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During 1825 the hydrocarbon Benzene was first isolated by Michael Faraday, nine years later Benzene’s molecular formula, C6H6 was calculated. This highly unusual and puzzling molecular formula consequently prompted vast speculation from chemists worldwide regarding the structure of the Benzene molecule. Then one dark German evening in 1862 a lone figure, haunted by his continuing failings to identify the illusive structure of the Benzene molecule, dozed exhausted by the fire. As he watched the flames flicker in the grate he suddenly “saw” snake like molecules “dancing” and “writhing” in the flames. Bemused he continued to stare until one of the snakes grasped its own tail in its mouth and formed a ring like structure. The man startled by the image awoke but the image of the ring remained vivid in his mind. Three years later, the same man, Freidrich August Kekulé proudly presented his proposed structure of the Benzene molecule – Kekulé’s model of Benzene was born.
Kekulé proposed that the benzene molecule had a cyclic ring structure with alternating double Carbon-Carbon bonds and single Carbon-Carbon bonds. This structure was accepted for many years however during this period many new pieces of evidence arose questioning Kekulé’s model of Benzene leading to the eventual proposal of a new model regarding the structure of Benzene.
Using single crystal X ray diffraction patterns the single Carbon-Carbon bond lengths in Cyclohexene (used due to its similar structure to Kekulé’s model of Benzene including possessing single Carbon-Carbon bonds, having a double Carbon-Carbon bond and a cyclic structure) were measured to be 0.154nm.
The bond length of the double Carbon-Carbon bond in Cyclohexene was also measured by X ray diffraction giving a value of 0.133nm. If indeed Kekulé’s structure was correct this indicates that the alternating double Carbon-Carbon and single Carbon-Carbon bonds would have different lengths but when the bond lengths in a molecule in Benzene were calculated using the same technique they were all calculated to have the same length of 0.139nm therefore they can-not be alternating single and double Carbon-Carbon bonds providing evidence against Kekulé’s model of Benzene.
If Kekulé’s cyclotriene structure of Benzene was assumed correct four disubstituted isomers
with a Chlorine molecule should be formed due to the reactive double Carbon-Carbon bonds. However when Ben-zene is reacted with a Chlorine molecule only 3 disubstituted isomers are formed and one monosubstituted product during experiments. This evidence suggests that Benzene does not contain double Carbon-Carbon bonds therefore disproving Kekulé’s structure of alternating double and single Carbon-Carbon bonds.
The calculated enthalpy of formation for one mole of gaseous Benzene with Kekulé’s structure from its constituent elements of Carbon and Hydrogen in their standard states is +252KJmol-1. However when the actual enthalpy of formation was tested it was considerably lower at a mere +82KJmol-1. This suggests that the structure of Benzene is significantly more stable than Kekulé’s model of Benzene.
Further thermo chemical evidence against Kekulé’s model originates from the theoretical enthalpy change for the hydrogenation of Benzene when cyclohexene, with its one double Carbon-Carbon bond, undergoes hydrogenation (see fig. 2) the enthalpy change is -119.6KJmol-1. The theoretical enthalpy change for the hydrogenation of Benzene can therefore be calculated by multiplying the enthalpy change for the hydrogenation of cyclohexene by three due to the fact Kekulé’s model of Benzene suggests Benzene has three double Carbon-Carbon bonds resulting in a value of -358.8KJmol-1.
However in practice the enthalpy change for the hydrogenation of Benzene is substantially lower at -208.4KJmol-1 suggesting the addition of hydrogen across a double Carbon-Carbon bond is not occurring thus disproving Kekulé’s model for the structure of Benzene.
The accumulation of evidence against Kekulé’s struc-ture of Benzene lead to a new structure to be pro-posed by Linus Pauling. His idea was to treat the struc-ture of the Benzene molecule as if it were half way between the 2 possible Kekulé structures. (fig.3)
His idea resulted the currently accepted structure of the Benzene molecule (fig.4) with a ring of delocalized electrons within a hexagonal skeleton of carbon atoms to which hydrogen atoms are attached accounting for the intermediate Carbon-Carbon bond lengths in Benzene, its relative lack of reactivity and its high stability.