A level Chemistry · Edexcel 9CH0 · Topic 6C
Alkenes
Key points
- Alkenes (CₙH₂ₙ) are unsaturated: the C=C is a σ bond plus a π bond, which is a region of high electron density.
- They undergo electrophilic addition with H₂ (Ni catalyst, 150 °C), halogens, hydrogen halides and steam (H₃PO₄ catalyst).
- Bromine water turns from orange to colourless — the test for C=C.
- Mechanism: the π bond attacks the electrophile, a carbocation intermediate forms, then a negative ion bonds to it.
- With unsymmetrical alkenes, the major product comes from the more stable carbocation (tertiary > secondary > primary), because alkyl groups release electrons.
- Acidified KMnO₄ oxidises alkenes to diols (purple to colourless).
- Addition polymers form from alkene monomers; disposal options include incineration, recycling, feedstock use and biodegradable polymers.
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The carbon–carbon double bond makes alkenes far more reactive than alkanes. Its electron-rich π bond attracts electrophiles, so alkenes take part in addition reactions that turn them into many other compounds, including polymers.
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