Edexcel A-Level Chemistry Paper 2, June 2018: Question 7

✅ Correct Answer

• Step 1: Ethanoyl chloride and aluminium chloride ( AlCl₃ ) catalyst with heat (Friedel-Crafts acylation). Forms intermediate A (phenylethanone / acetophenone).
• Step 2: Lithium tetrahydridoaluminate(III) ( LiAlH₄ ) in dry ether. Reduces intermediate A to form intermediate B ((1-phenylethanol)).
• Step 3: Concentrated phosphoric acid ( H₃PO₄ ) or concentrated sulfuric acid ( H₂SO₄ ) with heat. Dehydrates alcohol B to form phenylethene.

💡 Key Knowledge

• Friedel-Crafts acylation introduces a reactive acyl group onto the benzene ring, deactivating it slightly against further substitution compared to alkylation.
• LiAlH₄ is a powerful reducing agent used specifically to reduce ketones and aldehydes back to secondary and primary alcohols.
• Acid-catalysed elimination (dehydration) removes water from an alcohol to yield an alkene.

🧠 Exam Technique

• You must pick reagents strictly from the provided table list. Naming outside options results in immediate loss of marks for that step.
• Clearly label intermediate A as phenylethanone and intermediate B as (1-phenylethanol), either via name or unambiguous structural/displayed formula.

❌ Common Errors

• Forgetting the aluminium chloride catalyst in Step 1 (the acylation won't proceed without it).
• Attempting to use LiAlH₄ in aqueous conditions (it reacts violently with water; must use dry ether).
• Using oxidising agents like acidified potassium dichromate(VI) in Step 2 instead of a reducing agent.

✅ Correct Answer

B: acidified potassium manganate(VII)

💡 Key Knowledge

• Cold, dilute acidified KMnO₄ acts as an oxidising agent that hydroxylates the carbon-carbon double bond, turning an alkene into a diol (1,2-diol).

❌ Common Errors

• Choosing A (acidified potassium dichromate(VI)) which is used to oxidise alcohols, not alkenes.
• Choosing D (steam) which produces a single alcohol (hydration reaction), not a diol.

✅ Correct Answer

A polymer chain section displaying at least two consecutive repeat units derived from phenylethene:

-[CH(C₆H₅)-CH₂]₂- (or shown in displayed format showing two backbone carbons per unit with phenyl rings attached to alternating carbons).

🧠 Exam Technique

• Ensure the main carbon backbone has single bonds and extension bonds (brackets or trailing lines) cutting through the square brackets.
• Check that you have drawn two complete repeat units, not just one.

❌ Common Errors

• Leaving a double bond ( C=C ) in the polymer backbone.
• Missing CH₂ groups or misplacing the benzene ring attachment.

✅ Correct Answer

• Advantage: Release of energy (can be used to generate electricity) / avoids filling up landfill sites.
• Disadvantage: Release of toxic fumes, such as polycyclic aromatic hydrocarbons (or incomplete combustion products like carbon monoxide / carbon soot).

❌ Common Errors

• Vague statements like "it causes pollution" without specifying toxic fumes, carbon monoxide, or particulates.
• Incorrectly claiming incineration causes ozone depletion or acid rain (polystyrene contains no chlorine, fluorine, or sulfur).

📐 Step-by-Step Calculation

Formula of phenylethene: C₈H₈

1. Calculate Molar Mass (Mr) of phenylethene:
   Mr = (8 × 12.0) + (8 × 1.0) = 96 + 8 = 104
2. Calculate percentage by mass of carbon:
   Mass of carbon = 8 × 12.0 = 96
   Percentage = (96 ÷ 104) × 100 = 92.3076...%
3. Apply Significant Figures:
   Round to 2 or 3 significant figures: 92% or 92.3%.

❌ Common Calculation Traps

• Using incorrect molecular formula for phenylethene (e.g., forgetting it has a benzene ring containing 6 carbons plus a CH=CH₂ group, totaling 8 carbons).
• Failing to round to appropriate significant figures (2 or 3 sf as requested).