Electrophilic Substitution of Arenes (CIE A Level Chemistry)

• 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

Reaction type	Electrophile*
halogenation	X+, e.g. Cl+
nitration	NO2+
Friedel-Craft's alkylation	R+
Friedel-Craft's acylation	R-C=O+

 

• 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 AlBr₃ halogen carrier catalyst is used and during chlorination an AlCl₃ halogen 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