An introduction to chemical crystallography / by P. Groth, Authorised translation by Hugh Marshall.

  • Groth, P. (Paul), 1843-1927.
Date:
1906
Catalogue details

Licence: Public Domain Mark

Credit: An introduction to chemical crystallography / by P. Groth, Authorised translation by Hugh Marshall. Source: Wellcome Collection.

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    is impossible, owing to the lack of a density determination for the chlorine compound. The benzoic acid group would also be suitable for closer study. The crystals of this acid are pronouncedly pseudo-tetragonal, with a very large value for the parameter corresponding to the tetragonal principal axis, and it is rather remarkable that no cleavage takes place at right angles to this axis. Seeing that the density of the measured crystals was not determined, and that the existing determinations differ very greatly, this substance certainly requires further investigation as regards its crystal structure. The stable form of m-nitrobenzoic acid has retained the pseudo-tetragonal character and the lack of cleavage observed with benzoic acid, but possesses a much shorter principal axis. The calculation of the topic parameters of the two substances, which on the basis of old density determinations is only approximately possible, indicates (on the assumption of corresponding conditions of structure) that for the nitro-derivative the values of x and p are about one and a half times as large, whilst w is only about half as large, as for benzoic acid. Two chloronitrobenzoic acids are formed from the above-mentioned nitrobenzoic acid ; these have been investigated by Jaeger,1 and their topic parameters are : 2 5 i C0H3C1. N02. COOH % — 3*167 \f/ = 2*765 w = 16-620 4 5 = 3 1 C,.HoCl. N02. COOH ^ — 5.148 ^ = 4-022 w = 6-925. The typically pseudo-tetragonal crystals of the 2 : 5 acid have nearly the same values of x and p as benzoic acid has, but have a still larger value for w, and correspondingly complete cleavage on {001}; compared with nitrobenzoic acid, they exhibit diminu¬ tion of % and \f/ and very marked increase of w. The 4 : 5 acid shows greater similarity of habit with nitrobenzoic acid, and differs from it in that x and, more especially, a have undergone a not incon¬ siderable increase, whilst p has experienced a slight diminution. The same observer has also investigated two isomeric chloro- benzoic acids, and found for them : 2 1 C6H4Cl.COOH x = 5*638 1^ = 8-508 w —2-ii8 4 1 C6H4Cl.COOH x = 3-697 p = 2-902 07 = 9.662 In both of these pairs of cases it is evident that the morpho- 1 Zeits, f Krystf 1903, 38. 30 r„