Benzene Ring: Kekulé Structure, Reaction, Structural Stability and Bond Length

Benzene C₆H₆ had assumed so many fascinating suggested structures since it was first prepared in 1825 by Michael Faraday. Although the Story of the various Postulations are interesting to behold, it is how the quest for the actual structure of Benzene crept into the Cold War that is most interesting. The structure seemed to be bent on Defiance that it also misbehaved to the Kekulé's structure after it was generally seen as the most satisfactory of all the Suggestions. Now, follow me let's take a ride through the fascinating structure of Benzene.

The quest for the exact structure of the Benzene molecule had attracted a whole lot of Interests. Small wonder so many Scientists came up with an array of two and three dimensional Structures. But it was at first Kekulé, and later Thiele who were able to put forth more satisfactory explanations

The Kekulé Structure became the most satisfactory explanation of the structure of Benzene. It is of the Opinion that Benzene is made up of six Carbon Atoms in a Hexagonal ring. That the ring is made up of alternating double and single bonds with each carbon atom attached to one hydrogen atom.

As seemingly satisfactory as this explanation may be, the structure actually sparked up some Problems. This includes the question of the reaction of Benzene, bond Length and structural stability.

That Benzene is made up of three double bonds should have automatically confer an additional reaction like members of the Alkene group. But a reaction that would result to breaking any of the double bond would certainly destroy the Benzene ring. The truth is that, Benzene do not actually undergo an additional reaction but a substitution reaction where the hydrogen atoms are replaced by a reacting group.

The stability of Benzene was also put to test by virtue of thermochemical concept. The Enthalpy of Benzene when it is hydrogenated to Cyclohexane is -208KJmol^-1 as against what should be obtained when Enthalpy of three times Cyclohexene is calculated. Here, the Enthalpy of Cyclohexene, with only one double bond, is -120KJmol^-1 and would in an ideal case, result to -360KJmol^-1 for three double bonds that tallies with the structure of the Benzene molecule. That the Enthalpy of Benzene is far less than this shows a rather far more stable Benzene structure than as depicted by the Kekulé Structure.

Benzene would assume an unstable structure if the concept of alternating carbon-carbon double bond C=C and single bond C-C are true. This is because the single bond bond length 0.154nm is longer than the double bond C=C bond length 0.134nm. But the reality is that the Benzene structure is stable with Bond length of 0.139nm distributed equally around the six Carbon atoms. This tallies with the stable state of the Benzene Structure and depicts that the Benzene nine Bonds (three single bonds and three double bonds) are distributed equally as 1.5 Bonds around the six carbon atoms of the Benzene ring.