3.4.3Physical Chemistry
Reactions of Alkenes — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 78 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 6Why Alkenes Are Reactive
Why Alkenes Are Reactive
C=C bond
- C=C bond The double bond is a sigma bond plus a pi bond, and the pi bond is the weaker, more exposed part.
Electron density
- Electron density The pi electrons sit above and below the molecule, so the region is electron-rich and attracts positive species.
Electrophile
- Electrophile An electrophile is a species that accepts a pair of electrons, such as H⁺ or the δ+ end of a polar molecule.
Addition
- Addition Two reactants join to give one product, so the pi bond breaks and the atom economy is 100%.
Part 2 of 6Addition Reactions of Ethene
Each reagent adds across the double bond, turning C=C into a saturated C–C bond.
Hydrogen
- Hydrogen Ethene and H₂ with a nickel catalyst at about 150 °C give ethane, C₂H₆.
Bromine
- Bromine Ethene reacts with bromine at room temperature to give 1,2-dibromoethane, CH₂BrCH₂Br.
Hydrogen bromide
- Hydrogen bromide Ethene reacts with HBr at room temperature to give bromoethane, CH₃CH₂Br.
Steam
- Steam Ethene and steam react over a phosphoric acid catalyst at high temperature and pressure to give ethanol.
Sulfuric acid
- Sulfuric acid Cold concentrated H₂SO₄ adds to give ethyl hydrogensulfate, which water then converts to ethanol.
| Reagent | Conditions | Product from ethene |
|---|---|---|
| H₂ | Ni catalyst, about 150 °C | Ethane |
| Br₂ | Room temperature | 1,2-Dibromoethane |
| HBr | Room temperature | Bromoethane |
| H₂O(g) | H₃PO₄ catalyst, about 300 °C, high pressure | Ethanol |
Part 3 of 6Mechanism with Hydrogen Bromide
1
The H–Br bond is polar, so hydrogen carries a partial positive charge and acts as the electrophile.
2
A curly arrow from the C=C double bond goes to the δ+ hydrogen, forming a new C–H bond.
3
A second curly arrow shows the H–Br bond breaking heterolytically, with both electrons going to bromine to form Br⁻.
4
This leaves a carbocation, a carbon with only three bonds and a positive charge, as the reactive intermediate.
5
A lone pair on the bromide ion then attacks the positive carbon, forming a C–Br bond and the haloalkane product.
Part 4 of 6Unsymmetrical Alkenes
When the two carbons of C=C carry different groups, two products are possible and one is favoured.
Unsymmetrical Alkenes
Two intermediates
- Two intermediates Adding H⁺ to propene can give a primary or a secondary carbocation, leading to two different products.
Stability order
- Stability order Alkyl groups push electrons towards the positive carbon, so tertiary is more stable than secondary, which beats primary.
Major product
- Major product The more stable carbocation forms faster, so propene and HBr give mainly 2-bromopropane, CH₃CHBrCH₃.
Minor product
- Minor product The less stable primary carbocation gives a small amount of 1-bromopropane, CH₃CH₂CH₂Br.
Part 5 of 6Worked Example
Question Name the major organic product when 2-methylpropene, (CH₃)₂C=CH₂, reacts with HBr, and explain why it is the major product.
- H⁺ can add to the CH₂ carbon, leaving the positive charge on the carbon that holds two CH₃ groups: a tertiary carbocation, (CH₃)₃C⁺.
- H⁺ could instead add to the other carbon, giving a primary carbocation, (CH₃)₂CH–CH₂⁺.
- The tertiary carbocation has three electron-donating alkyl groups, so it is more stable and forms faster.
- Br⁻ attacks the tertiary carbocation to give (CH₃)₃CBr.
Answer The major product is 2-bromo-2-methylpropane, (CH₃)₃CBr, because it forms via the more stable tertiary carbocation.
Part 6 of 6Exam Checklist & Tips
3.4.3 – Key Points for the Exam
- Electrophile A species that accepts a pair of electrons to form a covalent bond.
- Electrophilic addition A reaction in which an electrophile attacks the C=C bond and two reactants combine into one product.
- Carbocation An organic ion with a positively charged carbon atom that has only three bonds.
- Heterolytic fission Breaking a covalent bond so both bonding electrons go to one atom, forming a positive and a negative ion.
- Inductive effect The pushing of electron density by alkyl groups towards a neighbouring positive carbon.
Common Mistakes
- Starting the curly arrow at the H atom of HBr instead of at the C=C double bond.
- Drawing the product of an unsymmetrical addition without saying which carbocation is more stable.
- Writing the steam hydration conditions as aqueous acid at room temperature rather than steam, H₃PO₄ catalyst, high temperature and pressure.
- Forgetting the δ+ and δ− charges on HBr in the mechanism.
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