The benzene derivatives admit of similar treatment,' which, however, is influenced by the fact that the relative position of the six carbon atoms is here not quite settled. We assume Kekulé's hypothesis, according to which double and single bonds alternate, and compare with benzene those analogous closed chains of five to eight carbon atoms, which may be assumed from the valence of carbon, namely, (CH)4, (CH),CH2, (CH), (CH)CH2, and (CH)。 ̧• To this end let us place side by side the sums of the angles which our theory requires when two carbon atoms are joined to a third (about 109° in the case of a single, and 125° in the case of a double bond), with the sums of the angles of a closed polyhedron: We see that, in fact, the greatest approximation occurs in the case of benzene, which accounts for the stability of this substance as well as for the fact that up to the present the others have not been prepared. 1 Wunderlich, Konfiguration organischer Moleküle; van 't Hoff Maandblad voor Natuurwetenschappen, 7, 150. CHAPTER VII NUMERICAL VALUE OF THE ROTATORY POWER WHEREAS thus far we have spoken only of the absence or presence of rotatory power, we have now to do with the magnitude of the rotation. It is already a considerable time since such determinations began to be made, and (as the expression of the quantitative relation) the so-called molecular rotation was chosen-that is, the specific rotation, a,1 multiplied by the molecular weight (and for shortness divided by 100). The chief results so obtained are, firstly, the statement of Mulder, Krecke, and Thomson2 that the molecular rotations within certain groups of substances bear a simple ratio to one another; and, secondly, the observation of Oudemans and Landolt that different salts of the same active base or acid in dilute aqueous solution possess the same molecular rotation. Such considerations have gained a new interest for stereochemistry since Guye3 and Crum Brown1 attempted 1 Rotation caused by 1 decim., the substance being supposed present in this column with the density one. 2 Zeitschr. f. Chemie, 1868, 58; Zeitschr. f. prakt. Chem. 1872, 5 6; Ber. 1880, 1881. * Thèses, 1891; Ann. Chim. et Phys. [6], 25, 145. 4 Proc. Roy. Soc. Edinb. 17, 181. to connect the magnitude of the rotation with the nature of the groups attached to the asymmetric carbon atom; accordingly the facts bearing on the question are here given in detail. I. COMPARISON OF THE NUMERICAL RESULTS. SOLUTION AND OF TAKING INTO ACCOUNT THE It was à priori certain that the relation between the groups attached to the asymmetric carbon and the rotation must be such that when two groups become identical the rotation vanishes; but in attempting to go beyond this we are at once met by the difficulty that the magnitude of the rotation depends on the wave-length of the light, on the solvent, and on the temperature. The first thing is, then, to determine the conditions in which comparable numbers may be obtained. And here it seems most essential to avail ourselves of the light thrown on the subject by the new conception of the nature of solutions. It is certainly inadmissible to use simply the figures obtained by an examination of the substance without special precautions, because the size of the molecule is then uncertain, and the magnitude of the rotation seems to be specially influenced by every change of constitution. In this connection it is important to remember the fact recently discovered by Ramsay,' that, of fifty-seven liquids examined, no 1 Chem. Soc. J. 1893, 1098. less than twenty-one possessed double molecules, among them the alcohols, acids, nitro-ethane, acetonitrile, and acetone. Another objection is that the rotation is generally influenced by the solvent, and, indeed, by every solvent differently, perhaps in consequence of the four groups attached to carbon being differently attracted. If the substance be used alone, without solvent, its own molecules may be supposed to exert a similar influence, an influence displayed most prominently in the formation of crystals, and which, in the case of strychnine sulphate, e.g., leads to the almost complete annihilation of the rotation. The objections mentioned disappear completely only when the substance is examined in the state of rarefied gas. As this is impracticable we are driven to adopt some other means, and thus arrive naturally at the state of dilute solution. It is also indispensable, of course, to take into account the molecular weight, which can then easily be determined; while the comparability of the results will evidently be by far the greatest when the same solvent is chosen for the different cases. The influence of wave-length and of temperature seems not to be important if the circumstances of each case are duly taken into account. The anomalous rotation-dispersion in the case of, say, tartaric acid in aqueous solution-which is such that the rotation changes its direction with the colour-is evidently connected with phenomena of equilibrium which affect the tartaric acid in the solution; it was also found by Biot in a mixture of right- and lefthanded substance. The same holds for the great alteration in the rotation of tartaric acid when the temperature, the concentration, or the solvent is changed. All these phenomena are connected together and only make necessary a careful use of the figures obtained, but are no argument against the existence of relations between rotation and constitution in general. II. ROTATORY POWER OF ELECTROLYTES. LAW OF OUDEMANS-LANDOLT Active bases. In perfect harmony with the new views of the nature of aqueous solutions-according to which electrolytes undergo, at a sufficient degree of dilution, a division into ions until, as Arrhenius pointed out, a limit is reached-stands Oudemans' observation concerning salts of active bases and acids. At a sufficient degree of dilution the molecular rotation of quinine, e.g., is independent of the salt observed. The following table (p. 137) gives the results obtained by Oudemans and also by Tykociner; it gives the specific rotation [a], observed at 16° C., and calculated for the base. 2 It may be remarked here that the equality of rotation which Wyrouboff3 recently showed to exist in solutions of isomorphous sulphates and selenates 1 Rec. des Trav. Chim. des Pays-Bas, 1, 18, 184. 2 l.c. 1, 144. For nicotine, Schwebel, Ber. 15, 2850; Carrara, Gazz. Chim. 23, [2], 593. 3 Compt. Rend. 115, 832. |