ROTATION OF ACTIVE COMPOUNDS IN ORGANIC SOLVENTS. 123 XIV.-Rotation of Opticully Active Compounds in Oyganic 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 b j the substance in solution is liable to enormous variations according to the pn~ticular solvent employed. Great importance, therefore, attaches to the discovery of any relationship between the real optical * The sign f is employed, as it has not yet been determined whether dextro- ethoxysuccinic acid is derived from dextro- or from levo-maic acid ; in either casr, lrowerer, our statement above is strongly supported by the facts.K 2124 FRANKLAND AND PlCKhKD : ROTATION OF 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 Breundler (ThBses pre’sente’s Ci la Faculte’ 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 follow- ing 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 compouud to undergo some change, and yields also abnormal cryoscopic and ebullioscopic figures.In this case, the concentration influences [a],, which departs from the normal value in proportion as the solution is more dilute.” The latter mode of behaviour is ascribed by Preundler to the operation of a dissoci:ition process in neutral organic solvents analo- gous 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 i t 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 con- sist, firstly, in the determination of the optical’ activity of a pure sub- stance in the liquid state, secondly in the determination of its activity a t different dilutions in solidifiable solvents, and, thirdly, in the cryoscopic determination of the molecular weight of the active sub- stlance in the same solvents a t similar dilutions. The optically active substance which served for the greater number of our experiments was met hylic dibenzoylglycerate, which cryatallises in beauxiful, slender needles melting a t 58--59O, and often upwards of an inch in length. The optical activity and molecular weight of methylic dibenzoyl- glycer ate were determined in the following solvents :-Benzene, acetic acid, ethylene dibromide, and nitrobenzene.Experirnerh with Benzene as Xolvmt. I n order to test the cryoscopic apparatus employed, which was of the ordinary Beckniann type with a thermometer graduated i n bnndreths of a degree Cent.., the following determinations were made with naphthalene.OPTICALLY ACTIVE COMPOUNDS IN ORGANIC SOLVENTS. 125 0 -3; 0 0 -865 0 -945 1 -360 1 '710 2 '100 2 '595 3 -010 3.380 , 4.320 C~yoscopic Determinntioir s with Naphthalene in Benzene. Xolecular weight of Naphthalene = 128. Molecular depression for Benzene = 49 (Raoult, Aimales d. Chim. et Phys., [6] 2, 1884). Weight of benzene in grams. 37 '4965 9 , 9 , 37 .;020 37 -4965 37 -2020 37 * 4965 Weight of naphthalene in grams.0.6345 0'7555 1 -1842 1 -6545 1 'ti800 2 5320 3 '2540 8 *3070 Grams of naph- thalenein 100 grams of solu- tion." 1 -6 1 -9 3 -0 4 -2 4.3 6 -3 8 -0 8 -1 Depression of freezing-point in "C. 0.685 0 -810 1 -285 1 9.45 1 -770 2 -660 3 -395 3-440 Molecular weight deduced. 121 '1 121 '8 120 '4 128 -9 125 -0 124 *4 126 '3 125 -6 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 methjlic dibenzoylglycerate (derived from dextrorotatory glyceric acid). Cryoscopic Determinations with Active Methylic Bihenzoylglycerate i i ~ Benzene.Molec ttlar weight of Methglic Dibenzoylglycerate = 328. Weight of benzene in grams. Weight of methylic diben- zoylglycerate in grams. 0 -1567 0 -3489 0 .3787 0 -5559 0 -7144 0 %8€3 1-1010 1.2920 1 -4439 1 *8644 Grams of subst,ance in 100 grams of solution. 2 -3 5 -0 5 -4 7 *8 9 *8 11 -0 14 '3 16 -4 18 *O 22 -0 Depression of freezing-point in "C. Indicated molecular weight. 315 -5 300 *5 298 -6 304 -2 311 '2 314 '4 316 -0 319 *8 318 *2 321 ' 5 * The percentage of substance employed is, throughout the paper, given in The terms of 100 parts of the solution and not of the solvent, as is usually done.126 FRANKLAND AND PICKARD : ROTATION OF Granis of solvent (benzene). In this series i t 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 solu- tions, 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 corre- sponding inactive '' racemate." This question was discussed many years ago by Perkin (Trans., 186'7, 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 t h e boiling points of the correspond- ing ethereal salts of the tartaric and racemjc 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 cryoRcopic examination directly comparable with that of the solutions examined in the polarimeter.* Since making these experiments, we End that K. Auwers (Zeit. physikal. Chem., 1894, 15, 51) has cryoscopically examined methylic and ethglic lactates (in- active Gf course) in benzene solution, with the following results. Meth,ylic lactate (inactive), C4H8O3 = 104. Grams of Grams of substance Observed Molecular weight substance. to 100 grams solvent.! depression. deduced. 15 -00 J Y J Y J J YJ Y ? ?, Y > Y Y 97 J Y J J Y 9 0 '1050 0 2220 0 *5555 1 -0205 1 '3920 0 -70 1 '48 3 -70 6 -80 9.28 0 '347 0 -662 1 '430 2 -271 2 '801 EthyZic lactate (inactive).C6H1,,03 = 118. 0 -0600 0 -1090 0 -3258 0 *6225 0 -9862 1 -3265 1 -5350 1-7725 0 -40 0 -73 2 -17 4 -15 6 -57 8 -84 10 -23 11 -82 0 -183 0 -302 0.831 1 *463 2.101 2 -535 2 -945 3 -294 98 9 110 -0 127 -0 14'7 -0 162 '0 107 -0 118.0 128 -0 139 '0 153.0 171.0 170.0 176.0 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 hydroxy- compounds in general. This conclusion can, however, obviously be only provision- ally drawn in the absence of any information as t,o the cryoscopic behavionr of the corresponding active compounds.OPTICALLY ACTIVE COMPOUNDS IN ORGANIC SOLVENTS.127 of cryoscopic determinations with inactive methylic dibenzoylglyce- rate, a substance which also crystallises in long, slender needles melting at 44-46', whilst the active body melts at 58-59'. The solubility of the active and inactive compounds in alcohol was also found to be very different ; thus at 1 2 * 8 O , 100 parts by weight of methylated spirit dissolves 1.96 part of active, and 5.33 parts of inactive methy lie diben zoy lgl ycerate. Cryoscopic Determinations with Inactive Methylic Dibenzoylglycerate in Benzene Solution. Molecular weight of inactive methjlic dibenzoylglycerate (calculated as a racemate) = 328 x 2 = 656.Weight benzene grame of in 5 -8980 8 *2791 8 -2791 5 .:980 Weight of methylic diben. zcylglycemte in grams. ~~ 0.2281 0 -2350 0 *4602 0 -4559 0.6542 0 -8111 1 -0280 1 -3430 1 -5917 1.8346 Grams of substance in 100 grams of solution. 3 . 7 3 ' 8 5 -2 7.1 7 -3 8 -9 11.0 13.9 16.1 18 -2 Depression of freezing point in O C. ---- I 0.600 1 0-640 ~ 0'915 i 1.240 1 '285 1 570 1 -980 2 '560 3 -015 ! 3.445 Indicated molecular weight. 308 *7 309 ' 8 297 -7 305 -4 301 -3 305 -7 307 -3 310 -5 312 *4 315 -2 The above figures f o r the molecular weight are obviously substan- tially the same as those obtained with active methylic dibenzoylgly- ccrate, and clearly indicat.e that in the benzene solution the molecules of the oppositely active ethereal salts, which give rise to the inactive com- POUnd, are not i n coinbination. The figures afford, moreover, in their divergence from the ca.lculated weight of the single molecule, a con- firmation of the results recorded above for active methylic dibenzoyl- gl ycerate.It will be interesting now to compare with these cryoscopic measurements the polarimetric determinations made with similar solutions of active methylic dibenzoylglycerate in benzene. The results are recorded in the following table (p. 128). These figures show that the rotation [aID of the ethereal salt in benzene solution is greatly in excess of its rotation in the pure state, and, further, that the rotation increases slowly, but unmistakably, with the dilution of the solution. From the diagram given on p.140, it will be seen that the rate of increase in rotation proceeds very steadily wit.h the dilution until the highest dilution is reached, when128 FRANKLAND AND PICKARD : ROTATION OF an abrupt rise in the rotation was observed. But, on extrapolation, it is found that., even with infinite concentration, the rotation is con- siderably in excess of that experimentally obtained with the pure substance in a state of fusion. Thus, on producing the straight line beyond the diagram t.0 100 per cent. concentration, the value [aD] = + 3 3 * 3 O is obtained, whilst the rotation of the pure substance at 15" is [a]D = +26.89'. Rotatiova of Benzene SINutims of Actice Methy& Dibenzoylglycerate. [a],, for methylic dibenzoylglycerata in the pure state a t 15' C.= -I- 26.89". Weight of benzene solution in grams. 4 "1633 3 -4963 3 -3259 6 *2430 4 09486 Weight of substnnce in grams. 0 -1435 0.1691 0 *2914 1 - 2191 1 *6916 Grams of aubstsnce in 100 grams of solution. 3 -0 4 - 7 7.2 19'5 39.1 empera- Density of Observed ture a t solution rotation, at tempem- which a,,, in rotation of 100.47 mm. o~~~~~~~ and densitj tube. withLwahr were deter, at mined. + 1-23O 0.8929 14.5" 1.86 0.8962 16.5 2.87 0.9017 16.5 7 -80 0.9411 15.0 13.76 0.9847 145 Specific rotation, ca3D. + 45*70° 44 '01' 43 *66 42 -26 40 '72 EXPERIMENTS WITH ETHYT~EX E DIBROMIDE AS SOLVENT. A similar series of experiments was made both with the active and inactive methylic dibenzoylglycerates i n ethylene dibromide solution (see next page). The ethylene dibromide employed, both in the cryo- scopic and polaritnetric determinations, was dried with calcium chloride and redistilled (b.p. 129'). Thus, in the case of the ethylene dibromide solution there is little or no evidence of dissociation, even with the highest dilutions employed, whilst in the more concentrated solutions the indicated values for the molecular weight are distinctly excessive. The values obtained in the case of the inactive compound are substantially the same as those for the active. The melting point was 10". Jt In order to ascertain wliethcr the rotation is affected by the solution being kept, this particular solution was preserved 18 hours before examination, whilst the other solutions vere examined a t once. The result shows that no change appears to take place, as the figure obtained falle into line with the others.OPTICALLY ACTIVE COMPOUNDS IN ORGANIC SOLVENTS.129 Depression of the frcezing-point in "C. Cryoscopic Detewninatioias wiih Active and Inactive Jfeethylic Dibenzoyl- glycemtes in Ethylene Dibromide Sokition. Molecular weight of methylic dibenzoylglycerate = 328. Molecular depression for ethylene dibromide = 118. Indicated molecular weight. Weight of ethylene dibromide in grams. 9 '7573 25 -2479 19 9 9 *Y573 25 $479 9 %73 ?t t ? 3 9 3 ) 1 ) 15 -7778 5 5932 15 "7778 5.5932 7 9 7? 1 ) t f >> Weight of methjlic diben- zoy lglycerate in grams. (0 0 2078 0 -6784 0.9805 1 -2457 1 -4982 0 -6807 0 -7809 0 '9238 1 -0589 2 * 7747 2 -9323 3 -1297 3,3903 1 *3553 1 -7050 0 *2125 0 '0963 0 -4595 0 -3107 0 -4031 0 -4643 0 -5596 0 -7146 0 -9003 (bj Grams of substance in 100 grams of solution.2 *o 2 -6 3 -7 4 '7 5 -6 6 -5 7 - 4 8 '6 9 -8 9.9 10 -4 11 *o 11 -9 12 *2 14 *9 lnactive Corn2 1 '3 1 -7 2 ' 8 5 '3 6 -7 7 *6 9 '0 11 '3 13 -8 0.780 0 -985 1 -415 1 -755 2 -090 2 *470 2 -740 3 *240 3 -695 3 *725 3 *885 4 *155 4 -525 4 '630 5 -745 0 -475 0 -580 1 *040 1 -900 2 '450 2.870 3 -460 4 -290 5 -290 ound. 322 -2 321 -9 323 -8 331 9 336 -0 333 -3 344.7 344.8 346 -6 348 -1 352 -7 352 * 1 350' 1 354 -0 358 -8 334 *6 350 -3 330 -5 345 *o 347 -1 341 '3 341.2 351 *4 359 -0 The results given in the table on p. 130 were obtained on examining similar solutions in the polarimeter. Thus, the rotation for all concentrations is greatly inferior to that exhibited by the pure substance in a state of fusion ; there is, more- over, very little variation with the concentration, but such as there is leads to the stronger solutions having-a distinctly higher rotation than the weaker ones.This slightly greater rotation is thus obtained in the case of those solutions which yield the markedly high values for the molecular weight as determined by the cryoscopic method. The relationship between the rotation and concentmtion is best seen by reference to the diagirtm 011 p. 140. By producing the straight line beyond the diagram until a coccentration of 100per cent. is reached, the value [a]D = + 3 2 O is obtained, and which is con- siderably in excess of that actually pielded by the pure substance.130 Weight of substance, in grams. FRANKLAND AND PICKARD : ROTATIOX OF Grams of substance in 100 grams solution.Rotation of Ethylene Dibrornide Solutions qf Actice Methylic Dib enzo y l g lcerate . [ a ] D for methylic dibenzoylglycerate in the pure state at 15" = + 26.89'. 0 -3695 0 -8982 1.1146 2.5857 3.0936 Weight of ethylene dibromide solution, in grams. 3 '3 6 -6 10.9 15.4 22-3 10 -9492 13 -5373 10 -1979 16 *8104 13.8365 Observed rotation, a=, in 100.47 mm. tube. Density of solution at tempera- ture of observation compared with water s t 4O. Tempera- ture at which rotation was ob- served. + 1 -38" 2 -78 4 a36 6 '33 9 '03 2 * 1286 2 -0749 2 '0103 1 '9494 1 *a538 CQID i 19 .IS0+ 20 -09" 19 -77 21 -02 21 -69 EXPERTMFNTS WITH NITROBENZENE AS Sor,vmiT. A similar series of cryoscopic determinations was made both with active and inactive methylic diben~oylglycerat~e in nitrobenzene solu- tion.The nitrobenzene employed bothin these and in the mbsequent polarimetric measurements had a constant boiling point of 20i0 and melted at 5.4". The cryoscopic determinations gal-e very irregular results ; wc have, however, recorded them in the following table. The figures from the cryoscopic determinations (next page), although disappointingly irregular, show that the indicated mole- cular weight is distinctly greater in nitrobenzene than in benzene, many of the values found approximating closely to the theoretical, irrespectively of the concentration. In spite of their irregularity, tfhe general tendency of the figures is to show that with the greatest dilution the molecular weight is lower than the theoret,ical, and with the highest concentration that it rises above the theoretical.As regards the indicated molecular weight of the inactive com- pound, all the values obtained very closely approximate to the theo- retical weight of a siagle molecule, and thus confirm. the conclusion arrived at in the case of tbe benzene and ethylene dibromide solu- tions, that the oppositely active molecnles of which the inactive compound consists are not in combination in the solution. On now turning to the polarimetric determinations made with the nitrobenzene solution of the active compound, the following results were obtained (see second table on next page). meter. * These solutions were made up 18 hours before examination with the polari-OPTICALLY ACTIVE COYPOUNDS 1N ORGANIC SOLVENTS.131 Cryoscopic Determinations with Active a d In,actire Me fhylic Dibenzoyl- glycerate i n Nitrobe.lzzene SiCOlutioiB. (a). Active Compound. Molecular weight of methylic dibenzoylglycerate = 328. Molecular depression for nitrobenzene = 72. 4 '4 6 '5 '7.4 8 '8 9-7 11 '5 14 '9 17 '2 Weight of nitrobenzene in grams. 1 -000 1 *545 1 -760 2 -160 2 -390 2 -860 3 -785 4.500 5 -2072 5 '4957 5 * 2072 5 '2072 5 -4957 5 -0482 5 '2072 5 -4957 5 *id72 5 '0482 6 '2072 5 '4957 5 '0764 5 4 5 7 97 9 ) 7 9 9 9 9 9 9 ) 97 Weight of sitbstance in grams. Weight. OE substance in grams. Density of G~~~~ of Observed solution at in 100 QD in Of mm. observatior compared grams solution. tube. wit,, ~ a t e r a t 4". substance rOtat,iOn 0 *0925 0 '2247 0 -2919 0 '3780 0 -4702 0 '4681 0 -6501 0 -6333 0 -6580 0 -7367 0 .8?8S 0 -9519 0 -9642 1 -0019 1-1507 0 -23'36 0 -3536 0 - 4064 0 -4936 0 *5452 0 -6623 0.8919 1 -0563 (b) * 0 *1668 0 -2632 0.6795 0.9950 1.8694 Grams of substance in 100 grams of solution.2.4 +0'58 1.2079 5 -5 1-40 1.2082 11.3 2.94 1.2082 17.4 4-60 1'2096 28-1 7-54 1.2164 1 -7 3 '9 5 '3 6.7 7.8 8 '3 10 -5 11 '1 11 '2 11-8 13 '7 15 -4 36.0 16 '1 17 -3 Depression of freezing point in O C. 0 -420 0.900 1 -270 1 -655 1 -910 2 -050 2 '500 2.665 2 -810 2 -885 3.495 4 -015 4 -030 4 -290 4 -475 Indicated molecular weight. 304'5 327 -1 317% 315 -8 322 -5 315 -7 340 -7 338 -9 323 -8 334 *4 329 -2 327 '8 341 -2 329 '8 336 *9 331 -3 324 -6 327 -5 324 -1 323 -6 328 *4 334 '2 338 -9 Rotation of Nitrobenzene Solutions of Active Methylic nibenzoylglycerate.iaJD from ethylic dibenzoylglycerate in the pure state a t 15" C. = + 26.89". ~~ ~ Weight of nitro- benzene solution in grams. ~ ~~ 6 -9157 4.7130 5 * 9898 5 "7157 6 -6643 Temperu- ture a t which rotation was observed. bl, . 15'-0 15 *9 16 *3 16 '6 15 -5 + 16.83 20 -62 21 '33" 21 '75 21 -99 * This solution was made up 18 hour# before polarimetric examination.132 FRAXKLAND AND PICKARD : ROTATION OF From these figures it will be seen that, although the observations were made over a very wide range of concentration, the specific i~otat~ion only suffered comparatively slight change. The specific rotation was only aboilt one-half of that exhibited in benzene solu- tion, and was markedlyinferior to that possessed by the pure sub- stance in a state OP fusion.Moreovei-, whilst in benzene the rotation increases with the dilution, in nitrobenzene it diminishes. The relationship is best seen from the diagram on p. 140, from which it may also be shown tbat, by extrapolating, the value for infinite con- centration closely approaches, although it is sIightly below, the experimental value for [a]D obtained with the pure siibstance in n state of fusion. Thus, by producing the straight line beyond the diagram until it reaches n, concentration of 100 per cent., the value obtained for [a]D is + 25', whilst the pure substance actually gives 1231, = +26.89'. EXPERIMENTS WITH Acenc ACID AS SOLVENT. The acetic acid employed boiled at 118", and had a melting point of 10.4', a specimen with higher melting point not being available at thc time.C'ryoscopie Detemzinations with A d i r e a d Inrxctivc Meth ylic Dibenzoyl- glycerate in Acetic acid Solution. Molecular M eight of methylic dihenzoylgljcerate = 328. Moleciilar depression for acetic acid = 39. Weight of acetic acid in grams. Weight of substance in grams. Grams of substance in 100 grams of soliit ion. Depression of freezing point in "C. 1 ndicated niolecular weight. 3 '7355 5 -0224 3 '7500 3 -8540 5 *0565 3 "7500 4 -3326 5 * 0 2 4 3 -9021 3 -7500 5 -0224 3 -8540 3 *go21 4 -3326 5 -0224 0 -0785 0 *l806 0' 1636 0 '2089 0 -3016 0 *3197 0 -3609 0 '4212 0 '3645 0 -4362 0.6145 0 '5733 0 - 6324 0 -6998 0 -9775 (a). Actire Compound. 2 -0 3.4 4 - 1 5 -1 5 -6 7 -8 7 -7 7 -7 8 ' 5 10 -4 10 -9 12 -9 13 -3 13 -9 16 -2 0 -250 0 '460 0 '495 0 -690 0 -710 0 -970 1 *ooo 1 -045 1 * o w 1 '330 1 *515 1 '840 lmb30 1 *850 2 -345 327 -9 304 '9 343 *7' 306 '4 327 -6 342 *9* 324 -9 313 -0 337 '3 341 el+ 315 -0 315 '3 329 -0 340-5 823 .% * I t will be obser.red that these figures, wliic.1~ are the niost errtitic, were allOPTICALLY ACTIVE COXPOUNDS IN ORGAMC SOLVENTS.133 4 *4101 0 '1145 77 0 -2725 >? 0'3947 9, 0 -5317 Y9 0 -6869 2-5 0 '300 337 * 5 5 '8 0 -770 312 -9 8 . 2 1 *095 318 -8 10 -7 1 -335 339 *5 13 *4 1 "150 346 '8 From the above figures, it will be seen that the results were of a more erratic character than wit.h the other solvents, the values f o r the molecular weight being sometimes above and sometimes below the theoretical. This is doubtless to be accounted for partly by the molecular depression possessed by acetic acid beiug smaller than that of the other solvents, and partly to the very hygroscopic character of the glacial acid, both circumstances which would tend to diminish the accuracy of the determinations.Another circumstance which must have interfered with the accuracy of the results was that the quan- tities of both solvent and substame employed were exceptionally small. In the case of the inactive compound, again, the cryoscopic deter- minations negative the existence of a double molecule racemate i n the solution, the values obtained for the molecular weight being essentially similar to those obtained f o r the active compound. Solutions of active methylic dibenzoylgl.ycerate were also examined polarimetrically, with the following resu 1 ts.Rotntioa of Acetic acid Solutions of Actice Methylic Dibenzoylglyceyate. [.ID f w methylic dibenzoylglycerate in state of fusion a t 15" C. = + 2689". Weight of acetic acid solution in grams. 7.6560 5 -0756 5 -8478 7.6152 6 -030'7 Weight of substance in grams. 0 -1313 0 '2430 0 *5548 1 *0356 1 -1219 Grams of substance in 100 grams of Bolution. 1 . 7 4 - 7 9 ' 5 13 -6 18 -6 Obs en-ed rotation, a=, iu 100.47 mm tube. + 0 -@do 1-71 3 '31 4 *79 6 -55 Density of solution a t tempera- ture of observutior compared with water a t 4 O . ' 1 -0561 1-0694 1 -0699 1 '075G 1 -0820 Tempera- ture a t which rotation was observed. 1 6 ' 2 O 16 -7 15 -6 16 *8 16 -3 34 -34" 32 '27 32 *45* 32 '61 32'38* obtained in a single series of experiments in which successive additions of thcb substance were made to one and the same p a n f i f y (viz., 3.75 grams) of the solvent.* These solutions were prepared 18 hours before pdarinietric examination.134 PRANKLAND AND PICKARD : ROTATION OF .-- The figures obtained for [ a ] ~ are all in excess of that obtained for the piire substance in a st'at'e of fusion at 15O, but they change com- paratively little on varying the degree of concentration ; the highest dilutions, however, exhibit the greatest rotcation, and by reference to the diagram on p. 140 it willbe seen that the acetic acid solutio~i gives values for [ a ] ~ , which approximate more closely than in the case of the other solvents t o the value obtained for the pure substance, On extrapolation for infinite concentration, moreover, it is found that the value for [ a ] D almost exactly coincides with the value obtained for the pure substance.Thus on producing the acetic acid straight line beyond the diagram, the value [a]= = +27*2O is obtained for 100 per cent. concentration, the pure substance giving [a]= = +26.89' at 15". I-- EX PERIRIENTS MADE WITH SOLUTIONS OF ACTIVE ETHYLIC DIACETYT,- GLY C E RATE. The striking contrasts between the results, both cryoscopic and polarimetric, obtained with benzene and acetic acid solutions of active methylic dibenzoylglycerate, rendered it desirable to ascertain whether similar results would be yielded in the case of other active substances. To this end, cryoscopic and polarime tric determinations were made with benzene and acetic acid solutions of active ethylic diacetylglycerate.The preparation and properties of this compound have already been described by one of us (Percy Frankland and Macgregor, Trans., 1893, 63, 142.2), and the following results were now obtained with its solutions in benzene and acetic acid re- spectively. ethglic diacetyl- glycerate in grams. Cryoscopic. Determinations with Benzene Solutions of Active Ethy lic Diacety lg lycernte. Molecular weight of ethylic diacetylglycerate = 218. Molecular depression for benzene = 49. substance in 100 grams of solution. Weight of benzene in grams. 0.1261 0 -3700 0 *5208 0'6451 0 -7223 0 *7988 3 -4 9'4 12.7 16 *8 18 -3 i 35.2 Depression of freezing- point in "C. Indicated molecular weight . 0 ,830' 2 -380 ' 3-320 1 4-050 4 '410 4 *855 208 -7 214 -1 215 -5 ' 218-8 225 '0 226 '1OPTICALLY ACTIVE COMPOCNDS 1N ORGANIC SOLVENTS.135 The above figures show that the solutions of ethylic diacetyl- glycerate in benzene give nearly true crjoscopic values for the molecular weight. The values for the higher dilutions are somewhat below the theoretical, whilst with increasing concentration they rise somewhat above it. The rotation of these benzene solutions was now investigated. Rotation of Benzene Solutions of Active Ethylic: Diacetylglycerate. [a],, for pure ethjlic diacetylglycerate at 15" = -16.31. Density a t ture of observation compared with water at 40. tempem- Weight of benzene solution, in grams. Tempera- ture at which rotation was observed. 4 '0621 7 '4943 0.8948 0.9580 Weight of ethylic diacet,yl- glyccrat,e, in grams.15.3" 15.0 0 '2153 2 -2290 Grams of substnnce in 100 grams oE solution. 5 *3 29.8 Observed rotation, an. in 00-47 mm tube. - 0 '82" -4*24 - 17 -20" - 14 -83" Thus with a low concentration the value for [a]= is somewhat greater than that exhibited by the pure substance, whilst with a, high concentration it is somewhat below the latter. Similar experiments were then made with solutioris of ethylic diacetylglycerate in acetic acid. Cryoscopic Determinations with Acetic acid Solutions of Active Ethylic Diacetylglycerate. l\folecular weight of ethylic diacetylglycerate = 218. Molecular depression of acetic acid = 39. Weight of acetic acid, in grams. Weight of ethylic diacetyl. glgcerate, in grdm s. 0 *0531 0 '1208 0 '2337 0 -4678 0 -7709 0 '925'7 Grams of substance in 100 grams of solution.1 '0 2 -4 4-5 8 -8 13 9 16 -1 I Depression of freezing point, in "C. Indicated molecular weight. 0.310 0 -705 1 -270 2 -040 3 -250 3 -860 135 *? 138 -3 148 -6 185 '2 191 *6 193 *6 The results present a striking contrast to those obtained in the case of benzene, for with all the concentrations investigated the values * This solution stood €or 18 hours before it was examined.136 FRANKLAND AND PIGKARD : ROTATION OF Observed rotation, ~ D Y in 100.47 mm. tube. for the molecular weight were greatly below the theoretical, the values rising with the concentration. Thus whilst with benzene for a concentration of 15.2 per cent. the cryoscopic value for the mole- cular weight was normal, with acetic acid for a concentration of 16.1 per cent.the value for Ihe molecular weight was much lower than with a concentration of only 3.4 per cent. in the case of the benzene, Rotation o j Acetic acid Solutions (,f Actire Ethyl ic liiacetylglycerate. [alD for pure ethylic diacetylglycerate at 15' = - 16 -31. Density a t tempera- ture of obserratior compared with water Weight of acetic acid solution, in grams. 4 -8922 5 -4825 Weight of e thylic diacetyl- glycerate, in grame. 0 -1651 2 -3775 Grams of substance in 100 grams solution. 3 *4 25 -0 Tcmpera- ture of observatioi of rota- tion. -5.28 1.0783 15.4 - - L - O 3 O I 1-0599 I Calp -288*'74O - 19 '44* Thus, with a concentration of 3.4 per cent., the value for [ a ] D is much greater, and, even wit,h a Lmcentration of 25 per cent.? con- siderably greater than that exhibited by the pure substance. These results are of particular interest, as showing that even in the case of two compounds so closely allied as methylic dibenzoylglycerate and ethylic diacetylglycerate, there may be this great divergence in the optical and cryoscopic properties, according to the solvent, employed, The experimental results recorded in the previous pages may bo thus summarised :- 1.Cryoscopic determinations show that inactive methylic di- benzoylglycerate does not exist as a racemised molecule when dis- solved i n benzene, ethylene dibromide, nitrobenzene, or acetic acid respectively. The values for the molecular weight are in all cases similar to those obtained under the same conditions for that of active methylic dibenzoylglycerate.2. The cryoscopic values for the molecular weight of methylic di- benzoylglycerate vary according to the solvent and the concentration employed. With benzene, all the values are below the theoretical. With ethylene dibromide and with nitrobenzene, the values are with low concentration below, and with high concentration above the theoretical. I n the case of acetic acid, the values are, with all con- centrations, sometimes above and sometimes below the theoretical. 3. In all cases, the specific rotation [a]= of active methylic di. f This solution stood for 18 hours before examination.OPTICALLY ACTIVE COJIPOUNDS I N ORGAKIC SOLVENTS. 137 bcnzoylglycernte is more or less affected by the solvent. In the case of benzene, the values of [a]= are much in excess, and in that of ethylene dibromide and nitrobenzene they are much below the value of I n the case of acetic acid, the values for [a]= most closely approximate to that of the pure substance.4. Low cryoscopic values for the molecular weight of methylic dibeiizoylglycerate are accompanied by high values for the specific rotation, and vice versc2. 5. This relationship between specific rotation and indicated mole- cular weight is borne out by the behaTiour of ethylic diacetylglyceratc in benzene and acetic acid respectively, but i n this case the low molecular weights and high rotations are obtained in acatic acid, the high molecular weight and low rotations in benzene. 6. The real rotation of the active compoucd cannot be directly calculated from the rotation of its solution, even hen the cryoscopic examination of that solution shows the molecular weight to be normal.Thus for methylic dibenzoylglycerate in acetic acid solutions giving normal molecular weights, [a]D was too high, whilst. conversely in nitrobenzene and ethylene dibromide solutions, giving normal inolecular weights, the values of [ u J D were too low. By graphic cxtrapolation for infinite concentration, as in the diagram on p. 140, lrowever, all three solvents give 1-alues at any rate approximating to the actual specific rotation obtained with the pure substance. In the case of benzene, on the other liand, all the solutions examined gave molecular weights below the theoretical, and the produced rotation curve gives R value for [a]D a t infinite conceiitration which departs rnore widelyfrom the real value than do the produced curves for nitrobenzene, ethylene dibromide, and acetic acid.It would appear, therefore, that even a moderately accurate estimate of the real rotation can only be arrived at by the study of solutions giving normal molecrilar meights, and extrapolating for infinite conceiitrstioii on their rotation curves. Thus the real specific rotation of methylic dibenzoylglycerate is for the pure substance. [a]= = +26.89" a t 15" C., whiIst [a]D calculated by extrapolation from benzene solu- tion values.. ........................... = +33-3" calculated by extrapolation from acetic acid solution values.. ........................ = 3-27-2 calculated by extrapolation from nitrobenzene solution values ......................... = 3-25.0 calculated by extrapolation from ethylene di- bromide solution values.................. = + 32.0 calculated from mean of the above extrapola- tion values.. .......................... = +29*4 TOL. LXIX. L ,, ,, ,, ,,138 FRANKLAND AND PlCKhRD : ROTATION OF Moleculrtr Moleculer [ u ] froni weight weight benzene (theoretical). , (cryoscopic) . solution. Propylic diacetyl- + 1 - 2 O . - tar irate 7. Our experiments show that the rotation of an active substance may be either raised or depressed by solvents ; similarly, the mole- cular weight, ci~yoscopically measured, may be either raised or de- pressed by solvents, the variations in the rotation being doubtless dependent on the variations in molecular weight.Now, the varia- tions in molecular weight can be most consistently explained on the assumption of dissociation and association processes taking place, both of which may go on concurrently. The phenomenon of dissociation is most conspicuously exhibited i n the case of the benzene solution of methylic dibenzoylglgcerate and in that of the acetic acid solution of ethylic diacetylglycerate. I n both cases, the cryoscopic values for the molecular weight are markedly below the theoretical figures. The effect of this assumed dissociation is in both cases to greatly increase the rotation. The act.ive ion must, however, be different in the two cases, as the dibenzoyl- glycerate is dextrorotatory, whilst t'he diacetylglycerate is laevo- rotatory, although both are derived from one and the same active glyceric acid (dextro-) .By dissociation of the dibenzoylglpcerate, the rotation becomes more dextrorotatory, whilst by dissociation of the diacetglglycernte the rotation becomes more laevorotatory. It must not be supposed, however, that the effect of dissociation is invariably to increase the rotation ; thus, in Freundler's experiments on the tetra-substitut ed tartrates, the dissociation was accompanied by diminution in the value of [ o I ] ~ , thus [U]D real. ---- + 13 *4" (Loc. cit., 1). 114). If, in the case of methylic dibenzoylglycerate, high values for specific rotation are shown by cryoscopic measurement to be acconi- panied by dissociation, we should naturally infer that low values for [a],, must be due to the opposing influence of association.* Such low values for [a]D we find in the case of the ethylene dibromide and nitrobenzene solutions of methylic dibenzoylglycerate.The variation in [%ID f o r differences of concentration is compara- tively small in the case of these two solvents, but such as it is, t.his * Just as there is a t present no 6 priori means of ascertaining whcthei* dissocia- tion will be attended by increase or by decrease in the value of [U]D, 80 there is none for predicting the effect on [ a ] ~ of association.OPTICALLY ACTIVE COJIPOUNDS I N ORGAEU'IC SOLVEKTS. 139 variation takes place in the opposite sense to that which goes on in the dissociating benzene solution, for with high dilution in the case of ethylene dibromide and nitrobenzene there is a diminution in the values for [%ID (see the diagram on p.140). These low values for the specific rotation would thus find the readiest explanation on the bypothesis of association becoming more pronounced the greater the dilution of the solution. Of such association at high dilutions there is, howerer, no direct evideiice from the cryoscopic determinations, the indicated molecular weights in the case of ethylene dibromide, and still more so in the case of nitrobenzene being somewhat below the theoretical value. On the other hand, with these two solvents at high concentrations, the indicated values for molecular weight are considerably in excess of the theoretical. But if these high indicated molecular weights were tho result of association we ought to find the values f o r [a]D simultaneously falling, whilst, as a matter of fact, they rise with the coticen tration.The only polarimetric confirmation of this cryoscopic evidence of association which we can find in our experiments is in the case of the benzene solutions of ethylic dincetylglycerate. Here the most con- centrated solutions give indicated molecular weights in excess of the theoretical, the value of [a]D calculated from such solutions being less than the value of [a]D for the pure substance, and since the dilute solutions give cryoscopic evidence of dissociation accompanied by excessive vaiuea for [a]=, we can conclude that association and deficient values for [a]D are connected, and that the deficient value for [a]D with high concentration is confirniation of the association cryoscopically indicated.Amongst the experimental material furnished by Freundler (Zoc. cit., p. 117) there are the following cases exhibiting cryoscopic evidence of association. Benzene i3olzctions. Active compound. ---- Metliglic tartrate. . . Propglic ,, . .. Molecular weight (t,heoretical). 178 234 --- Molecular weight (found). [ u ] ~ measured [.]I, obtained on t,he with the pure substance. I solution. - 8 . F + 2 -14' + 20 -1 I Eth y Eerie Dibyornide So Itition. -0.6" I + 12'44p I 326 I I 234 Propylic tartrate. . . ~~ Thr;s, in all the above cases, the cryoscopic determinations afford140 F’RANKLAND AND PIOKARD : ROTATIOX OP strong evidence of association, and yet the effect on the rotation is quite irregular, for in the benzene solution the effect on the rotation of methylic tartrate is negative, whilst in that of propylic tartrate it is positive.There is of coume nothing surprising in t,hese results, a,s association with the molecules of the same solvent may produce opposite rotatory effects in the case of two different active com- pounds, a,nd association with the molecules of two different solvents mayproduce opposite rotatory effects in the case of one aad the same active compound. In his experiments on the tetra-substituted tartrates. Freundler finds evidence, in several cases, of dissociation in organic solvents, and is of opinion thatl the dissociation consist.s in the splitting off of the two acid mdicles* substitnting the two alcoholic hydrogen atoms of the t,artaric acid.Our experiments, however, clearly show that the molecules of the fully substituted glycerates dissociate otherwise. Thm, as already poiri ted out, methylic dibeneoyl- glycerate dissociates in bezzone, and the active ion is moye dextro- rotatory than the undissociat,ed molecule. Ethylic diacetylglycerate, on the other hand, dissociates i n acetic acid, and the activc ion is more powerfully lavorotatorg than the nndissociated compound. Thus, the two active ions difler not only in sign but enormously in degree, whilst, if the acid radicles were split off, the only differ- ence in their constitution would be the presence of methyl in the one and of ethyl in the other, thus 7 H,- o i C, H,O CH,*O/ I C2H,0 CH*O/C;H,O , . . . . . . .. . . . . . . .. . .. . QH* O/C,H,O , ................. . . bOOCH, COOC2H5 Methylic dibenzoylglycerate Ethylic diucetylglycerate (dextrorotatory). (levorotatory). But we know that the dextrorotation of the dibenzoylglycerates is conditioned by the benzoyl gi‘oups, and the methylglyceryl and ethylglyceryl ions could not possibly differ in rotation to the extent which the ions in question actually do, hence .me regard the dissocia- tion as almost certainly taking place as indicated below QH,*OC,H,O $IH,*OC2830 YH*OCqHjO , QH*OC.&B30 COO: CsHJ COOiCH, I Q This supposition is quite out of harmony with Yerkin’s experiments on diethylic benzoyltartrate, in which it was found that by hjdrolising with an insufficient quantity of alcoholic potash benzoyltartaric acid was formed, the ethyl groups being eliminat,ed, whilst, the benzoyl groups remained atAached (Trans., 1867, 20,141).SPECIFIC ROTATlON OF METHYL DIBENZOYLGLYCERATE IN VARIOUS SOLVENTS (BENZENE, ACETIC ACID, NITROBENZENE AND ETHYLENE OIBROMIDE. )OPTICALLY ACTIVE COMPOUNDS IK ORGAKIC SOLVENTS. I4 I substance in grams. This process would leave the active ions cliffering profoundly in eon- stitutiou as they do i u rotation. We have recently obtained further evidence bearing on this point., in connection with the rotation of ethylic monobenzoylglycerate. This substance which has been prepared by one of us in conjunction with Mr. MacGregor is in all probability the /3-compound, OF the formula $lHz*O C7H5O YH*OH CO 0 C2H5 I n a state of fusion, the specific rotation of this substance is [ a ] , -9*80", whilst, with a benzene solution, the following resuIt was - - grams solution. ---- obtained. Weight O f benzene solution, grams. PI_ 9 -5558 I 1*3701 1 14.3 0 bserved rotation aD in 1984 mm. tube. -__ - 0 '925O Density of ture. I ---- I 0.9194 1 15.5" Thus the activity of the substance in beiizene solntion was far less laevorotatory than in the purs state, a circumstance which is quite in harmony with the supposition t,hat in benzene solution the molecule is more or less dissociat'ed into a C2H5-ion and the complex active ion CloH,O,. Tor, as already pointed out, in these ethereal salts of glyceric acid, dextrorotation is conditioned by the piweence of benzopl groups in place of the hydroxylic hydrogen atonis ; whilst lsvorotation is conditioned by the positive radicles replaciiig the carboxylic hydrogeo; now if, in the above substance the benzoyl group were split off in the dissociation supposed to take place in the benzene solution, then the molecule should become more laevo- rotatory; on the other hand, if it is the ethyl group that is split off we should anticipate t h a t the molecule would become more dextro- rotatory by virtue of the more prepondernt'ing influence obtained by the benzoyl group; and this is precisely what takes place, for in benzene solution t,he substance actually becomes more dextro- rotatory, or rather its lm-orotation i8 greatly diminished. Mason College, Birmi?zgh an1 .