BRAINCAKEReaction with Hydrogen Halides
Alkenes react with hydrogen halides (HCl, HBr, HI) at room temperature to form halogenoalkanes. The H adds to one carbon of the C=C and the halogen to the other:
CH2=CH2 + HBr → CH3CH2Br (bromoethane)
The halogen is more electronegative than hydrogen, so the H–X bond is polar, Hδ+–Xδ–, and the δ+ hydrogen is the electrophile. The reaction follows the two-step electrophilic addition mechanism: the C=C attacks the H, the H–X bond breaks heterolytically to give a carbocation and X–, then a lone pair on X– bonds to the positive carbon.
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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
- Combustion
- Electrophiles
- Electrophilic Addition
- Reaction with Halogens
- Asymmetrical Alkenes
- Reaction of Alkenes with Halogens
- Reaction with COncnetrated Sulfuric Acid
- Asymmetrical Alkenes
All 23 lessons in Alkenes · All AQA AS-level Chemistry topics