The Two Criteria for Aromaticity

The two resonance contributors of benzene are identical, so we expect benzene to have a relatively large delocalization energy (Section 8.6). 

Cyclohexene, a compound with one double bond with localized tt electrons, has an experimental LtP = -28.6 kcal/mol for its reaction with H2 to form cyclohexane. Therefore, the LhP of cyclohexatriene, an unknown hypothetical compound with three double bonds with localized n electrons, would be three times that value (A// = 3 x -28.6 = -85.8) for the same reaction. Benzene, which has three double bonds with delocalized tt electrons, has an experimental LhP = -49.8 kcal/mol for its reaction with H2 to form cyclohexane. The difference in the energies of cyclohexatriene and benzene (36 kcal/mol) is the delocalization energy of benzene—the extra stability benzene has as a result of having delocalized electrons. 

Because of its large delocalization energy, benzene is an extremely stable compound. Therefore, it does not undergo the electrophilic addition reactions that are characteristic of alkenes except under extreme conditions. (Notice the conditions that Sabatier had to use in order to reduce benzene s double bonds on page 332.) Now we can understand why benzene s unusual stability puzzled nineteenth-century chemists, who did not know about delocalized electrons (Section 8.1). 

How can we tell whether a compound is aromatic by looking at its structure In other words, what structural features do aromatic compounds have in common To be classified as aromatic, a compound must meet both of the following criteria  

It must have an uninterrupted cyclic cloud ofir electrons (called a tt cloud) above and below the plane of the molecule. Let s look a little more closely at what this means  

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