chemistry module 6 benzene and aromatic compounds

what are the 2 models of benzene

Kekule, delocalised

describe the kekule model

a hexagonal ring of six carbon atoms with alternating single and double C-C bonds

It suggests that the π-electrons in benzene are localised within these three double bonds

describe the delocalised model of benzene and draw a diagram

It proposes that the p-orbitals of all six carbon atoms overlap sideways, both above and below the plane of the ring

This overlap forms a continuous ring of electron density, creating a delocalised π-system where the six π-electrons are spread over the entire ring

how do bond lengths support the delocalised model over the kekule model

x-ray diffraction analysis shows that the bond lengths of all 6 carbon-carbon bonds are equal. whereas the kekule model would have alternating short double bonds and long single bonds

how do molecular shape & bond angles support the delocalised model over the kekule model

x-ray diffraction shows that benzene is a perfectly planar hexagon. the perfect hexagon only exists if all 6 bonds are equal in length

how does enthalpy of hydrogenation support the delocalised model over the kekule model

The hydrogenation of cyclohexene (one C=C bond) has an enthalpy change of -120 kJ mol-1

The Kekulé structure (with three C=C bonds) would therefore be expected to have an enthalpy of hydrogenation of -360 kJ mol-1

However, the experimental value for benzene is only -208 kJ mol-1

This means benzene is significantly more stable (less exothermic) than the Kekulé structure suggests, due to the energy of the delocalised π-system.

halogenation in benzene

In benzene, the electron density of the six π-electrons is delocalised and spread out over the entire ring

This means the electron density at any one point is lower than in an alkene's C=C bond

So, it is not strong enough to polarise the Br2 molecule

Therefore, benzene does not react with bromine under normal conditions

Benzene resists addition reactions as this would disrupt the very stable delocalised ring

Instead, it undergoes substitution reactions

This requires a halogen carrier catalyst, such as AlBr3