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Topic: NBS Reactions  (Read 5089 times)

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Offline Cooper

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NBS Reactions
« on: January 28, 2013, 09:35:41 PM »
Hi,

I am doing some homework for my orgo class. There are two questions involving the addition of NBS to alkenes. I don't get why they are marking my answers wrong. Doesn't NBS add Br to the more highly substituted allylic carbon of an alkene?

I'll attach the questions with what I believe are the answers. I've put red X's threw them to show that they are not correct.
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Offline discodermolide

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Re: NBS Reactions
« Reply #1 on: January 28, 2013, 10:22:25 PM »
Substitution happens in the allylic position forming the compounds with the C=C having more substituents.
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Offline Cooper

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Re: NBS Reactions
« Reply #2 on: January 28, 2013, 10:25:59 PM »
Quote from: discodermolide on January 28, 2013, 10:22:25 PM
...with the C=C having more substituents.

Sorry, what do you mean at that part? The C=C bond doesn't play a role in the mechanism, does it?
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Offline discodermolide

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Re: NBS Reactions
« Reply #3 on: January 28, 2013, 10:32:00 PM »
Yes, it stabilised the radical.
In the case of the cyclohexene ring, move the C=C towards the methyl group and place the Br where the C=C was, in the allylic position, I'll draw the structures if you wish.
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Offline Cooper

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Re: NBS Reactions
« Reply #4 on: January 28, 2013, 10:46:09 PM »
I may be misunderstanding you, but are you saying the Br radical comes in and interacts with the other resonance form? Why would it do that, isn't the tertiary radical resonance form more favored?

EDIT: Oh, so the tri-substituted alkene is so favorable over the di-substituted one that the radical being on the secondary carbon doesn't matter as much?
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Offline Cooper

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Re: NBS Reactions
« Reply #5 on: January 28, 2013, 10:46:30 PM »
Sorry, re-post.
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Offline discodermolide

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Re: NBS Reactions
« Reply #6 on: January 28, 2013, 10:50:35 PM »
That's the correct product, more stable alkene, less reactive bromide.
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Offline Cooper

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Re: NBS Reactions
« Reply #7 on: January 28, 2013, 10:51:15 PM »
Thanks! Makes sense now. :)
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Offline Woopy

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Re: NBS Reactions
« Reply #8 on: January 28, 2013, 11:08:22 PM »
where do you get software to do the drawings?
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Offline discodermolide

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Re: NBS Reactions
« Reply #9 on: January 28, 2013, 11:47:37 PM »
It's ChemDraw.
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Offline Cooper

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Re: NBS Reactions
« Reply #10 on: January 29, 2013, 10:14:50 AM »
I use Marvin Sketch, which is free.
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Offline discodermolide

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Re: NBS Reactions
« Reply #11 on: January 29, 2013, 10:26:14 AM »
A good friend bought it for me so I could a project for him.
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Offline curiouscat

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Re: NBS Reactions
« Reply #12 on: January 29, 2013, 12:16:52 PM »
Quote from: discodermolide on January 29, 2013, 10:26:14 AM
A good friend bought it for me so I could a project for him.

Nice taste in friends.  ;D
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Offline souro10

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Re: NBS Reactions
« Reply #13 on: January 29, 2013, 04:11:04 PM »
The more stable alkene, involving migration of double bond is the major product at room temperature.
The products in your original question may be the major product at very very low temperatures and short reaction times. They are the kinetic products. These products will rearrange into the more stable alkene under the conditions of the reaction(room temp) to form the thermodynamic products, after which further rearrangement is not feasible.
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Offline Cooper

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Re: NBS Reactions
« Reply #14 on: February 02, 2013, 11:59:33 AM »
Thanks everyone, I've since become comfortable with kinetic and thermodynamic products involving radicals and carbocations :)
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