Electrophilic Addition Reactions of Alkenes HL Only
Quick Notes
- Alkenes have a π bond (from the double bond) that is electron-rich and attracts electrophiles.
- Electrophilic addition = addition of an electrophile across the C=C double bond.
- Typical reactions include:
- Alkene + halogen (e.g. Br2) to form a dihalogenoalkane
- Alkene + hydrogen halide (e.g. HBr) to form a halogenoalkane
- Alkene + water (via H+ catalyst) to form an alcohol
- For symmetrical alkenes (e.g. ethene), the same product forms regardless of which carbon is attacked.
- The mechanism typically involves:
- Electrophile attacking the π bond
- Formation of a carbocation intermediate
- Nucleophile attacking the carbocation
Full Notes:
Alkenes react with electrophiles because the C=C double bond is electron-rich.

In electrophilic addition reactions, heterolytic fission occurs – the covalent bond breaks unevenly, forming ions. One atom takes both electrons, leaving a carbocation (ion with a positively charged carbon) and an anion.
The more substituted the carbocation (i.e., the more alkyl groups attached), the more stable it is. Stability order: tertiary > secondary > primary.
Electrophilic Addition Mechanisms
The high electron density within a carbon–carbon double bond attracts electrophiles and an addition reaction mechanism follows three basic steps:

- Step 1: Electrophile Attraction
The C=C bond has high electron density and attracts electrophiles (electron pair acceptors). Electrophiles may include H+, Br2, and H2SO4 (see below). - Step 2: Formation of Carbocation Intermediate
The double bond breaks as one carbon gains a new bond with the electrophile. The other carbon becomes positively charged (carbocation). - Step 3: Forming final product
A negative species (e.g., Br−, HSO4−) bonds to the carbocation, forming the final product.
Example Electrophilic Addition mechanisms
You need to know the following mechanisms:
Bromine + Ethene

- Br2 molecule approaches C=C (polarised by electron density)
- Double bond breaks, Br+ forms bond giving a carbocation intermediate
- Br− ion attacks carbocation to form 1,2-dibromoethane
HBr + Ethene

- HBr is polar (Hδ+—Brδ−), and H+ acts as the electrophile
- H+ bonds to one carbon of the C=C bond, forming a carbocation
- Br− attacks the carbocation, forming a halogenoalkane
H2O + Ethene (in presence of acid)

- π electrons attack H+ from H3O+ (formed when H2O accepts H+ from the acid catalyst), giving a carbocation
- H2O then attacks the carbocation to form an oxonium ion
- Deprotonation gives the alcohol
Notes on Symmetrical Alkenes
A symmetrical alkene (like ethene or but-2-ene) gives the same carbocation regardless of which carbon atom from the double bond the electrophile first bonds to, so only one product is formed.
This is unlike unsymmetrical alkenes which can form two products in unequal amounts (major and minor products). This is covered in R3.4.12 ‘Major Product of Addition Reaction’.
Linked Course Question
Why is bromine water decolourised by alkenes in the dark, but not by alkanes?
Alkenes decolourise bromine water in the dark
(As outlined above) 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.
Summary
- Alkenes undergo electrophilic addition because their π bond is electron-rich.
- Mechanisms proceed via electrophile attack, carbocation formation and nucleophilic capture.
- Key reactions include addition of Br2, HBr and acid-catalysed hydration to alcohols.
- Symmetrical alkenes give a single product with no regioisomerism.