In 1888 J.E. Marsh published a paper (Phil. Mag. [V.], 26, p. 426) in which he discussed various stereo-chemical representations of the benzene nucleus. (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 Kekulé’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. This form, afterwards supported by Carl Graebe (Ber., 1902, 35, p. 526; see also Marsh’s reply, Journ. Chem. Soc. Trans., 1902, p. 961) shows the proximity of the ortho-positions, but fails to explain the identity of 1·2 and 1·6 compounds. Arrangements connected with Claus’ formula are obtained by placing six tetrahedra on the six triangles formed by the diagonals of a plane hexagon. The form in which the tetrahedra are all on one side, afterwards discussed by J. Loschmidt (Monats., 1890, II, p. 28), would not give stereo-isomers; and the arrangement of placing the tetrahedra on alternate sides, a form afterwards developed by W. Vaubel (Journ. Pr. Chem., 1894[2], 49, p. 308), has the advantage of bringing the meta-positions on one side, and the ortho- and para- on opposite sides, thus exhibiting the similarity actually observed between these series of compounds. Marsh also devised a form closely resembling that of Thomsen, inasmuch as the carbon atoms occupied the angles of a regular octahedron, and the diagonal linkages differed in nature from the peripheral, but differing from Thomsen’s since rupture of the diagonal and not peripheral bonds accompanied the reduction to hexamethylene.
We may also notice the model devised by H. Sachse (Ber., 1888, 21, 2530; Zeit. fur phys. Chem., II, p. 214; 23, p. 2062). Two parallel triangular faces are removed from a cardboard model of a regular octahedron, and on the remaining six faces tetrahedra are then placed; the hydrogen atoms are at the free angles. This configuration is, according to Sachse, more stable than any other form; no oscillation is possible, the molecule being only able to move as a whole. In 1897, J.N. Collie (Journ. Chem. Soc. Trans., p. 1013) considered in detail an octahedral form, and showed how by means of certain simple rotations of his system the formulae of Kekulé and Claus could be obtained as projections. An entirely new device, suggested by B. König (Chem. Zeit., 1905, 29, p. 30), assumed the six carbon atoms to occupy six of the corners of a cube, each carbon atom being linked to a hydrogen atom and by single bonds to two neighbouring carbon atoms, the remaining valencies being directed to the unoccupied corners of the cube, three to each, where they are supposed to satisfy each other.
Condensed Nuclei.
Restricting ourselves to compounds resulting from the fusion of benzene rings, we have first to consider naphthalene, C10H8, which consists of two benzene rings having a pair of carbon atoms in common. The next members are the isomers anthracene and phenanthrene, C14H10, formed from three benzene nuclei. Here we shall only discuss the structure of these compounds in the light of the modern benzene theories; reference should be made to the articles [Naphthalene], [Anthracene] and [Phenanthrene] for syntheses, decompositions, &c.
Naphthalene.—Of the earlier suggestions for the constitution of naphthalene we notice the formulae of Wreden (1) and (2), Berthelot and Balls (3), R.A.C.E. Erlenmeyer (4) and Adolf Claus (5).
The first suggestion is quite out of the question. C. Graebe in 1866 (Ann. 149, p. 20) established the symmetry of the naphthalene nucleus, and showed that whichever half of the molecule be oxidized the same phthalic acid results. Therefore formula (2), being unsymmetrical, is impossible. The third formula is based on Dewar’s benzene formula, which we have seen to be incorrect. Formula (4) is symmetrical and based on Kekulé’s formula: it is in full accord with the syntheses and decompositions of the naphthalene nucleus and the number of isomers found. In 1882 Claus suggested a combination of his own and Dewar’s benzene formulae. This is obviously unsymmetrical, consisting of an aliphatic and an aromatic nucleus; Claus explained the formation of the same phthalic acid from the oxidation of either nucleus by supposing that if the aromatic group be oxidized, the aliphatic residue assumes the character of a benzene nucleus. Bamberger opposed Claus’ formula on the following grounds:—The molecule of naphthalene is symmetrical, since 2.7 dioxynaphthalene is readily esterified by methyl iodide and sulphuric acid to a dimethyl ether; and no more than two mono-substitution derivatives are known. The molecule is aromatic but not benzenoid; however, by the reduction of one half of the molecule, the other assumes a benzenoid character.
If β-naphthylamine and β-naphthol be reduced, tetrahydro products are obtained in which the amino- or oxy-bearing half of the molecule becomes aliphatic in character. The compounds so obtained, alicyclic-β-tetrahydronaphthylamine and alicyclic-β-tetrahydronaphthol, closely resemble β-aminodiethylbenzene, C6H4(C2H5)·C2H4NH2, and β-oxydiethylbenzene, C6H4(C2H5)·C2H4OH. If α-naphthylamine and α-naphthol be reduced, the hydrogen atoms attach themselves to the non-substituted half of the molecule, and the compounds so obtained resemble aminodiethylbenzene, C6H3·NH2(C2H5)2 and oxydiethylbenzene, C6H3·OH(C2H5)2. Bamberger’s observations on reduced quinoline derivatives point to the same conclusion, that condensed nuclei are not benzenoid, but possess an individual character, which breaks down, however, when the molecule is reduced.
It remains, therefore, to consider Erlenmeyer’s formula and those derived from the centric hypothesis. The former, based on Kekulé’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. On the centric hypothesis two formulae are possible: (i) due to H.E. Armstrong, and (2) due to E. Bamberger.
