CCEA A Level Chemistry
CCEA A Level Chemistry 2.4 Alkenes Exam Questions
Free CCEA A Level Chemistry revision for 2.4 Alkenes: what you need to know, common mistakes and exam-style questions with answers. Covers sigma and pi bonds, tests for unsaturation, addition reactions, electrophilic addition, Markovnikov products and addition polymers.
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About CCEA A Level Chemistry 2.4 Alkenes
Section 2.4 of Unit AS 2 covers alkenes and introduces your first ionic mechanism. You need to describe the C=C double bond as one sigma and one pi bond, explain why it is a region of high electron density and why alkenes are more reactive than alkanes, and use bromine water as a test for unsaturation. Reactions include catalytic hydrogenation with nickel and addition of Cl2, Br2, HCl and HBr. You must draw the electrophilic addition mechanism with curly arrows, define electrophile, explain carbocation stability and predict major and minor products with unsymmetrical alkenes. Addition polymerisation of ethene and propene completes the section.
What you need to know
- Bonding A C=C contains a sigma bond (head-on overlap) and a pi bond (sideways overlap of p orbitals above and below the plane). The pi bond is weaker and electron rich, so alkenes react readily with electrophiles.
- Test Bromine water changes from orange to colourless with an alkene.
- Addition reactions Hydrogen with finely divided nickel catalyst gives an alkane. Halogens give dihalogenoalkanes; hydrogen halides give halogenoalkanes.
- Electrophilic addition An electrophile is an electron pair acceptor. The pi electrons attack the delta+ atom of HBr (or the induced dipole in Br2), forming a carbocation and Br-. Br- then attacks the carbocation.
- Markovnikov With an unsymmetrical alkene the major product comes from the more stable carbocation. Tertiary carbocations are more stable than secondary, which are more stable than primary, because alkyl groups release electrons. Propene + HBr gives mainly 2-bromopropane.
- Addition polymers Many alkene monomers join to form a saturated chain, for example poly(ethene) and poly(propene). Draw the repeat unit with the side group and extension bonds.
Common mistakes in 2.4 Alkenes
- Starting the curly arrow from the H of HBr. The arrow starts from the C=C bond and goes to the H; a second arrow shows the H-Br bond breaking towards Br.
- Saying bromine water is decolourised to clear. Clear means transparent; say colourless.
- Drawing poly(propene) with the CH3 in the main chain. The main chain is two carbons per repeat unit with CH3 as a side group.
- Explaining the major product without mentioning carbocation stability. State which carbocation is more stable and why.
- Saying the pi bond is stronger than the sigma bond. The pi bond is weaker, which is why it breaks in addition reactions.
Try three 2.4 Alkenes exam-style questions
1. Draw the mechanism for the reaction of ethene with hydrogen bromide. (4 marks)
Show the mark scheme answer
Arrow from C=C to H of HBr; arrow from H-Br bond to Br; carbocation CH3CH2+ and Br- drawn with lone pair; arrow from lone pair on Br- to C+, giving bromoethane.
2. Explain why 2-bromopropane is the major product of propene and HBr. (3 marks)
Show the mark scheme answer
Secondary carbocation forms more readily than primary; it is more stable because two alkyl groups push electrons towards the positive carbon; Br- adds to the secondary carbon.
3. Draw the repeat unit of poly(propene). (1 mark)
Show the mark scheme answer
-CH2-CH(CH3)- with extension bonds.
2.4 Alkenes FAQs
What does CCEA A Level Chemistry 2.4 Alkenes cover?
It covers sigma and pi bonds, tests for unsaturation, addition reactions, electrophilic addition mechanisms, carbocation stability and addition polymerisation.
Which unit is 2.4 Alkenes examined in?
It is examined in Unit AS 2: Further Physical and Inorganic Chemistry and an Introduction to Organic Chemistry, a 1 hour 30 minute paper with a multiple-choice section and structured questions. Related practical skills can also be assessed in Unit AS 3.
How do I draw curly arrows correctly?
Each arrow starts at a pair of electrons (a bond or lone pair) and ends where the pair goes. Show partial charges, lone pairs and the carbocation charge clearly.
