A Level Official Exam Material
FREE Past Paper Questions by Topic
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Unit 1: Structure, Bonding and Introduction to Organic Chemistry
Topic 5: Alkenes
For both AS and A Level students
Q1. Alkenes and cycloalkanes have the same general formula, but react very differently with halogens.
Give the general formula that applies to both alkenes and cycloalkanes. (1)
Q2. Which of the following pairs are cis-trans isomers?
A 1 and 2
B 1 and 4
C 2 and 3
D 3 and 4
(Total for question = 1 mark)
Q3. Which of the following compounds shows geometric (E-Z or cis-trans) isomerism?
A but-1-ene
B 2-methylbut-1-ene
C but-2-ene
D 2-methylbut-2-ene
(Total for Question = 1 mark)
Q4. Which of the following alkenes exhibits E/Z isomerism?
A But-1-ene
B But-2-ene
C 2-Methylpropene
D Propene
(Total for Question = 1 mark)
Q5. Which molecule does not exhibit E / Z isomerism?
(Total for question = 1 mark)
Q6. 2-methylbuta-1,3-diene can react with hydrogen bromide.
When 2-methylbuta-1,3-diene reacts with excess hydrogen bromide, several isomeric products are possible.
Give the structures of four isomeric products. (4)
(Total for question = 4 marks)
Q7. Some plants are able to make terpenes by linking together several molecules of 2-methylbuta-1,3-diene, also known as isoprene.
The skeletal formula of 2-methylbuta-1,3-diene is
Predict the number of isoprene molecules that would be needed to make a single geraniol molecule.
Justify your answer. (2)
............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(Total for question = 2 marks)
Q8. Alkenes are unsaturated hydrocarbons.
They are used in the industrial production of many organic compounds.
(a) Add structural formulae to the flowchart below to show the organic product formed in each addition reaction of 2-methylpropene.
(b) Suggest a mechanism for the reaction of 2-methylpropene with bromine, Br2(l). Include curly arrows. (3)
(c) Ethene, C2H4, was prepared from ethanol, C2H5OH, by the following reaction
C2H5OH → C2H4 + H2O
A chemist reacted 9.2 g of ethanol, C2H5OH, and obtained 4.2 g of ethene.
Calculate the percentage yield of ethene in the reaction. (2)
(Total for Question = 9 marks)
Q9. (a) Which of the following represents a step in the mechanism during the reaction between ethene and hydrogen bromide? (1)
A C2H4 + Br+ → C2H4Br+
B C2H4 + HBr → C2H5 + + Br−
C C2H4 + HBr → C2H5• + Br•
D C2H4 + HBr → C2H4Br− + H+
(b) The mechanism of the reaction between ethene and hydrogen bromide is (1)
A electrophilic addition.
B electrophilic substitution.
C nucleophilic addition.
D nucleophilic substitution.
(Total for question = 2 marks)
Q10. Which of the following equations shows the first step in the mechanism for the reaction between hydrogen bromide and ethene?
Q11. * An example of an alkene with six carbon atoms is 2-methylpent-1-ene.
It reacts with chlorine by means of an electrophilic addition reaction.
The diagram below shows a student's attempt at drawing the mechanism for this reaction.
(i) Identify the three errors in this student's drawing of the mechanism. (3)
Error 1 ............................................................................................................................................. .............................................................................................................................................
Error 2 ............................................................................................................................................. .............................................................................................................................................
Error 3 ............................................................................................................................................. .............................................................................................................................................
(ii) The structure of the carbocation intermediate is correctly drawn. Explain why the positive charge is on the carbon atom shown. (1)
............................................................................................................................................ ............................................................................................................................................. .............................................................................................................................................
Q12. Ethene reacts with bromine in the dark. (i) What is the classification of the mechanism for the reaction between ethene and bromine? (1)
A electrophilic addition
B electrophilic substitution
C nucleophilic addition
D nucleophilic substitution
(ii) Which of the following shows the formation of the intermediate in the mechanism for the reaction between ethene and bromine?
Q13. An electrophile is defined as a species that
A is an electron pair acceptor.
B is an electron pair donor.
C has a negative charge.
D has a positive charge.
(Total for Question = 1 mark)
Q14. Electrophiles are
A electron pair donors that are attracted to regions of high electron density.
B electron pair donors that are attracted to regions of low electron density.
C electron pair acceptors that are attracted to regions of high electron density.
D electron pair acceptors that are attracted to regions of low electron density.
(Total for question = 1 mark)
Q15. Which of the following molecules is formed on adding bromine water to ethene?
A ethanol
B ethane-1,2-diol
C bromoethane
D 2-bromoethanol
(Total for question = 1 mark)
Q16. This question is about isomers of C4H8.
(a) (i) Alkenes contain a carbon-carbon double bond, which consists of a σ bond and a π bond.
Show, and clearly label, the σ and π bonds on the diagram below. (2)
*(ii) Explain why the σ bond is stronger than the π bond. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(b) (i) Draw the structural formula of E-but-2-ene. (1) (ii) Explain why but-1-ene does not exhibit E-Z isomerism. (1) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(iii) Describe the result of the test for the presence of a C C bond in E-but-2-ene using bromine water.
Give the displayed formula of the organic product. (2)
Test result ............................................................................................................................................. .............................................................................................................................................
Displayed formula of organic product:
(c) Another test for C C bonds is the reaction with acidified potassium manganate(VII).
Describe the result of this test using but-1-ene and give the displayed formula of the organic product. (2)
Test result ............................................................................................................................................. .............................................................................................................................................
Displayed formula of organic product
(d) Another isomer of C4H8 has the structure shown below.
(i) Name this isomer. (1)
.............................................................................................................................................
(ii) This isomer forms an addition polymer. Show the structure of this polymer by drawing two repeat units. (1)
(Total for question = 12 marks)
Q17. Alkenes and cycloalkanes have the same general formula, but react very differently with halogens.
Using structural formulae, complete the overall equation for the reaction of an alkene of your own choice, containing fewer than four carbon atoms, with liquid bromine.
Name the product. (3) + Br2 → Name: ...........................................................
Q18. This question is about ethene and its reactions. Ethene is produced in industry by cracking.
(a) (i) Write the equation for the cracking of dodecane, C12H26, to produce one mole of ethene as the only alkene product. (1)
(ii) Draw a labelled diagram of the apparatus and materials you would use to crack dodecane and collect a sample of the gaseous alkene in the laboratory. (4)
(b) Draw a diagram to show the regions of electron density in both parts of the double bond between the carbon atoms in ethene. Label each region with appropriate symbols. (2)
(c) (i) Give the name and structural formula for the product of the reaction between ethene and bromine, Br2(l). (2)
Name .............................................................................................................................................
Formula
(ii) Give the mechanism for the reaction between ethene and bromine. (3)
(d) Give the displayed formula for the organic product of the reaction between ethene and acidified potassium manganate(VII). (1)
(e) (i) Write a balanced equation for the formation of poly(ethene) from ethene, showing the structure of the polymer clearly. (2)
(ii) Comment on the atom economy of the reaction in (e)(i). (1) ............................................................................................................................................. .............................................................................................................................................
(Total for Question = 16 marks)
Q20. The repeat unit of a polymer is shown below.
The systematic name of the alkene monomer that forms this polymer is
A 2-methyl-3-ethylpropene
B 2-methylpent-2-ene
C 2-methylpent-3-ene
D 4-methylpent-2-ene
(Total for Question = 1 mark)
If you need the markscheme, including the examiners' reports for each question, and more exam and teaching materials on this topic, contact me through the contact page