Electrophilic Substitution of Arenes (Cambridge International A-Level Chemistry)

Spec point — 30.1.4

Electrophilic Substitution of Arenes

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

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

Generation of an electrophile

• The delocalised π system is extremely stable and is a region of high electron density

• Consequently, the first step of an electrophilic substitution reaction involves the generation of an electrophile

  • The electrophile can be a positive ion or the positive end of a polar molecule

• There are numerous electrophiles which can react with benzene:

Table of electrophiles commonly used with benzene

• Typically electrophiles cannot simply be added to the reaction mixture

  • The electrophile is produced in situ, by adding appropriate reagents* to the reaction mixture

Electrophilic attack

• A pair of electrons from the benzene ring is donated to the electrophile to form a covalent bond
• This disrupts the aromaticity in the ring as there are now only four π electrons and there is a positive charge spread over the five carbon atoms

Regenerating aromaticity

• In the final step of electrophilic substitution, the aromaticity of the benzene ring system is restored

• This happens by heterolytic cleavage of the C-H bond

  • This means that the electrons in this bond go into the benzene π bonding system

Electrophilic substitution mechanism

• The halogenation and nitration of arenes are both examples of electrophilic substitution reactions

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

Bromination and nitration of benzene

During bromination, a hydrogen atom is substituted by a bromine atom and during nitration, a hydrogen atom is substituted by a nitro group

Step 1: Generating the Br⁺ and NO₂⁺ electrophiles

• For the halogenation reaction:

  • This is achieved by reacting the halogen with a halogen carrier
  • The halogen molecules form a dative bond with the halogen carrier by donating a lone pair of electrons from one of its halogen atoms into an empty 3p orbital of the halogen carrier

Step 1 of the halogenation of arenes

During bromination, an AlBr₃halogen carrier catalyst is used and during chlorination an AlCl₃halogen carrier catalyst is used

• For the nitration reaction:

  • The electrophile NO₂⁺ion is generated by reacting it with concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄)

Step 1 of the nitration of arenes

During nitration, concentrated nitric acid and concentrated sulfuric acid react to form the NO₂⁺electrophile

Step 2: Electrophilic attack by the Br⁺ and NO₂⁺ electrophiles

• 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

• A pair of electrons from the benzene ring is donated to the electrophile to form a covalent bond

  • This disrupts the aromaticity in the ring as there are now only four π electrons and there is a positive charge spread over the five carbon atoms

Step 2 of the halogenation of arenes

A pair of electrons from the benzene ring is donated to the Br⁺electrophile to form a covalent bond causing a loss in aromaticity

Step 2 of the nitration of arenes

A pair of electrons from the benzene ring is donated to the NO₂⁺electrophile to form a covalent bond causing a loss in aromaticity

Step 3: Regenerating / restoring aromaticity

• In the final step of the reaction, this aromaticity is restored by heterolytic cleavage of the C-H bond

  • This means that the bonding pair of electrons goes into the benzene π bonding system

Step 3 of the halogenation of arenes

Step 3 of the nitration of arenes

In both reactions, the C–H bond of the substituted carbon atom breaks and the electrons go back into the benzene π bonding system, restoring aromaticity

Addition reactions of arenes

• The delocalisation of electrons(also called aromatic stabilisation) in arenes is the main reason why arenes predominantly undergo substitution reactions over addition reactions

• In substitution reactions, the aromaticity is restored by heterolytic cleavage of the C-H bond

• In addition reactions, on the other hand, the aromaticity is not restored and is in some cases completely lost

  • The hydrogenation of arenes is an example of an addition reaction during which the aromatic stabilisation of the arene is completely lost
  • The cyclohexane formed is energetically less stable than the benzene