Structure of Benzene

Structure of BenzeneKekule's modelBenzene has a molecular formula \(C_6H_6\), Kekule's model fitted with the molecular structure. It consists of a ring of 6 carbon atoms with alternating double and single bonds which existed in equilibrium in two states. However, it failed to explain a number of chemical and physical properties of benzene.Delocalised modelDue to weaknesses in Kekule's structure the delocalised model was developed, this suggests benzene is a cyclic hydrocarbon with each carbon bonded to two other carbon atoms and one hydrogen atom (the same as Kekule's). The electron in the p-orbital of each carbon overlaps with that in the p-orbital of the carbons both above and below the plane of the ring. This forms a ring of high electron density and a system in which the p-orbital electrons are no longer shared between just two atoms but spread over the whole ring. EvidenceKekule's model was changed because of three main pieces of evidence:Bond legnths - His model was presented as symmetrical however C-C bonds and C=C bonds have different lengths so in reality it wouldn't be. X-ray studies show that benzene is symmetrical, ie. all the bonds should be the same length (somewhere between the C-C and C=C bond length) and all bond angles are the same. This suggested the alternating double and single bond idea was incorrect.Enthalpy change of hydrogenation - When an alkene reacts with hydrogen to become a saturated hydrocarbon it becomes more stable - energy is released. This energy change is called the enthalpy change of hydrogenation. One C=C has /\H = -120 kJmol-1 so as Kekule's structure had 3 C=C bonds the /\H should be -360kJmol-1 (3x120). However the actual value for the hydrogenation of benzene is -208kJmol-1, which doesn't support the model.Resistance of reaction - If C=C bonds were present then benzene would be expected to react in a similar way to alkenes, for example by decolourising bromine water. However benzene doesn't take part in many of the expected electrophilic additions - suggesting it did not contain double bonds.

Arene or Aromatic Hydrocarbon - A hydrocarbon containing at least one benzene ring.

Delocalised electrons - Electrons which are shared between more than two atoms.

Structure of Benzene