Alkenes contain a carbon-carbon double bond that makes them far more reactive than alkanes. The double bond can "open up" to accept additional atoms across it in a single step — an addition reaction — turning one reactant into one product with no atoms left over. This reactivity underpins industrial processes from margarine production to polymer manufacturing.
Why are alkenes reactive?
Alkenes belong to the homologous series with general formula CₙH₂ₙ. They contain at least one C=C double bond. This double bond consists of a strong σ (sigma) bond and a weaker π (pi) bond. The π electrons are exposed above and below the plane of the molecule, making them readily available to react with other species.
Because the C=C double bond is not fully satisfied with hydrogen atoms at the reaction sites, alkenes are described as unsaturated — they have the capacity to absorb more atoms. Alkanes, which have only single bonds, are saturated and are much less reactive.
What is an addition reaction?
In an addition reaction:
- The double bond in the alkene opens (the π bond breaks)
- Two fragments from the other reactant add to the two carbon atoms
- One product is formed from two reactants; nothing is left over
This is the key distinction from a substitution reaction (where one atom replaces another, and a second product is formed).
How does bromine react with alkenes?
Bromine water test: This is the most important addition reaction to know for GCSE.
When bromine water (orange-brown) is added to an alkene, the bromine reacts across the double bond:
Ethene + Bromine → 1,2-dibromoethane
CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br
The orange-brown colour of bromine water decolourises (becomes colourless), confirming the presence of a C=C double bond. Alkanes do not react with bromine water in the dark, so decolourisation is a reliable test for unsaturation.
In the laboratory: Bromine water can be added to the unknown compound in a test tube. If the mixture decolourises at room temperature without UV light, the compound is unsaturated.
How does hydrogen react with alkenes?
Hydrogenation is the addition of hydrogen (H₂) across a double bond:
Ethene + Hydrogen → Ethane (in the presence of a nickel catalyst, 150 °C)
CH₂=CH₂ + H₂ → CH₃–CH₃
The double bond is converted to a single bond, producing a saturated compound.
Industrial application: Hydrogenation converts vegetable oils (unsaturated) into solid fats such as margarine and cooking fat. A nickel catalyst is used. Partial hydrogenation (not all double bonds reacted) gives a semi-solid consistency — this is how spreads like vegetable shortening are made. The process is called hardening of oils.
How does steam react with alkenes?
Hydration is the addition of water (as steam) across a double bond:
Ethene + Steam → Ethanol (in the presence of a phosphoric acid catalyst, 300 °C, 60–70 atm pressure)
CH₂=CH₂ + H₂O → CH₃CH₂OH
This is the industrial route to ethanol — it is faster and produces purer ethanol than fermentation, but requires ethene derived from crude oil (a non-renewable resource). Fermentation (using yeast + sugars) is the sustainable alternative but produces dilute ethanol that must be distilled.
How do hydrogen halides react with alkenes?
Hydrogen halides (HCl, HBr, HI) add across the double bond:
Ethene + Hydrogen bromide → Bromoethane
CH₂=CH₂ + HBr → CH₃CH₂Br
The hydrogen atom bonds to one carbon; the bromine atom bonds to the other. This produces a halogenoalkane, which can be used as a starting material for further reactions.
Summary of alkene addition reactions
| Reactant added | Conditions | Product | Key application |
|---|---|---|---|
| Bromine (Br₂) | Room temperature; no catalyst | Dibromoalkane | Test for unsaturation |
| Hydrogen (H₂) | Ni catalyst, ~150 °C | Alkane (saturated) | Hardening oils to make margarine |
| Steam (H₂O) | H₃PO₄ catalyst, 300 °C, high pressure | Alcohol | Industrial ethanol production |
| Hydrogen halide (e.g. HBr) | Moderate temperature; no catalyst | Halogenoalkane | Organic synthesis |
Frequently asked questions
Why do alkenes decolourise bromine water but alkanes do not?
Alkenes decolourise bromine water because the π electrons in the double bond react with bromine molecules in an addition reaction, forming a colourless dibromo compound. Alkanes have no double bond and no exposed π electrons; in the dark and at room temperature, they cannot react with bromine water. (Under UV light, alkanes do react with bromine via a free-radical substitution mechanism, but this requires activation energy from light — very different from the spontaneous addition seen with alkenes.)
Why is industrial ethanol production from ethene preferred to fermentation for some uses?
The hydration of ethene produces pure ethanol continuously at a fast rate, suitable for industrial solvents, fuels (bioethanol in petrol blends), and pharmaceuticals. Fermentation is batch-process, slow, and produces a dilute ethanol solution (about 15% maximum before yeast die) that needs costly distillation to concentrate. However, fermentation uses renewable sugar crops whereas ethene comes from crude oil cracking, so for sustainable or food-grade production, fermentation is preferred. In practice, most industrial chemical ethanol is made via hydration; most drinking alcohol is from fermentation.
What happens when a symmetrical versus an unsymmetrical alkene reacts with a hydrogen halide?
For a symmetrical alkene such as ethene (CH₂=CH₂), the hydrogen and halide can only add in one way (one product). For an unsymmetrical alkene such as propene (CH₃–CH=CH₂), there are two possible ways to add HBr: HBr can add so that Br goes to carbon-1 (giving 1-bromopropane) or to carbon-2 (giving 2-bromopropane). Markovnikov's rule predicts which product predominates (the halide bonds to the more substituted carbon), but this detail is generally A-level rather than GCSE.
How do I distinguish between an alkane and an alkene in the laboratory?
Add a few drops of bromine water to the unknown compound in a test tube and shake. If the orange-brown colour disappears (decolourises) without UV light, the compound is an alkene (unsaturated). If the colour is retained, the compound is an alkane (saturated). You can also burn a small amount: alkenes tend to burn with a smokier, more orange flame than alkanes because of their higher carbon-to-hydrogen ratio, though this is a qualitative observation rather than a definitive test.
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