Prof. I. I. Kotyukov.

Index of Optically Active Substances.

(Theory of the Mesostructure of Organic Compounds II).

The theory of the mesostructure of organic compounds that we are developing is an attempt to reform the outdated and, in terms of factual material, inadequate methodology of so-called “stereochemistry.” In the preceding communication¹) the problem of “geometrical” isomerism was considered from an entirely new point of view, which made it possible both to outline ways toward its solution and to unite under the concept of “mesoisomerism” phenomena that are directly connected with the problem of “spatial” arrangement. Among the latter are cryptochemical polymorphism, steric hindrance, and the phenomena of diritiration in the benzene nucleus. However, the regions considered are by no means self-sufficient in the colossal theoretical conception of “stereochemistry”—they are only consequences of the famous postulate of van ’t Hoff concerning the so-called “asymmetric carbon atom.” This latter, and it alone, is the foundation upon which the whole fragile and inelegant edifice of “stereochemistry” rests. The theory of mesostructure proposes new theoretical premises for the phenomena of “mesoisomerism,” premises encompassing a far greater body of factual material than the classical theory and making it possible to predict precisely the number of mesoisomers. But if the new theory attempts to explain in another way phenomena that rest upon the postulate of the vectoriality of the four units of affinity of carbon, then naturally a doubt also arises as to the rationality of the postulate itself. If “geometrical” isomerism is to be reconsidered, then it is quite impossible to leave without attention the optical isomerism so closely related to it, especially since the latter, both theoretically and practically, is far more important than the former.

The first step in the study of any field is the gathering of the corresponding literature. Here, however, we encounter a rather unexpected and strange phenomenon. To be sure, the literature of stereochemistry itself does not shine with good monographs—the latest edition of Stewart’s Stereochemistry is dated 1919 and differs very little from the preceding edition of 1909. But the state of the literature on optical activity is still more discouraging. It turned out that a compilation on optically active substances in a more or less exhaustive form had not been made since Landolt, i.e., since 1898. Since the main proof of the rationality of the theory of mesostructure lies in the outlining of real paths toward solving the problem of the dependence of the magnitude of rotation of the plane of polarization on structure, and since the problem of optical activity is directly connected with the solved problem of “geometrical” isomerism, we decided to acquaint ourselves in detail with the exhaustive

¹) Izvestiya S. T. I., vol. 48 (1927).

factual material in this field. As a result of this enormous work we have also constructed an “index of optically active substances.”

The first exhaustive compilation on optically active substances was given by Landolt in the first edition of his Das optische Drehungsvermögen, 1877. There about 240 substances were collected. The second edition, 1879, contains about 400 substances, and the last, of 1898, 1220. In 1904 Walden, in his famous compilation on optical activity in the Berichte—a compilation that from that time onward drew the attention of outstanding chemists to the phenomenon of optical activity and gave impetus to the extraordinary development of this field—expresses the supposition that up to that time about 900 optically active substances had been prepared. An exhaustive collection of the material by us gave, by 1904, a total of about 3360 substances. At the present time, when the index has been completed and put in order, we have counted, through 1927 inclusive, about 12,000 substances, excerpted from more than 2500 original articles.

The index of optically active substances is at present on cards. The cards were filled out from the material available mainly in Chemisches Centralblatt, and only when there were ambiguities, omissions, or—as occurred especially often with respect to the most important English literature—references to the original source, did we turn to the context. For certain years inaccessible to us we used Chemical Abstracts. On the card, on the left side, were entered: the empirical formula, molecular weight, name, melting or boiling point, specific rotation with indication of wavelength, temperature and solvent, and finally the citation. For example:

\[ \begin{array}{lll} \mathrm{C}_{10}\mathrm{H}_{16}\mathrm{O} & \text{Camphor} & [\alpha]_{D}^{20}=-40.56\\ 156 & & \mathrm{CH}_{3}\mathrm{OH}\\ & 178.6 & 44.75\\ & & \mathrm{CS}_{2}\\ 329. & & 1912.\ \mathrm{II}.\ 1202 \end{array} \]

The number at the lower left indicates the series of the index.

On the reverse side of the card the structural formula was copied out; in individual cases, the meso-formula and the molecular rotation for all available conditions. For example,

\[ [M]_{D}^{20}=+84.2 \]

The advantages of the card system are generally known, and there is no point in justifying it here.

As I have already mentioned, there turned out to be about 12,000 cards. We classified them by years, by series, by the number of asymmetric carbon atoms, by the purpose for which the preparation was made, and by chemical nature.

By years, optically active substances are distributed as follows:

From the table given it is evident that the World War, as in all areas of our life, had a very sharp effect on the number of optically active substances—from 1914 to 1918 their number fell by more than half. After the conclusion of peace the figures again grow steadily. At the present time it may be considered that about 500 optically active substances are prepared per year, described on average in one and a half hundred communications. These figures correspond precisely to the prewar years 1908–1914.

All the cards were then divided into series, with each series containing substances of identical or very similar chemical nature. The overall picture obtained was as follows:

The great bulk of the above-listed preparations consists of substances prepared not for the investigation of their optical activity; here rotation was considered only as a constant permitting the substance to be individualized. These include carbohydrates, alkaloids, many terpenes, polypeptides, naphthenes, and so forth. The next category consists of preparations specially made for the investigation of rotation from natural products—derivatives of camphor, menthol, tartaric acid, and others. Finally, the fewest substances are those decomposed into optical isomers artificially, and their special derivatives. Thus, unfortunately, the enormous majority of the figures do not satisfy modern requirements for the rotational constant. Rotational spectra, in particular, have been very little studied.

For the same reasons, there are very few optically active substances of comparatively simple structure. Thus, with one asymmetric...

with a carbon atom there are about 1500 of them, and with two identical ones, 400.

As regards their chemical nature, the bulk consists of alkaloids ($\sim 1500$), camphor and its derivatives ($\sim 1500$), carbohydrates ($\sim 1500$), menthol derivatives ($\sim 800$), oxyacids ($\sim 700$), and amino acids with polypeptides ($\sim 500$). Almost all these substances are of a very complex nature, with a poorly determined structure that allows the possibility of various unexpected isomerizations and rearrangements. The very small number of such important representatives as aldehydes and ketones, tertiary alcohols, hydrocarbons, nitro compounds, and heterocycles is striking. All these categories are as yet little accessible, because of the technical conditions for resolution into antipodes.

The enormous work we have carried out, unfortunately, under present-day conditions cannot appear in print in context—it would occupy about 700 printed pages.

We hope to present the most important categories of substances in literary surveys of our experimental work in this field.