Reactions of Alkenes with Halogens
A concise revision guide to the addition reaction of alkenes with bromine or chlorine, including the functional group change, reaction conditions and the electrophilic addition mechanism.
- 5.4ii
- 5.6i-a
- 5.6ii-b
What these spec points say
- 5.4ii be able to describe the reactions of alkenes, limited to: the addition of halogens to produce a di-substituted halogenoalkane
- 5.6i-a be able to describe the electrophilic addition mechanism of bromine to ethene, including diagrams and evidence where possible
- 5.6ii-b use curly arrow notation in electrophilic addition mechanisms, with curly arrows starting from a bond or lone pair of electrons
Before You Start
This page builds on GCSE work on halogen molecules, the bromine water test and addition reactions of alkenes.
- Chlorine and bromine exist as diatomic molecules, Cl2 and Br2.
- A covalent bond is a shared pair of electrons between two atoms.
- Alkenes are unsaturated and decolourise orange bromine water; alkanes do not.
- In an addition reaction, atoms add across the C=C double bond to give one product.
GCSE Recap: Halogens and the Bromine Test
Four quick GCSE questions on halogen molecules and testing for alkenes before you look at the reaction in detail.
Watch: how a non-polar bromine molecule becomes an electrophile
Bromine is non-polar, so how can it be an electrophile? Watch it become one as it approaches the double bond.
The Overall Reaction
Alkenes react with halogens such as bromine, Br2, or chlorine, Cl2, in an addition reaction. The C=C double bond opens and one halogen atom adds to each carbon from the original double bond.
The functional group changes from an alkene to a dihalogenoalkane. For example, ethene reacts with bromine to form 1,2-dibromoethane.
Key idea: The reaction is addition because atoms are added across the C=C double bond and only one organic product is formed from the alkene.
Bromine or chlorine adds across the C=C at room temperature, without UV light, and the orange-brown colour of bromine disappears.
Quick Check: Predict the Product
Work out what chlorine does to an alkene that is not shown on this page.
Why Bromine Acts as an Electrophile
The π bond in an alkene is a region of high electron density. As a Br2 molecule approaches the alkene, the electron-rich π bond repels the shared electron pair in the Br-Br bond.
This repulsion induces a dipole in the bromine molecule. One bromine atom becomes slightly positive, Brδ+, while the other becomes slightly negative, Brδ–.
The slightly positive bromine atom acts as the electrophile because it is attracted to the electron-rich π bond.
Electrophile: An electron-pair acceptor. In this reaction, the polarised bromine molecule provides Brδ+, which accepts electron density from the alkene π bond.
The π electrons repel the Br–Br bonding pair, inducing a dipole: the nearer bromine atom becomes Brδ+ and acts as the electrophile.
Quick Check: The Induced Dipole
In each round, choose the one statement that is accurate.
Bond Fission and Carbocation Formation
The Br-Br bond breaks by heterolytic fission. This means both electrons from the covalent bond go to one bromine atom, forming a bromide ion, Br–.
At the same time, the alkene π bond forms a new bond to Brδ+. One of the carbon atoms from the original double bond becomes positively charged, producing a carbocation intermediate.
Remember: A carbocation is an organic ion with a positively charged carbon atom.
The π bond polarises Br2, Brδ+ adds first, then Br– attacks the carbocation to give the dibromo product.
Quick Check: Charges, Atoms and Amounts
Answer as quickly as you can, using new alkenes each time.
The Mechanism in Three Steps
Step 1: The π bond polarises Br2
The high electron density in the C=C bond repels the Br-Br bonding pair, making the nearer bromine atom Brδ+.
Step 2: The electrophile adds
The alkene π bond donates electron density to Brδ+, forming a C-Br bond and a carbocation intermediate.
Step 3: Bromide attacks
The Br– ion donates a lone pair to the carbocation, forming a second C-Br bond and producing the dihalogenoalkane.
Quick Check: Chlorine with Cyclopentene
Drag the words and numbers into place to build the account of the reaction.
Electrophilic Addition in Motion
Choose a step to watch HBr and then Br₂ add across the π bond of ethene, one curly arrow at a time.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
Bromine and Chlorine Give Similar Products
The same addition pattern applies to chlorine. With Cl2, the product is a dichloroalkane. With Br2, the product is a dibromoalkane.
| Halogen | Product type | Example from ethene |
|---|---|---|
| Br2 | Dibromoalkane | 1,2-dibromoethane |
| Cl2 | Dichloroalkane | 1,2-dichloroethane |
Exam focus: Do not describe this as substitution. The C=C double bond breaks and two atoms are added, so this is an addition reaction.
Quick Check: Work It Out, Then Flip
Work each answer out on paper first, then turn the card over to check your reasoning.
Quick Check: Halogen Addition from Start to Finish
Each round covers one part of the page, from the product to the mechanism. Choose the one accurate statement.
FAQs
These questions secure the electrophile, conditions and naming points examiners test on halogen addition.
Why does a non-polar Br2 molecule act as an electrophile?
As Br2 approaches, the π electrons repel the electron pair in the Br-Br bond. This induces a dipole, so the nearer bromine atom becomes Brδ+ and accepts an electron pair from the π bond.
Why should the reaction be carried out away from UV light?
In UV light, bromine splits into radicals that can substitute hydrogen atoms in the molecule. At room temperature in the dark, the reaction stays as electrophilic addition across the C=C bond.
What forms when bromine adds to propene?
1,2-dibromopropane, CH3CHBrCH2Br. One bromine atom adds to each carbon of the original C=C bond. Both added atoms are the same, so, unlike HBr, there is only one possible product.
Can I write “dibromoethane” for the product from ethene?
Give the full name, 1,2-dibromoethane, because the two bromine atoms sit on different carbon atoms. 1,1-dibromoethane is a different compound and is not formed by this addition.
Does chlorine react with alkenes in the same way as bromine?
Yes. Cl2 is polarised by the π bond in the same way and adds across the C=C bond by electrophilic addition. Ethene gives 1,2-dichloroethane, another dihalogenoalkane.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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