ABSTRACT
The first cyclic chemical species to be studied in detail was the benzene ring. Although even Dalton had constructed hexagons of atoms [23], it was not until 1854 that Laurent published the first illustration of the benzene ring [28]. A leprodnction of tins illustration is presented in Figure 5(a). In 1861 Loschmidt [29] published graphic representations for the structural formulas of over 350 different molecules. In this system the hydrogen atonas were represented by small circles, carbon atoms by larger circles, and the benzene ring by one very large circle. Loschmidt’s representation of the benzene molecule is reproduced in Figure 5(b). His work was of great significance since it depicted for the first time both double and triple bonds as well as offering a representation of the benzene nucleus as a single ring. The now famous structural formula for benzene published by Kekulé [30] in 1866 is shown in Figure 5(c). Letters were placed at the vertices of his hexa^n to assist in the enumeration of the isomers formed when the hydrogen atoms were substituted by various heteroatoms. Shortly after the appearance of Kekulé’s paper [30], some surprisingly modem looking representations of the naphthalene molecule [31] and anthracene molecule [32] were published by Graebe.
