Benzene (HL) (HL IB Chemistry)

Structure of Benzene

• The structure of benzene was determined many years ago, by a chemist called 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 that an unsaturated alkene does
  • However, this is not the case

The structure of benzene

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
  • Carbon-carbon bond lengths from X-ray diffraction
  • Saturation tests
  • Infrared spectroscopy

Enthalpy changes of hydrogenation

• 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^ꝋ = -120 kJ mol^-1

• Hydrogenation of benzene
  • The Kekulé structure of benzene as cyclohexa-1,3,5-triene has three double C=C bonds:

Structural formula of benzene

The structural formula of benzene shows the alternating single and double 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^ꝋ = 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^ꝋ = -208 kJ mol^-1 
  • This means that 152 kJ mol^-1 less energy is produced than expected
  • Therefore, the actual structure of benzene is more stable than the theoretical Kekulé model

Carbon-carbon bond lengths

• Cyclohexene contains two different carbon-carbon bonds 
  • The single carbon-carbon bond (C-C) has a bond length of 154 pm
  • The double carbon-carbon bond (C=C) has a bond length of 134 pm
• The Kekulé structure of benzene as cyclohexa-1,3,5-triene has three single C-C and three double C=C bonds
  • It would be expected that benzene would have an equal mixture of bonds with lengths of 134pm and 154 pm

Comparing carbon-carbon bond lengths

The bond lengths observed in benzene are intermediate of single and double carbon-carbon bond lengths

• All of the carbon-carbon bond lengths are 140 pm suggesting that they are all the same and also intermediate of the single C-C and double C=C bonds

Saturation tests

• Cyclohexene will decolourise bromine water as an electrophilic addition reaction takes place
• The Kekulé structure of benzene as cyclohexa-1,3,5-triene has three double C=C bonds
  • It would, therefore, be expected that benzene would easily decolourise bromine water
• Benzene does not decolourise bromine water suggesting that there are no double C=C bonds

Infrared spectroscopy

• Cyclohexene shows a peak in the range of 1620 - 1680 cm^-1 for the double C=C bond within its structure
• The Kekulé structure of benzene as cyclohexa-1,3,5-triene has three double C=C bonds
  • It would, therefore, be expected to also show a peak at 1620 - 1680 cm^-1 for the double C=C bonds
• Benzene does not show a peak in this range for the double C=C bonds, instead, peaks are seen at around 1450, 1500 and 1580 cm^-1 which are characteristic of double C=C bonds in arenes