17.4 : 苯的结构：凯库勒模型

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.