Electrophilic Addition to Alkenes
Quick Notes:
- Alkenes contain a carbon–carbon double bond (C=C) with high electron density.
- This makes alkenes susceptible to attack by electrophiles.
- These reactions are called electrophilic addition reactions.
- Common electrophilic additions:
- Alkenes + H2O → alcohols
- Alkenes + halogens (Cl2, Br2) → dihalogenoalkanes
- Alkenes + hydrogen halides (HCl, HBr) → halogenoalkanes
- Bromine water is decolourized by alkenes but not by alkanes.
- Alkenes are often called “starting molecules” in industry due to their reactivity.
Full Notes:
Why Alkenes Are Reactive with Electrophiles?
Alkenes are susceptible to electrophilic attack because of the high electron density of the carbon–carbon double bond. These reactions lead to electrophilic addition.

Electrophilic Addition Reactions
These reactions involve breaking the double bond and adding atoms across it. You need to know the following examples:
Reaction with steam:
Alkene + H2O (steam) → alcohol

Phosphoric acid catalyst, high pressure (60 to 70 atm) and temperature (300 °C)
Reaction with halogens (e.g. Br2):
Alkene + Br2 → dihalogenoalkane

This is useful as it forms the basis of the bromine water test for alkenes (bromine water turns from orange to colourless when mixed with an alkene).
Reaction with hydrogen halides (e.g. HBr):
Alkene + HBr → halogenoalkane

Unsymmetrical alkenes give major and minor products due to carbocation stability.
Why is bromine water decolourised by alkenes in the dark, but not by alkanes?
Alkenes decolourise bromine water in the dark: Alkenes contain a C=C double bond with a region of high electron density. This can attract electrophilic Br2 molecules, causing an electrophilic addition reaction. The Br–Br bond breaks, and each Br atom adds across the double bond — forming a colourless dibromoalkane and the orange colour of bromine disappears.

Alkanes do not react in the dark: Alkanes lack a C=C bond and are relatively unreactive. They only react with bromine via a radical substitution reaction, which requires UV light to initiate homolytic fission of Br2. No reaction occurs in the dark, so the orange colour remains.
Industrial Significance of Alkenes
Alkenes are readily reactive, especially in addition reactions making them useful in industry.
- They are used to synthesise polymers, alcohols, halogenoalkanes, and many other compounds.
- They are often referred to as “starting molecules” or “feedstock chemicals” in organic synthesis and the petrochemical industry.
Summary
- Alkenes react with electrophiles due to the high electron density of their double bond.
- Electrophilic addition involves breaking the C=C bond and adding atoms across it.
- Reactions include addition with steam, halogens, and hydrogen halides.
- Bromine water test identifies alkenes by decolourisation.
- Alkenes are important industrial feedstock chemicals.