Aromaticity and Its Chemical Manifestations

Department of Chemistry, Yale University, New Haven, CT 06520 1. HISTORICAL PRELUDE 

In the 18th century, a number of naturally occurring compounds were isolated and described as aromatic because of their distinctive odor.1 When the structural theory of organic chemistry was developed in the 19th century, it became apparent that most of these compounds were benzene derivatives. As a result, they became known as aromatic compounds, in contrast to aliphatic compounds. 

The benzene derivatives presented an enigma to structural chemists in that although the benzene rings had three double bonds, they underwent substitution rather than addition when treated with reagents such as bromine and nitric acid. No adequate explanation for their behavior was presented prior to the development of quantum mechanics. In the early 1930 s, two explanations were presented. One was by Pauling making use of valence bond theory,2 and the other was by E. Huckel making use of molecular orbital theory.3 

Pauling s use of valence bond theory had a direct connection with the types of structures commonly used by organic chemists, and was relatively easy to understand, provided one did not delve too deeply into its details. The basic postulate was that compounds having n-electron systems that can be described by more than one structure will be stabilized by resonance and will have a lower energy than any of the contributing structures. Thus, for benzene one would write 

This type of formulation was easily extended to reactions.4 Thus, considering 

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K. Wiberg,Aromaticity and its chemical manifestations, in Pauling s legacy Modem modelling of the chemical bond, Vol. 6, ed. Z. B. Maksic and W. J. Orville-Thomas (Elsevier, Amsterdam, 1999). 

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