IB Chemistry HL

Revision Notes

20.1.3 Electrophilic Substitution Reactions

The Structure of Benzene

  • 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

Organic Chemistry - Nomenclature of Functional Groups Aromatic Compounds, downloadable AS & A Level Chemistry revision notes

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

Structure of benzene, downloadable AS & A Level Chemistry revision notes

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

bond lengths in benzene, downloadable AS & A Level Chemistry revision notes

The Delocalisation of Benzene Model

Electrophilic Substitution Mechanism

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:

General electrophilic substitution mechanism 1, downloadable AS & A Level Chemistry revision notesGeneral electrophilic substitution mechanism 2, downloadable AS & A Level Chemistry revision notes

Exam Tip

Make sure you understand the general steps of the electrophilic substitution mechanism and that you can explain what is happening – the same steps happen every time, the only difference is the electrophile used in the reaction!

  • 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 mechanism of electrophilic substitution

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

Hydrocarbons - Overall Nitration, downloadable AS & A Level Chemistry revision notes

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

Nitration of benzene mechanism, downloadable AS & A Level Chemistry revision notes

Nitration of Benzene

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