Allylic bromination of alkanes using NBS

by James

Description: When treated with N-bromosuccinimide (NBS) and light (hν) alkyl groups adjacent to alkenes will be converted into alkyl bromides.
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Notes: The position next to a double bond is called the “allylic” position.

Examples: 

Notes: Note that in example 3, peroxides (that’s “ROOR”)  and heat has the same effect as “light”.

Mechanism: NBS provides a constant, low-level concentration of Br2, which is present when HBr reacts with NBS.

When treated with light, Br2 fragments homolytically to bromine radicals (Step 1, arrow A). At any given time only a small concentration of these radicals are present.

Bromine radical then removes a hydrogen from the allylic carbon, leaving behind the resonance-stabilized allylic radical (Step 2, arrows B and C). The allylic radical then reacts with Br2, giving the allylic bromide (Step 3, arrows D and E).

Notes: The KEY to this whole process is that the allylic C-H bond is weak and allylic radicals are greatly stabilized due to resonance. This reaction does not work on ordinary alkanes unless they are tertiary (see also).

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{ 12 comments… read them below or add one }

Ola

In the subject of this post you have written alkanes and these are, I think alkenes.

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DIVYA

what is the reagent used in allylic bromination of CH3CH2CH=CH2

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james

NBS, just like in this example.

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Remo

Hey I think you are wrong because in the third and fourth reaction from top you forgot allylic rearrangement

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james

Good point! This should be updated to reflect the potential for allylic rearrangement.

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Raj

Hey from where exactly are you getting the HBr

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james

The dirty secret with this reaction is that it works best with slightly impure NBS, which contains HBr.

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remo

In the subject of this post you have written alkanes and these are, I think alkenes.

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James

I chose “alkanes” because it’s an alkyl (sp3) carbon which is being brominated but I should probably just remove the “alkanes” part and keep it as “allylic bromination”. : – )

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Frederick Karayan

Greetings! I have found your Oxidation Ladder, part of your Org 2 study sheets to be invaluable. I have “added” allylic bromination to your oxidation ladder. You should definitely add it into a future version.

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James

Hi Frederick – thanks for writing and I’m very glad you found that sheet useful – it’s one of my favorites. I agree that allylic bromination is an oxidation but I wasn’t sure where to put it in the oxidation ladder that would make sense in the context of other reactions. For instance one could do an elimination to make a diene, but there are no dienes on the sheet. So I left it off in the name of simplicity. Where did you add it, if you don’t mind me asking?

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Felipe

I couldn’t understand very well if 1 mole of NBS will give me, in theory, 1 mole of brominated product. Besides, will I get only 0.5 moles of brominated product when I use pure bromine (due to the formation of HBr)? Any help would be appreciated.

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