The structure of benzene 6-7

Question 1

Explain how Kekule’s equilibrium model of benzene accounted for the fact that Kekule’s proposed structure did not react in the same way as alkenes ( for example, decolourising bromine water. ).

Answer 1

Kekule suggested that the two forms of benzene were in such a rapid equilibrium that an approaching bromine molecule could not be attracted to a double bond before the structure changed. Hence, bromine could not react with the double bonds.

Question 2

Why did the x-ray studies of the carbon-carbon bond lengths in benzene suggest that the Kekule structure of benzene was incorrect?

Answer 2

X-ray studies revealed that all six of the carbon-carbon bond lengths in benzene are the same length: 0.139nm. This is between the C-C bond length of 0.153nm and the C=C bond length of 0.134nm. This was evidence that suggested Kekule was incorrect.

Question 3

How did the hydrogenation of benzene allow scientists to discover that the structure of benzene is more stable than a structure containing C=C bonds?

Answer 3

• When an alkene reacts with hydrogen the energy change is called the enthalpy change of hydrogenation. When cyclohexene, with one C=C bond, reacts with hydrogen, the enthalpy change of hydrogenation is -120 KJ mol^-1. Kekule’s benzene, with three C=C bonds, would be expected to have an enthalpy change of hydrogenation of -360KJ mol^-1, three times that of cyclohexane.
• When benzene is hydrogenated, the experimental value for the enthalpy change is -208 KJ mol^-1. This enthalpy change is -152 KJ mol^-1 less than the expected enthalpy change of hydrogenation of cyclohexa-1,3,5-triene.
• The most important conclusion is that the actual structure of benzene has much less energy than the proposed Kekule structure. The real structure of benzene is 152 KJ mol^-1 more stable than the Kekule structure. This energy is known as the delocalisation energy, or resonance energy, of benzene.

This evidence sugests that the real structure of benzene is more stable than a structure containing C=C bonds. It helps explain why benzene is less reactibe than alkenes.