Not a Straight-Chained Species

• Kekulé Postage Stamp; Quadell, commons.wikimedia.org
  
  Carbon forms four single bonds, one double bond and two single bonds, two double bonds, or one triple bond and one single bond. None of these formulae could be applied to a straight-chained structure with success. That is because benzene is not a straight-chained structure. That fact would become clear in a most unusual way.
  
  

Kekulé Had a Dream

• Figure 1. Benzene Representations; Vladsinger, commons.wikimedia.org
  
  In the 1850s, German chemist August Kekulé, who had been intensely interested in the problem of benzene, became tired and had a daydream. He visualized a snake with its tail in its mouth that was spinning around. That snake was benzene, biting itself in the tail. Benzene was a ring structure. It made sense if it possessed a alternating single and double bonds. Two such identical structures could be drawn (see Figure 1). Kekulé needed to find proof his daydream was correct.
  
  

Verification

• August Kekulé came up with a simple method of verification. He realized only one mono-derivative of benzene could be formed. This implied that all benzene's carbons are equivalent, since no matter which one reacted, the result was the same. This would only be possible if benzene exists as a ring.
  
  As for di-derivatives, only three isomers could be made. One with substituents on carbons one and two, one with substituents on carbons one and three and one with substituents on carbons one and four (later called ortho, meta and para, respectively). This again pointed to a single six-carbon ring.
  
  

Criticism

• Kekulé's method of structure determination was criticized, since a ring with alternating single and double bonds should form two ortho structures, one with substituted carbons separated by a single bond and another ortho structure with substituted carbons separated by a double bond.
  
  

An Important Tweak

• Kekulé recognized the legitimacy of this argument. To account for it, he suggested (in 1872) that benzene bonds switch back and forth, thus making all six carbon-carbon bonds equal, each being single half the time and double half the time. Although the switching mechanism is no longer considered correct (thanks to quantum mechanics), the explanation of equivalent bonds is.