Topic 14 of 34
An alkene offers a reactive pi bond that an alkane lacks
Reagent and conditions determine whether the double bond adds, reduces or oxidatively breaks.
A-Level 9476 (2026-2027)
An alkene offers a reactive pi bond that an alkane lacks
Reagent and conditions determine whether the double bond adds, reduces or oxidatively breaks.
Complete combustion of ethane is 2C2H6 + 7O2 → 4CO2 + 6H2O. With inadequate oxygen, carbon monoxide and/or carbon can form. Halogenation is a different reaction: Cl2 or Br2 under UV at room temperature replaces C-H by C-X through the radical chain described earlier.
| Reagent and essential conditions | Main organic change | Example product |
|---|---|---|
| Steam, H3PO4 catalyst, high temperature and pressure | Electrophilic hydration: H and OH add across C=C. | CH2=CH2 + H2O → CH3CH2OH. |
| HX gas, under the polar addition conditions | H and X add; apply the carbocation/Markovnikov explanation for an unsymmetrical alkene. | Ethene + HBr → bromoethane. |
| Br2 or Cl2 in CCl4, room temperature, no UV needed | Halogen addition across C=C. | Ethene + Br2 → BrCH2CH2Br. |
| Aqueous halogen, room temperature | Rapid addition consumes the halogen colour. Water can compete as a nucleophile. | In bromine water, a bromohydrin such as HOCH2CH2Br can form alongside the dibromide; solvent affects product composition. |
| H2 gas, Ni catalyst and heat | Catalytic hydrogenation reduces C=C to C-C. | Ethene → ethane. |
| Cold, dilute alkaline KMnO4 | Mild oxidation adds OH to both double-bond carbons. | Ethene → ethane-1,2-diol, HOCH2CH2OH; purple manganate(VII) is consumed and brown MnO2 commonly forms. |
The specified Br2/CCl4 mechanism gives a vicinal dibromide; do not silently treat water as an inert solvent in all halogen additions. For a bromine-water test, the rapid disappearance of colour is the useful observation, but it is not unique to alkenes because activated aromatic compounds can also consume bromine.
Worked example
Predict a new alkene product
What is the major product when but-1-ene reacts with HBr by the usual polar mechanism, and what changes with H2/Ni?
- HBr protonation that produces a secondary rather than primary carbocation is favoured.
- Bromide attacks the secondary carbon, giving predominantly 2-bromobutane.
- Hydrogenation instead adds one H to each double-bond carbon without adding bromine.
HBr gives mainly 2-bromobutane; H2/Ni gives butane. The first product can be formed as an enantiomeric pair in an achiral reaction environment.