Topic 3 of 9

Alkenes: a double bond creates new reactions

Distinguish substitution, addition and cracking.

O-Level 6092 (2026) / SEC G3 K324 (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 C_nH_2n. Ethene is CH₂=CH₂; propene is CH₂=CH-CH₃. Each carbon still has four bonds in total.

Pure only

Pure: C4 alkene structures
Connectivity	Condensed structure
But-1-ene	CH₂=CH-CH₂-CH₃
But-2-ene	CH₃-CH=CH-CH₃
Branched C₄H₈	CH₂=C(CH₃)₂

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.

Answer

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

C2H4. Count four bonds at every carbon, two at oxygen and one at hydrogen. The flat layout shows connectivity, not molecular shape.

Displayed propene

C3H6. Count four bonds at every carbon, two at oxygen and one at hydrogen. The flat layout shows connectivity, not molecular shape.

Pure only

Displayed but-1-ene

C4H8. Count four bonds at every carbon, two at oxygen and one at hydrogen. The flat layout shows connectivity, not molecular shape.

Pure only

Displayed but-2-ene

C4H8. Count four bonds at every carbon, two at oxygen and one at hydrogen. The flat layout shows connectivity, not molecular shape.

Pure only

Displayed branched C4 alkene

CH2=C(CH3)2. The central carbon has a double bond and two single bonds, so it has no hydrogen.

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.

Ethene reaction pathways
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)

Pure only

Pure: steam adds across C=C with an acid catalyst such as phosphoric acid, at elevated temperature and pressure: C₂H₄(g) + H₂O(g) -> C₂H₅OH(g). The ethanol is condensed after the reaction. Exact numerical temperatures and pressures are not required.

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.

Check your understandingC₈H₁₈ cracks to C₆H₁₄ and one other molecule. Deduce its formula.Think it through, then reveal the answer

Subtract atoms: two C and four H remain, so C₂H₄, ethene. Check both carbon and hydrogen totals rather than guessing from the word cracking.