Bromination of alkenes with Br2 to give 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.
(Advanced) References and Further Reading:
- First example
The Halogenation of Ethylenes
Irving Roberts and George E. Kimball
Journal of the American Chemical Society 1937 59 (5), 947-948
One of the earliest descriptions in the literature of a three-membered bromonium ion, accounting for the anti stereochemistry of this reaction.
- Mechanistic studies
The question of reversible formation of bromonium ions during the course of electrophilic bromination of olefins. 2. The crystal and molecular structure of the bromonium ion of adamantylideneadamantane
Slebocka-Tilk, R. G. Ball, and R. Stan Brown
Journal of the American Chemical Society 1985 107 (15), 4504-4508
This paper describes the crystal structure of an isolated, stable bromonium ion. This is significant because it proves the intermediacy of these three-membered cyclic bromonium ions in the electrophilic addition of bromine to alkenes.
- Reference To An Experimental Procedure
Rufine Akué-Gédu and Benoît Rigo
Org. Synth. 2005, 82, 179