Edexcel A-Level Chemistry Paper 2, June 2025: Question 6

💡 Target Functional Groups & Diagnostic Tests

• Benzaldehyde (C₆H₅CHO): Aromatic aldehyde.
  • Tollens' reagent: forms a silver mirror / grey precipitate.
  • Acidified potassium dichromate(VI): orange turns to green.
  • Fehling's / Benedict's solution: blue solution forms a red precipitate (though aliphatic aldehydes react faster, accepted by Edexcel mark scheme).
• Phenylethanone (C₆H₅COCH₃): Methyl ketone (contains CH₃C=O bonded to carbon).
  • Iodoform / Haloform test: Iodine + NaOH(aq) (alkaline iodine) gives a pale yellow precipitate (triiodomethane, CHI₃).
• Benzoic acid (C₆H₅COOH): Carboxylic acid.
  • Carbonate test: Add Na₂CO₃(aq) or NaHCO₃(aq) → effervescence / bubbles of CO₂ gas.

✅ Model 6-Mark Answer Structure

Test 1 — For Benzaldehyde:
Add Tollens' reagent and warm in a water bath.
Positive result: Silver mirror (or grey precipitate) forms. Only benzaldehyde gives this result.

Test 2 — For Phenylethanone:
Add iodine and aqueous sodium hydroxide (alkaline iodine) and warm gently.
Positive result: Pale yellow precipitate of triiodomethane forms. Only phenylethanone contains the methyl carbonyl group.

Test 3 — For Benzoic Acid:
Add aqueous sodium carbonate (or sodium hydrogencarbonate).
Positive result: Effervescence / fizzing is observed. Only benzoic acid is acidic enough to liberate CO₂ gas.

🧠 Exam Technique & Mark Scheme Criteria

This is a 6-mark extended response marked on Indicative Content (IP) and Structure/Linkage:

• 6 IP points total: 3 correct reagents (IP1, IP3, IP5) paired with their 3 matching positive observations (IP2, IP4, IP6).
• Indicative marks: 6 points = 4 marks; 4–5 points = 3 marks; 2–3 points = 2 marks; 1 point = 1 mark.
• Reasoning marks (up to 2 marks): Full marks require sustained, logically structured links between each test, the specific compound it identifies, and the distinct observation.

❌ Common Errors & Examiner Traps

• Violating the prompt rule: The prompt explicitly states: "Do not use tests which give positive results for more than one of these compounds". Using 2,4-DNPH gives an orange precipitate with both benzaldehyde and phenylethanone, scoring zero indicative marks.
• Incomplete reagent names: Writing just "dichromate" instead of acidified dichromate, or just "iodine" without sodium hydroxide (or alkaline) loses the reagent mark.
• Vague carbonate tests: Writing "add carbonate ions" is insufficient; specify a named reagent such as sodium hydrogencarbonate or calcium carbonate.

✅ Correct Answer

B — condensation

Selected option: B

💡 Why Condensation?

A condensation reaction occurs when two molecules join together with the simultaneous elimination of a small molecule, in this case water (H₂O):

RCHO + H₂N-NH-Ar → RCH=N-NH-Ar + H₂O

• It is also described mechanistically as addition-elimination.
• A (addition) is incorrect because addition produces a single product with no eliminated molecule.
• C (hydrolysis) is incorrect because water is a product, not a reacting reactant split across bonds.
• D (polymerisation) is incorrect because no continuous repeating chain is formed.

✅ Correct Product Structure

Replace the R group in the general hydrazone structure with a phenyl ring ( -C₆H₅ ):

O₂N H \ / (Ar)-NH-N=C / \ O₂N (C₆H₅)

Formula representation: (NO₂)₂C₆H₃-NH-N=CH-C₆H₅

🧠 Examiner Drawing Guidance

• What to keep: Leave the entire 2,4-dinitrophenylhydrazine moiety unchanged up to the -NH-N=C group.
• What to swap: Look at the given general formula: -N=C(H)(R) . For benzaldehyde (C₆H₅CHO), simply substitute R with a phenyl group (benzene ring or -C₆H₅).
• Acceptable variations: Drawing a skeletal benzene ring or writing -C₆H₅ is fully credited. Both cis and trans (E/Z) geometrical isomers about the C=N bond are accepted.
• Examiner allowance: Connectivity slips between the two nitrogens ( N-N ) are ignored in this specific question.