AlkenesEdexcel International A Level Chemistry: Topic test

1. 1
   A chemist is given four alkenes, each with the molecular formula C₆H₁₂: hex-1-ene, 2-methylpent-2-ene, 3-methylpent-2-ene and 2,3-dimethylbut-2-ene.

   (a)

   Which of these alkenes shows E-Z (geometric) isomerism?

   [1 mark]

   • A3-methylpent-2-ene
   • B2-methylpent-2-ene
   • Chex-1-ene
   • D2,3-dimethylbut-2-ene

   (b)

   Which statement about the C=C double bond in these alkenes is correct?

   [1 mark]

   • AIt consists of two σ bonds, each formed by head-on overlap of orbitals
   • BIt consists of a σ bond formed by sideways overlap and a π bond formed by head-on overlap
   • CIt consists of a σ bond formed by head-on overlap of orbitals and a π bond formed by sideways overlap of p orbitals
   • DIt consists of a σ bond and a π bond, with the π electron density lying along the line joining the two carbon nuclei

   (c)

   In one isomer of 3-methylpent-2-ene the CH₃ group on carbon 2 and the CH₂CH₃ group on carbon 3 lie on the same side of the double bond. State whether this is the E or the Z isomer and justify your answer using the priorities of the groups.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A chemist reacts but-1-ene, CH₃CH₂CH=CH₂, with a range of reagents in separate experiments.

   (a)

   What is the organic product when but-1-ene is shaken with cold, dilute, acidified potassium manganate(VII) solution?

   [1 mark]

   • Abutan-2-ol
   • Bbutane
   • C1,2-dibromobutane
   • Dbutane-1,2-diol

   (b)

   Which observation is made when but-1-ene is shaken with bromine water?

   [1 mark]

   • AThe solution stays orange
   • BThe orange solution becomes colourless
   • CThe colourless solution turns orange
   • DA white precipitate forms

   (c)

   But-1-ene reacts with steam in the presence of a phosphoric acid catalyst to form two isomeric alcohols. Name the major product and explain why it is formed in the greater amount.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A chemist converts 5.60 g of but-1-ene, C₄H₈, into 2-bromobutane by reacting it with hydrogen bromide. Relative atomic masses: H = 1.0, C = 12.0, Br = 80.0.

   (a)

   The chemist obtains 9.59 g of 2-bromobutane. Calculate the percentage yield.

   [3 marks]

   (b)

   Describe the mechanism of the reaction of but-1-ene with hydrogen bromide, stating where each curly arrow starts and finishes, and explain why 2-bromobutane is the major product.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Pent-2-ene, CH₃CH=CHCH₂CH₃, exists as two geometric isomers and reacts with bromine in an inert solvent.

   (a)

   Explain, with reference to the bonding in the C=C bond, why pent-2-ene exists as two geometric isomers, and use the E-Z system to name the isomer in which the CH₃ and CH₂CH₃ groups lie on opposite sides of the double bond.

   [6 marks]

   (b)

   Describe, in words, the mechanism of the reaction of pent-2-ene with bromine, stating where each curly arrow starts and finishes, and name the product.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   Poly(phenylethene), commonly called polystyrene, is used for disposable cups and packaging. Its monomer is phenylethene, C₆H₅CH=CH₂.

   (a)

   Which is the repeat unit of poly(phenylethene)?

   [1 mark]

   • A–CH(C₆H₅)–CH(C₆H₅)–
   • B–CH₂–CH₂–C₆H₅–
   • C–CH₂–CH(C₆H₅)–
   • D–CH₂–C(C₆H₅)₂–

   (b)

   Which statement about the formation of poly(phenylethene) is correct?

   [1 mark]

   • AIt forms with no other product, so every atom in the monomer appears in the polymer
   • BIt forms by loss of a small molecule such as water between monomers
   • CThe polymer contains C=C bonds and so decolourises bromine water
   • DEach monomer loses a hydrogen atom as the chain forms

   (c)

   A sample of poly(phenylethene) has an average relative molecular mass of 52 000. Calculate the average number of repeat units in each chain. Relative atomic masses: H = 1.0, C = 12.0.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   2-Methylbut-2-ene, (CH₃)₂C=CHCH₃, reacts with hydrogen bromide to form two isomeric bromoalkanes by an electrophilic addition mechanism.

   (a)

   Which is the major product of this reaction?

   [1 mark]

   • A2-bromo-3-methylbutane
   • B1-bromo-2-methylbutane
   • C2,3-dibromo-2-methylbutane
   • D2-bromo-2-methylbutane

   (b)

   Which statement best explains why the intermediate leading to the major product is more stable than the one leading to the minor product?

   [1 mark]

   • AIt has fewer alkyl groups, so the positive charge is concentrated on one carbon
   • BIts three alkyl groups release electron density towards the positively charged carbon
   • CThe positively charged carbon carries more hydrogen atoms
   • DThe positively charged carbon has a full outer shell of eight electrons

   (c)

   Name the minor product of this reaction and explain why it is formed in a smaller amount.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   A manufacturer makes hot-water pipes from poly(but-1-ene) and drain pipes from poly(chloroethene), PVC. Relative atomic masses: H = 1.0, C = 12.0.

   (a)

   Give the repeat unit of poly(but-1-ene) and calculate the relative molecular mass of a chain that contains 2000 repeat units.

   [3 marks]

   (b)

   Offcuts of PVC are sometimes burned. Explain why the waste gases must be treated, how this is done, and why chemists are developing biodegradable polymers as an alternative to disposal.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   A chemist compares the reactions of hex-3-ene, CH₃CH₂CH=CHCH₂CH₃, and hex-2-ene, CH₃CH=CHCH₂CH₂CH₃.

   (a)

   Each alkene reacts with hydrogen bromide. Explain, with reference to carbocation intermediates, why hex-3-ene forms only one product whereas hex-2-ene forms a mixture of two, and name the products.

   [6 marks]

   (b)

   Explain why hex-3-ene shows geometric isomerism and name the isomer in which the two ethyl groups lie on the same side of the double bond. State the organic product when hex-3-ene reacts separately with (i) hydrogen and a nickel catalyst and (ii) cold, dilute, acidified potassium manganate(VII), and give the colour change seen in (ii).

   [6 marks]

   Total for question 8: 12 marks

End of questions