Topic 3 of 7
Alkenes: a double bond creates new reactions
Distinguish substitution, addition and cracking.
O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)
Alkenes: a double bond creates new reactions
Distinguish substitution, addition and cracking.
Alkenes are unsaturated hydrocarbons containing a carbon-carbon double bond. For acyclic molecules with one C=C bond, their general formula is CnH2n. Ethene is CH2=CH2; propene is CH2=CH-CH3. Each carbon still has four bonds in total.
Worked example
Expand an alkene without adding too many hydrogens
Draw propene from CH2=CH-CH3.
- Join three carbon atoms. Put the double bond between the first two; this uses two of each participating carbon's four bonds.
- Carbon 1 needs two C-H bonds. Carbon 2 already has three bonds in total to carbons, so needs only one H. Carbon 3 needs three H.
- Count the result: C3H6, with four bonds at each carbon.
Use the displayed propene below as a check. A double bond counts twice for valency but still connects the same two carbon atoms.
Displayed ethene
Complete displayed structure with 2 carbon, 4 hydrogen atoms: C2H4. Every bond and hydrogen is shown.
Displayed propene
Complete displayed structure with 3 carbon, 6 hydrogen atoms: C3H6. Every bond and hydrogen is shown.
Aqueous bromine distinguishes an alkene from an alkane under the usual test conditions without UV. The alkene decolourises orange/brown bromine water because bromine adds across C=C. Ethene forms CH2Br-CH2Br. The alkane does not show this addition reaction. The colour change is an observation; unsaturation is the inference.
| Reagent or process | Essential conditions | Product/change |
|---|---|---|
| Oxygen: complete combustion | Ignition, sufficient oxygen | C2H4 + 3O2 -> 2CO2 + 2H2O |
| Aqueous bromine | Room conditions; no UV required | C2H4 + Br2 -> C2H4Br2 |
| Hydrogen | Nickel catalyst, heat | C2H4 + H2 -> C2H6 |
| Addition polymerisation | Suitable temperature, pressure and catalyst | Many ethene molecules form poly(ethene) |
Cracking breaks larger hydrocarbons into smaller molecules using strong heating, often with a catalyst. Products can include smaller alkanes, alkenes and hydrogen. Examples of possible balanced changes are C10H22 -> C8H18 + C2H4, and C2H6 -> C2H4 + H2. Actual cracking produces a mixture; use atom conservation to deduce a missing product.
Polyunsaturated food molecules contain more than one C=C bond. Hydrogenating unsaturated vegetable oils with hydrogen and a nickel catalyst reduces the number of double bonds, making the product more solid and useful in margarine manufacture. This is addition, not polymerisation.