9701 · 34.2
Phenylamine and azo compounds flashcards
Revision flashcards for Cambridge 9701 Phenylamine and azo compounds (syllabus 34.2). Flip, recall, then mark a real past-paper question.
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What are the reagents and conditions for the preparation of phenylamine from nitrobenzene?
Reagents: Tin (Sn) and concentrated hydrochloric acid (HCl). Conditions: Heat under reflux. An alkali like NaOH is then added to liberate the free amine.
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Why is phenylamine a weaker base than ammonia ($NH_3$)?
The lone pair of electrons on the nitrogen atom in phenylamine is delocalised into the pi-electron system of the benzene ring, making it less available to accept a proton ($H^+$).
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Why is ethylamine ($CH_3CH_2NH_2$) a stronger base than ammonia?
The ethyl group is an electron-donating alkyl group (positive inductive effect), which increases the electron density on the nitrogen atom, making the lone pair more available to accept a proton.
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Place ammonia, ethylamine, and phenylamine in order of increasing basicity.
Phenylamine < Ammonia < Ethylamine
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What is diazotisation?
The reaction of a primary aromatic amine (like phenylamine) with nitrous acid ($HNO_2$) at a low temperature (below 10°C) to form a diazonium salt.
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What are the reagents used to make the nitrous acid for diazotisation?
Sodium nitrite ($NaNO_2$) and a strong acid, typically dilute hydrochloric acid (HCl). The nitrous acid is generated in situ (in the reaction mixture).
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Why must the temperature be kept below 10°C during diazotisation?
The diazonium salt formed is unstable and will decompose at higher temperatures, typically forming phenol, nitrogen gas, and HCl.
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What is a coupling reaction in this context?
The reaction where a diazonium salt acts as an electrophile and substitutes into another activated aromatic ring (like phenol or another amine) to form an azo compound.
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What is a chromophore? Give an example.
A part of a molecule responsible for its colour. The azo group, $-N=N-$, is the chromophore in azo dyes.
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What conditions are required for the coupling of benzenediazonium chloride with phenol?
Cold conditions (below 10°C) and alkaline conditions (e.g., by dissolving the phenol in NaOH solution).
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What is the observation when aqueous bromine is added to phenylamine?
A white precipitate of 2,4,6-tribromophenylamine is formed, and the bromine water is decolourised. This shows the high reactivity of the phenylamine ring.
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What is the structure of the benzenediazonium ion?
$C_6H_5-N^+\equiv N$