📘 IB Understanding
Alkenes are unsaturated hydrocarbons that undergo electrophilic addition reactions. The electron-rich C=C double bond attracts electrophiles, breaking the pi bond and forming two new sigma bonds.
What is an Electrophile?
An electrophile is an electron-pair acceptor -- a species attracted to areas of high electron density. Examples include H+, Br+, NO2+, and polar molecules like HBr.
Key Reactions of Alkenes
| Reagent | Product | Type |
|---|---|---|
| H2 (Pt or Ni catalyst) | Alkane | Hydrogenation |
| Br2 | Dibromoalkane | Halogenation |
| HBr | Bromoalkane | Hydrohalogenation |
| H2O (H+ catalyst) | Alcohol | Hydration |
Mechanism: Electrophilic Addition of HBr to Ethene
Test for Unsaturation
Adding bromine water to an alkene causes it to decolourise (orange → colourless), as Br2 adds across the double bond. This is a classic test for C=C bonds.
⚠️ Exam Tip
When drawing curly arrow mechanisms, the arrow must start from a source of electrons (lone pair or bond) and point towards the electrophilic centre. Show both steps: formation of the carbocation, then nucleophilic attack by the halide.