Organic chemistry was especially developed by the publication of Gerhardt's Traite de chimie organique in 1853-1856, and of Kekule's Lehrbuch der organischen Chemie in 1861-1882.
If we accept Kekule's formula for the benzene nucleus, then we may expect the double linkages to be opened up partially, either by oxidation or reduction, with the formation of di-, tetra-, or hexa-hydro derivatives, or entirely, with the production of open chain compounds.
More important are Kekule's observations that nitrous acid oxidizes pyrocatechol or [I.2]-dioxybenzene, and protocatechuic acid or [3.4]- dioxybenzoic acid to dioxytartaric acid, (C(OH) 2 COOH) 2 (Ann., 1883, 221, p. 230); and 0.
The value of this ringed structure was readily perceived, but objections were raised with respect to Kekule's disposal of the fourth valencies.
CH CH HC, N CH HC CH HC HC HC Hcch Hc Ch Ch Kekule t i Dewar Ladenburg One of the earliest and strongest objections urged against Kekule's formula was that it demanded two isomeric ortho-di-substitution derivatives; for if we number the carbon atoms in cyclical order from i to 6, then the derivatives 1.2 and 1.6 should be different.'
This implied that in the benzene complex there was at least one carbon atom linked to three others, thus rendering Kekule's formula impossible and Ladenburg's and Claus' possible.
The formation of this substance readily follows from Kekule's formula, while considerable difficulties are met with when one attempts an explanation based on Ladenburg's representation.
By projecting Ladenburg's prism on a plane and numbering the atoms so as to correspond with Kekule's form, viz.
That 1.2 and 1.6 should be ortho-positions, 1.3 and 1.5 meta-, and 1.4 para-, and following out the transformation on the Ladenburg formula, then an ortho-dioxyterephthalic acid (IV) should result, a fact denied by experience, and inexplicable unless we assume a wandering of atoms. Kekule's formula (III), on the other hand, is in full agreement (Baeyer).
But the formation of the A2.5 acid as the first reduction product is not fully consistent with Kekule's symbol, for we should then expect the or the acid to be first formed (see also Polymethylenes).
The stronger argument against the ethylenoid linkages demanded by Kekule's formula is provided by the remarkable stability towards oxidizing and reducing agents which characterizes all benzenoid compounds.
It is difficult to determine which configuration most accurately explains the observed facts; Kekule's formula undoubtedly explains the synthetical production of benzenoid compounds most satisfactorily, and W.
Stohmann of Leipzig; and the new data and the conclusions to be drawn from them formed the subject of much discussion, Briihl endeavouring to show how they supported Kekule's formula, while Thomsen maintained that they demanded the benzene union to have a different heat of combustion from the acetylene union.
(The stereo-chemistry of carbon compounds has led to the spatial representation of a carbon atom as being situated at the centre of a tetrahedron, the four valencies being directed towards the apices; see above, and Isomerism.) A form based on Kekule's formula consists in taking three pairs of tetrahedra, each pair having a side in common, and joining them up along the sides of a regular hexagon by means of their apices.
Formula (4) is symmetrical and based on Kekule's formula: it is in full accord with the syntheses and decompositions of the naphthalene nucleus and the number of isomers found.
The former, based on Kekule's symbol for benzene, explains the decompositions and syntheses of the ring, but the character of naphthalene is not in keeping with the presence of five double linkages, although it is more readily acted upon than benzene is.
Kekule's main importance lies in the far-reaching contributions which he made to chemical theory, especially in regard to the constitution of the carbon compounds.
Japp, in the Kekule memorial lecture he delivered before the London Chemical Society on the 15th of December 1897, declared that three-fourths of modern organic chemistry is directly or indirectly the product of Kekule's benzene theory, and that without its guidance and inspiration the industries of the coal-tar colours and artificial therapeutic agents in their present form and extension would have been inconceivable.
Many of Kekule's papers appeared in the Annalen der Chemie, of which he was editor, and he also published an important work, Lehrbuch der organischen Chemie, of which the first three volumes are dated 1861, 1866 and 1882, while of the fourth only one small section was issued in 1887.