Alkenes react with water, halogens, and hydrogen halides by electrophilic addition across the carbon-carbon double bond. The π bond breaks while the σ bond remains, converting the alkene into a saturated product.
The Mechanism
The π bond has high electron density, so it attracts an electrophile, which accepts an electron pair. A new bond forms, followed by the addition of a second atom or group to the other carbon of the original double bond. This links molecular structure to reactivity: the electron-rich double bond provides the reactive site.
| Reagent and conditions | Product and mechanism |
|---|---|
| Water or steam with an acid catalyst | Hydration produces an alcohol. The alkene is protonated, water attacks the carbocation, and deprotonation regenerates the catalyst. For example, ethene forms ethanol: . Because hydration is reversible, steam and an acid catalyst establish equilibrium. |
| A halogen such as bromine | A dihalogenoalkane forms. The π bond induces a dipole in ; a cyclic bromonium ion forms and is attacked by . Bromine is decolourized, providing a test for unsaturation. The orange-brown colour disappears rapidly at room temperature. |
| A hydrogen halide such as | A halogenoalkane forms. The π bond attacks , producing a carbocation, which is then attacked by . |
For an unsymmetrical alkene, the major product usually forms through the more stable carbocation. For example, propene reacting with mainly produces 2-bromopropane because the secondary carbocation is more stable than the primary carbocation.
A common misconception is that the entire double bond breaks. Only the π component breaks; the carbon-carbon σ bond remains.
IB Exam Technique
The general electrophilic-addition concept is relevant at SL, while detailed mechanisms and carbocation stability are AHL content in Reactivity 3.4. In a mechanism question, draw curly arrows from the π bond or a lone pair toward where the new bond forms, show charges clearly, and identify the electrophile and nucleophile. For “explain”, connect electron density to electrophile attraction.
