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, Benzene
Benzene, C₆H₆, is a colourless, sweet smelling,
flammable liquid found naturally in crude oil. It is
classified as a carcinogen (can cause cancer)& is
classed as an aromatic arene.
The Kekulé model
For many years scientists attempted to establish a
structure for benzene. The German chemist Kekulé
suggested that the structure of benzene was based
on a 6 membered ring of carbon atoms joined by
alternate single & double bonds. The Kekulé structure
could therefore be given the name cyclohexa-1,3,5-
triene.
, Evidence to disprove Kekulé’s model
• If benzene contained the C=C bonds, it should decolourise bromine in an
electrophilic addition reaction. However benzene does not undergo
electrophilic addition reactions or decolourise bromine under normal
conditions.
• Using a technique called x-ray diffraction, it is possible to measure bond
lengths in a molecule. It was found that all bonds in benzene were
0.139nm in length. This bond length was between the length of a single
bond, 0.153nmm, & a double bond 0.134nm.
• If benzene did have the Kekulé structure it would be expected to have an
enthalpy change of hydrogenation that is 3X of that of cyclohexene.
When cyclohexene is hydrogenated one double bond reacts with
hydrogen. The enthalpy change of hydrogenation is -120 KJmolˉ¹. as the
Kekulé structure is predicted to contain 3 double bonds the expected
enthalpy change for reacting 3 double bonds with hydrogen would be 3 x -
120= -360 KJmolˉ¹
The actual enthalpy change of hydrogenation of benzene is only
-208KJmolˉ¹, meaning that 152KJmolˉ¹ less energy is produced than
expected. The actual structure of benzene is therefore more stable
than the theoretical Kekulé's model of benzene.
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