The Benzene Ring

Unsaturated aliphatic hydrocarbons presented few problems in terms of structure and bonding. The Scottish chemist Alexander Crum Brown had shown a double bond in ethene in 1864.                                                                      

However the structure of Benzene (C6H6) was presenting serious problems. It would be Kekule who would make a vital contribution to the solving of this problem and which also aid the development  of organic chemistry.

Kekule's first paper on the cyclic structure of benzene appeared in 1865. He represented the structure using one of his "sausage" formulae. Each long thin carbon atom was shown to overlap with its neighbour on one side by two affinity units And with one Affinity unit with its neighbour on the other side.

                                               

The spare affinity units at either end were stabalised by combining with each other in the cyclic structure. Later in 1865 he again attempted to represent Benzene, but this time he did so by means of a simple hexagon.

                                                                                   

 Then in 1866 he published a diagram of the model he had made of benzene. It was the first structure to show alternating double and single bonds,

                                                                           

After he published this structure, many chemists began to doubt and object to Kekule’s idea saying that benzene did not show the kind of reactivity associated with a compound containing 3 double bonds. Also, such a view leads to the prediction that two 1,2-disubtituted benzene’s should exist.

                                                                  

But in practise, it was found that 3 di-substituted benzenes could exist, but when methods were devised for assigning structures to them only one was shown to be 1,2-isomer.

Despite these criticisms, the hexagonal structure of benzene, which Kekule had proposed solved many problems the chemists of the time faced and so it was widely accepted. The reasons for the equivalence of the carbon – carbon bonds and for the lack of reactivity remained matters for speculation, but the fundamental concept of the hexagonal ring encountered very little opposition. The only serious alternative for the structure was Ladenburg’s (1842-1911) prism formula shown below.

                                                                                       

While this structure could explain the number of di- and tri-substituted isomers, it could not account for the formation from benzene of dihydro and tetrahydro derivatives that clearly contained double bonds.

The problems of bonding in benzene still puzzle chemists till this day. But it is widely accepted that the idea put forward by Kekule in 1872 was a stroke of genius. He proposed that the atoms in the molecule were oscillating rapidly and they were colliding with their neighbours. The number of collisions made by an atom in unit time was equal to its valency.

                                               

Thus C1 in one oscillation might collide with C2, C6, H or C2 but in the course of the next oscillation the sequence might be reversed to give C6, C2, H or C6.

This meant that in the first cycle the double bond would be located between C1 and C2, and in the second between C1 and C6. This concept of valency being related to atomic collisions was not generally accepted, but the idea of oscillating double bonds was frequently used, and can be regarded as the forerunner of the modern concept of Resonance.

Below are several other attempts that were made to explain the equivalence of all six positions in the benzene ring with the concept of the tetravalent carbon atom.