The Historical Development of Aromatic Structures

No problem in organic chemistry has attracted interest so unfailingly as that of accounting for the properties of aromatic compounds by means of chemical structures. Many accounts have been given of the protracted feelings towards the true solution, but only in the last thirty years have the grounds for that solution been established. 

Three phases are discernible in the history of aromatic theory. From Kekule s perception of the cyclic nature of benzene until 1922, classical valency theory was exploited to its limits. The decade 1920-30 saw the emergence of electronic theories, and the confusion of electronic and classical ideas presented by the chemical literature of that time is difficult to resolve in retrospect. From about 1930 onwards, the significant ideas of the previous decade assumed clearer outlines and crystallized in a quantum theory of valency which to a great extent has solved the problem of aromatic stability and reactivity. 

Every gain in our understanding of benzene has enriched our insight into the nature of heteroaromatic compounds such as pyridine and pyrrole, prototypical of all those to be discussed in this book. It is interesting, therefore, to consider the development of structures for pyridine and pyrrole against the background of the benzene problem in the first phase of its growth and as it was affected by some of the ideas from the second phase. 

---