A LITTLE LIGHTBULB MOMENT
Alkenes, alkynes and addition reactions
Halogen addition gives vicinal dihalides
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Bromine snaps onto both carbons of a double bond.
The idea behind the cue
Addition of Br2 or Cl2 across an alkene produces a vicinal dihalide, with one halogen on each carbon of the former double bond. This is a standard electrophilic addition reaction of alkenes.
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Short & simple
Bromine snaps onto both carbons of a double bond.
Acronym
HVD — Hungry Vampires Dance.
Story
A red bromine molecule meets an alkene like a pair of magnets. The pi bond opens, and each carbon ends up wearing one halogen like matching boots.
Connect the pieces
- H (Hungry) → Halogenation: Br2 or Cl2 adds across an alkene by electrophilic addition.
- V (Vampires) → Vicinal: The two halogens end up on neighboring carbons of the former double bond.
- D (Dance) → Dihalide: The product is a dihalide, with one halogen on each alkene carbon.
- Hungry · Halogenation: Br2 or Cl2 adds across an alkene by electrophilic addition.
- Vampires · Vicinal: The two halogens end up on neighboring carbons of the former double bond.
- Dance · Dihalide: The product is a dihalide, with one halogen on each alkene carbon.
Know the limits
This card covers halogen addition to isolated alkenes; aromatic halogenation is a different reaction class.
Further reading · OpenStax Organic Chemistry ↗Check your understanding
What is formed when Br2 adds across an alkene double bond?
A. An alkane
B. A bromoalcohol
C. A vicinal dibromide
D. A geminal dibromide
Reveal explanation
A vicinal dibromide
Halogenation of an alkene places one bromine on each carbon of the former double bond, giving a vicinal dibromide.