Bonding (Oxford AQA International A Level Chemistry)

The 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

• 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

  • So, they have both single and double bond character

• The bonds all being the same length is evidence for the delocalised ring structure of benzene

The bonding in 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    ΔH^ꝋ= -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    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

Why does benzene undergo electrophilic substitution reactions?

• The delocalisation of electrons (also called aromatic stabilisation) in arenes is the main reason why arenes predominantly undergo substitution reactions over addition reactions

  • In substitution reactions, the aromaticity is restored

  • In addition reactions, on the other hand, the aromaticity is not restored and is in some cases completely lost

Nomenclature of Aromatic Compounds

• The method used to name aromatic compounds is similar to that of aliphatic compounds

• Students are required to use systematic nomenclature of simple aromatic molecules with one benzene ring and one or more simple substituents

Nomenclature of simple aromatic organic molecules with functional groups table

Functional group	Example	Name
Arene		Propyl benzene
Chlorobenzene		2-methylchlorobenzene
Phenol		2,3-dimethyl phenol