20.1.3 Electrophilic Substitution Reactions

• In normal, everyday conversation the word 'aromatic' is used to refer to pleasant, fragrant smells
• However, in chemistry, it is used to describe molecules that contain one or more benzene rings, i.e. a ring with conjugated π systems
  • Conjugated π systems arise from alternating double and single bonds in which the electrons are delocalised

• Benzene is found in many useful pharmaceuticals, pesticides, polymers and dyes
  • The common painkillers aspirin, paracetamol, ibuprofen and morphine all contain benzene rings

Examples of aromatic compounds including benzene table

Structure of Benzene

• The structure of benzene was determined many years ago, by the German chemist Friedrich August Kekulé
• The structure consists of 6 carbon atoms in a hexagonal ring, with alternating single and double carbon-carbon bonds
  • This suggests that benzene should react in the same way as an unsaturated alkene
  • However, this is not the case

Like other aromatic compounds, benzene has a planar structure due to the sp² hybridisation of carbon atoms and the conjugated π system in the ring

• Each carbon atom in the ring forms three σ bonds using the sp² orbitals
• The remaining p orbitals overlap laterally with p orbitals of neighbouring carbon atoms to form a π system
• This extensive sideways overlap of p orbitals results in the electrons being delocalised and able to freely spread over the entire ring causing a π system
  • The π system is made up of two ring shaped clouds of electron density - one above the plane and one below it

• Benzene and other aromatic compounds are regular and planar compounds with bond angles of 120 ^o
• The delocalisation of electrons, as shown below, means that all of the carbon-carbon bonds in these compounds are identical and have both single and double bond character
  • Single covalent bonds have a bond order of 1 and double covalent bonds have a bond order of 2
  • The covalent bonds within benzene have a bond order of 1.5

• The bonds all being the same length is evidence for the delocalised ring structure of benzene

The Delocalisation of Benzene Model

Reactions of Benzene

• The main reactions which benzene will undergo involve the replacement of one of the hydrogen atoms from the benzene ring
  • This is different to the reactions of unsaturated alkenes, which involve the double bond breaking and the electrophile atoms 'adding on' to the carbon atoms

• These reactions where benzene hydrogen atoms are replaced, are called electrophilic substitution reactions
  • The delocalised π system is extremely stable and is a region of high electron density
  • The hydrogen atom is substituted by an electrophile, which is either a positive ion or the positive end of a polar molecule

General Electrophilic Substitution Mechanism:

• There are numerous electrophiles which can react with benzene
  • However, they usually cannot simply be added to the reaction mixture to then react with benzene
  • The electrophile has to be produced in situ, by adding appropriate reagents to the reaction mixture

Nitration of Benzene

• You must be able to provide the mechanism for the nitration of benzene via electrophilic substitution
• The electrophilic substitution reaction in arenes consists of three steps:

  1. Generation of an electrophile
  2. Electrophilic attack
  3. Regenerating aromaticity

Nitration of Benzene Mechanism

• The nitration of benzene is an example of electrophilic substitution as a hydrogen atom is replaced by a nitro (-NO₂) group

The overall reaction of nitration of arenes

• Step 1: Generation of an electrophile
  • The electrophilic nitronium ion, NO₂⁺, is generated by reacting concentrated nitric acid, HNO₃, and concentrated sulfuric acid, H₂SO₄
  • The sulfuric acid is a catalyst

• Step 2: Electrophilic attack
  • Once the electrophile has been generated, it will carry out an electrophilic attack on the benzene ring
  • The nitrating mixture of HNO₃ and H₂SO₄ is refluxed with the arene at 25 - 60 ^oC

• Step 3: Regenerating aromaticity
  • The aromaticity is restored by the heterolytic cleavage of the C-H bond

• For the nitration of benzene, there is an extra step involving the regeneration of the sulfuric acid catalyst

The different stages in the nitration of benzene

Chlorination of Benzene Mechanism

• The chlorination, or halogenation, of benzene is another example of electrophilic substitution

The overall reaction of chlorination of arenes

• Step 1: Generation of an electrophile
  • The electrophilic chlorine cation, Cl⁺, is generated by reacting chlorine with anhydrous aluminium chloride, AlCl₃
    • The aluminium chloride is electron deficient and acts as a Lewis acid by accepting a lone pair from one of the chlorine atoms
    • As the aluminium forms a dative covalent bond with one of the chlorine atoms, the other chlorine atom becomes a chlorine cation, Cl⁺

• Step 2: Electrophilic attack
  • Once the electrophile has been generated, it will carry out an electrophilic attack on the benzene ring

• Step 3: Regenerating aromaticity
  • The aromaticity is, once again, restored by the heterolytic cleavage of the C-H bond

• For the chlorination of benzene, there is an extra step involving the regeneration of the aluminium chloride catalyst

The different stages in the chlorination of benzene