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A-Level Chemistry May/June 2025 Q9(a)(iii): Identify a single test that will distinguish between C6H5NH2 and CH3CH2CH2NH2 by produc…
A-Level Chemistry · Paper 9701/41 · May/June 2025 · Question 9(a)(iii) · [2 marks]
Identify a single test that will distinguish between C6H5NH2 and CH3CH2CH2NH2 by producing a white precipitate with only one of these amines. Draw the structure of the compound that is precipitated.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
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Test and Observation
Test: Add aqueous bromine (bromine water) to each amine and shake.
Result: A white precipitate is formed with C₆H₅NH₂ (phenylamine). No precipitate is formed with CH₃CH₂CH₂NH₂.
Structure of Precipitate
The compound is 2,4,6-tribromophenylamine.
(The candidate is expected to draw the skeletal or displayed formula for this compound, showing a benzene ring with an -NH₂ group and three -Br atoms bonded at the 2, 4, and 6 positions relative to the -NH₂ group.)
How the marks are awarded
- M1 — Correctly identifying the reagent as 'aqueous bromine' (or bromine water) and linking it to the formation of a precipitate with phenylamine / C₆H₅NH₂.
- M2 — Drawing the correct structure of the product, 2,4,6-tribromophenylamine. The structure must clearly show a benzene ring, one NH₂ group, and three Br atoms in the correct 2, 4, and 6 positions.
Common mistakes
- Stating that the aliphatic amine (propylamine) gives the precipitate instead of the aromatic amine (phenylamine).
- Drawing the incorrect number or position of bromine atoms on the benzene ring, for example, showing only one bromine atom (monosubstitution) or placing them at incorrect positions like 3,5.
- Suggesting a reagent that does not distinguish between the two compounds, such as universal indicator or a strong acid, as both are bases.
- Forgetting to specify that the bromine must be aqueous (in water), as this is crucial for the reaction to proceed as a simple laboratory test producing a precipitate.
Examiner tip: Recognise that the highly activated benzene ring in phenylamine (and phenol) undergoes rapid, multiple electrophilic substitution with aqueous bromine, a key characteristic reaction for both synthesis and identification.
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