6.4.10Additional lessons
Addition to Unsymmetrical Alkenes — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 156 of 172
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 4Two Possible Products
The product mixture is unequal because the two carbocation intermediates differ in stability.
Unsymmetrical alkene
- Unsymmetrical alkene An alkene such as propene has different groups on the two carbons of the double bond.
Two routes
- Two routes Hydrogen bromide could add H to either carbon, giving 1-bromopropane or 2-bromopropane.
Major product
- Major product 2-bromopropane is formed in larger amount, so it is the major product.
Markovnikov's rule
- Markovnikov's rule The hydrogen adds to the carbon that already has more hydrogen atoms, and the halogen goes to the other carbon.
Part 2 of 4Explaining the Rule
1
The pi electrons attack the hydrogen of H–Br, and H can bond to either carbon of the double bond.
2
Adding H to the end carbon gives a secondary carbocation, CH₃CH⁺CH₃.
3
Adding H to the middle carbon gives a primary carbocation, CH₃CH₂CH₂⁺.
4
Alkyl groups donate electron density by the inductive effect, so the secondary carbocation is more stable and forms faster.
5
The bromide ion then attacks this carbocation, giving 2-bromopropane as the major product.
| Carbocation type | Alkyl groups | Relative stability |
|---|---|---|
| Tertiary | 3 | Most stable |
| Secondary | 2 | Intermediate |
| Primary | 1 | Least stable |
Part 3 of 4Applying the Rule
Applying the Rule
Propene + HBr
- Propene + HBr The major product is CH₃CHBrCH₃, 2-bromopropane.
Propene + H₂O
- Propene + H₂O Hydration gives mainly propan-2-ol, CH₃CH(OH)CH₃, with H₃PO₄ catalyst.
2-methylpropene + HBr
- 2-methylpropene + HBr The tertiary carbocation leads to 2-bromo-2-methylpropane as the major product.
Symmetrical alkenes
- Symmetrical alkenes Alkenes such as but-2-ene give only one product, so no rule is needed.
Part 4 of 4Exam Checklist & Tips
6.4.10 – Key Points for the Exam
- Unsymmetrical alkene An alkene with different substituents on the two carbons of the double bond.
- Markovnikov's rule In addition to an alkene, the hydrogen adds to the carbon with more hydrogens.
- Inductive effect Donation or withdrawal of electron density through sigma bonds.
- Major product The product formed in the greatest amount.
Common Mistakes
- Putting the halogen on the carbon with more hydrogens.
- Saying the primary carbocation is more stable.
- Forgetting that a minor product is still formed in small amounts.
- Applying the rule to a symmetrical alkene where it has no effect.
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