Structure of Benzene (AQA A Level Chemistry): Revision Note

Structure of Benzene

Aromatic Compounds

• In everyday language, the word aromatic is used to describe pleasant or fragrant smells

• In chemistry, however, aromatic refers to molecules that contain one or more benzene rings, meaning rings with conjugated π systems

• Conjugated π systems arise from alternating single and double bonds, in which the π electrons are delocalised over the ring

• Benzene is found in many useful compounds, including pharmaceuticals, pesticides, polymers, and dyes

  • Common painkillers such as aspirin, paracetamol, ibuprofen, and morphine all contain benzene rings

• Examples of common aromatic compounds include:

The Kekulé structure of benzene

• The structure of benzene was proposed in 1865 by the Belgian chemist August Kekulé

• Benzene consists of six carbon atoms arranged in a hexagonal ring, with alternating single and double carbon–carbon bonds.

  • This structure suggests that benzene should react like an unsaturated alkene

The Delocalisation Model

• However, this is not the case, as 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

  

• Each carbon atom in the ring forms three σ bonds

• The remaining p orbitals overlap laterally with the 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 spread over the entire ring

  • The π system is made up of two ring-shaped clouds of electron density - one above the plane and one below it

• The modern representation of benzene is a hexagon with a circle inside

  • The circle represents the delocalised ring system

Electrophilic Substitution

Reactions of Benzene

• The main reactions of benzene involve the replacement of one of the six hydrogen atoms on the benzene ring

• This is different from the reactions of unsaturated alkenes, which involve breaking the C=C double bond and adding electrophile atoms across it

• Reactions in which one or more hydrogen atoms in benzene are replaced are called electrophilic substitution reactions

  • In these reactions, a hydrogen atom is substituted by an electrophile

General Electrophilic Substitution Mechanism:

• The delocalised π system in benzene is extremely stable and forms a region of high electron density

• Electrophilic substitution reactions involve an electrophile, which may be a positively charged ion or the positive end of a polar molecule

  • Although many electrophiles are capable of reacting with benzene, they usually cannot be added directly to the reaction mixture

• Instead, the electrophile must be generated in situ by adding appropriate reagents to the reaction mixture