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6.4.3Reactivity 1 — What drives chemical reactions?

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

5 min read 7 parts Lesson 8 of 8 Unit quiz

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Part 1 of 7Why Alkenes React

6.4.3 Electrophilic Addition Reactions

Electrophilic addition is the characteristic reaction of alkenes.

Electron-rich
  • The C=C double bond has a region of high electron density from the π bond.
Electrophiles
  • Electron-deficient species such as H⁺ and the δ+ end of HBr are attracted to this region.
Addition
  • The π bond breaks and each carbon forms a new bond, so one product forms.
Unsaturation
  • Alkenes are unsaturated and more reactive than alkanes.

Part 2 of 7Addition Reactions of Ethene

Addition Reactions of Ethene
Hydrogen bromide
  • CH₂=CH₂ + HBr → CH₃CH₂Br, giving a halogenoalkane.
Bromine
  • With bromine the product is a dihalogenoalkane: CH₂=CH₂ + Br₂ → CH₂BrCH₂Br (1,2-dibromoethane).
Steam
  • CH₂=CH₂ + H₂O → CH₃CH₂OH, using a phosphoric acid catalyst at about 300 °C and 60 atm.
Test
  • Bromine water changes from orange to colourless when shaken with an alkene.

Part 3 of 7Mechanism with Bromine

1
The π electrons of the alkene polarise the Br–Br molecule, making the nearer bromine δ+.
2
A curly arrow goes from the C=C bond to the δ+ bromine, and another goes from the Br–Br bond to the far bromine.
3
This forms a carbocation intermediate and a Br⁻ ion.
4
Bromide acts as a nucleophile, donating a lone pair to the positive carbon to complete the addition.

Part 4 of 7Symmetry and Products

Symmetrical alkenes
  • Ethene gives only one product with HBr, since both carbons are equivalent.
But-2-ene
  • Butene with bromine adds across the bond: CH₃CH=CHCH₃ + Br₂ → CH₃CHBrCHBrCH₃ (2,3-dibromobutane).
Hydrogen
  • Hydrogenation is also an addition reaction, giving an alkane.
ReagentProduct classConditions
H₂AlkaneNickel catalyst, heat
Br₂DihalogenoalkaneRoom temperature
HBrHalogenoalkaneRoom temperature
H₂O(g)AlcoholAcid catalyst, 300 °C, 60 atm

Part 5 of 7Practical Use of the Reactions

Practical Use of the Reactions
Test for alkenes
  • Adding bromine water drop by drop to an unknown shows unsaturation if it decolourises quickly.
Ethanol manufacture
  • Hydration of ethene gives ethanol, but the equilibrium and the temperature limit the single-pass conversion.
Industrial recycling
  • Unreacted ethene is recycled to improve overall use of the feedstock.
Safety
  • Bromine is toxic and corrosive, so dilute bromine water is generally used instead.

Part 6 of 7Worked Example

Question Write the equation and product name when but-2-ene reacts with bromine, and describe the observation.

  1. Both carbons of the C=C bond each gain one Br atom.
  2. CH₃CH=CHCH₃ + Br₂ → CH₃CHBrCHBrCH₃.
  3. Bromine is used up, so the orange colour disappears.

Answer The product is 2,3-dibromobutane and the orange solution becomes colourless.

Part 7 of 7Exam Checklist & Tips

6.4.3 – Key Points for the Exam

  • Electrophile An electron-deficient species that accepts an electron pair.
  • Addition reaction A reaction in which two molecules combine to give one product.
  • Carbocation A positively charged ion with a carbon atom with only six electrons in its valence shell.
  • Unsaturated Containing at least one carbon–carbon multiple bond.

Common Mistakes

  • Describing the bromine water result as colourless to orange.
  • Drawing the curly arrow from bromine to the double bond.
  • Saying the alkene acts as an electrophile; it acts as an electron-rich nucleophile here.
  • Stating that substitution occurs; alkenes undergo addition.

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