AQA A-Level Chemistry Paper 3, 2019: Question 4

✅ Correct Answer [1 Mark]

nucleophilic addition

❌ Common Errors

• Writing nucleophilic addition-elimination (confusing aldehydes with acyl chlorides/acid anhydrides).
• Writing electrophilic addition (confusing C=O with C=C alkenes).
• Writing simply addition or nucleophilic alone (0 marks).

✅ Full Mark Answer Breakdown [5 Marks]

• Name [M1]: Racemic mixture or racemate.
• Explanation Point 1 [M2]: The carbonyl group (C=O) / carbon-oxygen double bond is planar.
• Explanation Point 2 [M3]: Attack by cyanide (:CN⁻) has an equal probability from either side (above or below the plane).
• Structure [M4]: Correct connectivity of 2-hydroxypropanenitrile: CH₃-CH(OH)-CN .
• 3D Isomers [M5]: Correct 3D tetrahedral drawing showing non-superimposable mirror image relationship.

🧠 Exam Technique • Precision Phrasing

• The "Planar" Trap: You MUST say "planar around the C=O group" or "the carbonyl bond is planar". Writing "the ethanal molecule is planar" loses M2 completely because the methyl group (-CH₃) is tetrahedral!
• Equal Probability: Always specify that attack from both faces/sides is equally likely.

💡 How to Draw 3D Enantiomers [M4 & M5]

Each chiral carbon must show a 3D tetrahedral arrangement (two bonds in-plane at an angle < 180°, one wedge pointing forward, one dash pointing backward). Draw either a direct mirror image or swap two groups:

❌ Common Misconceptions

• Drawing flat 2D crosses (displayed structures) instead of 3D wedge-and-dash conventions.
• Connecting through nitrogen in the nitrile group (writing -NC instead of -CN when attached to the chiral carbon, unless mirrored where C must still link to C).
• Omitting the chiral hydrogen atom or confusing ethanal's chain length.

✅ Correct Answer [2 Marks]

• Reagent [M1]: Concentrated sulfuric acid ( conc H₂SO₄ ) or concentrated phosphoric acid ( conc H₃PO₄ ).
• Conditions [M2]: Heat or 170 °C or reflux.

💡 Reaction Classification

This is an acid-catalysed elimination (dehydration) of an alcohol:

CH₃-CH(OH)-CN → CH₂=CHCN + H₂O

An -OH from C2 and an -H from C1 (methyl) are eliminated to form the C=C double bond.

❌ Examiner Pitfalls

• Omitting the word concentrated for the acid (loses M1 unless given in conditions).
• Stating "ethanolic" or "alcoholic" (this is the reagent condition for haloalkane elimination with KOH, not alcohol dehydration!).
• Writing "warm" instead of heat or temperature ≥ 100 °C (loses M2).

✅ Correct Structure [1 Mark]

Also accepted in condensed form:
—CH₂—CH(CN)—CH₂—CH(CN)—CH₂—CH(CN)—

🧠 Exam Technique • Polymer Traps

• Trailing Bonds: You must extend the single covalent bonds outside the ends of the chain to show continuous repeating.
• No Double Bonds: During addition polymerisation, the C=C opens into a C—C saturated chain.
• Connectivity: The C-C bond in the chain must attach to the carbon atom of the CN group (not to N).
• Repeating Unit Count: The question specified three repeating units (6 carbons in the continuous backbone). Supplying fewer or more loses the mark.