Bromination of alkenes with Br2 to give dibromides

by James

Description: Treatment of alkenes with bromine (Br2) gives vicinal dibromides (1,2-dibromides). 

Notes: The bromines add to opposite faces of the double bond (“anti addition”). Sometimes the solvent is mentioned in this reaction – a common solvent is carbon tetrachloride (CCl4). CCl4 actually has no effect on the reaction, it’s just to distinguish this from the reaction where the solvent is H2O, in which case a bromohydrin is formed (see also).

Notes: Again note that in the first example CCl4 is merely the solvent and in these cases has no effect on the reaction (unlike when water or alcohols are the solvent – see bromohydrin formation page).


Attack of the alkene on bromine (Step 1, arrows A and B) gives the bromonium ion, which is attacked at the backside by bromide ion to give the trans-dibromo product. Note that the bromines are delivered to opposite sides of the alkene (“anti” addition).

Notes: Note that a 1:1 mixture of enantiomers will be formed in this reaction. The enantiomer formed will depend on which face of the alkene the bromine adds to.


{ 3 comments… read them below or add one }

Chan May 23, 2013 at 6:09 am

Unlike Bromohydrin. Why isn’nt there any chlorohydrin?Is that not possible?


Eric March 13, 2014 at 2:53 pm

Great post here, one question- could you discuss what would happen when 1,2 dimethylcyclohexene reacts with Br2 (1 mole)/light? The answer key shows a double bond shift and only one Br attaching. Everywhere I’ve read, in an alkene reaction both Br’s attache to the ring in trans.. Could you show the mechanism involved?


cam April 14, 2014 at 9:29 pm

I believe only 1 Br attaches because it is a radical reaction, hence the “light”. Being radical means only 1 attaches during propagation


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