AQA A Level Chemistry
AQA A Level Chemistry 3.3.4 Alkenes Exam Questions
Free AQA A Level Chemistry exam questions for 3.3.4 Alkenes, with a mark scheme for every pack. Covers structure and reactivity of the C=C bond, electrophilic addition mechanisms, carbocation stability, major and minor products and addition polymers.
3.3.4 Alkenes exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About AQA A Level Chemistry 3.3.4 Alkenes
Alkenes are unsaturated hydrocarbons with a reactive C=C double bond. On the AQA A Level course you need to explain why alkenes react with electrophiles, draw electrophilic addition mechanisms with HBr, Br2 and concentrated sulfuric acid, and explain major and minor products in terms of carbocation stability. You also use bromine water as a test for unsaturation and study addition polymers such as poly(ethene), poly(propene) and PVC, including how their properties are modified and why they are hard to dispose of. Examiners commonly ask for full mechanisms, predictions of major products and repeat units of polymers.
What you need to know
- Structure and reactivity The C=C bond is a region of high electron density made of a σ bond and a π bond. The π bond attracts electrophiles and is weaker than the σ bond, so alkenes undergo addition.
- Addition of HBr The π electrons attack the δ+ H of HBr and the H-Br bond breaks heterolytically. A carbocation forms, then the Br- lone pair attacks the positive carbon to give a bromoalkane.
- Addition of bromine Br2 is polarised by the π electrons to give an induced dipole. The mechanism is like HBr, forming a dibromoalkane. Bromine water changes from orange to colourless, which is the test for a C=C.
- Sulfuric acid Cold concentrated H2SO4 adds across the C=C to form an alkyl hydrogensulfate, which is hydrolysed by water to an alcohol, regenerating the acid.
- Carbocation stability Tertiary carbocations are more stable than secondary, which are more stable than primary, because alkyl groups push electron density towards the positive carbon. The more stable carbocation leads to the major product.
- Addition polymers Many alkene monomers join to form a saturated chain. Draw repeat units with extension bonds. PVC can be made flexible by adding plasticisers. Addition polymers are unreactive, so they are not biodegradable.
Common mistakes in 3.3.4 Alkenes
- Drawing the first curly arrow from a carbon atom instead of from the C=C bond, or pointing it to the wrong bromine atom.
- Forgetting the δ+ and δ- on HBr or the induced dipole on Br2.
- Predicting the major product without reference to carbocation stability. Say which carbocation is more stable and why.
- Describing bromine water as becoming 'clear'. It goes from orange to colourless.
- Drawing a polymer repeat unit with a double bond still present, or without the extension bonds.
Try three 3.3.4 Alkenes exam-style questions
1. Outline the mechanism for the reaction of propene with hydrogen bromide to form the major product. Name the product. (4 marks)
Show the mark scheme answer
Curly arrow from C=C to H of H-Br, with arrow from H-Br bond to Br; secondary carbocation shown on the middle carbon; curly arrow from a lone pair on Br- to the positive carbon; 2-bromopropane.
2. Explain why 2-bromopropane is the major product of this reaction. (2 marks)
Show the mark scheme answer
It forms via a secondary carbocation, which is more stable than the primary carbocation; because the two alkyl groups push electron density towards the positive carbon.
3. Draw the repeat unit of poly(chloroethene) and explain why it does not biodegrade easily. (2 marks)
Show the mark scheme answer
Repeat unit -CH2-CHCl- with extension bonds on each side; the C-C and C-H bonds in the saturated chain are non-polar and unreactive, so it is not broken down by organisms.
3.3.4 Alkenes FAQs
What does AQA A Level Chemistry 3.3.4 Alkenes cover?
It covers the structure of the C=C bond, electrophilic addition of HBr, Br2 and sulfuric acid, major and minor products, the bromine water test and addition polymers.
Which paper is 3.3.4 Alkenes examined in?
It is examined in Paper 2, and Paper 3 can also test it.
How do I avoid losing marks on electrophilic addition mechanisms?
Draw the first arrow from the middle of the double bond to the δ+ atom, show the bond breaking to the δ- atom, draw a clear positive charge on the correct carbon, and start the last arrow from a lone pair on the bromide ion.
