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Learn9701Paper 2 · AS Level Structured QuestionsHydrocarbonsAlkenes and electrophilic addition
Hydrocarbons · Subtopic 2 of 2

Alkenes and Electrophilic Addition

The C=C bond has a σ bond and a weaker π bond with high electron density, which attracts electrophiles. In electrophilic addition, the π electrons attack the electrophile (e.g. H in HBr, or an induced-dipole Br₂), forming a carbocation; the anion then bonds to it. Alkenes add H₂ (Ni, 150 °C), steam (H₃PO₄, 300 °C, 60 atm), HX and X₂. Markovnikov: with unsymmetrical alkenes, the more stable carbocation (tertiary > secondary > primary, because alkyl groups are electron-donating) forms the major product. Cold dilute acidified KMnO₄ gives a diol; hot concentrated KMnO₄ cleaves the C=C.

Cambridge 9701 Paper 1Cambridge 9701 Paper 2Edexcel YCH11 WCH11AQA 7405 Paper 2≈ 50 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Describe addition reactions of alkenes.
  • Draw the electrophilic addition mechanism.
  • Use carbocation stability to predict major and minor products.
  • Describe oxidation of alkenes by KMnO₄.
Pause and think

Why does bromine, a non-polar molecule, react with alkenes as an electrophile?

Section 2 of 14

Core idea

Start hereAn electron-rich target

The π bond in an alkene is a cloud of electrons sitting above and below the molecule, an inviting target for anything positive.

The big idea

Electrophiles attack the π bond, forming a carbocation that is then captured by a nucleophile. The more stable carbocation gives the major product.

1. The mechanism

Electrophilic addition of HBr

Two steps through a carbocation.

Worked examplePredicting the major product

2-methylpropene + HBr.

Goal
Major product.
Strategy
Compare possible carbocations.
  1. (CH3)3C+ (3∘) vs (CH3)2CHCH2+ (1∘)\mathrm{(CH_3)_3C^+}\ (3^\circ)\ \text{vs}\ \mathrm{(CH_3)_2CHCH_2^+}\ (1^\circ)
    Two options.
  2. 3∘ more stable3^\circ\ \text{more stable}
    Three alkyl groups donate electron density.
Answer2-bromo-2-methylpropane.
✓ CheckH added to the CH₂ end, which had more H ✓.
Key takeawayKMnO₄ cleavage Hot concentrated acidified KMnO₄: =CH₂ → CO₂; =CHR → RCOOH; =CR₂ → R₂C=O. Products reveal where the double bond was.
Pause and think

Why are alkenes more reactive than alkanes?

Summary

  • Electrophilic addition.
  • Carbocation stability.
  • KMnO₄ tests and cleavage.
Key words
Carbocation
Ion with a positively charged carbon.
Markovnikov's rule
H adds to the carbon with more H atoms in unsymmetrical additions.
Inductive effect
Electron donation or withdrawal along σ bonds.
HistoryMarkovnikov. Vladimir Markovnikov stated his rule in 1870, long before carbocations were understood; the explanation came about 60 years later.
Fun factRipening in transit. Fruit shipments include potassium manganate(VII) sachets that oxidise ethene given off by the fruit, slowing ripening.
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