BRAINCAKEReaction with Halogens
Alkenes react with halogens at room temperature by electrophilic addition. The two halogen atoms add to the two carbons of the C=C, giving a dihalogenoalkane with the halogens on adjacent carbons:
CH2=CH2 + Br2 → CH2BrCH2Br (1,2-dibromoethane)
Br2 is a non-polar molecule, so how can it be an electrophile? As a Br2 molecule approaches the C=C, the high electron density of the double bond repels the electron pair in the Br–Br bond. This induces a dipole: the Br atom nearer the C=C gains a partial positive charge (δ+) and the further one becomes δ–.
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Key terms in this lesson
- carbocation
- A positively charged carbon atom formed as a reactive intermediate when a bond breaks heterolytically.
- electrophile
- A species that accepts a pair of electrons to form a new covalent bond, attracted to regions of high electron density.
More in Alkenes
- Physical Properties of Alkenes
- Combustion
- Electrophiles
- Electrophilic Addition
- Reaction with Hydrogen Halides
- Asymmetrical Alkenes
- Reaction of Alkenes with Halogens
- Reaction with COncnetrated Sulfuric Acid
All 23 lessons in Alkenes · All AQA AS-level Chemistry topics