AQA A-Level Chemistry Paper 3, 2017: Question 18

1 mark · Medium difficulty · Multiple Choice

Identify which pair of alkene monomers could produce the given random addition copolymer structure.

Practise this question

Question

Question 18 presents a continuous polymer backbone of 12 carbon atoms with various substituents attached to each carbon: C1 has H and H; C2 has CF3 and H; C3 has F and H; C4 has H and H; C5 has H and H; C6 has F and H; C7 has CF3 and H; C8 has H and H; C9 has H and CF3; C10 has H and H; C11 has H and H; C12 has H and F. Four multiple choice options list pairs of monomers: A (CH2=CHF and CH2=CHCF3), B (CH2=CH2 and CHF=CHCF3), C (CH2=CH2 and CH2=CHCF3), and D (CH2=CHF and CHCF3=CHF).
Question text

18 This structure shows a section of a polymer chain formed from the random

polymerisation of two different monomers.

Which pair of monomers could produce this polymer?

[1 mark]

A CH2=CHF and CH2=CHCF3

B CH2=CH2 and CHF=CHCF3

C CH2=CH2 and CH2=CHCF3

D CH2=CHF and CHCF3=CHF

Mark scheme

Show the mark scheme Mark scheme showing question number 18 with the correct answer designated as option A.

18 A

How to answer it

Deducing Monomers from an Addition Copolymer Chain

What this question tests

This question assesses your ability to reverse-engineer addition polymerisation for a copolymer. Key skills include: breaking down a continuous carbon backbone into 2-carbon repeating units, accounting for random orientation (head-to-tail and head-to-head/tail-to-tail linkages), and reconstructing the alkene monomers by reintroducing the C=C double bond.

Question 18 • 1 Mark

Identifying the Monomer Pair

Multiple Choice (Addition Polymers)

✅ Correct Answer

A: CH₂=CHF and CH₂=CHCF₃

1 mark awarded for correctly identifying option A.

💡 Key Knowledge

  • Alkene Polymerisation: Each alkene monomer (C=C) contributes exactly 2 carbon atoms to the continuous main polymer backbone.
  • Random Copolymerisation: When two alkenes polymerise together, monomer units can join in any order and either orientation (e.g., -CH₂-CH(R)- or -CH(R)-CH₂-).
  • Reconstructing Monomers: Divide the 12-carbon chain into six 2-carbon units, then place a double bond between the two carbons in each pair.

📐 Step-by-Step Chain Deconstruction

Number the main backbone carbons from left to right (1 to 12) and pair them in twos:

  1. Carbons 1 & 2: -CH₂-CH(CF₃)-
    Has one -CH₂- and one -CH(CF₃)- . Restoring the double bond gives: CH₂=CHCF₃
  2. Carbons 3 & 4: -CH(F)-CH₂-
    Has one -CH(F)- and one -CH₂- . Restoring the double bond gives: CHF=CH₂ (CH₂=CHF)
  3. Carbons 5 & 6: -CH₂-CH(F)-
    Has one -CH₂- and one -CH(F)- . Restoring the double bond gives: CH₂=CHF
  4. Carbons 7 & 8: -CH(CF₃)-CH₂-
    Has one -CH(CF₃)- and one -CH₂- . Restoring the double bond gives: CH(CF₃)=CH₂ (CH₂=CHCF₃)
  5. Carbons 9 & 10: -CH(CF₃)-CH₂-
    Has one -CH(CF₃)- and one -CH₂- . Restoring the double bond gives: CH(CF₃)=CH₂ (CH₂=CHCF₃)
  6. Carbons 11 & 12: -CH₂-CH(F)-
    Has one -CH₂- and one -CH(F)- . Restoring the double bond gives: CH₂=CHF

All six 2-carbon segments consistently originate from only two distinct monomers: CH₂=CHF and CH₂=CHCF₃.

❌ Common Errors & Traps

  • The Ethene Trap (Options B & C): Spotting adjacent unsubstituted -CH₂-CH₂- units (such as carbons 4 & 5, or 10 & 11) leads many students to guess that CH₂=CH₂ (ethene) is a monomer.
    Why it's wrong: These adjacent -CH₂- groups are formed by a "tail-to-tail" addition of two substituted monomers (e.g. -CH(F)-CH₂- joined to -CH₂-CH(F)- ).
  • Misidentifying Disubstituted Monomers (Option D): Trying to pair C2 and C3 creates a fictional unit with both -F and -CF₃. Always check whether the entire chain breaks cleanly into consistent 2-carbon pairs starting from carbon 1.

🧠 Exam Technique

  • Count the Carbons: The chain shown has 12 carbons. 12 ÷ 2 = 6 monomer units. Draw vertical dividing lines between carbons 2|3, 4|5, 6|7, 8|9, and 10|11.
  • Check Substituent Symmetry: Notice that every single pair contains exactly one substituted carbon (with either -F or -CF₃) and one unsubstituted -CH₂- carbon. This immediately rules out ethene ( CH₂=CH₂ ) and 1,2-disubstituted alkenes ( CHF=CHCF₃ ).

Topics

Organic Chemistry · 3.3.4 Alkenes

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.