AQA A-Level Chemistry AS Paper 2, June 2025: Question 4

📐 Structural Deduction Step-by-Step

1. Degree of Unsaturation: A saturated 4-carbon chain has the formula C₄H₁₀. C₄H₆O₂ is missing 4 H atoms, meaning it has 2 degrees of unsaturation (2 rings/double bonds).
2. Reaction with Na₂CO₃: Forms a gas (CO₂). Both A and B contain a carboxylic acid group ( -COOH ). The carbonyl C=O accounts for 1 degree of unsaturation.
3. Remaining unsaturation: Exactly 1 unsaturation remains (either 1 C=C bond OR 1 ring) plus 3 carbon atoms.
4. Compound A: Decolourises bromine water → contains an alkene (C=C) group. It is an unsaturated straight-chain carboxylic acid.
5. Compound B: Does not decolourise bromine water → contains no C=C bond. Therefore, the remaining degree of unsaturation must be a ring (cyclopropane ring).

✅ Acceptable Structures (Mark Scheme)

Compound A (any one of the following alkenoic acids):

Compound B:

🧠 Exam Technique & Tips

• Always check the molecular formula count: count every C, H, and O to prevent losing careless marks.
• When asked for functional group isomers, ensure that the functional groups differ or are assembled in distinct structural classes. Here, A is an alkenoic acid, while B is a saturated cycloalkanoic acid.
• Display your structures clearly. In skeletal or structural formulae, make sure bonds connect to the carbon of the -COOH group, not to the acidic oxygen.

❌ Common Misconceptions & Pitfalls

• Assuming B cannot be an acid: Students often forget that a ring constitutes a degree of unsaturation. Failing to deduce a ring leads to impossible acyclic structures without C=C.
• Drawing an ester for B: Esters do not react with sodium carbonate to evolve gas under standard conditions. Both must be carboxylic acids.
• Incorrect hydrogen counts: Writing pentanoic acid derivatives or drawing extra double bonds violating the C₄H₆O₂ formula.

💡 Infrared Data Analysis

• 3230–3550 cm⁻¹: Characteristic of an O-H (alcohol) stretch.
• 1620–1680 cm⁻¹: Characteristic of a C=C (alkene) stretch.
• NO absorption at 1680–1750 cm⁻¹: Confirms the complete absence of a C=O (carbonyl) group (no aldehyde, ketone, ester, or carboxylic acid).
• Formula C₄H₆O₂: Contains 2 oxygen atoms and 2 degrees of unsaturation. Since there is no carbonyl group, both oxygens must belong to alcohol groups ( -OH ) or ether linkages.

✅ Acceptable Structures

Any valid structure of C₄H₆O₂ containing either:

• An acyclic chain with 2 C=C bonds and 2 -OH groups:
  e.g., HO-CH=CH-CH=CH-OH (buta-1,3-diene-1,4-diol)
  or CH₂=CH-C(OH)=CH(OH)
• A cyclic compound with 1 C=C bond and 2 -OH groups:
  e.g., cyclobut-2-ene-1,2-diol (a 4-membered ring with one C=C double bond and two -OH groups attached).
• Alternative allowed: Cyclic ethers containing one -OH and one C=C group (e.g., 2,5-dihydrofuran derivatives with an OH).

❌ Common Errors in 04.2

• Including a C=O bond: Drawing an aldehyde or ketone. The prompt explicitly states no absorption in the range 1680–1750 cm⁻¹. Any carbonyl immediately invalidates the answer.
• Missing the second degree of unsaturation: Drawing a mono-alkene diol like HO-CH₂-CH=CH-CH₂-OH . That has formula C₄H₈O₂, which has 2 too many hydrogens! A diene or a cyclic alkene is mandatory.

🧠 Exam Technique: Negative Evidence

In AQA organic questions, negative IR evidence ("NO absorption at...") is just as critical as positive peaks. Use it as a strict elimination rule before building your structure.

💡 Monomer Structure & Polymerisation Rule

Monomer is vinyl acetate: CH₂=CH-O-CO-CH₃

• Addition polymerisation breaks the C=C double bond to form a continuous single-bonded carbon backbone.
• All other groups ( -O-CO-CH₃ and hydrogens) remain attached as side chains / substituents.
• The repeating unit must strictly contain 2 carbon atoms in the backbone chain.

✅ Correct Repeating Unit Representation

The standard repeating unit shows the two backbone carbons with single bonds extending beyond the ends:

Note: Mark scheme guidance explicitly states: Ignore any 'n' or brackets, but trailing extension bonds must pass through or emerge from the two backbone carbons.

❌ Common Traps to Avoid

• Retaining the double bond: Leaving a C=C bond in an addition polymer repeating unit scores 0.
• Incorporating ester atoms into the backbone: Drawing oxygen or carbonyl carbons as part of the main polymer chain (which happens in condensation polymers, NOT addition polymers).
• Missing extension bonds: Failing to show the open end-bonds extending out of the repeating unit.

🧠 Quick Check Checklist

• ✓ Does the backbone consist of exactly two C atoms joined by a single bond?
• ✓ Are extension bonds clearly shown at both ends?
• ✓ Is the ester side chain connected via the oxygen atom ( -O-CO-CH₃ ) and not directly via the carbonyl carbon?