Halogenoalkanes and nucleophilic substitution
Halogenoalkanes
- A halogenoalkane is an alkane with a halogen in place of hydrogen (a C–X bond).
- The C–X bond is polar, which makes these compounds react.
- They come in three classes.
Practice
A halogenoalkane is:
A halogenoalkane has a polar C–X bond, which is the reactive site.
Three classes
- primary, secondary or tertiary, set by how many carbons are joined to the carbon holding the halogen (one, two or three).
- Made by: free-radical substitution of an alkane, electrophilic addition to an alkene, or substitution of an alcohol.
Practice
A tertiary halogenoalkane has the halogen on a carbon joined to:
Primary = 1 carbon attached, secondary = 2, tertiary = 3, on the carbon bearing the halogen.
Nucleophilic substitution
- The C–X carbon is slightly positive, so a nucleophile (lone-pair species) attacks and replaces the halogen.
| Reagent | Product |
|---|---|
| $\text{NaOH(aq)}$, heat | an alcohol |
| $\text{KCN}$ in ethanol, heat | a nitrile (adds a carbon) |
| $\text{NH}_3$ in ethanol, heat + pressure | an amine |
Practice
Warming a halogenoalkane with aqueous NaOH produces:
NaOH(aq) substitutes the halogen for –OH, giving an alcohol.
Practice
Reaction with KCN in ethanol gives a nitrile, which is useful because it:
The CN group adds a carbon, lengthening the chain; NH₃ instead gives an amine.
Identifying the halogen
- Warm with silver nitrate in ethanol: a silver halide precipitate forms — white AgCl, cream AgBr, yellow AgI.
You've got it
Key idea
- a halogenoalkane has a polar C–X bond; classes are primary/secondary/tertiary
- nucleophilic substitution: NaOH(aq) → alcohol; KCN → nitrile (+1 carbon); NH₃ → amine
- identify the halogen with silver nitrate (white/cream/yellow precipitate)