A level Chemistry · Edexcel 9CH0 · Topic 6D
Halogenoalkanes
Key points
- Halogenoalkanes are primary, secondary or tertiary depending on the carbon bonded to the halogen.
- The C–X bond is polar (Cδ+), so nucleophiles (OH⁻, CN⁻, NH₃, H₂O) attack the carbon in nucleophilic substitution.
- Reactivity is controlled by bond enthalpy, not polarity: C–I is weakest so iodoalkanes react fastest.
- Compare rates by warming with aqueous silver nitrate in ethanol: yellow AgI appears first, then cream AgBr, then white AgCl.
- Products: aqueous KOH → alcohol; ethanolic KCN → nitrile (adds one carbon); excess ethanolic NH₃ under pressure → amine.
- Elimination with hot ethanolic KOH gives an alkene. Tertiary halogenoalkanes react by the SN1 route via a carbocation; primary by SN2 in one step.
- Name the mechanism and show curly arrows, partial charges and the lone pair on the nucleophile.
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Halogenoalkanes contain a carbon atom bonded to a halogen. That polar bond makes the carbon a target for nucleophiles, so halogenoalkanes are useful starting points for making alcohols, nitriles and amines, and under different conditions…
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