AQA A-Level Chemistry Paper 2, 2018: Question 7

✅ Mark Scheme Model Answer

Equation for electrophile generation (1 Mark):

AlCl₃ + CH₃CH₂COCl → CH₃CH₂–C⁺=O + AlCl₄⁻

(Positive charge allowed on C or O in the equation, but in the mechanism it MUST reside on the carbonyl C).

Mechanism for Electrophilic Substitution (3 Marks):

• M2 (Curly arrow): Arrow starts from inside the delocalised π-ring of benzene and points directly to the positively charged carbonyl carbon of ⁺COCH₂CH₃ .
• M3 (Intermediate structure): A six-membered ring with both –H and –COCH₂CH₃ bonded to C1. A broken "horseshoe" delocalised system with the opening facing C1 (extending over 5 carbons from C2 to C6, strictly not past them), and a positive charge ( + ) located inside the ring cavity.
• M4 (Aromatic restoration): Curly arrow starts on the C–H single bond at C1 and points directly back into the partially delocalised ring to complete the 6-electron aromatic system.

🧠 Exam Technique & Drawing Details

• Drawing the Horseshoe: Make sure the open ends of the horseshoe do not wrap past carbons 2 and 6. If your horseshoe touches carbon 1, you automatically lose mark M3.
• Location of the '+' Charge: The positive charge must be in the centre of the ring or biased towards carbons 3, 4, or 5. Do not place it right next to carbon 1 or on the hydrogen atom!
• Arrow Origin: For M4, ensure your arrow touches the C–H bond itself. If it starts from the H atom or from empty space, no mark is awarded.

💡 Key Knowledge

AlCl₃ acts as a Lewis acid catalyst. It has an empty p-orbital on the Al atom that accepts a lone pair from the chlorine atom of the acyl chloride, forming the powerful acylium ion electrophile ( R–C⁺=O ).

Friedel–Crafts acylation is preferred over alkylation in synthesis because the resulting aromatic ketone is deactivated towards further substitution, preventing poly-acylation.

❌ Common Mistakes to Avoid

• Writing an electrophile as CH₃CH₂CO⁺Cl (failing to remove the chloride).
• Directing the first arrow (M2) to the methyl/ethyl carbon instead of the carbonyl carbon.
• Drawing a full circle with a + in the intermediate rather than an open horseshoe.
• Using AlCl₄⁻ or Cl⁻ to pluck off the proton in M4—while examiners ignore this, drawing incorrect curly arrows from the base can risk losing clarity.

✅ Mark Scheme Model Answer

• IUPAC Name (1 Mark): 1-phenylpropan-1-ol
  (Both numbers are required. "Phenylpropanol" or "1-phenylpropanol" scores 0).
• Reagent (1 Mark): NaBH₄ (or LiAlH₄ , or H₂ with Ni/Pd/Pt ).
• Name of Mechanism (1 Mark):
  • If NaBH₄ / LiAlH₄ chosen: Nucleophilic addition
  • If H₂ / catalyst chosen: Addition or Hydrogenation

🧠 Exam Technique

• Numbering IUPAC chains: The principal functional group is the alcohol ( –OH ), so the longest continuous carbon chain containing the –OH group is propane (3 carbons). The –OH is on carbon 1, and the phenyl ring substituent is also on carbon 1: hence 1-phenylpropan-1-ol.
• Reagent-Mechanism Pair: Ensure your mechanism name strictly matches your chosen reagent! Naming "electrophilic addition" or mixing up hydrogenation with nucleophilic addition is an immediate mark loss.

💡 Key Knowledge

NaBH₄ supplies the hydride ion ( :H⁻ ), which acts as a nucleophile attacking the electron-deficient carbonyl carbon ( δ+ ). Because carbonyls have a polar C=O double bond, they undergo nucleophilic addition, unlike the non-polar C=C alkenes which undergo electrophilic addition.

❌ Common Mistakes to Avoid

• Writing "phenylpropan-1-ol" omitting the locant "1-" for the phenyl group.
• Writing "reduction" as the name of the mechanism (Reduction is the reaction type, not the mechanism).
• Confusing the hydride nucleophile: stating "electrophilic addition".

✅ Mark Scheme Model Answer

• Observation (1 Mark): Misty fumes / steamy fumes / white fumes
  (Allow: sweet/fruity smell. Reject: "smoke" or "white smoke").
• Name of Mechanism (1 Mark): (Nucleophilic) addition-elimination
• Structure of the Ester (1 Mark):
              CH₂CH₃
      O       |
      ||      |
  CH₃–C – O – CH – C₆H₅

  (Ethanoyl group CH₃CO– attached to the oxygen of the original secondary alkoxy group –O–CH(CH₂CH₃)(C₆H₅) ).

🧠 Exam Technique: Fumes vs. Smoke

In A-Level chemistry mark schemes, fumes ≠ smoke.

• Fumes: Tiny droplets of liquid / condensed gas (e.g. gaseous HCl interacting with moisture in the air forming droplets of hydrochloric acid).
• Smoke: Solid unburnt carbon particulate matter.
• Describing HCl as "white smoke" or "smoky fumes" loses the mark every single time! Stick to misty fumes or steamy fumes.

💡 Key Knowledge

Reaction equation:

CH₃COCl + CH₃CH₂CH(OH)C₆H₅ → CH₃COOCH(C₆H₅)CH₂CH₃ + HCl

Unlike esterification of carboxylic acids with alcohols, acyl chloride reactions:

• Occur vigorously at room temperature without need for an acid catalyst.
• Are irreversible and go to completion (giving a far higher yield).
• Release gaseous HCl instead of H₂O .

❌ Common Mistakes to Avoid

• Writing "electrophilic substitution" or just "substitution" for acyl chloride esterification (the correct mechanism is addition-elimination).
• Misconnecting the ester linkage—for example, attempting to attach the benzene ring directly to the ethanoyl carbonyl.
• Forgetting that ethanoyl chloride has 2 carbons ( CH₃CO– ), not 3 carbons.