Thus von Groth showed first that benzene, C6H6, crystallises in the rhombic system with axial ratios a : b : c = 0.891 : 1 : 0.977. Next, that when one or two of the hydrogen atoms are replaced by hydroxyl OH groups the substances produced, phenol C6H5.OH and resorcinol C6H4(OH)2, are found also to crystallise in the rhombic system, and in the second case, for which alone the axial ratios could be determined, the ratio a : b proved to be very similar, but the ratio c : b was different, the actual values being a : b : c = 0.910 : 1 : 0.540. Pyrocatechol, the isomer of resorcinol, also crystallises in the rhombic system, but the crystals have not been obtained sufficiently well formed to enable any deductions to be made from any measurements carried out with them.
Similarly, the nitro-derivatives of phenol, orthonitrophenol C6H4.OH.NO2, dinitrophenol C6H3.OH.(NO2)2, and trinitrophenol C6H2.OH.(NO2)3, also crystallise in the rhombic system, and with the following respective axial ratios: 0.873 : 1 : 0.60; 10.933 : 1 : 0.753; 0.937 : 1 : 0.974. Again, the value for the ratio a : b is not very different from that of benzene itself, while the ratio c : b differs considerably in the first two cases. Similar relations were also found to hold good in the cases of meta-dinitrobenzene, C6H4(NO2)2, axial ratios 0.943 : 1 : 0.538, and trinitrobenzene, C6H3(NO2)3, which possesses the axial ratios 0.954 : 1 : 0.733.
The introduction of a chlorine or bromine atom or a CH3 group in place of hydrogen was found by von Groth to produce more than the above effect, the symmetry being often lowered to monoclinic, a fact which had also been observed to occur in the cases of certain isomers of the substances quoted above, ortho-dinitrobenzene for instance. But it was nevertheless observed that the angles between the faces in the prism zone remained very similar, the angles between the faces of the primary prism (110) and (1̄10), for instance, only varying in eight such derivatives of all three types, whether rhombic or monoclinic, from 93° 45′ to 98° 51′.
The crystallographic relationships of organic substances, however, are very much complicated by the possibilities of isomerism, the ortho, meta, and para compounds—corresponding to the replacement of the two hydrogen atoms attached to two adjacent, alternate, or opposite carbon atoms respectively, of the six forming the benzene ring—generally differing extensively and sometimes completely in crystalline form. Consequently, the phenomenon of morphotropy is best considered quite independently of isomorphism.
An interesting intermediate case between morphotropy and true isomorphism was investigated by the author in the year 1890, namely, a series of homologous organic compounds differing by regular increments of the organic radicle CH3. They were prepared by Prof. Japp and Dr Klingemann, and consisted of the methyl, CH3, ethyl, C2H5, and propyl, C3H7, derivatives of the substance triphenyl pyrrholone, all of them being solids crystallising well. The problem was somewhat complicated by the development of polymorphism, the methyl, ethyl, and propyl compounds having each been found to be dimorphous, and not improbably trimorphous, but only two varieties of each salt were obtained in crystals adequately perfect for measurement. That the production of these different forms was due to polymorphism and not to chemical isomerism (different arrangement of the chemical atoms in the molecule) was shown by the fact that one variety could be obtained from the other by simply altering the conditions of crystallisation from the same solvent. Their identical chemical composition was established by direct analysis.
The methyl (CH3) compound crystallised in rhombohedra and in triclinic prisms. The ethyl (C2H5) derivative was deposited in triclinic prisms exactly resembling those of the methyl compound in habit and disposition of faces. A crystal of the triclinic methyl derivative which would represent equally well the ethyl compound is shown in Fig. 56. The angles also of the crystals of the two substances are so similar that one might infer the existence of true and complete isomorphism. The actual angular differences rarely exceeded three degrees.
Fig. 56.—Crystal of Methyl Triphenyl Pyrrholone.
Besides the triclinic form the ethyl derivative was also obtained in monoclinic crystals, one of which is represented in Fig. 57. This illustration might serve equally well, however, for a corresponding monoclinic form of the propyl (C3H7) derivative, and the angles of these two monoclinic ethyl and propyl compounds are even closer than those of the triclinic methyl and ethyl derivatives, the closeness increasing with the advent of symmetry.
