11.5 Alkenes
- Syllabus
- 0620–2026–2027
- Topic
- 11.5
- Level
- —
Learning objectives
11.5.1Bonding in alkenes includes a double• State: the bonding in alkenes includes a double carbon–carbon covalent bond and that alkenes are unsaturated hydrocarbons11.5.2Manufacture of alkenes and hydrogen by• Describe manufacture of alkenes and hydrogen by the cracking of larger alkane molecules using a high temperature and a catalyst11.5.3Reasons for the cracking of larger• Describe reasons for the cracking of larger alkane molecules11.5.4Test to distinguish between saturated• Describe test to distinguish between saturated and unsaturated hydrocarbons by their reaction with aqueous bromine11.5.5In an addition reaction only one• State: in an addition reaction only one product is formed11.5.6Alkene addition reactions with• Describe alkene addition reactions with bromine/aqueous bromine, hydrogen with nickel catalyst, and steam with acid catalyst; draw structural/displayed formulae of products
Recognise alkenes as unsaturated hydrocarbons
Alkenes are unsaturated hydrocarbons containing a carbon–carbon double covalent bond, C=C.
| Term | Meaning for an alkene |
|---|---|
| hydrocarbon | contains carbon and hydrogen only |
| unsaturated | contains a C–C bond that is not single |
| double covalent bond | two shared pairs of electrons join the carbon atoms |
Ethene, CH₂=CH₂, and propene, CH₃CH=CH₂, are alkenes. The C=C bond is the reactive part of each molecule.
A molecule containing C=O is not thereby an alkene. The defining double bond must be between two carbon atoms.
Manufacture alkenes and hydrogen by cracking
Cracking breaks larger alkane molecules into smaller molecules using a high temperature and a catalyst. The products include alkenes and may include hydrogen and shorter alkanes.
| Requirement | Role |
|---|---|
| large alkane feedstock | molecule to be split |
| high temperature | supplies energy to break bonds |
| catalyst | speeds the reaction |
| products | smaller alkane(s), alkene(s) and sometimes hydrogen |
Example: C₁₀H₂₂ → C₂H₄ + C₈H₁₈. Another possible cracking pattern is an alkane → alkene + hydrogen, provided the equation is balanced.
For a missing product, subtract the atoms already present in the known products from the atoms in the starting alkane, then check that every product is a valid molecule.
Cracking is not fractional distillation: distillation separates existing molecules, while cracking chemically changes large molecules into smaller ones.
Explain why large alkanes are cracked
Large alkane molecules are cracked because smaller hydrocarbons are more useful and often in greater demand, while alkenes are needed as chemical feedstocks.
| Product of cracking | Why it is wanted |
|---|---|
| shorter-chain alkanes | useful, more volatile fuels with high demand |
| alkenes | reactive feedstocks for addition reactions and addition polymers |
| hydrogen | useful product where the cracking equation produces it |
Cracking converts a surplus of less useful large molecules into products whose properties and chemical reactivity better match industrial demand.
Cracking does not create more total carbon or hydrogen atoms; it rearranges the atoms already present into smaller molecules.
Use aqueous bromine to test for unsaturation
| Sample | Observation after shaking with aqueous bromine | Conclusion |
|---|---|---|
| unsaturated hydrocarbon | orange/brown/yellow → colourless | C=C or C≡C present |
| saturated hydrocarbon | no colour change; bromine colour remains | no carbon–carbon multiple bond detected |
Add aqueous bromine (bromine water) to the sample and observe the initial and final colour. State both the reagent and the observation.
An alkene decolourises bromine because bromine adds across the C=C bond. An alkane has no C=C bond, so there is no rapid reaction under the test conditions.
Do not say bromine water changes from colourless to orange. The positive result is decolourisation: orange/brown/yellow to colourless.
Define an addition reaction
In an addition reaction, two reactant molecules join to form only one product molecule.
For an alkene, the C=C double bond becomes a C–C single bond and new atoms attach to the two carbon atoms.
alkene + small molecule → one saturated product
Do not count catalysts as products. If two different product molecules are formed, the reaction does not fit this addition definition.
Predict the three required alkene addition reactions
| Reagent and condition | Change across C=C | Product family | Ethene example |
|---|---|---|---|
| bromine or aqueous bromine | Br and Br add | dibromoalkane | CH₂=CH₂ + Br₂ → CH₂BrCH₂Br |
| hydrogen with nickel catalyst | H and H add | alkane | CH₂=CH₂ + H₂ → CH₃CH₃ |
| steam with acid catalyst | H and OH add | alcohol | CH₂=CH₂ + H₂O → CH₃CH₂OH |
Locate the two carbon atoms of C=C, change the double bond to a single bond, and attach one part of the reagent to each carbon. Keep the original carbon skeleton and check every carbon has four bonds.
With propene, bromine forms 1,2-dibromopropane, CH₃CHBrCH₂Br, and the aqueous bromine is decolourised.
Addition of steam to an unsymmetrical alkene can give positional alcohol isomers. For propene, the syllabus question evidence includes propan-1-ol and propan-2-ol; use an acid catalyst.
Nickel is the catalyst for hydrogen addition; an acid is the catalyst for steam addition. Bromine adds two bromine atoms across C=C rather than replacing a hydrogen.