Ch 2 · Phenylamine and azo compounds · AS & A Level Chemistry 9701
Topic 2 of 4 in Nitrogen compounds — AS & A Level Chemistry 9701 — 57 lessons.
Introduction
Phenylamine, C_6H_5NH_2, has its -NH_2 group joined directly to a benzene ring, exactly as phenol has its -OH group joined to a ring. The same delocalisation that makes phenol's ring so reactive also operates here, but this time it makes phenylamine a far weaker base than ammonia, and it sets up the diazonium chemistry behind one of the most important classes of synthetic dye.
Key ideas — Preparation of phenylamine
Benzene is first nitrated with a mixture of concentrated nitric acid and concentrated sulfuric acid at about 50 °C, giving nitrobenzene. The nitro group is then reduced: nitrobenzene is heated under reflux with tin and concentrated hydrochloric acid, and the resulting salt is treated with excess sodium hydroxide to liberate free phenylamine. C_6H_5NO_2 + 6[H] arrow C_6H_5NH_2 + 2H_2O
🧑🏫 Teacher's explanation
Think of phenylamine (aniline) as a benzene ring that has been 'activated' by adding an amino group (-NH_2). This group is a powerful electron donor, making the ring much more reactive toward electrophiles than benzene itself. It's like adding a high-powered turbocharger to a car engine; the car (the benzene ring) is now much more sensitive to any input.
📖 Definitions
🔍 Worked example — Standard
Explain why phenylamine is a weaker base than ethylamine (CH_3CH_2NH_2).
🔍 Worked example — Exam-style
A sample of phenylamine is reacted with bromine water. Describe the observation and write the chemical equation for the reaction.
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🎯 Exam question
Key ideas — Reactions of phenylamine with bromine water and nitrous acid
Like phenol, phenylamine reacts instantly with bromine water at room temperature, with no catalyst, giving a white precipitate of 2,4,6-tribromophenylamine and decolourising the bromine water. Below 10 °C, phenylamine reacts with nitrous acid (generated in situ from sodium nitrite and dilute hydrochloric acid) to form benzenediazonium chloride, a pale yellow solution that decomposes above about 10 °C, releasing nitrogen gas: C_6H_5NH_2 + HNO_2 + HCl xrightarrow<10 ^circC C_6H_5N_2^+Cl^- + 2H_2O
🧑🏫 Teacher's explanation
Think of phenylamine (aniline) as a benzene ring with a very 'generous' amino group attached. Because the lone pair of electrons on the nitrogen is heavily involved in the ring's resonance, it is much less available to act as a base or to react with electrophiles compared to something like ethylamine. However, the ring itself becomes extremely electron-rich because that lone pair is being pushed into the pi-system.
📖 Definitions
🔍 Worked example — Easy
Predict the products and observation when phenylamine is added to bromine water at room temperature. Write the balanced equation for the substitution reaction.
🔍 Worked example — Standard
Calculate the mass of 2,4,6-tribromophenylamine produced from 0.100 mol of phenylamine in the reaction with bromine water, assuming 100% yield.
🔍 Worked example — Exam-style
A student reacts phenylamine with NaNO_2 and HCl at 5^° C. If the temperature rises to 40^° C, what is the primary observation and what gas is evolved?
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🎯 Exam question
Key ideas — Relative basicities of ammonia, ethylamine and phenylamine
The order of base strength is ethylamine > ammonia > phenylamine. Ethylamine's electron-donating ethyl group increases the electron density on nitrogen relative to ammonia, making the lone pair more available to accept a proton, so ethylamine is a stronger base than ammonia. In phenylamine, the nitrogen lone pair instead overlaps with the aromatic ring's π system and becomes delocalised into it, considerably reducing its availability to bind a proton; phenylamine is therefore a much weaker base than ammonia, despite also having an alkyl-type group attached to nitrogen.
🧑🏫 Teacher's explanation
Think of a base as a person trying to hand out a gift (a lone pair of electrons) to a friend (a proton). The 'strength' of the base depends on how easily that person can let go of the gift. If the person is holding the gift tightly, they are a weak base. If they are holding it loosely, they are a strong base.
📖 Definitions
🔍 Worked example — Easy
Explain why ethylamine (C_2H_5NH_2) is a stronger base than ammonia (NH_3).
🔍 Worked example — Standard
Predict the order of basicity for NH_3, CH_3NH_2, and C_6H_5NH_2 and justify the position of C_6H_5NH_2.
🔍 Worked example — Exam-style
A student claims that because phenylamine has a nitrogen atom like ammonia, it should be a stronger base due to the presence of the benzene ring. Explain why this is incorrect.
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🎯 Exam question
Key ideas — Coupling with phenol to form an azo compound
Below 10 °C, benzenediazonium chloride reacts with an alkaline solution of phenol in a coupling reaction, an electrophilic substitution, forming an intensely coloured azo compound joined by an azo group, -mathrmN=N-: C_6H_5N_2^+ + C_6H_5O^- arrow mathrmC_6H_5-N=N-C_6H_4-OH The azo group, -mathrmN=N-, links the two aromatic rings and is responsible for the strong colour of the product.
🧑🏫 Teacher's explanation
Imagine you are building a complex Lego model. You have two separate, large pieces (the benzene rings), and you need a very specific, small connector piece to snap them together. In this reaction, the diazonium ion acts as the 'connector' that seeks out electron-rich areas to bond with.
📖 Definitions
🔍 Worked example — Easy
Identify the functional group present in the product of the reaction between benzenediazonium chloride and phenol.
🔍 Worked example — Standard
Predict the product of the reaction between C_6H_5N_2Cl and C_6H_5OH in alkaline conditions.
🔍 Worked example — Exam-style
Calculate the molar mass of the azo dye formed: C_12H_10N_2O. (Use C=12, H=1, N=14, O=16)
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🎯 Exam question
(a) Write the chemical equation for this reaction, including state symbols. [2] (b) Explain why the reaction must be kept below 10 °C. [1] [3 marks]
Key ideas — Azo compounds as dyes
The extended conjugation created by the azo group linking two aromatic rings allows the compound to absorb light in the visible region, giving azo compounds intense, characteristic colours; this is why they are widely manufactured as dyes and pigments. Different azo dyes are made by the same general coupling route, reacting a diazonium salt with a different aromatic amine or phenol to vary the colour and properties of the product.
🧑🏫 Teacher's explanation
Imagine a long, alternating chain of single and double bonds, like a series of connected trampolines. In a regular molecule, the electrons are stuck in small, localized loops. But in an azo compound, the nitrogen-nitrogen double bond (- extN= extN-) acts as a bridge that connects two large benzene rings. This creates a massive, continuous 'highway' for electrons to move across, a phenomenon called extended conjugation.
📖 Definitions
🔍 Worked example — Easy
Identify the functional group present in the molecule $ ext{C}_6 ext{H}_5- ext{N}= ext{N}- ext{C}_6 ext{H}_4 ext{OH}$ and state its general use.
🔍 Worked example — Standard
A diazonium salt reacts with phenol to form an azo dye. If the diazonium salt is benzene diazonium chloride and the phenol is phenol, write the balanced equation for the coupling reaction.
🔍 Worked example — Exam-style
Explain why an azo compound is colored while benzene is colorless.
⚠️ Common errors
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🎯 Exam question
(a) Identify the functional group that gives the resulting compound its intense color. [1] (b) Explain, in terms of electronic structure, why this compound is colored while the starting materials are colorless. [2] [3 marks]
Worked example
Question. Phenylamine and ethylamine are each shaken with excess dilute hydrochloric acid. (a) Explain, in terms of bonding, why ethylamine reacts more readily than phenylamine with the acid. [3] (b) Cold benzenediazonium chloride solution is then added to an alkaline solution of phenol. State what is observed and name the class of product formed. [2]
🔍 Worked example — Easy
Compare the basicity of ammonia (NH_3) and ethylamine (CH_3CH_2NH_2). Which is a stronger base and why?
🔍 Worked example — Standard
A sample of phenylamine (C_6H_5NH_2) is reacted with dilute HCl. Predict the change in the lone pair availability on the nitrogen atom.
🔍 Worked example — Exam-style
A solution of benzenediazonium chloride is added to a solution of phenol in an alkaline medium. State the observation and the type of compound formed.
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Practice
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