Kekulé’s Oscillating D3h Cyclohexatriene Structure of Benzene

Kekulé postulated that neighbouring carbon atoms in benzene undergo incessant collision with each other, thereby leading to the interchange of double and single bonds, which amounts to an oscillation between two cyclohexatriene structures in dynamic equilibrium. It has been claimed that Kekulé arrived at a fully symmetric D6h structure of benzene and that the oscillation hypothesis should not be attributed to him. However, Clausius’ collision theory, which was known at the time, implies that, when the absolute temperature approaches zero, the collision frequency will tend toward zero too, i.e. collisions will stop, and a static, D3h cyclohexatriene obtains. The classical collision theory did not allow Kekulé to construct the desired D6h structure as the energy minimum. The theory of harmonic oscillators would have allowed it, but that was not attempted at Kekulé’s time. Kekulé’s attempts in 1872 to arrive at a fully symmetrical (D6h) benzene structure by means of intramolecular C−C collisions only resulted in the oscillating D3h cyclohexatrienes, which would have become static at zero K.