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Topic 3.3.4: Alkenes - Exam Practice

AQA A-Level Chemistry • Organic Chemistry • Exam-Style Questions

Exam Practice

Topic 3.3.4: Alkenes Exam Practice

Test your understanding of alkene structures, electrophilic addition mechanisms, carbocation intermediates, and polymers with exam-style questions.

Alkenes › Exam Practice

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📋 Structured Questions

Complete each question on paper, then check your answers against the mark scheme.

Question 1: Electrophilic Addition Mechanism

5 marks

(a) Write the mechanism for the reaction of propene with hydrogen bromide (HBr) to form the major product. Include all dipoles, lone pairs, and curly arrows. [4]

(b) State the IUPAC name of the major product. [1]

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(a)

  • Nucleophilic attack: Curly arrow from the C=C double bond to the hydrogen atom of the H-Br molecule. [1]
  • Leaving group: Partial charges shown as H delta+ and Br delta-. Curly arrow from the H-Br bond to the Br atom. [1]
  • Carbocation: Structure of the secondary carbocation intermediate (CH3-CH+-CH3) with a positive charge on carbon-2. [1]
  • Final step: Curly arrow from the lone pair of the bromide ion (Br-) to the positive carbon atom. [1]

(b)

  • 2-bromopropane. [1]
Examiner tip: Always start your curly arrow exactly at the double bond (not from carbon atoms) and point it to the hydrogen atom. Draw the secondary carbocation clearly, showing the positive charge on the middle carbon.

Question 2: Carbocation Stability

3 marks

Explain why the reaction of propene with HBr forms 2-bromopropane as the major product rather than 1-bromopropane. Reference carbocation intermediates and stability factors in your response. [3]

Show Mark Scheme
  • The reaction to form 2-bromopropane goes via a secondary carbocation intermediate (CH3-CH+-CH3), whereas the reaction to form 1-bromopropane goes via a primary carbocation (CH3-CH2-CH2+). [1]
  • The secondary carbocation is more stable than the primary carbocation. [1]
  • This is because the secondary carbocation has two electron-donating alkyl (methyl) groups which push electron density towards the positive carbon (inductive effect), helping to stabilise the charge. The primary carbocation has only one alkyl group. [1]
Examiner tip: Ensure you mention both carbocations (primary and secondary) by name. Use the term "inductive effect" or "electron-donating group" to explain stability.

Question 3: Polymers and Plasticisers

4 marks

(a) Draw the repeating unit of the addition polymer formed from methylpropene. [1]

(b) Addition polymers are highly non-biodegradable. Explain why they are chemically inert. [1]

(c) Explain how plasticisers modify the physical properties of a polymer like poly(chloroethene) (PVC). [2]

Show Mark Scheme

(a)

  • Structure: A carbon-carbon single bond backbone, with one carbon bonded to two H atoms, and the other bonded to two methyl (-CH3) groups. Square brackets and extension bonds must be present:
    -[CH2-C(CH3)2]- [1]

(b)

  • The polymer chain has a saturated hydrocarbon backbone consisting of strong, non-polar C-C and C-H bonds that are not easily attacked by chemical reagents or enzymes. [1]

(c)

  • Plasticiser molecules get between the polymer chains, pushing them further apart. [1]
  • This reduces/weakens the intermolecular (van der Waals) forces between the chains, allowing them to slide past each other more easily, making the polymer more flexible and soft. [1]
Examiner tip: For drawing polymer repeating units, do not include the double bond in the backbone. Ensure you draw the extension bonds passing through the brackets.

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