What are the reagents and catalyst for bromination of benzene?

Br₂ with anhydrous FeBr₃ (or AlBr₃).

How is the electrophile formed in bromination?

Br₂ + FeBr₃ → Br⁺ + FeBr₄⁻.

How is the catalyst regenerated in bromination?

H⁺ + FeBr₄⁻ → HBr + FeBr₃.

What are the reagents and conditions for nitration?

Concentrated HNO₃ and concentrated H₂SO₄ at about 50 °C.

What is the electrophile in nitration and how is it formed?

NO₂⁺: HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻.

Why is the nitration temperature kept at or below 55 °C?

To prevent further substitution forming dinitrobenzene.

What is the electrophile in sulfonation?

SO₃, from fuming sulfuric acid.

What is the equation for sulfonation of benzene?

C₆H₆ + SO₃ → C₆H₅SO₃H.

Why does benzene burn with a smoky flame?

It has a high proportion of carbon, so combustion in air is incomplete and soot forms.

What catalyst is used in Friedel–Crafts reactions?

Anhydrous aluminium chloride, AlCl₃.

Why must AlCl₃ be anhydrous?

It reacts with water, so it would no longer act as a catalyst.

Electrophilic substitution of benzeneEdexcel International 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 ring.
What type of species attacks benzene?: Electrophiles.
What are the reagents and catalyst for bromination of benzene?: Br₂ with anhydrous FeBr₃ (or AlBr₃).
How is the electrophile formed in bromination?: Br₂ + FeBr₃ → Br⁺ + FeBr₄⁻.
How is the catalyst regenerated in bromination?: H⁺ + FeBr₄⁻ → HBr + FeBr₃.
What are the reagents and conditions for nitration?: Concentrated HNO₃ and concentrated H₂SO₄ at about 50 °C.
What is the electrophile in nitration and how is it formed?: NO₂⁺: HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻.
Why is the nitration temperature kept at or below 55 °C?: To prevent further substitution forming dinitrobenzene.
What is the electrophile in sulfonation?: SO₃, from fuming sulfuric acid.
What is the equation for sulfonation of benzene?: C₆H₆ + SO₃ → C₆H₅SO₃H.
Why does benzene burn with a smoky flame?: It has a high proportion of carbon, so combustion in air is incomplete and soot forms.
What catalyst is used in Friedel–Crafts reactions?: Anhydrous aluminium chloride, AlCl₃.
What is the electrophile in the reaction of benzene with CH₃COCl and AlCl₃?: CH₃CO⁺.
Why must AlCl₃ be anhydrous?: It reacts with water, so it would no longer act as a catalyst.

Exam questions on Electrophilic substitution of benzene

Benzene is warmed with a mixture of concentrated nitric acid and concentrated sulfuric acid at 50 °C to make nitrobenzene, C₆H₅NO₂.
Write an equation for the formation of the electrophile in this reaction and explain why the temperature is kept at or below 55 °C.2 marks
Benzene reacts with bromine to form bromobenzene, C₆H₅Br, in the presence of anhydrous iron(III) bromide, FeBr₃, as a catalyst.
Write the overall equation for the reaction, and an equation to show that the catalyst is regenerated.2 marks
Benzene is heated under reflux with ethanoyl chloride, CH₃COCl, in the presence of anhydrous aluminium chloride to make phenylethanone, C₆H₅COCH₃.
Give an equation for the formation of the electrophile, the overall equation for the reaction, and one reason why the aluminium chloride must be anhydrous.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).