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
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 4Why Alkenes Are Reactive
Alkenes undergo addition reactions with electrophiles, unlike alkanes.
Double bond
- Double bond An alkene has a C=C bond made of a sigma bond and a pi bond.
Pi electrons
- Pi electrons The pi bond is a region of high electron density above and below the plane of the carbon atoms.
Electrophiles
- Electrophiles Positive or partially positive species are attracted to the pi bond and react with it.
Addition
- Addition The pi bond breaks and two new sigma bonds form, so a saturated product results.
Section 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.
Section 3 of 4Features of the Reaction
Features of the Reaction
Mechanism type
- Mechanism type Electrophilic addition is the name, because an electrophile attacks the electron-rich double bond.
Polarity
- Polarity Br₂ is nonpolar, but it becomes polarised as it approaches the electron density of the double bond.
Product
- Product Only one product forms, so the atom economy of addition is 100 percent.
Industrial use
- Industrial use Hydration of ethene is a major route to ethanol, and hydrogenation is used to make margarine.
| Reagent | Conditions | Product from ethene |
|---|---|---|
| H₂ | Ni, 150 °C | Ethane |
| Br₂ | Room temperature | 1,2-dibromoethane |
| HBr | Room temperature | Bromoethane |
| H₂O | Steam, H₃PO₄ catalyst | Ethanol |
Section 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.
Last updated:
\n