OCR A Level Chemistry

Revision Notes

6.1.1 Benzene

Test Yourself

Comparing Models of Benzene

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 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   ΔHΘ = -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   ΔHΘ = 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, ΔHΘ = -208 kJ mol-1

Nomenclature of Aromatic Compounds

  • 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

Organic Chemistry - Nomenclature of Functional Groups Aromatic Compounds, downloadable AS & A Level Chemistry revision notes

Benzene Resistance to Halogenation

  • 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

You've read 0 of your 0 free revision notes

Get unlimited access

to absolutely everything:

  • Downloadable PDFs
  • Unlimited Revision Notes
  • Topic Questions
  • Past Papers
  • Model Answers
  • Videos (Maths and Science)

Join the 80,663 Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Philippa

Author: Philippa

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.