Jacobus Henricus van 't Hoff

“ Dealing with organic compounds, it is metamerism that deserves chief attention, as it has largely developed our notions as to molecular structure. Polymerism required no particular explanation, since this was given by the difference in molecular magnitude. One general remark, however, may be made here. There are polymers which have hardly any inter-relations other than identity in composition; on the other hand, there are others which are related by the possibility of mutual transformation ”

“ Mutual Transformation of Antipodes.—Thus far we have supposed the molecule to be stable with atoms in fixed places, as may be the case at absolute zero ; in reality, at ordinary temperatures, atoms probably are endowed with movement, and this may be supposed to take place along the fixed places just mentioned as centres, which movement can go so far as to lead to total transformation, the one stereo-isomer changing over into the other. ”

“ The Asymmetric Carbon Atom.—Though stereo-chemistry is based on the notion of atoms, there is not the least danger that it may break down when newer notions about those atoms are intro- duced. Even admitting that they are of a compound nature, i.e. built up from smaller electrical particles or anything else and able to split up under given conditions, their average lapse of existencs is long enough to consider them as reliable building-stones of the molecule, though these building-stones may give way now and then, as our best ordinary ones by the action of an earthquake. ”

“ It is the limit where the possibility of spontaneous separation begins, and is relatively rare, so that this way of separation is an exceptional one, most antipodes forming a racemic compound stable at all temperatures that come into question. ”

“ 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 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. ”

“ 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. ”

“ 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. ”

“ The use of optically active compounds in separating antipodes is of the greatest value. The general principle is that the compounds which the d- and l-form give with a different active compound, for instance d producing dd and ld, are by no means antipodes and so exhibit the ordinary differences, e.g. in solubility, which allow separation. It was in this way that Pasteur split up racemic acid by cinchonine. ”

“ 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. ”

“ One of the most simple examples is offered by the trimethylene- dicarboxylic acids
for which three formulae can be deduced:—
the first, where the carboxyl groups ⋅CO2H lie on the same side of the carbon ring, called, as Von Baeyer proposed, the cis-form, the others trans-forms. The trans-forms show enantiomorphism and correspond to optical antipodes, whereas the first symbol may be considered as corresponding to mesotartaric acid, symmetrical in configuration and inactive; this third stereo-isomer has also been met with. ”

“ Phenomena analogous to those observed in carbon compounds might also exist in derivatives of other quadrivalent elements ; and only the relative stability of carbon-compounds makes every form of isomer, which often is unstable, more easily obtainable in organic chemistry. Nevertheless it has been possible to obtain stereo-isomers with different elements, but, as expected from the above, especially in derivatives containing carbon. Some of them have the character of optical antipodes and are more easily considered from a theoretical point of view ; others have not. ”

“ But the crystalline form, which may show enantiomorphism, indeed shows this difference in the isomers in question; and especially the behaviour (in the amorphous state) towards polarized light differs in the sense that the plane of polarization is turned to the left by the one isomer, and exactly as much to the right by the other, so that they may be termed " optical antipodes." All these differences disappear with the asymmetric carbon, and the succinic acid, HO2C·CH2·CH2·CO2H, from tartaric acid is optically inactive and shows no stereo-isomerism. ”

“ 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 ”

“ The method called “spontaneous separation” was first observed by Pasteur with racemic acid, which in its double sodium and am- monium salt crystallized from its aqueous solution in two enantio- morphic forms, which could be separated on examination. One of the two proved to be the ordinary sodium-ammonium-tartrate, the other its laevogyre antipode; thus /-tartaric acid wasdiscovered, and racemic acid proved to be a combination of d- and /-tartaric acid. ”

“ A special feature, however, wanting with carbon, is that compounds with one atom only of the element in question have been obtained as antipodes. A second observation of some interest is that the compounds in question are electrolytes and that, as in solutions, where they are split up into ions, activity must be due to the last, the ionic complex, for instance, RiR 2 R s S, must cause optical rotation. ”

“ 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. ”

“ 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. ”

“ Configuration of Stereo-isomers.—The conception of asymmetric carbon not only opens the possibility of determining when and how many stereo-isomers are to be expected, but also allows a deeper insight into the relative position of atoms in each of them. The chief indication here lies in the configuration of the meso-type, already given for mesotartaric acid ”

“ Unsaturated compounds generally show larger rotative powers; amyl alcohol with −5° produces an aldehyde with 15 ; succinic (diamyl) ether with −9° produces fumaric ether with 15°, &c. Cyclic configuration especially leads to the highest values known: the lactic acid with 3 leads to a lactone with −86°,
H3C-CH-COO
I I
OOC-HC-CH 3 ;
mannosaccharic acid, H0 2 C (CHOH) 4CO 2 H, to a dilactone (with two rings, formed by the loss of two molecules of water) with 202°, whereas the original acid only shows a small rotation. ”

“ The formation of a racemic compound built up from dd and ld has also been observed in the so-called partial racemate. An example is the racemate of strychnine. It is in this case also that the transition-point forms the limit of possible separation, determined by Ladenburg and G. Doctor to be 30°. ”

“ A different way of using active compounds in producing antipodes consists in the so-called asymmetric synthesis. The method consists in the introduction of an active complex before that of the asymmetric carbon ; both stereo-isomers need not then form in the same quantity. W. Marckwald and A. McKenzie, who chiefly worked out this method, found, for example, that the salt of methylethylmalonic acid, C (CH3) (C2H5) (CO2H) 2 , with the active brucine forms on heating the corresponding salt of d- and /-methylethylacetic acid C (CH 3 ) (C 2 H 5 ) H (C0 2 H) , with the /-antipode in slight excess. ”

“ These isomers, having some relation to those in cyclic compounds, may be also denoted as cw- (maleic) and trans- (fumaric) forms, a close analogy existing indeed in those ring systems of which the simplest type is:—
this has been realized in the 1, 3-tetramethylene dicarboxylic acids, which exist in a trans- and cis-form:—
When two double carbon Unkings are present, as in H 2 C:C:CH 2 , the four hydrogen atoms form the summits of a tetrahedron according to the development in fig. 4 ; and consequently the introduction of different groups may bring enantiomorphism and optical antipodes. ”

“ The last group of stereo-isomers, in which insight is most difficult yet, is that of Werner's complex metallic compounds, observed with cobalt, platinum and chromium. No enantiomorphous character throws light here, and there is no relation to ethylene derivatives. ”

“ Could it be that the web of treachery and destruction the Kaiser’s busy agents were weaving had its deadly strands fastened even here—at West Point?
CHAPTER X
CARTER’S DISCOVERY
“It’s the young man I’m after,” said Chief Fleck. “We have the goods on old Hoff, but we have nothing incriminating against Frederic yet. The very fact that he holds aloof from his uncle’s activities makes me think he is engaged in more important work. He’s just the type the Germans would select as a director.” ”

“ Even at the luncheon table he got up once and walked to the window to look out, making some clumsy excuse about the beautiful view.
Determined to press the opportunity, Jane endeavored to turn the conversation into personal channels.
“You are an American,” she said turning to Hoff, “are you not? I’m surprised that you are not in uniform, too.”
“A man does not necessarily need to be in uniform to be serving his government,” he replied. ”

“ As a political sect, the Jacobins can be compared to none that ever existed, for none but themselves ever thought of an organized, regular, and continued system of murdering and plundering the rich, that they might debauch the poor by the distribution of their spoils. They bear, however, some resemblance to the frantic followers of John of Leyden and Knipperdoling, who occupied Munster in the seventeenth century, and committed, in the name of religion, the same frantic horrors which the French Jacobins did in that of freedom. ”

“ Next morning Hoffman had found himself practically a prisoner. Nearly the whole of the Chinese sided with the good-looking scoundrel, who had so often led them on their forays, and the German had to clear out, and was lucky to find a junk whose crew remained faithful to him. That is how he first came to Tinghai, and it was there that he saw Hobbs and his daughter for the first time. ”

“ The real Hopi should not live in town and cater after the fleshpots of the white man. He should not strive for big cars, go in debt or be obligated to any one in a manner which would make it difficult for him to be a true Hopi. ”

“ And also it is possible that Captain Joseph Joffre had not then begun to develop in himself those qualities which made him ready for greatness when the opportunity came.
If, however, any one has ever expressed surprise at Ferdinand Foch's attainment, I have not heard of it. He seems always to have impressed people with whom he came in contact as a man of tremendous energy, application, and thoroughness. ”