Alkenes and Addition Reactions

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CognitoAlkenes and addition reactions

Key facts & methods

Alkene structure and properties

General formula: CnH2n.
Ethene C2H4, Propene C3H6, Butene C4H8.
Contain a C=C double bond (unsaturated). More reactive than alkanes.
Test for unsaturation: decolourise bromine water (orange to colourless).

Addition reactions

Hydrogenation: C=C + H2 → C-C. Ni catalyst, 150°C. Hardens vegetable oils.
Halogenation: C=C + Br2 → C(Br)-C(Br). Explains bromine water test.
Hydration: C=C + H2O → alcohol. Steam + H3PO4 catalyst. Ethene + H2O → ethanol.
All addition reactions: 100% atom economy (only one product).

Addition polymerisation

Many alkene monomers join via the C=C double bond: n(C2H4) → -(CH2CH2)n-
The C=C bond opens — monomers link into a long chain.
Alkanes CANNOT polymerise (no double bond).
Most addition polymers are non-biodegradable — disposal problem.
Poly(ethene) from ethene, poly(propene) from propene.

Exam questions — 4 questions · 10 marks · AQA 8462 style

1 markAlkene testAQA 8462

What is the test for an alkene and what is observed?

A Add acidified KMnO4 — turns colourless

B Add bromine water — decolourises from orange to colourless

C Add universal indicator — turns green

D Burns with no soot

3 marksAddition reactionsAQA 8462

Ethene reacts with steam to form ethanol. (a) Write the word equation. (b) What type of reaction is this? (c) Why does this reaction have 100% atom economy? (3 marks)

+30 XP

1 markPolymer requirementAQA 8462

Why can alkanes NOT undergo addition polymerisation?

A Alkanes are too large

B Alkanes are gases

C Alkanes have no C=C double bond — no site for addition reactions

D Alkane monomers repel each other

3 marksCompare alkane and alkeneAQA 8462

Compare alkanes and alkenes: (a) general formula, (b) bonding, (c) reaction with bromine water. (3 marks)

+30 XP

Alkenes and Addition Reactions

Alkene structure and properties

Addition reactions

Addition polymerisation

Module complete! 🎉