Reactions of Benzene
Lajoy Tucker & Dr. Davinder Bhachu
Teachers
Contents
Reactions of Benzene
Benzene reacts according to an electrophilic substitution mechanism
Benzene has a delocalised ring of π-electrons above and below the plane of carbon atoms.
This makes benzene particularly stable
Addition reactions would break this stability, so benzene favours substitution, which preserves the aromatic system.
The π-electrons attack electrophiles. Unlike alkenes, the electron density in benzene is not high enough to react with neutral electrophiles. Generation of a positively charged electrophile is therefore required.
Electrophilic substitution: General mechanism steps
Step 1 – Formation of the electrophile
Benzene reacts with strong electrophiles (positively charged or electron-deficient species),
The electrophile is often generated by a catalyst (e.g. ).
Step 2 – Attack on the ring
The π-electrons in benzene are attracted to the electrophile.
A new bond forms between one carbon atom and the electrophile.
This breaks the delocalised system temporarily, giving a positively charged arenium ion (carbocation intermediate).
Step 3 – Restoration of aromaticity
A hydrogen atom is lost from the same carbon.
The electrons from the C–H bond return to the ring, reforming the stable aromatic system.
The catalyst is regenerated if used.
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Reaction of Benzene Explained
Nitration
Reaction Overview
This is done using a mixture of concentrated nitric acid and concentrated sulfuric acid at a temperature of around .
1. Generation of the Electrophile
Concentrated sulfuric acid acts as a catalyst, initially reacting with the nitric acid to generate the nitronium ion :
The ion is the electrophile, which attacks the benzene ring.
2. Mechanism: Electrophilic Substitution
The π-electrons of the benzene ring are attracted to and attack the NO₂⁺ electrophile
A sigma complex (arenium ion) forms with a delocalised positive charge.
A proton is lost from the intermediate.
The aromaticity is restored, and nitrobenzene is formed
3. Regeneration of catalyst
Further reactions:
This is an important first step in the synthesis of aromatic amines. Nitrobenzene is converted to phenylamine through reduction with Sn/HCl
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Friedel-Crafts acylation of Benzene
Friedel–Crafts Acylation is a type of electrophilic substitution reaction where an acyl group (RCO-) is introduced onto an aromatic ring
It is usually carried out in the presence of a catalyst such as aluminium chloride
This process is commonly used to synthesise aromatic ketones
Reaction overview:
1. Generation of the electrophile -
To carry out this reaction, we need an electrophile (RCO⁺) to attack the benzene ring.
The electrophile is created using an acyl chloride and a halogen carrier, aluminium chloride:
The electrophile is a carbocation
2. Mechanism - Electrophilic Substitution
The π-electrons of the benzene ring are attracted to and attack the RCO⁺ electrophile
A sigma complex (arenium ion) forms with a delocalised positive charge.
A proton is lost from the intermediate.
The aromaticity is restored, and the aromatic ketone is formed.
3. Regeneration of catalyst
Further reactions:
The product can then go on to react as a ketone in nucleophilic addition reactions such as addition of hydrogen cyanide and reduction to a secondary alcohol using .
Question 1:
Write the equation for the Friedel–Crafts acylation of benzene using ethanoyl chloride as the acylating agent.
Answer:
Question 2:
Name and outline the mechanism for the reaction of benzene with propanoyl choride. Include an equation for the generation of the electrophile from propanoyl chloride and an appropriate catalyst.
Answer: 
