7.4.1 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

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

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

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