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6.4.3Reactivity 1 — What drives chemical reactions?

Electrophilic Addition Reactions — IB Chemistry HL Revision Notes & Topical Questions

5 min read 4 parts Lesson 10 of 12 Unit quiz

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Part 1 of 4Why Alkenes Are Reactive

6.4.3 Electrophilic Addition Reactions

Alkenes undergo addition reactions with electrophiles, unlike alkanes.

Double bond
  • An alkene has a C=C bond made of a sigma bond and a pi bond.
Pi electrons
  • The pi bond is a region of high electron density above and below the plane of the carbon atoms.
Electrophiles
  • Positive or partially positive species are attracted to the pi bond and react with it.
Addition
  • The pi bond breaks and two new sigma bonds form, so a saturated product results.

Part 2 of 4Key Reactions of Ethene

1
With hydrogen and a nickel catalyst at about 150 °C ethene forms ethane: C₂H₄ + H₂ → C₂H₆.
2
With bromine at room temperature it forms 1,2-dibromoethane: C₂H₄ + Br₂ → CH₂BrCH₂Br.
3
With hydrogen bromide it forms bromoethane: C₂H₄ + HBr → CH₃CH₂Br.
4
With steam and a phosphoric acid catalyst, hydration forms ethanol: C₂H₄ + H₂O → CH₃CH₂OH.
5
Bromine water is decolourised from orange to colourless, which is the test for unsaturation.

Part 3 of 4Features of the Reaction

Features of the Reaction
Mechanism type
  • Electrophilic addition is the name, because an electrophile attacks the electron-rich double bond.
Polarity
  • Br₂ is nonpolar, but it becomes polarised as it approaches the electron density of the double bond.
Product
  • Only one product forms, so the atom economy of addition is 100 percent.
Industrial use
  • Hydration of ethene is a major route to ethanol, and hydrogenation is used to make margarine.
ReagentConditionsProduct from ethene
H₂Ni, 150 °CEthane
Br₂Room temperature1,2-dibromoethane
HBrRoom temperatureBromoethane
H₂OSteam, H₃PO₄ catalystEthanol

Part 4 of 4Exam Checklist & Tips

6.4.3 – Key Points for the Exam

  • Electrophile A species that accepts a pair of electrons.
  • Addition reaction A reaction in which two reactants join to give a single product.
  • Unsaturated Containing at least one carbon-carbon multiple bond.
  • Pi bond A bond formed by sideways overlap of p orbitals.

Common Mistakes

  • Saying alkenes undergo substitution with bromine water; they undergo addition.
  • Calling bromine water test positive when it turns brown rather than colourless.
  • Forgetting that the product of adding Br₂ to ethene has a bromine on each carbon.
  • Describing the pi bond as stronger than the sigma bond.

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