Centric formula

A theoretical understanding of furan and its congeners followed in the wake of the progress with the benzene problem, but for the five-membered heterocyclic compounds Bamberger s centric formula was always the most reasonable one, as Kekule structures could not be written at all. It is not now clear to what extent the heterocycles were really regarded as benzene analogues, but a remark made by Hantzsch, that coumarone is the furfurane of the naphthalene series was certainly percipient. Furan is variously said to be aromatic, superaromatic or not aromatic at all, for the debate continues. 

It is true there are still more configurations possible such as the centric formula and three Ladenburg prism formulae projected on to a plane, but it can be shown that these can be regarded as superpositions of the five configurations mentioned above. 

Thus in the benzene molecule, one will be able to speak of neither single, nor double, nor diagonal (para) bonds in the ordinary sense of the word. Of the structural formulas [proposed] for benzene up to now, the concept which comes closest to this one is that which von Baeyer designated as the centric formula, of course, in terms of the usual theory. ... 

Claus, Armstrong and Baeyer.—Still other formulas have been suggested on account of this difficulty, each endeavoring to avoid the necessity of oscillation in order to satisfy the four valencies of carbon. One of these was suggested by Claus and called the diagonal formula and another by Armstrong and Baeyer and known as the centric formula. 

In the formula of Kekule the carbon atoms are joined alternately by double bonds. According to Claus, each carbon atom is in combination with the carbon atom opposite to it in the molecule. The so-called centric formula of Armstrong and Baeyer is meant to indicate that the combining powers of the carbon atoms which are not utilized in the ring formation and... 

As thiophene resembles closely benzene in chemical properties, a centric formula similar to that of benzene has been proposed for it —... 

These syntheses indicate that the relation between the structure of quinoline and that of naphthalene is similar to that between the structures of pyridine and benzene in both cases one CH group may be considered as replaced by a nitrogen atom. On account of the aromatic properties of quinoline the centric formula is usually assigned to it. The substitution-products are indicated, by assigning a number to each atom in the ring —... 

Baeyer showed that reduced benzene rings acquire aliphatic properties. At first he adopted a centric formula for benzene in place of Kekule s (see p. 804), but as a result of this mass of work he announced in 1892 that no conclusion as to the structure of benzene derivatives can be drawn from their behaviour on reduction . In the course of this work he prepared cis- and mw -derivatives of hexahydrophthalic acid and introduced the names cis-and tram-. 

Following Graebe and Liebermann (1868, see p. 789) Armstrong proposed a quinonoid theory of colour. He proposed a theory of benzene substitution, and a centric formula of benzene (see p. 804). His work with F. P. Worley on reaction velocities in the catalytic hydrolysis of cane sugar and esters by acids ... 

A centric formula for benzene was proposed by Lothar Meyer, who says The model of Kekule is almost identical with the assumption of a ring-shaped linking and a free affinity on each atom ... 

Bamberger carried out important researches on hydro-aromatic (alicyclic) compounds. He extended the centric formula of benzene (see p. 804) to naphthalene ... 

The benzene molecule is a hybrid of Kekule and Dewar structures shown in B and C, Fig. 91. Since the molecule is planar, Ladenburg s prism formula is excluded. The Claus and the centric formulae (see p. 802) can be formed from the Kekule and Dewar structures and are not what are called canonical structures . ... 

---