Why does benzene burn with a smoky flame?

High carbon to hydrogen ratio, so incomplete combustion gives carbon particles that glow.

Write the equation forming NO₂⁺ from the nitrating mixture.

HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻

Why is the nitration temperature kept at about 50 °C?

Higher temperatures give further substitution and dinitrobenzene.

What catalyst is used for bromination of benzene and what does it do?

FeBr₃ (or AlBr₃): a halogen carrier that accepts a lone pair from Br₂ to generate the electrophile.

Write the equation forming the electrophile in the bromination of benzene.

Br₂ + FeBr₃ → Br⁺ + FeBr₄⁻

What are the reagents for Friedel-Crafts acylation of benzene?

An acyl chloride (e.g. CH₃COCl) with anhydrous AlCl₃.

Write the equation forming the electrophile from CH₃Cl and AlCl₃.

CH₃Cl + AlCl₃ → CH₃⁺ + AlCl₄⁻

What happens to the catalyst at the end of a Friedel-Crafts reaction?

It is regenerated: AlCl₄⁻ + H⁺ → AlCl₃ + HCl.

What is seen when phenol is added to bromine water?

The bromine water is decolourised and a white precipitate of 2,4,6-tribromophenol forms.

Why is phenol brominated more easily than benzene?

An oxygen lone pair delocalises into the ring, increasing its electron density so it polarises Br₂ more strongly.

Electrophilic substitution of benzeneEdexcel A-Level Chemistry: Flashcards

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Why does benzene undergo substitution rather than addition?

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Why does benzene undergo substitution rather than addition?: Addition would destroy the stable delocalised π system; substitution keeps it.
Why does benzene burn with a smoky flame?: High carbon to hydrogen ratio, so incomplete combustion gives carbon particles that glow.
What is the electrophile in the nitration of benzene?: The nitronium ion, NO₂⁺.
Write the equation forming NO₂⁺ from the nitrating mixture.: HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻
Why is the nitration temperature kept at about 50 °C?: Higher temperatures give further substitution and dinitrobenzene.
What catalyst is used for bromination of benzene and what does it do?: FeBr₃ (or AlBr₃): a halogen carrier that accepts a lone pair from Br₂ to generate the electrophile.
Write the equation forming the electrophile in the bromination of benzene.: Br₂ + FeBr₃ → Br⁺ + FeBr₄⁻
What are the reagents for Friedel-Crafts acylation of benzene?: An acyl chloride (e.g. CH₃COCl) with anhydrous AlCl₃.
Write the equation forming the electrophile from CH₃Cl and AlCl₃.: CH₃Cl + AlCl₃ → CH₃⁺ + AlCl₄⁻
What happens to the catalyst at the end of a Friedel-Crafts reaction?: It is regenerated: AlCl₄⁻ + H⁺ → AlCl₃ + HCl.
What is seen when phenol is added to bromine water?: The bromine water is decolourised and a white precipitate of 2,4,6-tribromophenol forms.
Why is phenol brominated more easily than benzene?: An oxygen lone pair delocalises into the ring, increasing its electron density so it polarises Br₂ more strongly.

Exam questions on Electrophilic substitution of benzene

Nitrobenzene is manufactured by reacting benzene with a mixture of concentrated nitric acid and concentrated sulfuric acid. The reaction is an electrophilic substitution and the reaction mixture is kept at about 50 °C.
Write an equation for the formation of the electrophile in this reaction, and explain why the temperature is kept at about 50 °C rather than being allowed to rise much higher.2 marks
Phenol, C₆H₅OH, decolourises aqueous bromine at room temperature and forms a precipitate, whereas benzene does not react with aqueous bromine. In an experiment, 0.940 g of phenol was added to an excess of bromine water. Relative atomic masses: H = 1.0, C = 12.0, O = 16.0, Br = 79.9.
Calculate the maximum mass of 2,4,6-tribromophenol, C₆H₂Br₃OH, formed in this experiment. Give your answer to three significant figures.2 marks
Phenylethanone, C₆H₅COCH₃, is made by reacting benzene with ethanoyl chloride, CH₃COCl, in the presence of anhydrous aluminium chloride, AlCl₃. This is a Friedel-Crafts reaction.
Write an equation for the formation of the electrophile in this reaction. Explain the role of aluminium chloride and why it is classed as a catalyst.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).