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rotation, which thus undergoes modifications which differ with every change in the relative proportions of the three components. In such cases several series of solutions must be prepared, such, that whilst in each series the active substance bears some one constant proportion by weight to one of the inactive substances, the
proportion of the other inactive substance undergoes variation.
In this way Biot? determined the combined action of water and boracic acid on dextro-tartaric acid. In presence of each of these substances singly, as by fusion along with increasing quantities of boracic acid on the one hand, or solution in increasing volumes of water on the other, the rotatory power of tartaric acid increases. The same is the case when a mixture of the two acids is brought into the presence of water, and then the amount of increase is found to depend upon two conditions :-(1) On the ratio between tartaric acid and boracic acid in the mixture. The property of increasing the rotation of tartaric acid possessed by boracic acid depends on the formation of an unstable compound (represented, according to Dubrunfaut,” by H, B 03 + 2 C, HOb) of higher rotatory power. The ratio between the tartaric acid and water remaining constant, the increase of rotation caused by the addition of different quantities of boracic acid (B parts by weight per unit-weight of solution) may be represented by the formula
BB [a] = A +
of which the three constants A B C must be determined by a series of observations. (2) On the amount of water. The latter exerts its inherent property of increasing the rotatory power of the tartaric acid ; on the other hand, it tends to break up the combination between the two acids, thereby causing a decrease of rotatory power. Experiment shows that so long as the mixture contains less than 0.088 part of boracic acid to 1 part tartaric acid, the rotation undergoes a continuous increase on the addition of increasing proportions of water. When the ratio between the tartaric and boracic acid is exactly 1:0.088, the specific rotation remains unchanged at each dilution, the decrease of optical activity due to the progressive breaking-up of the compound being exactly counterbalanced by the increase caused by the action of increasing volumes of water upon the acid set free. Lastly, should the mixture contain more than 0.088 part of boracic acid to 1 part tartaric acid, the influence of the first of the two factors predominates, and the observed rotation declines with successive additions of water. In such cases in general the variations may be expressed by the formula [a] = A + B q, in which q represents the proportion of water per unit-weight of mixture. If all three ingredients vary, certain weight proportions are found to give a maximum of rotation. The extensive series of researches by Biot show what great difficulty such ternary systems oppose to a correct appreciation of the rotation-phenomena.
| Biot : Ann. Chim. Phys. , 11, 82; 29, 341, 430; 59, 229. 2 Dubrunfaut: Comptes Rend. 42, 112.
Similar phenomena are exhibited in other cases, as, for example, when alkaloids are dissolved in varying proportions of acids and water, or in mixtures of alcohol and chloroform in different proportions. The variations in specific rotation in these cases are perfectly obscure, and we may meet with curves showing a maximum at any point whatever.
In polariscopic investigations it is therefore necessary to employ solvents in the purest possible state, or, at any rate, in using mixtures like aqueous alcohol, to see that the composition of the solvent is kept perfectly constant.
$ 27. A peculiar case of variation of specific rotation occurs in crystallized milk-sugar, C12H,2 011+ H, 0, and also in glucoses having the formula C6 H1206+ H, 0 (honey-sugar, grape-sugar, starch-sugar, and sugars from glucosides, as salicin, amygdalin, and phlorhizin). When a freshly prepared aqueous solution of one of these substances is placed under observation, it is found to exhibit a gradually decreasing rotatory power, after a certain length of time, however, becoming constant. The decrease begins immediately after solution, so that observations must be made with all speed if we wish to determine the maximum of rotation with anything like accuracy. At ordinary temperatures the decrease stops after the lapse of about twenty-four hours. Heat accelerates the change, and boiling brings it to a conclusion in the space of a few minutes; cold, on the contrary, delays it, as well as addition of alcohol, wood-spirit, or acids.
The extent to which such variations may occur in certain cases is shown by the subjoined results of experiments made by Hesse :
1 Oudemans : Liebig's Ann. 182, 51; 166, 70. 2 Hesse : Ann. d. Chem, 176, 99 and 105.
As the initial rotation is nearly twice as great as the constant rotation, the name of bi-rotation has been applied to this peculiarity.
As Dubrunfaut, Erdmann, and Béchamp' have shown, the phenomenon is connected with the fact that these sugars can assume two distinct modifications, the one crystalline, the other amorphous; the latter being produced by fusion. The crystalline form alone gives the initial maximum of rotatory power; the amorphous gives the minimum at once, and hence it is inferred that when the former is brought into solution, it undergoes a gradual conversion into the latter modification. The higher rotatory power observed at first is probably due to the presence in the liquid of certain groups of molecules (so-called crystalmolecules) possessing optically-active structures, still remaining intact after the solution of the crystals has taken place, and so superadding the rotation due to crystalline form, to that which is possessed by the individual molecules themselves. In course of time the perfect separation of these crystalline combinations into their constituent chemical molecules takes place, and then we have simply the rotation due to the latter.
Another instance of bi-rotation just after solution is found in the case of the crystals belonging to the rhomboidal system, formed by the union of grape-sugar and sodium chloride, 2 C6H1206. Na CI + H, O (Pasteur?). But no other instances of the phenomenon have hitherto been observed.
$ 28. From this inconstancy of specific rotation of substances in solution, it follows that we must no longer, as formerly, assume the complete indifference of the active to the presence of inactive
1 Dubrunfaut, Erdmann, and Béchamp : Jahresb. für Chem. 1855, 671 ; 1856, 639. ? Pasteur: Ann. Chim. Phys. , 31, 95.
molecules, since it is clear that the latter do exert some influence on the former, and it becomes a question what the nature of this influence is.
The statement by Biot in this connection is that the rotatory power of a substance may be modified either by changes in its chemical constitution, or, where this remains unaltered, by the formation of molecule-groups of variable composition, arising from the perfect distribution of the solvent through the active substance, the latter imparting to them that amount of rotatory power, endowed with which they constitute the active elements of the mass.
It is a conceivable view that when between the molecules of some active substance, such as oil of turpentine, in which the molecular attractions throughout the mass are in equilibrium, we introduce the molecules of some other substance, as alcohol, which set up mutual attractions different in degree from the former, some modification in the structure of the active molecule ensues—some change in the mutual distances of the atoms composing it, their arrangement in space, and mode of vibration. In this way would be produced a modification of that non-symmetry in the density of the ether on which the phenomenon of optical activity depends, and the result would be more marked in proportion to the number of inactive molecules taking part in the reaction. The solutions of an active substance in different inactive liquids ought in this way to.give different specific rotations, inasmuch as each species of molecule exerts an attraction peculiar to itself.
Of the theories proposed by Biot, the second alternative which supposes a transfer of rotatory power from the active molecules to a number of inactive molecules combined with them, is quite inconceivable.
we possibly suppose that in diesolving such bodies as oil of turpentine in alcohol, sugar in water, &c., the mere solution produces any real alteration of chemical constitution. If we purposely bring about such alteration by the addition of some strong re-agent, we shall at once see how different the two cases are. In the first case we were dealing with modifications of rotatory power of a temporary character; and it was possible by removing the disturb. ing agent to restore the rotation to its original intensity ; but in the latter case this is no longer possible—the modification is permanent. Whatever the true explanation may be, these variations of optical activity point to some mobile condition of the arrangement of atoms in the molecule. How considerable in amount they may at times
become, is evident from the fact, as we have already seen (16, towards the end), that in some cases the alteration is so great as to cause complete reversal of the original direction of rotation, lasting only so long as the disturbing substance is present.
C. Determination of the True Specific Rotation of Active Substances from the Rotatory Power of their Solutions.
§ 29. As we have seen (824), the value [a], calculated from a solution of an active substance, is never the true specific rotation of the substance itself, in a state of purity. It is necessary first to ascertain the law of the variation brought about by the inactive liquid, by examining a number of solutions of different strengths, and then the true specific rotation may be approximately obtained by calculating the value of constant A in the formula [a] = A + B4, or [a] = A + B + Cq?, &c. (see $ 24).
+ c Whether this method is really reliable or not can only be proved by experiments on substances, the real specific rotation of which can be determined independently. A series of solutions of such substances of different strengths must be prepared, from the observed specific rotations of which the value of constant A must be determined, and the result compared with that obtained by direct observation. Up to the present, the only investigations of this kind have been made by Biot with cane-sugar and tartaric acid, the latter of which, when cast in solid plates, was found to give nearly the same specific rotation as had been deduced from its solutions by application of the formula [a] = A + B
It was, therefore, of importance that further researches of the same kind should be instituted, and more particularly upon liquids, since it is only such substances that admit of the specific rotation of the absolute substance being accurately determined, as well as of a complete examination of the changes produced by the addition of successive quantities of various inactive solvents.
In the following pages are given the results of the author's investigations of the rotatory powers of right- and left-handed oil of turpentine, nicotine, and tartrate of ethyl, taken first alone, and afterwards in
| Biot: Mem de l'Acad., 13, 39 ; Ann. Chim. Phys. , 10, 175.