6.4.9Reactivity 1 — What drives chemical reactions?
The Mechanisms of Electrophilic Addition Reactions — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 12 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 4Mechanism With a Hydrogen Halide
Electrophilic addition is a two-step mechanism with ions as intermediates.
Step 1
- Step 1 The pi electrons of the C=C bond attack the δ+ hydrogen of H–Br, while the H–Br bond breaks heterolytically.
Intermediate
- Intermediate This gives a carbocation, CH₃CH₂⁺, and a bromide ion Br⁻.
Step 2
- Step 2 A lone pair on the bromide ion attacks the positive carbon, forming the C–Br bond.
Product
- Product The final product is bromoethane, CH₃CH₂Br, in which the double bond has become a single bond.
Section 2 of 4Mechanism With a Halogen
1
As bromine approaches the alkene, the pi electrons repel the electrons of the Br–Br bond and induce a dipole.
2
The nearer bromine atom becomes δ+, so it behaves as an electrophile and is attacked by the pi electron pair.
3
The Br–Br bond breaks heterolytically, forming a bromide ion and a carbocation with a bromine on the adjacent carbon.
4
The bromide ion then attacks the carbocation using a lone pair, giving 1,2-dibromoethane.
5
The orange colour of bromine disappears as the product forms, which is why bromine water tests for alkenes.
Section 3 of 4Drawing the Curly Arrows
Drawing the Curly Arrows
First arrow
- First arrow Starts at the C=C double bond and ends at the δ+ hydrogen or bromine atom.
Second arrow
- Second arrow Starts at the middle of the H–Br or Br–Br bond and ends at the more electronegative atom.
Third arrow
- Third arrow Starts at a lone pair on the halide ion and ends at the positively charged carbon.
Charges
- Charges Show the partial charge on the polarised molecule and the full positive charge on the carbocation.
| Species | Role in the mechanism |
|---|---|
| C=C pi bond | Nucleophile, electron-rich |
| H–Br or Br–Br | Electrophile, after polarisation |
| Carbocation | Intermediate |
| Br⁻ | Nucleophile in step 2 |
Section 4 of 4Exam Checklist & Tips
6.4.9 – Key Points for the Exam
- Electrophilic addition A reaction in which an electrophile attacks a double bond and two groups add across it.
- Carbocation An ion containing a positively charged carbon atom, formed as an intermediate.
- Induced dipole A temporary charge separation created in a molecule by a nearby charge.
- Curly arrow An arrow showing the movement of an electron pair in a mechanism.
Common Mistakes
- Starting the first curly arrow from the electrophile rather than from the double bond.
- Leaving out the positive charge on the carbocation.
- Drawing a one-step mechanism, when it has two steps.
- Showing the bromide ion attacking without using a lone pair.
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