Master Organic Chemistry Reaction Guide

Bromination of alkenes with Br2 to give dibromides


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).

Mechanism:

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).

bromination of alkenes with br2 showing formation of bromonium ion intermediate then attack by bromide ion and formation of trans dibromide

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.

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(Advanced) References and Further Reading:

  1. First example
    The Halogenation of Ethylenes

    Irving Roberts and George E. Kimball
    Journal of the American Chemical Society 1937 59 (5), 947-948
    DOI:10.1021/ja01284a507
    One of the earliest descriptions in the literature of a three-membered bromonium ion, accounting for the anti stereochemistry of this reaction.
  2. 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
    DOI: 10.1021/ja00301a021
    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.
  3. Reference To An Experimental Procedure
    Rufine Akué-Gédu and Benoît Rigo
    Org. Synth. 2005, 82, 179
    DOI: 10.15227/orgsyn.082.0179

 

Comments

Comment section

33 thoughts on “Bromination of alkenes with Br2 to give dibromides

  1. 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?

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

    2. Yeah I just looked this up because I was confused too. When you have light plus Br2 plus an alkene you get allylic bromination, which means a bromine radical removes a hydrogen (and one electron from its bond) from a carbon adjacent to the double bond (in allylic position), leaving only one electron on that carbon which reacts with another Br2 to take one of the bromines and one of its electrons to form a halogenated alkene and another bromine radical in the propagation step. If you have an alkene plus light plus HBr, the light forms a bromine radical which forms a bond to the least substituted carbon of the alkene and puts a radical on the most substituted carbon, which then grabs the hydrogen and one electron from another HBr molecule, forming another bromine radical and making it a propagation step.

      When you don’t have light but have Br2 and an alkene and a non-nucleophilic solvent the bromines add to both carbons that formerly held the double bond- the first bromine is grabbed by the pi bond and leaves its electrons to form a ring with the alkane (halonium ion), which the other bromine (which has extra electrons and is thus nucleophilic) then attacks from the back, which breaks the ring and forms an alkane with two bromines attached to adjacent carbons. If you add HBr with a non-nucleophilic solvent and without light the same thing happens, but the first atom grabbed by the pi bond is the hydrogen without its electrons. (In this process the hydrogen doesn’t form a ring, just a carbocation, so rearrangement can occur to generate different di-halogenated alkanes.)

      So basically radical addition of halogens to a pi bond (that gives the anti-Markovinov product) only happens if you have HBr and light (and peroxides to create the bromine radical), not Br2 and light. If you have Br2 and light, allylic bromination occurs, which adds Br not to the double bond but to the carbon adjacent to the doubly bonded carbons. Or the bromine can just add across the double bond to each of the formerly double bonded carbons, as our dear Ochem master says on his page:
      Allylic bromination: https://www.masterorganicchemistry.com/2013/11/25/allylic-bromination/
      Free Radical Addition of HBr to Alkenes: https://www.masterorganicchemistry.com/2013/04/12/a-fourth-alkene-addition-pattern-free-radical-addition/

  2. Is the first Br considered slightly positive just before it is attacked by the electron dense double bond? I’m just trying to understand why the electrons from the double bond are attracted to the Br. Thanks!

    1. No, the Br-Br bond is perfectly covalent. But what does happen is that one of the Br atoms (at random) is attacked by the pi bond, and something called a “pi complex intermediate” is formed. This breaks down (losing Br-) to give the 3-membered ring.

  3. What would happen if bormination were to occur with a diene? There is no other intermediate besides the bromonium ion.
    Specifically how does the rearrangement occur?

  4. Which are the solvents that can dissolve both bromine and NaBr to give solutions containing both Br2 and NaBr?

  5. After the cyclic intermediate, why the other Bromine ion attacks at the point why has more +I effect? Shouldn’t it attack at the other end?

  6. if in a reaction ,only br2 is mentioned than will the product would be cis or trans???
    i’m so confused that when and where ,in how many rxns in organic there is syn addn and elimination and where its trans??

  7. I take issue with your incomplete curly arrows for the formation of the cyclic bromonium ion. You’ve missed a third curly arrow going from a bromine lone pair back onto one of the alkene’s carbon atoms. The MO theory model of Br2 bromination of an alkene shows that there is concomitant donation of the alkene HOMO into the bromine sigma-antibond, and a sort of “backbonding” of the bromine pi-antibond HOMO (filled!) into the pi-antibond LUMO of the alkene.

    You should correct the above incomplete mechanism, especially seeing as a lot of students use this website as a resource!

  8. This is fine… but will the reaction take the same course even in the presence of chloroform instead of carbon tetra chloride..? Does chloroform involve in any way in the reaction?

  9. hi!
    for the compound (C(CH3)3)2C=CH2
    on the addition of br2 will form first the cyclic bromonium ion
    now since the other carbon is sterically hindered how will br- attack that carbon.

    1. Running this reaction neat (i.e. without solvent) is generally not recommended, as it will generate heat. Solvent helps to keep the temperature uniform and the reaction rate under control. Bromination is a fast reaction even at 0°C so heating is not necessary. Heating it is generally overkill. On anything but the simplest molecules the result will likely be a mess.

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