Electrophilic Substitution (Oxford AQA International A Level Chemistry)

Electrophilic Substitution Mechanism

• Benzene undergoes electrophilic substitution reactions

  • The hydrogen atom is substituted by the electrophile

• You must be able to provide the mechanisms for specific examples of the electrophilic substitution of benzene including:

  • Nitration

  • Sulfonation

  • Friedel–Crafts Reactions

• The electrophilic substitution reaction in arenes consists of three steps:

  • Generation of an electrophile

  • Electrophilic attack

  • Regenerating aromaticity

General Electrophilic Substitution Mechanism:

Nitration

• The nitration of benzene is one example of an electrophilic substitution reaction

  • A hydrogen atom is replaced by a nitro (-NO₂) group

• This is an important step in the:

  • Manufacture of explosives, like trinitrotoluene (TNT)

  • Formation of amines, which, in turn, are used to make dyes and pharmaceuticals

Nitration of benzene

• In the first step, the NO₂⁺ electrophile is generated

  • This is done by reacting concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄)

• The nitrating mixture of HNO₃ and H₂SO₄ is refluxed with the arene at 25 - 60 ^oC

• Once the electrophile has been generated, it will carry out an electrophilic attack on the benzene ring

• The intermediate formed loses a proton to regenerate its aromaticity

  • The proton is then used to reform the H₂SO₄ catalyst

Nitration of Benzene Mechanism:

Sulfonation

• Another example of a substitution reaction is the sulfonation of arenes

• In these reactions, a sulfonyl group (-SO₃H) replaces a hydrogen atom on the arene

Sulfonation of benzene

• In the first step, an SO₃ electrophile is generated by:

H₂SO₄ ⇌ SO₃ + H₂O

• The benzene is warmed with a fuming sulfuric acid at a temperature of 40 ^oC for around 30 minutes

  • Fuming sulfuric acid is made by dissolving sulfur trioxide in concentrated sulfuric acid

• Due to the electronegativity of the oxygen atoms, the SO₃ is polarised

• Once the electrophile has been generated, it will carry out an electrophilic attack on the benzene ring

• The intermediate formed loses a proton to regenerate its aromaticity

The mechanism of sulfonation

Friedel–Crafts Reactions

• Friedel-Crafts reactions are also electrophilic substitution reactions

• Due to the aromatic stabilisation in arenes, they are often unreactive

• To use arenes as starting materials for the synthesis of other organic compounds, their structure, therefore, needs to be changed to turn them into more reactive compounds

• Friedel-Crafts reactions can be used to substitute a hydrogen atom in the benzene ring for an alkyl group (Friedel-Crafts alkylation) or an acyl group (Friedel-Crafts acylation)

• Like any other electrophilic substitution reaction, the Friedel-Crafts reactions consist of three steps:

  • Generating the electrophile

  • Electrophilic attack on the benzene ring

  • Regenerating aromaticity of the benzene ring

Alkylation and Acylation Reactions

Friedel-Crafts Acylation

• In the Friedel-Crafts acylation reaction, an acyl group is substituted into the benzene ring

  • An acyl group is an alkyl group containing a carbonyl, C=O group

• The benzene ring is reacted with an acyl chloride in the presence of an AlCl₃ catalyst

• An example of an acylation reaction is the reaction of methylbenzene with propanoyl chloride to form an acyl benzene

  • Note that the acyl group is on the 4 position due to the -CH₃ group on the benzene

The mechanism of Friedel-Crafts acylation