Phenylamine and azo compounds
Cambridge A-Level Chemistry (9701) · Unit 34: Organic nitrogen compounds · 7 flashcards
Phenylamine and azo compounds is topic 34.2 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 34 — Organic nitrogen compounds , alongside Primary and secondary amines, Amides and Amino acids. In one line: Warming benzenediazonium chloride with water leads to the formation of phenol, nitrogen gas (N₂), and HCl. The diazonium group is replaced by a hydroxyl group (-OH).
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 7 flashcards — 2 definitions and 5 key concepts — covering the precise wording mark schemes reward. Use the 2 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
What product is formed when benzenediazonium chloride is warmed with H₂O
Warming benzenediazonium chloride with water leads to the formation of phenol, nitrogen gas (N₂), and HCl. The diazonium group is replaced by a hydroxyl group (-OH).
What the Cambridge 9701 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Describe how phenylamine is prepared: benzene is nitrated to nitrobenzene, which is reduced with hot Sn and concentrated HCl, followed by NaOH(aq).
- Describe: (a) what happens when Br₂(aq) is added to phenylamine at room temperature; (b) phenylamine treated with HNO₂ (or NaNO₂ plus dilute acid) below 10 °C to give a diazonium salt, and the phenol formed when this salt is then warmed with water.
- Describe and explain how the basicities of ethylamine, phenylamine and aqueous ammonia compare.
- Recall these facts on azo compounds: (a) describe how phenol and benzenediazonium chloride couple together in NaOH(aq), which gives an azo compound as product; (b) identify an azo group; (c) state the frequent use of azo compounds as dyes; (d) know that a similar route gives other azo dyes.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Outline the three-step process for preparing phenylamine from benzene.
1. Nitration of benzene using concentrated HNO₃ and H₂SO₄ to form nitrobenzene. 2. Reduction of nitrobenzene with hot Sn/concentrated HCl. 3. Neutralization with NaOH(aq) to release phenylamine.
Describe the reaction of phenylamine with bromine water (Br₂(aq)).
Phenylamine reacts rapidly with Br₂(aq) at room temperature to form a white precipitate of 2,4,6-tribromophenylamine. This occurs because the lone pair of electrons on the nitrogen atom in phenylamine is delocalized into the benzene ring, activating it towards electrophilic substitution.
Outline the reaction of phenylamine with HNO₂ (or NaNO₂ and dilute acid) at temperatures below 10 °C.
Phenylamine reacts with nitrous acid (HNO₂, generated in situ from NaNO₂ and dilute HCl) at <10°C to form a diazonium salt (benzenediazonium chloride). This reaction requires low temperatures to prevent decomposition of the diazonium salt.
What product is formed when benzenediazonium chloride is warmed with H₂O?
Warming benzenediazonium chloride with water leads to the formation of phenol, nitrogen gas (N₂), and HCl. The diazonium group is replaced by a hydroxyl group (-OH).
Explain the relative basicities of aqueous ammonia, ethylamine, and phenylamine.
Ethylamine > Ammonia > Phenylamine. Ethylamine is the strongest base due to the electron-donating ethyl group increasing electron density on the nitrogen. Phenylamine is the weakest base, as the lone pair on nitrogen is delocalized into the benzene ring, making it less available for protonation.
What is an azo compound and how are they formed?
Azo compounds contain the functional group R-N=N-R', where R and R' are aryl groups. They are typically formed by the diazo coupling reaction, where a diazonium salt reacts with an activated aromatic compound (
Describe the reaction to create an azo dye, including the reagents and conditions.
Azo dyes are formed by reacting a diazonium salt with a coupling agent such as phenol or phenylamine under alkaline conditions (typically pH 9-10) and low temperature (0-5°C). This electrophilic substitution reaction forms the -N=N- bond that characterizes azo dyes.
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Phenylamine and azo compounds sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
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