AQA A-Level Chemistry Paper 2, 2019: Question 8

✅ Correct Answer

Electrophilic substitution

❌ Common Errors

• Writing just "Nitration" (this is the reaction type, not the mechanism name).
• Writing "Nucleophilic substitution" or "Electrophilic addition" (benzene rings resist addition due to loss of aromatic stability).

✅ Correct Answer

4-nitrophenol + 3H₂ → 4-aminophenol + 2H₂O

OR using [H] representation:

4-nitrophenol + 6[H] → 4-aminophenol + 2H₂O

🧠 Exam Technique

Always balance the oxygen atoms first by adding water to the right-hand side. A nitro group (-NO₂) has 2 oxygens, so it produces 2H₂O. Then balance the hydrogens:

• Right side: -NH₂ (2 H) + 2H₂O (4 H) = 6 H atoms needed.
• Left side: write either 3H₂ or 6[H]. Never write just 3H or 6H.

✅ Mechanism Breakdown

Step 1: Nucleophilic Attack

• Arrow from the lone pair on the :N atom of RNH₂ to the carbonyl carbon atom (C=O).
• Arrow from the C=O double bond onto the electronegative O atom.

Step 2: Tetrahedral Intermediate Structure (Mark 1)

• Must draw: CH₃–C(O⁻)(Cl)–N⁺H₂R
• Crucial: Must display a formal negative charge on O (O⁻) and a formal positive charge on N (N⁺).

Step 3: Elimination & Deprotonation (Mark 2)

• Lone pair on O⁻ reforms the double bond: arrow from lone pair on O⁻ to the C–O bond.
• Chloride leaves: arrow from C–Cl bond to the Cl atom.
• Loss of proton: arrow from one of the N–H bonds to the N⁺ atom (or attacked by another RNH₂ molecule).

❌ Common Examiner Traps

• Penalised: Using Cl⁻ to remove the proton from the nitrogen. (At this level, AQA strictly penalises Cl⁻ acting as a base in acylations; the proton is either lost spontaneously to the medium or abstracted by another amine molecule).
• Missing charges: Forgetting the positive charge on nitrogen in the intermediate loses M1 immediately.
• Imprecise curly arrows: Arrows must start cleanly from a lone pair or bond and point directly to an atom or bond.

✅ Any One Acceptable Reason

• Ethanoyl chloride produces corrosive / toxic HCl fumes (ethanoic anhydride produces ethanoic acid).
• Ethanoyl chloride reacts too violently / vigorously / exothermically.
• Ethanoyl chloride is more expensive.
• Ethanoyl chloride is more vulnerable to hydrolysis by atmospheric moisture.

❌ Vague Answers (0 Marks)

Do NOT write just "It is toxic", "It is harmful", or "It is dangerous". These are too vague for A-Level Chemistry. You must name the hazard specifically (e.g., "releases corrosive fumes of hydrogen chloride").

💡 Chemical Context

4-aminophenol contains two nucleophilic functional groups:

• A primary amine group ( –NH₂ ), which reacts to form an amide (paracetamol).
• A phenolic hydroxyl group ( –OH ), which can also react with an acylating agent to form an ester.

✅ Accepted Structures (Draw either one)

Option 1 (Ester formed at phenol group only):

Benzene ring with an ester group at position 1 and an unreacted amine at position 4:
CH₃COO–C₆H₄–NH₂ (4-aminophenyl ethanoate)

Option 2 (Di-acylated product):

Benzene ring with both groups acylated: an ester at position 1 and an amide at position 4:
CH₃COO–C₆H₄–NHCOCH₃

✅ Balanced Equation

Hydroquinone + CH₃COONH₄ → Paracetamol + 2H₂O

(Ammonium ethanoate can also be written as CH₃COO⁻ NH₄⁺ or NH₄CH₃COO)

🧠 How to Balance

• Hydroquinone provides: 1 benzene ring + 2 OH groups.
• Paracetamol contains: 1 benzene ring + 1 OH group + 1 –NHCOCH₃ group.
• Reactant needed must supply the ethanoate and nitrogen: ammonium ethanoate (CH₃COONH₄).
• Account for remaining atoms: 1 oxygen from hydroquinone + 1 oxygen from ammonium ethanoate + 4 hydrogens from ammonium = 2H₂O.

3 Marks: M1 for Mᵣ(paracetamol) = 151.0; M2 for finding moles (or ratio setup); M3 for final answer with unit handling (minimum 2 significant figures).