Jacobus Henricus van 't Hoff

“ This connexion of isomerism with resistant linking, and of this with high valency, explains, in considerable measure, why inorganic compounds afforded, as a rule, no phenomena of this kind until the systematic investigation of metallic compounds by Werner brought to light many instances of isomerism in inorganic compounds. Whereas carbon renders isomerism possible in organic compounds, cobalt and platinum are the determining elements in inorganic chemistry, the phenomena being exhibited especially by complex ammoniacal derivatives. ”

“ Isomerism.—Certain investigations on isomerism which have become especially prominent in recent times bear on the possibility of the mutual transformation of isomers. As soon as this reversibility is introduced, general laws related to thermodynamics are applicable (see Chemical Action; Energetics) . These laws have the advantage of being applicable to the mutual transformations of isomers, whatever be the nature of the deeper origin, and so bring polymerism, metamerism and polymorphism together. ”

“ C – is one of the most difficult to loosen.
The conception of metamerism, or isomerism in restricted sense, has been of the highest value for the development of our notions concerning molecular structure, i.e. the conception as to the order in which the atoms composing a molecule are linked together. In this article we shall confine ourselves to the fatty compounds, from which the fundamental notions were first obtained; reference may be made to the article Chemistry: Organic, for the general structural relations of organic compounds, both fatty and aromatic. ”

“ That this particular feature belongs to carbon compounds is due to a property of carbon which characterizes the whole of organic chemistry, i.e. that atoms attached to carbon, to express it in the atomic style, cling more intensely to it than, for instance, when combined with oxygen. ”

“ In the divalent oxygen we meet with the modification called ozone, which, although unstable, changes but slowly into oxygen. Similarly the trivalent phosphorus in the ordinary white form shows such resistance as if it were practically stable; on the other hand the red modification is in reality also stable, being formed, for instance, under the influence of light. In the case of the quadrivalent carbon, diamond seems to be the stable form at ordinary temperatures, but one may wait long before it is formed from graphite. ”

“ These phenomena were quite in accordance with the atomic conception of matter, since a compound containing the same number of atoms of carbon, nitrogen, oxygen and hydrogen as another in the same weight might differ in internal structure by different arrangements of those atoms. ”

“ In ordinary isomers indications of mutual transformation often occur; and among these the predominant fact is that denoted as tautomerism or pseudomerism. It exhibits itself in the peculiar behaviour of some organic compounds containing the group – C·CO·C – , e.g. CH3CO·CHX·CO2C2H5, derivatives of acetoacetic ester. These compounds generally behave as ketones; but at the same time they may act as alcohols, i.e. as if containing the OH group; this leads to the formula H3C·C (OH) :CX·CO2C2H5. ”

“ This temperature is somewhat different from the ordinary melting-point, the latter corresponding to atmospheric pressure, the former to the maximum vapour-pressure; and so we come to a third relation for polymorphism. Just as the melting-point changes with pressure, the transition-point also changes; even the same quantitative relation holds for both, as L. J. Reicher proved with sulphur: aT/aP = AvT/q, v being the change in volume which accompanies the change from rhombic to prismatic sulphur, and q the heat absorbed. ”

“ Consequently, of each pair of isomers we may establish beforehand which is the more stable; either in particular circumstances, a direct change taking place, as, for instance, with maleic acid, which when exposed to sunlight in presence of a trace of bromine, yields the isomeric fumaric acid almost at once, or, indirectly, one may conclude that the isomer which forms under greater heat-development is the more stable, at least at lower temperatures. Now, whether a real, though undetected, change occurs is a question to be determined from case to case ”