Benzene (OCR A Level Chemistry)

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

Evidence for delocalisation

• This evidence of the bonding in benzene is provided by data from enthalpy changes of hydrogenation and carbon-carbon bond lengths
• Hydrogenation of cyclohexene
  • Each molecule has one C=C double bond
  • The enthalpy change for the reaction of cyclohexene is -120 kJ mol^-1

C₆H₁₀ + H₂ → C₆H₁₂   ΔH^Θ = -120 kJ mol^-1

• Hydrogenation of benzene
  • The Kekule structure of benzene as cyclohexa-1,3,5-triene has three double C=C bonds
  • It would be expected that the enthalpy change for the hydrogenation of this structure would be three times the enthalpy change for the one C=C bond in cyclohexene 

C₆H₆ + 3H₂ → C₆H₁₂   ΔH^Θ = 3 x -120 kJ mol^-1 = -360 kJ mol^-1

• When benzene is reacted with hydrogen, the enthalpy change obtained is actually far less exothermic, ΔH^Θ = -208 kJ mol^-1

• 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

• Alkenes tend to undergo bromination easily which can be observed in cyclohexene

C₆H₁₀ + Br₂ → C₆H₁₀Br₂ 

• As the π bond contains localised electrons, it produces an area of high electron density allowing it to repel the electron in the bromine molecule
• Therefore a dipole is introduced making one bromine atom δ+ and one δ- bromine atom 
• The δ+ bromine is attracted to the π bond in the cyclohexene 
• This then leaves a carbocation in the intermediate molecule which the negative bromide ion is attracted to, hence forming 1,2-dibromocyclohexane by electrophilic addition
• In benzene, there are no localised areas of high electron density, preventing it from being able to polarise the bromine moelcule 
• In order for benzene to undergo electrophilic substitution with bromine, a halogen carrier must be present in the reaction e.g. AlBr₃