Alkenes

Summary: Alkenes are unsaturated hydrocarbons with C=C double bond (CnH2n). Ethene (C2H4) is the simplest. They undergo addition reactions (bromine, hydrogen, steam) and polymerisation. Tags: igcse chemistry organic-chem Created: 2026-07-14 Last Updated: 2026-07-16

What are Alkenes?

Alkenes are a homologous series of unsaturated hydrocarbons with the general formula:

CnH2n

Unsaturated means the molecule contains at least one carbon-carbon double bond (C=C). The double bond consists of one sigma bond and one pi bond. Because of the double bond, alkenes have two fewer hydrogen atoms than the corresponding alkane.

Alkenes are much more reactive than alkanes because the C=C double bond can open up, allowing atoms to add across it.

The First Three Alkenes

There is no “methene” — the simplest alkene has 2 carbon atoms (ethene) because at least two carbons are needed to form a C=C double bond.

Displayed formulas (showing all atoms and bonds):

• Ethene (C2H4): H2C=CH2 — two carbon atoms joined by a double bond, each carbon bonded to two H atoms
• Propene (C3H6): CH3-CH=CH2 — three carbon chain with a double bond between C1 and C2
• But-1-ene: CH3CH2CH=CH2 — the double bond is between C1 and C2

Test for Unsaturation: Bromine Water Test

This is the distinguishing test between alkanes and alkenes:

Why? The C=C double bond in alkenes undergoes an addition reaction with bromine. The bromine adds across the double bond, forming a colourless dibromoalkane:

C2H4 (g) + Br2 (aq) → CH2BrCH2Br (l)
Ethene + Bromine → 1,2-dibromoethane

     Orange/brown       Colourless

This reaction happens without UV light — unlike the substitution reaction of alkanes. The double bond is already reactive enough.

Addition Reactions of Alkenes

In all addition reactions, the C=C double bond opens up and atoms add across it. Only one product is formed.

Addition of Hydrogen (Hydrogenation)

Alkene + H2 → Alkane

C2H4 (g) + H2 (g) --(Ni catalyst, 150 °C)--> C2H6 (g)
Ethene + Hydrogen → Ethane

Conditions: Nickel catalyst, 150 °C Use: Converting unsaturated oils to saturated fats (margarine manufacture — making oils solid at room temperature)

Addition of Halogens (Bromination/Chlorination)

Alkene + Halogen → Dihaloalkane

C2H4 (g) + Br2 (aq) → CH2BrCH2Br (l)
Ethene + Bromine → 1,2-dibromoethane

C2H4 (g) + Cl2 (g) → CH2ClCH2Cl (l)
Ethene + Chlorine → 1,2-dichloroethane

No catalyst or UV light required — the reaction is spontaneous.

Addition of Steam (Hydration)

Alkene + H2O (steam) → Alcohol

C2H4 (g) + H2O (g) --(H3PO4 catalyst, 300 °C, 60 atm)--> C2H5OH (l)
Ethene + Steam → Ethanol

Conditions: Phosphoric acid (H3PO4) catalyst, 300 °C, 60 atmospheres pressure Use: Industrial production of ethanol (hydration of ethene)

Addition of Hydrogen Halides

Alkene + Hydrogen halide → Halogenoalkane

C2H4 (g) + HCl (g) → CH3CH2Cl (g)
Ethene + Hydrogen chloride → Chloroethane

C2H4 (g) + HBr (g) → CH3CH2Br (l)
Ethene + Hydrogen bromide → Bromoethane

No catalyst or UV light required.

Addition Polymerisation

Polymerisation is the process in which many small monomer molecules (alkenes) join together to form a very large polymer molecule.

Key points:

• The monomers are small alkene molecules (e.g., ethene)
• The C=C double bond in each monomer opens up
• Monomers join by single covalent bonds to form a long chain
• The polymer is saturated (contains only C-C single bonds in the main chain)
• No other products are formed — this is an addition polymerisation

Common Addition Polymers

Drawing the polymer repeating unit from a monomer:

Example: Poly(ethene) from ethene

• Monomer: CH2=CH2
• Open the double bond: -CH2-CH2-
• Repeating unit: -(CH2-CH2)-

Example: Poly(propene) from propene

• Monomer: CH3CH=CH2
• Open the double bond: -(CH(CH3)-CH2)-
• The -CH3 group is a side branch, not part of the main chain

When drawing repeating units, show the open bonds extending through the brackets (one on each side) to indicate the chain continues.

Distinguishing Alkanes from Alkenes

Key Points

• General formula: CnH2n
• Alkenes are unsaturated (contain C=C double bond)
• First three: ethene (C2H4), propene (C3H6), butene (C4H8)
• Bromine water test: orange/brown → colourless with alkenes (addition); stays orange with alkanes
• Hydrogenation: alkene + H2 → alkane (Ni catalyst, 150 °C)
• Hydration: alkene + H2O → alcohol (H3PO4, 300 °C, 60 atm)
• Bromination: alkene + Br2 → dibromoalkane (no catalyst needed)
• Addition polymerisation: many monomers join via opening of C=C bonds to form a long saturated chain
• Common polymers: poly(ethene), poly(propene), PVC, PTFE

Key Concepts from Past Papers

• Fractional distillation depends on boiling point
• Petroleum is heated and vaporised; vapour enters at bottom of fractionating column
• Bromine water test: alkanes stay orange, alkenes decolourise

Keywords from Past Papers

methane, ethene, point, bonds, carbon, fractions, alkenes, energy, hydrocarbons, boiling, idea, structure, water, alcohol, addition

Related Notes

• Organic Chemistry Fundamentals
• Alkanes
• Alcohols
• Carboxylic Acids
• Polymers

Sources

• OpenStax Chemistry 2e — [Chapter 20: Organic Chemistry (Alkenes)], Rice University (free, CC BY 4.0)
• BBC Bitesize GCSE Chemistry — [Alkenes], BBC (free educational resource)
• Cambridge IGCSE Chemistry 0620 — Syllabus Section 11: Organic Chemistry (Alkenes), Cambridge Assessment International Education
• CK-12 Chemistry for High School — [Chapter 25: Alkenes], CK-12 Foundation (free, CC BY-NC 3.0)

Past Paper Sources

• 0620/32 Feb/March 2017: Q33(b)(iv) (1m), Q33(c)(iii) (1m)
• 0620/32 Feb/March 2018: Q66(a)(i) (1m)
• 0620/32 Feb/March 2019: Q55(d)(iii) (1m)
• 0620/32 Feb/March 2020: Q11(a)(ii) (1m), Q11(a)(v) (1m)
• 0620/32 Feb/March 2021: Q66(c)(i) (1m)
• 0620/32 Feb/March 2022: Q22(a)(ii) (2m), Q77(b)(i) (1m)
• 0620/32 Feb/March 2023: Q66(a)(ii) (1m), Q66(a)(iv) (2m)
• 0620/32 May/June 2018: Q11(a)(v) (1m), Q33(a)(iv) (1m), Q33(c)(ii) (0m) (+1 more)
• 0620/33 May/June 2016: Q44(c)(iv) (1m), Q66(e)(i) (1m)
• 0620/33 May/June 2017: Q44(a)(ii) (1m)
• 0620/33 May/June 2019: Q11(a)(i) (2m), Q11(a)(iv) (1m), Q11(a)(vi) (1m) (+3 more)
• 0620/33 May/June 2021: Q44(a)(iii) (1m), Q44(d)(i) (1m), Q44(d)(ii) (1m) (+1 more)

Common Misconceptions