Halogenoalkanes
A halogenoalkane (haloalkane) has a halogen atom in place of a hydrogen atom, for example chloroethane C₂H₅Cl. They are classified as primary, secondary or tertiary by the number of alkyl groups on the carbon bonded to the halogen. The C–X bond is polar (carbon δ⁺), so it is attacked by nucleophiles: species with a lone pair that attack positive centres.
| Bond | Bond enthalpy (kJ/mol) | Reactivity |
|---|---|---|
| C–F | 484 | Least reactive |
| C–Cl | 338 | |
| C–Br | 276 | |
| C–I | 238 | Most reactive |
Nucleophilic substitution
| Reagent | Nucleophile | Product |
|---|---|---|
| Aqueous NaOH, heat | OH⁻ | Alcohol |
| KCN in ethanol, heat | CN⁻ | Nitrile (lengthens the carbon chain) |
| Excess ammonia in ethanol, pressure | NH₃ | Amine |
| Water, slowly | H₂O | Alcohol |
Two mechanisms are possible. SN2 (bimolecular) happens in one step: the nucleophile attacks from the back as the halide leaves. It is common for primary halogenoalkanes and its rate depends on both the halogenoalkane and the nucleophile. SN1 (unimolecular) happens in two steps: the halide leaves first to give a carbocation, then the nucleophile attacks. It is common for tertiary halogenoalkanes and its rate depends on the halogenoalkane only.
Choose the factor by which the halogenoalkane concentration and the nucleophile concentration change. See the change in rate for each mechanism.
Elimination and uses
With ethanolic (alcoholic) potassium hydroxide, a halogenoalkane loses HX and forms an alkene: this is elimination. Aqueous hydroxide gives substitution; ethanolic hydroxide gives elimination. The test for halide ions in a halogenoalkane is to warm with aqueous NaOH, acidify with nitric acid and add silver nitrate: AgCl is white, AgBr cream and AgI yellow.
| Compound | Use | Concern |
|---|---|---|
| CFCs (chlorofluorocarbons) | Old refrigerants and aerosols | Break down ozone in the stratosphere; banned |
| Trichloromethane, dichloromethane | Solvents | Toxic |
| PVC (poly(chloroethene)) | Pipes, cables, floor covering | Disposal |
| DDT | Insecticide against malaria mosquitoes | Persists in the environment |
Key terms
- Nucleophile
- A species with a lone pair of electrons that attacks a positive centre.
- SN1
- Two-step nucleophilic substitution through a carbocation; rate depends on the halogenoalkane only.
- SN2
- One-step nucleophilic substitution; rate depends on both reactants.
- Elimination
- Loss of a small molecule such as HX to form a double bond.
Summary
- Halogenoalkanes react by nucleophilic substitution: reactivity depends on C–X bond strength (I > Br > Cl > F).
- SN2 for primary, SN1 for tertiary halogenoalkanes.
- Ethanolic KOH causes elimination to an alkene; aqueous NaOH causes substitution.
- Uses include solvents, plastics and insecticides, but CFCs damage the ozone layer.