6.1.2 Electrophilic Substitution

Reactions of Benzene

• The main reactions which benzene will undergo include the replacement of one of the 6 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 at least one of the H atoms from benzene are replaced, are called 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

General Electrophilic Substitution Mechanism:

• The delocalised π system is extremely stable and is a region of high electron density
• Electrophilic substitution reactions involve an electrophile, which is either a positive ion or the positive end of a polar molecule
• 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

• The electrophilic substitution reaction in arenes consists of three steps:
  • Generation of an electrophile
  • Electrophilic attack
  • Regenerating aromaticity

Mechanism of electrophilic substitution

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

The overall reaction of nitration of arenes

• In the first step, the electrophile is generated
  • The electrophile NO₂⁺ ion is generated by reacting concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄)

• 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

Nitration of Benzene Mechanism:

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
• 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

Halogenation

• The nature of benzene is different to other unsaturated compounds such as alkenes and halogenation via electrophilic addition is not possible
• Therefore aromatic compounds will react with halogens in the presence of a metal halide carrier
  • iron(III) bromide
  • aluminium chloride 
• The reaction of the metal halide carrier acts as a catalyst and creates the electrophile, X⁺ (where X represents a halogen atom)
• At the end of the reaction, it is regenerated

AlCl₃ + Cl₂ → AlCl₄^- + Cl⁺

FeBr₃ + Br₂ → FeBr₄^- + Br⁺

• The overall equation for halogenation is

C₆H₆ + X₂ → C₆H₅X + HX

• Remember that one hydrogen atom on the benzene ring has been substituted for one halogen atom, therefore HX will be a product
• The electrophilic substitution reactions follow the same pattern as the general mechanism 

The different stages in the chlorination of benzene

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

• A metal halide catalyst is needed to generate the necessary alkyl electrophile
• The benzene ring is reacted with an acyl chloride in the presence of an AlCl₃ catalyst
• This complex then reacts with the benzene ring in a similar manner as we have seen before
• 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

Example of a Friedel-Crafts acylation reaction