AQA A Level Chemistry

Revision Notes

7.4.1 Structure of Benzene

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 compounds, for example in 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 a chemist called Kekule
  • 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 that an unsaturated alkene does
    • However, this is not the case

Organic Chemistry - Shape and 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 means that all of the carbon-carbon bonds in these compounds are identical and have both single and double bond character
  • The bonds all being the same length is evidence for the delocalised ring structure of benzene

The Delocalisation Model

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

 

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!

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