Edexcel A-Level Chemistry Paper 2, November 2021: Question 9

✅ Correct Answers

• 4 peaks for 4-nitrophenol (para isomer, symmetrical).
• 6 peaks for 2-nitrophenol (ortho isomer, unsymmetrical).

💡 Key Knowledge

The number of peaks in a 13C NMR spectrum corresponds to the number of non-equivalent carbon environments. Planes of symmetry halve the number of distinct signals.

🧠 Exam Technique

Count carefully across the benzene ring and substituents. 4-nitrophenol has a vertical line of symmetry down C1 and C4, leaving only 4 unique carbon environments. 2-nitrophenol lacks this symmetry, resulting in 6 distinct environments.

📐 Step-by-Step Calculation

1. Calculate moles of paracetamol desired:
   1.00 kg = 1000 g
   Moles = 1000 ÷ 151.0 = 6.6225 mol
2. Determine overall percentage yield:
   0.32 × 0.85 × 0.70 = 0.1904 (or 19.04%)
3. Calculate theoretical moles of phenol required without loss:
   Theoretical moles = 6.6225 ÷ 0.1904 = 34.782 mol
4. Convert moles of phenol to mass:
   Mass = 34.782 mol × 94.0 g mol⁻¹ = 3269.5 g = 3.27 kg

❌ Common Errors & Traps

• Additive yields: Adding percentages together instead of multiplying fractional yields sequentially.
• Wrong direction multiplier: Multiplying target mass by yields instead of dividing when scaling back from product to reactant.
• Unit mix-ups: Failing to convert between grams and kilograms.

✅ Correct Answer

C — oxidation

💡 Examiner Commentary

Students must compare the structures of paracetamol and compound Z. Looking closely at the structures, hydrogen atoms are lost from the OH and NH groups to form the quinone-imine ring structure. Loss of hydrogen = oxidation.

✅ Correct Answer

Circle the two central alpha-carbon atoms that are bonded to four different groups: the carbon adjacent to the first -COOH and -NH₂ group, and the carbon adjacent to the second internal peptide backbone chain.

🧠 Exam Technique

A chiral carbon has 4 completely different groups attached. Scan the molecule for sp³ hybridized carbons with four distinct substituents. Do not circle carbons in CH₂ groups, terminal carbonyl carbons, or achiral backbone carbons.

✅ Correct Answer

B — glutamic acid and cysteine

❌ Common Errors

Distractor D (methionine) was frequently chosen by students who failed to notice that methionine contains a thioether methyl group ( -S-CH₃ ), whereas the side chain in glutathione features a free thiol ( -SH ) group characteristic of cysteine.

✅ Correct Answers

• Amino acids exist as zwitterions (dipolar ions with both positive and negative charges) in the solid state.
• There are strong electrostatic forces of attraction (ionic bonds) between oppositely charged zwitterions, requiring significant energy to break.

❌ Common Errors

A common mistake is attributing high melting/boiling points and crystalline states to hydrogen bonding alone. While hydrogen bonds exist, the primary reason amino acids are high-melting crystalline solids at room temperature is the presence of ionic bonding (electrostatic attractions between zwitterions).