5.4.1 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   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:

 

• • 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

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

• 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