Structure and electrophilic addition reactions of alkenesAQA A-Level Chemistry: Revision notes
Section 1
Structure of alkenes
Alkenes are unsaturated hydrocarbons containing a carbon–carbon double bond, C=C, with the general formula CₙH₂ₙ. They are unsaturated because they can add atoms across the double bond.
The C=C bond contains four electrons and is a centre of high electron density. This makes alkenes attractive to electrophiles, which is why their typical reaction is electrophilic addition.
Section 2
Electrophilic addition of bromine
Bromine is non-polar, but it becomes polarised (an induced dipole) as it approaches the electron-rich C=C bond.
- A pair of electrons from C=C attacks the δ+ bromine atom.
- The Br–Br bond breaks heterolytically, forming Br⁻ and a carbocation intermediate.
- Br⁻ attacks the carbocation, forming the dibromoalkane.
Ethene gives 1,2-dibromoethane. This reaction is used as the test for unsaturation: bromine water changes from orange to colourless when shaken with an alkene.
The result is orange to colourless, not clear. A saturated alkane leaves the bromine water orange.
Section 3
Addition of HBr and H₂SO₄
Hydrogen bromide is polar. Its δ+ hydrogen is attacked by the C=C electrons, forming a carbocation and Br⁻, and Br⁻ then attacks the carbocation:
CH₂=CH₂ + HBr → CH₃CH₂Br
Cold concentrated sulfuric acid reacts in the same way, with the δ+ hydrogen attacked and HSO₄⁻ the attacking species in step 2:
CH₂=CH₂ + H₂SO₄ → CH₃CH₂OSO₂OH (ethyl hydrogensulfate)
In mechanisms, draw the curly arrow from the middle of the C=C bond to the H, and the arrow breaking H–Br from the bond to the Br.
Section 4
Major and minor products
With an unsymmetrical alkene such as propene, H–Br can add in two ways, giving two isomeric products.
- H adds to the terminal carbon, giving a secondary carbocation CH₃CH⁺CH₃, which leads to 2-bromopropane (major).
- H adds to the middle carbon, giving a primary carbocation CH₃CH₂CH₂⁺, which leads to 1-bromopropane (minor).
Alkyl groups release electrons and stabilise a carbocation. The order of stability is tertiary > secondary > primary, and the more stable carbocation is formed in greater amount.
The stability that matters is that of the carbocation intermediate, not of the final product. Name the type of carbocation in your answer.
Must Know
- Alkenes are unsaturated; C=C is a centre of high electron density
- Electrophilic addition with Br₂, HBr, H₂SO₄
- Bromine water: orange to colourless with alkenes
- Mechanism: C=C electrons attack the electrophile, forming a carbocation
- Stability: tertiary > secondary > primary
- More stable carbocation gives the major product
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Structure and electrophilic addition reactions of alkenes
- Propene, CH₃CH=CH₂, is an unsaturated hydrocarbon. A student shakes a sample of propene gas with bromine water, and in a separate experiment reacts propene with bromine in an inert solvent.Write an equation for the reaction of propene with bromine, using structural formulae, and name the organic product.2 marks
- Carbocations are intermediates in electrophilic addition reactions of alkenes. They are classified as primary, secondary or tertiary according to the number of alkyl groups attached to the positively charged carbon atom.Explain why a tertiary carbocation is more stable than a primary carbocation.2 marks
- Alkenes react with electrophiles such as bromine and concentrated sulfuric acid. A student is asked to describe the mechanisms of these reactions using ethene as the example.Describe the mechanism for the reaction of ethene with bromine, referring to the movement of electron pairs.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).