Overview

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.

Procedure

In 1865, August Kekulé proposed the first definite structure for benzene.

He suggested that benzene has a cyclic structure of six carbon atoms with alternating single and double bonds. Each carbon atom is attached to one hydrogen atom.

This model satisfied the tetravalency of carbon, making all the hydrogens equivalent.

Kekulé's model predicted two forms of 1,2 -dibromobenzene, which differed in the positions of the double bonds. But, only one form of 1,2-dibromobenzene was isolated.

Kekulé defended it by proposing a rapid equilibrium between the two inseparable forms of benzene, now known as Kekulé structures. However, it is now known that no such equilibrium exists.

Moreover, it failed to explain the unusual chemical behavior of benzene, which undergoes a substitution reaction with bromine, as opposed to alkenes, which undergo an addition reaction.