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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Cooper on February 18, 2013, 08:17:29 PM

Title: Product of NBS Reaction
Post by: Cooper on February 18, 2013, 08:17:29 PM: Hi,

I was told that NBS always gave the most stable product. Is that true?

Even if it doesn't always, shouldn't you get the most stable (thermodynamic) product if you're adding heat, as in the reaction below?

AKA, why is my book telling me the lesser substituted alkene is the major product of this reaction?

Thanks
Title: Re: Product of NBS Reaction
Post by: orgopete on February 18, 2013, 09:43:26 PM: Can someone verify the product of ths reaction?
Title: Re: Product of NBS Reaction
Post by: Mocook on February 19, 2013, 12:05:42 AM: It is correct , allylic bromination :)
Title: Re: Product of NBS Reaction
Post by: discodermolide on February 19, 2013, 01:34:09 AM: I think the other product is the major product.
Title: Re: Product of NBS Reaction
Post by: orgopete on February 19, 2013, 08:28:25 AM: Quote from: Mocook on February 19, 2013, 12:05:42 AM
It is correct , allylic bromination :)

In that case, could you give the journal reference for us.
Title: Re: Product of NBS Reaction
Post by: Cooper on February 19, 2013, 01:55:38 PM: I found out that one of my fellow students said they asked the professor about this and he said it is an unfair question. It has something to do with the orientation of the molecule and since there are 2 of the tertiary radicals possible on either side, it is the major product.
Title: Re: Product of NBS Reaction
Post by: willug on February 19, 2013, 05:39:41 PM: This looks like another question where the reaction hasn't ever been done, and the 'major product' isn't (scifinder) a known compound!
Title: Re: Product of NBS Reaction
Post by: Mocook on February 20, 2013, 08:07:05 PM: is this not basic allylic bromination? this is pure theory due, bromine has a higher selectivity for tertiary carbons over secondary, therefore the major product is accurate.
Title: Re: Product of NBS Reaction
Post by: Cooper on February 20, 2013, 08:09:59 PM: Quote from: Mocook on February 20, 2013, 08:07:05 PM
is this not basic allylic bromination? this is pure theory due, bromine has a higher selectivity for tertiary carbons over secondary, therefore the major product is accurate.

NBS operates through radicals, which means high temperature. Therefore, the thermodynamic product should be the major product in normal cases. My professor said this is just because of weird substituent effects/compound shape and its an exception.
Title: Re: Product of NBS Reaction
Post by: discodermolide on February 20, 2013, 08:31:08 PM: Just to clarify, radicals do not automatically mean high temperature, you can generate them at low temperature as well.
Title: Re: Product of NBS Reaction
Post by: Cooper on February 21, 2013, 02:13:42 PM: Quote from: discodermolide on February 20, 2013, 08:31:08 PM
Just to clarify, radicals do not automatically mean high temperature, you can generate them at low temperature as well.

My mistake. What conditions would you need to create radicals at low temperature? Light?
Title: Re: Product of NBS Reaction
Post by: discodermolide on February 21, 2013, 02:26:02 PM: Yes, photochemical generation does not require temperature.
Also there are radical initiators which generate radical at quite low temperatures, by virtue of their instability.
Remember that biological radicals , mainly O₂ are one of the initiators of allergic reactions, amongst other symptoms, ultimately causing death if not treated.
Title: Re: Product of NBS Reaction
Post by: Cooper on February 21, 2013, 02:34:46 PM: Does NBS operate at high temperatures? I was told it always gives the thermodynamic product, why is this?
Title: Re: Product of NBS Reaction
Post by: discodermolide on February 21, 2013, 02:53:46 PM: You usually reflux in carbon tetrachloride. Or shine a strong light on it at lower temperatures.
The product I suggested is the most stable alkene and the most stable bromide.
Title: Re: Product of NBS Reaction
Post by: Cooper on February 21, 2013, 02:54:31 PM: Okay, thanks :)