Topic 4 of 12
Hydrocarbon reactions
Follow the bond that changes and account for every product.
A-Level 8873, revised syllabus (2026-2027)
An alkane substitutes; an alkene adds
Follow the bond that changes and account for every product.
Alkanes have only strong C-C and C-H sigma bonds and no especially reactive functional group. They are generally unreactive under ordinary conditions, but burn and undergo substitution with chlorine under ultraviolet light. Alkenes contain a pi bond, which can be replaced by two new sigma bonds in an addition reaction. Addition joins reactants into a product without eliminating a small molecule; substitution replaces an atom or group with another.
| Starting compound and conditions | Equation | What changes |
|---|---|---|
| Ethane; oxygen, ignition | 2C2H6 + 7O2 → 4CO2 + 6H2O | Complete combustion oxidises carbon and reduces oxygen. Hydrogen remains at oxidation number +1. Limited oxygen can give CO and/or carbon instead. |
| Ethane; Cl2, ultraviolet light, room temperature | C2H6 + Cl2 → C2H5Cl + HCl | Substitution replaces H by Cl. Further substitutions can occur, so one equation does not promise a single pure product. |
| Ethene; oxygen, ignition | C2H4 + 3O2 → 2CO2 + 2H2O | Complete combustion. |
| Ethene; Br2 in CCl4 | CH2=CH2 + Br2 → CH2BrCH2Br | Addition across C=C gives 1,2-dibromoethane. Bromine colour is discharged; no HBr is a product of this addition. |
| Ethene; H2, Ni catalyst, heat | CH2=CH2 + H2 → CH3CH3 | Hydrogenation is addition and reduction. A pi bond is replaced by two C-H bonds. |
Worked example
Transfer the reaction to a longer alkene
Predict the product when but-2-ene reacts with Br2 in CCl4.
- Locate the C=C between carbon 2 and carbon 3: CH3CH=CHCH3.
- Change that C=C to C-C and attach one Br to each of those carbons.
- The product is CH3CHBrCHBrCH3, 2,3-dibromobutane. No carbon is added or removed, and both Br atoms appear in the product.
Answer
CH3CH=CHCH3 + Br2 → CH3CHBrCHBrCH3.
Check your understandingWhy is decolourisation evidence for reaction but not, by itself, a complete structural identification?Think it through, then reveal the answer
It shows that coloured bromine has been consumed under the stated conditions. Combine it with the formula and other chemical evidence to identify a compound. A reaction observation does not uniquely identify one particular alkene.