Thus in the above series such a change as that from dimethylic acetylmalate to dimethylic chloracetylmalate is essentially similar to the change from methylic diacetylglycerate to methylic diphenylacetylglycerate; and in both cases the effect on rotation is remarkably small. In malic acid, the hydroxyl group being directly attached to the asymmetric carbon atom, corresponds to the a-hydroxyl group in glyceric acid, and replacement of this hydrogen in both cases produces a profound change in the rotation. Still more profound is the change in the rotation if the whole of the OH-group is substituted; as, for example, in the passage from dimethylic malate to dimethylic chlorosuccinate.

Again, it appears that the replacement of the hydroxylic hydrogen by a hydrocarbon radicle produces far more effect on the rotation than the substitution of the same atom of hydrogen by an acid radicle containing the same number of carbon atoms; thus a comparison may be made between

XIV.-Rotation of Optically Active Compounds in Organic Solvents.

By PERCY FRANKLAND, Ph.D., F.R.S., and ROBERT HOWSON PICKARD, B.Sc.

IN pursuing the study of the connection between optical activity and chemical composition, the investigation is frequently hampered by the circumstances that the active compounds under examination are solid at those temperatures at which polarimetric observations can be conveniently made, and that the optical activity displayed by the substance in solution is liable to enormous variations according to the particular solvent employed. Great importance, therefore, attaches to the discovery of any relationship between the real optical

*The signis employed, as it has not yet been determined whether dextroethoxysuccinic acid is derived from dextro- or from lævo-malic acid; in either case, however, our statement above is strongly supported by the facts.

activity of a particular substance and the variable activity which it exhibits in different solvents.

This problem has been attacked in a very suggestive way by Freundler (Thèses présentés à la Faculté des Sciences de Paris, 1894) in connection with his interesting researches on the derivatives of tartaric acid, and his conclusions are summarised in the two following statements.

"When a solvent gives normal figures for the molecular weight of the dissolved active compound, it does not alter its rotatory power for any concentration.

"On the contrary, if the solvent gives abnormal figures for the rotatory power, it causes the compound to undergo some change, and yields also abnormal cryoscopic and ebullioscopic figures. In this case, the concentration influences [a]D, which departs from the normal value in proportion as the solution is more dilute."

The latter mode of behaviour is ascribed by Freundler to the operation of a dissociation process in neutral organic solvents analogous to that which takes place in saline solutions.

The double importance of this subject, from the point of view of optical activity on the one hand, and from that of the dissociation theory on the other, appeared to render it highly desirable that the validity of these conclusions should be tested by further experiments, and for which some of the optically active compounds prepared by one of us were particularly well adapted.

The experiments which we have carried out with this object consist, firstly, in the determination of the optical activity of a pure substance in the liquid state, secondly in the determination of its activity at different dilutions in solidifiable solvents, and, thirdly, in the cryoscopic determination of the molecular weight of the active substance in the same solvents at similar dilutions.

The optically active substance which served for the greater number of our experiments was methylic dibenzoylglycerate, which crystallises in beautiful, slender needles melting at 58-59°, and often upwards of an inch in length.

The optical activity and molecular weight of methylic dibenzoylglycerate were determined in the following solvents:-Benzene, acetic acid, ethylene dibromide, and nitrobenzene.

Experiments with Benzene as Solvent.

In order to test the cryoscopic apparatus employed, which was of the ordinary Beckmann type with a thermometer graduated in hundreths of a degree Cent., the following determinations were made with naphthalene.

The above figures, which approximate to the theoretical molecular weight of naphthalene, show that the arrangement employed was capable of yielding accurate results.

In the next instance, a similar series of cryoscopic determinations was made with active methylic dibenzoylglycerate (derived from dextrorotatory glyceric acid).

Cryoscopic Determinations with Active Methylic Dibenzoylglycerate in Benzene.

Molecular weight of Methylic Dibenzoylglycerate

= 328.

The percentage of substance employed is, throughout the paper, given in terms of 100 parts of the solution and not of the solvent, as is usually done. The

In this series it will be seen that the indicated molecular weights of the ethereal salt are in all cases decidedly below the theoretical, the values being, on the whole, smallest for the most dilute solutions, and rising with increased concentration.

The molecular weight of an optically active compound at once raises the question as to what is the molecular weight of the corresponding inactive "racemate." This question was discussed many years ago by Perkin (Trans., 1867, 20, 149), who was, however, unable to obtain the molecular weights of the ethereal salts of tartaric and racemic acids by vapour density determination, but came to the conclusion that their molecular weights must be identical in consequence of the identity of the boiling points of the corresponding ethereal salts of the tartaric and racemic acids. As far as we are aware, however, this point has not yet been investigated by means of cryoscopic and ebullioscopic methods for racemates dissolved in organic liquids. We proceeded, therefore, to make a similar series object of this is to render the percentage composition of the solutions submitted to cryoscopic examination directly comparable with that of the solutions examined in the polarimeter.

Since making these experiments, we find that K. Auwers (Zeit. physikal. Chem., 1894, 15, 51) has cryoscopically examined methylic and ethylic lactates (inactive of course) in benzene solution, with the following results.

There is no reference made to the possibility of the lactates being present as racemised molecules, and the very high values obtained for the molecular weights are attributed exclusively to the abnormal behaviour which is exhibited by hydroxycompounds in general. This conclusion can, however, obviously be only provisionally drawn in the absence of any information as to the cryoscopic behaviour of the corresponding active compounds.