Benzene - Structure & Stability
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
- 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 ΔHꝋ = -120 kJ mol-1
- Hydrogenation of benzene
- The Kekulé 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 ΔHꝋ = 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, ΔHꝋ = -208 kJ mol-1
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
- 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 at around 1650 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 around 1650 cm-1 for the double C=C bonds
- Benzene does not show a peak at around 1650 cm-1 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