resonance, my guess is that it is more stable now because of the electron withdrawing group so it is harder to do for sure, anything can happen with enough energy id guess. but in theory wise, using NBS and a lot of energy, can it happen twice ? what if they put cyclohexene ---> (Excess NBS/High energy), how many times can it happen? is it anywhere in the books?
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Offline Babcock_Hall

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Re: Can NBS happen more than once?
« Reply #6 on: February 07, 2024, 10:28:36 AM »
https://www.scientificupdate.com/process-chemistry-articles/now-you-see-me-now-you-dont-a-judicious-work-around-for-over-bromination-at-benzylic-carbon/

My gut feeling is that a second reaction is likely, but I don't have first-hand knowledge.  The places that I would start are textbooks like Jerry March's Advanced Organic Chemistry. In the third edition (p. 625), he has one passage that may be pertinent:  "It is possible to substitute both sides of the double bond.  Because of the electron withdrawing nature of bromine, the second bromine substitutes on the other side of the double bond rather than alpha to the first bromine."  You can check the references that he provided.

Offline sharbeldam

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Re: Can NBS happen more than once?
« Reply #7 on: February 07, 2024, 05:10:16 PM »
i really appreciate that thx babock!
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