Benzene - Structure & Stability (Edexcel International A Level Chemistry)

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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

Structure of benzene, downloadable AS & A Level Chemistry revision notes

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 = -120 kJ mol-1

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

Benzene 2 

    • 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 = 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 = -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

bond lengths in benzene, downloadable AS & A Level Chemistry revision notes

  • 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

Bromination Resistance

  • 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

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Richard

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Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.