Master Organic Chemistry Reaction Guide

Allylic bromination of alkanes using NBS

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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26 thoughts on “Allylic bromination of alkanes using NBS

    1. Because it’s a bromination of the allylic carbon I think it still qualifies as an alkane-centric reaction, as the double bond isn’t involved in the mechanism beyond resonance stabilization of the carbon radical.

      1. Hi James, what do you mean by impure NBS. Do you mean that for best reaction we should add little HBr to the reaction mixture with pure NBS?

        1. Pure NBS is gleaming white crystals. After sitting on the shelf for awhile, it often gains a yellow color due to trace Br2 and HBr. That grade of NBS is perfect for allylic bromination because it already contains trace Br2.

    1. 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”. : – )

  1. 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.

    1. 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?

  2. 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.

    1. Hi Felipe, Low yield is the main issue in this reaction. I think if we use little HBr with NBS so can improve the yield. My opinion is that adding HBr can prevent NBS to Make HBr initially.

  3. In your example using NBS in combination with peroxide and heat what, if anything, would happen differently if NBS was used in combination with only peroxide?

    1. In the absence of heat, the O-O bond of the peroxide won’t be broken, and the free-radical reaction won’t start. Heat is required for initiation.

  4. Hi James,
    Will this reaction brominate at the benzylic position as well?
    More specifically, can NBS brominate toluene?

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