Why Are Alkenes Susceptible to Electrophilic Attack?
Alkenes contain a C=C double bond with a pi (π) bond. A region of high electron density above and below the carbon skeleton. This electron-rich region attracts electrophiles (electron-pair acceptors).
Unlike alkanes, alkenes are unsaturated and highly reactive. The π bond is weaker than the σ bond and readily breaks to form new bonds.
The Electrophilic Addition Mechanism
General mechanism (e.g. HBr + alkene)
Step 1. Electrophilic attack:
- The electrophile (e.g. HBr) is polarised: Hδ+−Brδ−
- The π electrons of C=C attack the Hδ+ end (curly arrow from C=C to Hδ+)
- The H−Br bond undergoes heterolytic fission → carbocation intermediate formed
Step 2. Nucleophilic attack:
- The Br⁻ ion (nucleophile) attacks the positively charged carbon (curly arrow from Br⁻ to C⁺)
- Final addition product formed. Both atoms added across the double bond
Markovnikov's Rule
The Rule
When HX adds to an unsymmetrical alkene, the H adds to the carbon with more H atoms already attached. This forms the more stable (more substituted) carbocation.
Why? Alkyl groups are electron-donating (inductive effect). They stabilise the positive charge on the carbocation. Stability order: 3° > 2° > 1°.
Example: Propene + HBr
Major Product (Markovnikov)
CH₃CHBrCH₃
2-bromopropane
Via secondary carbocation (more stable)
Minor Product (Anti-Markovnikov)
CH₃CH₂CH₂Br
1-bromopropane
Via primary carbocation (less stable)
Halogenation of Alkenes (Br₂ Test)
Bromine Water Test for Unsaturation
Observation: Bromine water changes from orange → colourless
Mechanism: The π electrons of C=C polarise the approaching Br₂ molecule (induced dipole). The Brδ+ is attacked by the π electrons.
For the IB syllabus, students should be able to show the electrophilic addition mechanism for Br₂ addition to an alkene producing a dihalogenoalkane.
Symmetrical vs Asymmetrical Electrophiles
Symmetrical (e.g. Br₂, H₂)
Only one product possible. Markovnikov's rule does not apply
Br₂ + ethene → 1,2-dibromoethane
Asymmetrical (e.g. HBr, HCl, H₂O)
Markovnikov's rule applies. two possible products (major and minor)
HBr + propene → 2-bromopropane (major)
Summary of Electrophilic Addition Reactions
| Reagent | Product | Type |
|---|---|---|
| H₂ (+ Ni, 150 °C) | Alkane | Hydrogenation |
| Br₂ | Dihalogenoalkane | Halogenation |
| HBr | Halogenoalkane | Hydrohalogenation |
| H₂O (+ H₃PO₄, 300 °C) | Alcohol | Hydration |
Think About It
Why is bromine water used to test for unsaturation, and what is the mechanism?
Br₂ is polarised by the electron-rich C=C bond. The electrophilic Brδ+ attacks the π bond, forming a carbocation intermediate. Br⁻ then attacks → 1,2-dibromoalkane (colourless). The colour change from orange to colourless = positive test for alkenes.
⚠️ Common Exam Mistakes
- Not drawing the curly arrow starting from the C=C π bond. The electrons come from the double bond, not from the electrophile
- Applying Markovnikov's rule to symmetrical electrophiles (e.g. Br₂). It only applies to asymmetrical electrophiles like HBr
- Forgetting that the mechanism has two steps (not one) with a carbocation intermediate
- Not explaining why the major product forms. Link to carbocation stability (inductive effect of alkyl groups)