Edexcel A-Level Chemistry Paper 2, June 2017: Question 6

✅ Correct Answer

• But-1-ene polymer: Backbone chain of 4 carbons repeated twice (8 carbons total in the main chain backbone), with ethyl side chains pointing off alternate carbons, enclosed in square brackets with bonds extending outside.
• Cyclohexene polymer: Backbone containing two joined ring units with opening/closing bonds extending through the brackets.

💡 Key Knowledge

• Addition polymers form when unsaturated double bonds (C=C) open up and link together.
• When drawing "two repeat units", the backbone carbon chain must be double the length of a single monomer unit (e.g., 4 carbons in the monomer become an 8-carbon backbone for 2 repeat units).
• End bonds (extension bonds) must cut through the square brackets.

❌ Common Errors & Examiner Pitfalls

• Showing only one repeat unit when the question explicitly requests two (limits the score to a maximum of 1 mark overall).
• Omitting the extension bonds passing through the square brackets, or adding them outside the brackets incorrectly.
• Failing to show the ethyl group properly attached as a side chain for but-1-ene.

🧠 Exam Technique

• Always redraw the monomer with the C=C bond written horizontally as C=C to make conversion to the polymer straightforward.
• Check your carbon count before moving on: 2 carbons per monomer × 2 units = 4 backbone carbons for ethene-derivatives, but here but-1-ene has 4 carbons per monomer, so 2 units require 8 backbone carbons!

✅ Correct Answer

• Dicarboxylic acid / Diacyl dichloride: HOOC-(CH₂)₄-COOH (or ClOC-(CH₂)₄-COCl )
• Diamine: H₂N-(CH₂)₄-NH₂

💡 Key Knowledge

• Polyamides are formed by condensation polymerisation between a dicarboxylic acid (or diyl chloride) and a diamine, releasing water (or HCl).
• To find the monomers, locate the peptide/amide link ( -CO-NH- ) and hydrolyse (break) the bond down the middle to reveal the functional groups at each end.

❌ Common Errors & Examiner Pitfalls

• Forgetting to include the terminal functional groups ( -COOH and -NH₂ ), leaving dangling carbon chains instead of complete molecules.
• Writing monofunctional species instead of di-functional monomers.

🧠 Exam Technique

• Count the carbon chain length carefully between the functional groups in the repeat unit diagram to avoid miscounting CH₂ groups.
• Monomers can be given in either order; clearly separate them with the word "and" or a comma.

✅ Correct Answer

H₂N-(CH₂)₄-COOH (an amino acid containing both an amine and a carboxylic acid group).

💡 Key Knowledge

• A single monomer can form a polyamide if it possesses both a -NH₂ group and a -COOH group (an amino acid).
• Alternatively, a cyclic lactam structure is also accepted by the mark scheme.

✅ Correct Answer

Two 3D tetrahedral structures drawn around a central chiral carbon atom, shown as non-superimposable mirror images using wedges (bonds coming out) and dashes (bonds going into the page).

🧠 Exam Technique & 3D Drawing Rules

• Mandatory 3D representation: You must use wedge and dashed bonds to indicate spatial arrangement. Simple flat line structures score 0 marks.
• Draw one isomer completely, then draw its direct reflection across an imaginary mirror plane for the second enantiomer.
• The four groups attached to the central C are: -H , -OH , -CH₃ , and -COOH .

✅ Correct Answer

Use a polarimeter to measure how they rotate the plane of plane-polarised light in opposite (equal and opposite) directions.

💡 Key Knowledge

• Enantiomers are optically active.
• One enantiomer rotates plane-polarised light clockwise (+), and the other rotates it counter-clockwise (-).

✅ Correct Answer

They do not accumulate in the environment / do not occupy landfill space / avoid toxic waste disposal methods like incineration.

💡 Key Knowledge

• Biodegradable polymers can be broken down by microorganisms into small natural molecules (like CO₂ and water).
• This reduces the long-term pollution crisis associated with non-biodegradable addition polymers (like polyalkenes).