17.4: Structure of Benzene: Kekulé Model
In 1865, August Kekule suggested the structure of benzene according to the structural theory of organic chemistry based on the three assertions—formula of benzene is C6H6, all the hydrogens of benzene are equivalent, and each carbon must have four bonds due to its tetravalency.
He proposed that benzene has a cyclic structure of six carbon atoms attached to one hydrogen atom each, with three alternating pi bonds.
According to his model, there must be two isomers for the 1,2-disubstituted benzene derivatives, as shown in Figure 2, wherein in one structure, the two substituted carbons are connected by a double bond but by a single bond in another.
This notion of Kekule was proven wrong as only one structure is found for 1,2-disubstituted benzene. To account for this, he stated that the two forms of benzene are in rapid equilibrium and that it is not possible to separate the two compounds.
However, no such equilibrium exists, and only one structure for 1,2-disubstituted benzene derivatives is found.
Benzene has four degrees of unsaturation, constituting one ring and three pi bonds. However, benzene does not undergo addition reactions, unlike other unsaturated hydrocarbons such as alkenes and alkynes. For example, ethylene undergoes an addition reaction with bromine to form ethylene dibromide, but benzene does not.
Benzene does react with bromine to form bromobenzene via substitution reaction in the presence of Lewis acid ferric bromide.
The main drawback of Kekule’s model is that it failed to explain this unusual reactivity of benzene compared with other unsaturated hydrocarbons such as alkenes and alkynes.