3.7.5Physical Chemistry
Electrophilic Addition — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 94 of 102
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.
Part 1 of 5Why Alkenes Undergo Addition
Why Alkenes Undergo Addition
Electron-rich bond
- Electron-rich bond The C=C double bond has a high electron density that attracts electrophiles.
Single product
- Single product Two reactants combine to form one product, so the atom economy is 100%.
Reagents
- Reagents Alkenes react with hydrogen halides, halogens, and steam with an acid catalyst.
Intermediate
- Intermediate Each mechanism forms a carbocation intermediate, followed by attack of a nucleophile.
Part 2 of 5Hydrogen Bromide and Ethene
1
HBr is polar, so hydrogen is δ+ and bromine is δ−, and the hydrogen acts as the electrophile.
2
A curly arrow goes from the C=C double bond to the δ+ hydrogen atom.
3
A second arrow from the H–Br bond moves both electrons to bromine, forming Br⁻.
4
This leaves a carbocation, CH₃CH₂⁺, which is attacked by a lone pair on Br⁻.
5
The result is the single addition product, bromoethane, CH₃CH₂Br.
Part 3 of 5Bromine and Steam
Induced dipole
- Induced dipole Bromine is non-polar, but the electron-rich double bond repels electrons in Br–Br, making the nearer Br atom δ+.
Bromine addition
- Bromine addition The δ+ bromine accepts the pair from C=C, Br⁻ leaves, and Br⁻ attacks the carbocation to give 1,2-dibromoethane.
Acid catalyst
- Acid catalyst In hydration, an H⁺ ion from the acid, such as H₃PO₄ or H₂SO₄, acts as the electrophile.
Water attack
- Water attack A lone pair on water attacks the carbocation, and the oxonium ion then loses H⁺ to form the alcohol.
Catalyst regenerated
- Catalyst regenerated The H⁺ released in the final step is recycled, so the acid is a catalyst.
Part 4 of 5Major and Minor Products
Major and Minor Products
Unsymmetrical alkene
- Unsymmetrical alkene HBr and propene can form either a primary or a secondary carbocation.
Stability
- Stability Tertiary carbocations are more stable than secondary, which are more stable than primary, because of electron-donating alkyl groups.
Major product
- Major product The more stable carbocation forms in greater amount, so 2-bromopropane is the major product from propene.
Minor product
- Minor product The less stable carbocation gives 1-bromopropane in a smaller quantity.
| Alkene + HBr | Major product | Minor product |
|---|---|---|
| Propene | 2-Bromopropane | 1-Bromopropane |
| But-1-ene | 2-Bromobutane | 1-Bromobutane |
| 2-Methylpropene | 2-Bromo-2-methylpropane | 1-Bromo-2-methylpropane |
Part 5 of 5Exam Checklist & Tips
3.7.5 – Key Points for the Exam
- Electrophilic addition A reaction in which an electrophile attacks the C=C bond and two reactants form one product.
- Induced dipole A temporary dipole produced in a non-polar molecule by a nearby electron-rich region.
- Carbocation An ion with a positively charged carbon atom and only three bonds.
- Major product The product formed in the greater amount, via the more stable intermediate.
Common Mistakes
- Starting the first curly arrow at the H–Br bond instead of at the C=C double bond.
- Forgetting to show the induced dipole on bromine before drawing the mechanism.
- Naming the primary-carbocation product as the major product from an unsymmetrical alkene.
- Writing the acid catalyst as used up in hydration, when the H⁺ is regenerated in the final step.
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