THE OPTICAL ROTATION OF LACTIC ACID* BY WILDER D. BANCROFT A I D HERBERT L. DAVIS
A recent study' of malic acid led us to the conclusion that its optical rotatory power is due not only to the rotations of its ions and molecules as their formula is ordinarily written but also to the rotation of another tautomeric form whose rotation is in the opposite direction to that of the ions or molecules and which has an ethylene oxide formula. It was shown that solutions of 1-malic acid contain mixtures of the normal form and of the ethylene oxide form. Form I (Levo-)
Ho\ O=C
Form I1 (Dextro-)
H O yC Ho\
HOCH
I
HCH
I
COOH
I I
HCH COOH
The levorotatory form is the one which predominates in dilute aqueous solutions and ionizes to give levorotatory ions; while the dextrorotatory form predominates in optical effect in concentrated solutions causing such solutions of the free acid or its salts to be dextrorotatory. There has been no confusion in the designation of the malic acids, the acid whose properties have just been described being always called 1-malic acid. This is by no means true of the lactic acids and even today what acid any given author means by d-lactic acid can only be determined by a careful study of the properties reported. Sarcolactic acid in aqueous solutions which are not too concentrated is dextrorotatory while its salts and esters show levorotation. This acid should most properly be designated l(+)lactic acid. The present paper will show that the properties of lactic acid are exactly analogous to those of malic acid and that the rotations are to be explained in the same manner. The study of lactic acid is further complicated by the existence of anhydride and lactide forms. Such forms have not been reported for malic acid and if they exist at all are far less stable than those of lactic acid. The lactic acids occur in the usual three modifications: the dextrorotatory alpha-hydroxy-propionic acid called also dextrolactic, sarcolactic or paralactic acid; levolactic acid; and finally the inactive or dl-lactic acid commonly called Bancroft and Davis: J. Phys. Chem., 34, 897 (1930). the programme now being carried out a t Cornell University under a grant from the Heckscher Foundation for the Advancement of Research established hy August Heckscher a t Cornell University.
* This work is part of
OPTICAL ROTATION OF LACTIC ACID
2 509
fermentation lactic acid. The commercial lactic acid is usually a synthetic product or a fermentation product. I n the latter case we have confirmed the observations of various authors and find that the commercial lactic wid does show some activity due to a slight excess of one form of the active acid over the other. Thus a sample from Kahlbaum was found to be weakly dextrorotat,ory. In order t,o avoid confusion we shall in the present paper treat the subject entirely in terms of sarcolactic acid even though some of the data reported have been obtained from investigation of the antipodal acid or its salts. There is no authentic’ evidence of any difference other than the direction of optical rotation between the two acids, so that anything which may be said of one optically active form may be equally said of the other form with the sign of rotation changed. The sarcolactic acid with which we deal, then, is dextrorotatory in dilute aqueous solutions, while it’s salts and esters are levorotatory. As has been pointed out by us before2 and, as will be more fully shown in this paper, this acid should be called l(+)lactic acid to indicate that its real rotation is levorotatory even though, because of abnormalities in its properties, it happens to give dextrorotatory aqueous solutions. The necessity of considering the salts and esters as giving the real rotation of such alpha-hydroxy acids as lactic acid was pointed out by van’t HofP and has gradually been working into accepted practice, as is shown in the note to a paper by Frankland and Garner.‘ “In considerations of this kind in which the sign of optical activity is concerned, much confusion often arises owing to the rotation of an acid and its salts or esters being of opposite sign. In all such cases, it is most satisfactory to denote the rotation of the acid by the sign exhibited by its salts in dilute aqueous solution, that is, the sign of the active acid ion. This convention will be adopted consistently in the present paper. Thus by 1-lactic acid will be denoted the acid which gives levo-lactates.” X more recent example of the confusion which is sometimes arising in this connection is the reply of Clough5 to a criticism from Levene. Levene and Haller ( 1 9 2 7 ) showed that “the optically active form of a-hydroxybutyric acid which is dextrorotatory in aqueous solutions a t ordinary temperatures, but which yields levorotatory salts and esters, possesses the same configuration as d-lactic acid or 1-t’artaric acid.” Clough had previously said that, 1-lactic acid was the acid of analogous configuration and now replies: ”In order to distinguish the enantiomorphous forms of a compound from one another, the conventional symbols d and 1 have usually been assigned to them in a somewhat arbitrary manner. Thus d-lactic acid is the form which is deutrorotatory in aqueous solution at ordinary temperature but which gives levorotatory salts and exters, while 1-aspartic acid which may be prepared from
3
5
Herzog and Slansky: Z. physiol. Chem., 73, 240 ( 1 9 1 1 ) . Bancroft and Davis: J. Phys. Chem., 35, 1624 ( 1 9 3 1 ) . van’t Hoff: “The Arrangement of Atoms in Space”, 141 (1898). Frankland and Garner: J. Chern. SOC., 105, I I O ( 1 9 1 4 ) . Clough: J. Biol. Chern., 75, 489 (1927).
2510
WILDER D. BANCROFT AND HERBERT L. DAVIS
natural 1-aspargine, is dextrorotatory in aqueous solution. Guye and Jordan who resolved z-hydroxybutyric acid into its optically active forms, termed that variety of this acid, the salts and esters of which were levorotatory, ‘l’acide alpha-oxy-butyrique gauche’. Beilstein refers to the same compound as ‘1-alpha-oxy-buttersaure’. The present author also designated this compound 1-alpha-hydroxybutyric acid and expressed the view that it was configurationally related to 1-tartaric acid.” Probably no better example could be cited to show the confusion resulting from a failure to understand the principle enunciated by van’t Hoff and to apply it. The numerous investigations designed to correlate the configurations of various optically active compounds have brought this question much nearer a solution and have shown clvarly the necessity of thus considering the acids in terms of their salts and esters. Patterson and Lawson’ have reported such a study for the derivatives of that form of lactic acid which is levorotatory. The derivatives are dextrototatory and their change in rotation with change in temperature classifies them with the derivatives of d-tartaric acid and the conclusion is reached that this form of lactic acid should be called d-lactic acid.
The Forms of Sarcolactic Acid The references just cited demonstrate clearly the wisdom of an adequate system of nomenclature and designation for the lactic acids. Probably the most familar form of lactic acid is the sarcolactic acid which is dextrorotatory in dilute aqueous solutions and which gives levorotatory salts and esters. This acid should be designated as 1 (+) lactic acid and will be referred to hereafter as sarcolactic acid. The literature includes some discussion of a t least three apparently well-authenticated forms of sarcolactic acid and to these we must now add a fourth form, an ethylene oxide form analogous to that shown to exist in the case of malic acid. These four forms take part in equilibria and will be referred to by these names. Normal form CH3.CHOH.COOH
e
Anhydride form CH3.CHOH.CO
I I
0
CH,.CH,COOH
Ethylene oxide form CH3.CH.C/OH LOA ‘OH Patterson and Lawson: J. Chem.
SOC., 1929, 2024.
Lactide form CH,. CH. CO
I I
0
OC.
I 1
0
CH.CH3
OPTICAL ROTATION O F LACTIC ACID
2511
The evidence indicates that the first three familiar forms of sarcolactic acid are all levorotatory while the ethylene oxide form is dextrorotatory and this evidence will now be set forth. Wislicenus' made an early, extensive, and classic study of the lactic acids, their properties, and their formula. The zinc salts are the ones most commonly prepared and the active lactic acids give zinc lactates with two mole~ ) ~ the inactive zinc salt crystallizes with three cules of water ( 1 2 . 8 8 ~while molecules of water (18.187~).The water content of the zinc salts together with their optical activity serve as tests of their purity. Wislicenus extracted sarcolactic acid from meat and found that when this acid was heated in a retort in a current of dry air and at I~o'-I~o', the lactide distilled over and crystallized, having a melting point of 124.jo. This lactide was found to be practically inactive but on removing the water from a lactic acid solution stored over sulphuric acid at room temperature, Wislicenus obtained a mixture which he believed to contain only anhydride and lactide and which in Jungfleisch and Godchot realcohol showed a specific rotation of -85.9'. peated many of the experiments of Widicenus and they were able to show* that the inactivation discovered by Wislicenus was due to the prolonged heating necessary for the distillation of large samples of lactic acid. If smaller samples were taken, dehydrated at 70°, and then distilled over in a vacuum at 150'-15 ,'j an optically active lactide possessing a different crystalline form and melting point (9j') from the dl-lactide was obtained. I n benzene solution this lactide was found to possess the following rotations : Grams of lactide per Specific rotation
IOO
cc of solution
166j -298'
I
o j832 -280'
o 2916 - 246'
Because of the difference in solvents, these rotations are not directly comparable with those of Wislicenus but demonstrated that Jungfleisch and Godchot undoubtedly had in hand a much purer preparation of the lactide than did Wislicenus. This lactide, which they call the dilactide, was dissolved (0.117g. of dilactide in 30 cc of aqueous solution) and the hydrolysis followed with time by means of the rotations at 13'. Hours Specific rotation
0
-192.8'
6
2
-141'
-111'
I2
-59.8'
48 -42.7'
72 -8'
This is merely the usual reversal of the dehydration and if permitted to reach equilibrium would have shown a dextrorotation. If the lactide were diluted successively in aqueous solution,the decrease in rotatory power observed would be at once attributed to the formation of the more weakly rotatory anhydride or normal form. This explanation is not valid however to explain the same phenomenon in the benzene solutions above for there is no water present. The change must therefore be due to Wislicenus: Ann., 167, 302 (1873). Jungfleisch and Godchot: Compt. rend., 141,
111
(1904).
WILDER D. BANCROFT AND HERBERT L. DAVIS
2512
some other cause such as the existence in equilibrium of such an ethylene oxide form as is here proposed. This change may be quite simply achieved by the mere breaking of t'he oxygen bonds of the lactide, producing the ethylene oxide forms minus one molecule of water.
CHS
C&
I
0 = C-O--CH
I
HC-O--C
I
CH3
CH
I =0
.1-
o =HC Y>+Oate
C N a, (a), (AI),
47.17
23.58
4.21
2.10
-7.83
2.37' 10.62
-8.78
11.9
-3.50'
11.79 1.05 2.80'
11.87 13.3
j.89 0.526 1.46' 12.4
13.9
2.95
1.47
0.74
0.263
0,131
0.74'
0.36'
o.06j 0.18'
12.53 14.05
12.23 13. 7
12.20
13.68
Again these first four points lie practically on a straight line which then bends over to what amounts to a constant value for the optical rotatory poiver for the solution of o.5K and less. It is this value of - 14' which represents the real rotatory power of the lactate ion from whatever source it may be derived. Further evidence for this and for the abnormal character of the zinc salt is shown by similar data for zinc lactate. .1 sample was weighed out and dissolved in hot water, the solution being then cooled and diluted. The first three solutions were supersaturated. All were examined in 2 dm tubes. T.4BLE
111
Rotation-Dilution of Zinc Lactate 14.07 1.0
- I . 51' -5.37
-6.
j~
7.03
3.51
I.jj
0.j
0 . 2 j
0.97' 6.89 8.35
0.56' 7.96 9.65
0.12j 0.29'
8.27 10.05
0.85 o.062j 0.16'
9.13 11.1
WILDER D. BANCROFT A S D HERBERT L. DAVIS
2520
In Table I11 is introduced a new term, (E)Dthe equivalent rotation and this is equal to half the molecular rotation in the case of zinc lactate. The specific rotation is the rotation that would be observed in a tube one decimeter in length and filled with a solution containing one gram of solute per cubic centimeter of solution and is calculated on the assumption that the rotation is independent of the concentration. The molecular rotation is the specific rotation multiplied by the molecular weight divided by 100. The equivalent rotation is the specific rotation multiplied by the equivalent weight divided by 100. This enables one to compare directly solutions containing the same number of the active lactate radicals.
0
FIG.I Rotation-Dilution of Sarcolactic Acid and Lactates
The graph of these rotation-dilution data shown in Fig. I demonstrates that both sodium lactate and zinc lactate approach the value of -14' in the dilute solution and that for each concentration the rotation of the zinc salt is below that of the sodium salt, the former changing much more rapidly with concentration. This may be attributed to the lower dissociation of the zinc salt or to the hydrolysis resulting in the formation of some of the free acid and hence the dextrorotatory ethylene oxide form, or what seems most likely, to a markedly increased tendency in concentrated solutions of the zinc salt to form the ethylene oxide form of the ion or of the salt molecule. This might go according to the scheme; Dextrorotatory Levorotatory O= C-0-Zn-0-C =0 HO-C-0-Zn-0-C-OH / l>O HOCH HLOH 0\cH HC
I I
CHs
I CH3
s
I
CH3
I
CH3
OPTICAL ROTATION OF LACTIC ACID
2521
These data agree with those of Purdiel who found the rotation of the Na, 0.1RI solutions,while lactates of six bivalent metals were below this value but extrapolated to it. The abnormality in the case of the rotation of zinc lactate may be associated with the observation of Irvine.2 “Reference may be made here to experiments which are in progress in this laboratory on the solubilities of the zinc lactates. I t has been found that although active zinc lactate when crystallized in the usual manner readily loses its water of crystallization a t IIO’, the residue left on evaporating an aqueous solution of the salt on a water bath does not become completely anhydrous even at 150’. I t is evident, therefore, that in estimating the specific rotation of active zinc lactate the concentration of the solution cannot be determined by the evaporation of a measured volume of the solution.” There appear to be two possible explanations for this observation. I t may very well be that heating brings about the hydrolysis of the zinc lactate, the consequent loss of some of the lactic acid and a residue of a basic zinc lactate or even the hydrous oxide losing water with much greater difficulty. The other possibility is that the hot saturated solution of zinc lactate formed quantities of the ethylene oxide form of the salt and that this form for some reason loses water much more slowly than does the normal zinc lactate in crystal. The present section established the fact that the normal form of lactic acid is levorotatory and probably of about the same rotatory power as the sodium salt derived from it. I n the field of the indicators it has been established that dilute colored acids give ions and dilute salt solutions of the same color unless some internal warrangement takes place. The same thing is probably true of the rotatory power of the acid, its ions and salts. Failure of these three to show the same direction of rotation will in general be an indication of some internal rearrangement. Since, therefore, the normal form of sarcolactic acid, the anhydride form, and the lactide forms are all levorotatory, the explanation of the dextrorotation of its aqueous solutions must be referred to a rearrangement into a dextrorotatory ethylene oxide form. The next section will demonstrate how these forms enter into the various equilibria and how they affect the rotatory power of the aqueous solutions.
K, and NHI salts 14’ in
Equilibria in Lactic Acid Solutions It has now been shown that solutions of sarcolactic acid may contain mixtures of three levorotatory forms and one dextrorotatory form. The equilibria involving these forms are reached with differentvelocities and may be followed by means of the optical rotatory poiyer of the solution. The data indicate that if one starts with a dilute solution containing only the normal and the ethylene oxide form the rotation will be to the right as a consequence of the greater rotatory effect of the ethylene oxide form. As one withdraws water from such a solution, the equilibrium between these two tautomeric forins l
Purdie: J. Chem. SOC.,67, 616 (189j). Irvine: J. Chern. SOC.,89, 935 (1906).
2522
WILDER D. BANCROFT AND HERBERT L. DAVIS
is displaced in favor of the ethylene oxide form and the dextrorotation increases. As concentration of the solution increases there begins another very slow reaction, the conversion of the normal form or of the ethylene oxide form to the relatively strongly levorotatory anhydride and to the very strongly levorotatory lactide. A rapid concentration of the lactic acid solution leads to increasing dextrorotation, while, if the concentration is carried far enough, this dextrorotation is found to diminish with time and to change to fairly large levorotations. TTe shall thus have for freshly prepared solutions a regular increase in dextrorotations while, if these solutions are permitted to come to equilibrium, there will be with increasing concentration, first an increasing dextrorotation t o a maximum, and then for solutions sufficiently concentrated to develop the anhydride or lactide forms, decreasing dextrorotations leading finally to levorotatory solutions of a power several times that of the original lactic acid. This change in the rotatory power of lactic acid solutions was demonstrated by the first preparation of lactic acid already reported. The tube containing some of this original most concentrated (8.4 S ) solution was examined from time to time. TABLE
IT7
Time-Rotation of Sarcolactic Acid Time in days Specific rotation
0
f5.26'
I
j
5 o 4.03
7 IO 3.54 z . 7 j
I2
15
2.27
1.93
I7 1.37
The final value did not represent equilibrium but the change in this tube was not followed beyond this point. I t is of interest to note that the most concentrated sodium lactate solution did not change appreciably in seven days while the acid dropped to 7oyc of its initial value in that period. Xow in the acid solution the decrease in rotation is to be attributed to the formation of the anhydride with its levorotation. The constancy of the rotation of the sodium salt shows the absence of such an effect there, for of course neither anhydride nor lactide can form in the sodium lactate solution. In other words, in the salt solution, only the normal-ethylene oxide equilibrium is added to the ionization effect while in the free acid these and the anhydride and lactide must be considered. Another evidence of this change is shown in the dilution-time-rotation runs on this same original acid. On the day of the original dilutions this acid diluted with fifteen volumes of water in steps gave a solution for which (M)D = +0.763', while when the original acid had stood in the concentrated form for five days and was then diluted from 8.4 K to 0 . 5 2 ?; the resultant solution showed ( h l ) = ~ -0.286'. The explanation of this is that during the five day period there had formed in the concentrated solution sufficient of the levorotatory anhydride form so that when on dilution the equilibrium was a t once adjusted to a relatively low ethylene oxide concentration, this was not sufficient to overcome the levorotation of the anhydride form. Certainly the latter situation did not represent equilibrium while the dilution of the
OPTICAL ROTATION O F LACTIC ACID
2523
freshly prepared solution did represent very nearly an equilibrium condition. If the levorotatory solution had been permitted to stand it would have changed toward or to the dextrorotation shown by the same concentration of lactic acid freshly prepared. This demonstrates a t the same time the possibility of preparing levorotatory dilute solutions of sarcolactic acid even though they are surely not in equilibrium and also the necessity of having careful regard to the history of any lactic acid solution if one is to draw any conclusions from its rotation. A very striking confirmation of these explanations was obtained from the study of another sample of sarcolactic acid. This second preparation was as the first, the final filtrate being evaporated in the vacuum still to 47 cc and then stored over sulphuric acid for a week in a desiccator until it,s volume !%-as 2 0 cc (as compared to 3 j cc for the first preparation). At' the end of that time the reacidification observed on letting the neutralized solution stand at room temperature for five hours was about 0.5 of one per cent of the total alkali required so that no large amaunt of anhydride had developed. This concentrated sarcolactic acid was placed in a 0.94; dm tube, anot,her portion was diluted to twice its volume with water and a third sample was made by diluting the first dilution to twice its volume. The ratio of concentrations then was I : 0.j : 0.25. The first t\yo solutions were placed in 0.947 dm tubes while the third was examined in a 2 dm tube, all being left at room temperature and being observed from time to time.
TABLE V Time-Rotation-Dilution of Sarcolactic Acid 0
Original (0.947 dm tube) First dilution (0.947 dm tube) Second Dilution ( 2 dm tube)
+3.16' + o . 77' -o.zIo
4 days - j.0 I 0 $0.84' +o.Ijo
41 days -19.36'
+
1.170
--_
These data are a t first glance quite incomprehensible but on further examination are found to agree with the explanations previously made. The first dilution solution reached a constant value within the 41 days and did not change further in eighteen months. h similar behavior would have been shown by the second dilution but this tube developed a leak and was lost, The behavior of the original concentration solution is most interesting, for even 41 days was not sufficient to enable this solution to reach equilibrium, and after eighteen months it showed a reading of - z 1.8', having changed but 0.5' in the previous year. It appears quite probable that in agreement with the results of Eder and Kutter something like one hundred days are required for the attainment of equilibrium in such systems. It is noted that temperature control of this experiment was inadequate, only after 41 days were the tubes placed in a constant temperature room at about 23' where they were free from the daily and seasonal fluctuations of temperature and in which they could also be examined polarimetrically. A repetition of this experiment is
WILDER D. BANCROFT AND HERBERT L. DAVIS
2524
planned but the qualitative results leave no doubt as to their interpretation. These experiments were carried out without the guidance of a satisfactory theory and hence failed to show all that they might have shown. Thus it is very probable that if the evaporation of the lactic acid solution had been carried out all the way in the vacuum still and the most concentrated solution examined a t once under these same conditions it might well have shown a positive rotation of something like +IS' as this is the value obtained by extrapolating back the curve of rotation for this solution to six days before the beginning of these observations. We should then have the phenomenon of a lactic acid solution changing its rotation from + Iso to - 2 I O on standing. Truly such a remarkable indication of a fundamental change demands explanation. In view of what has been shown before it is clear that the process of concentrating the solution builds up a t each step very rapidly a concentration of the ethylene oxide form which is directly proportional to the total concentration of the solution. A consequence of the low rate of reaction to the anhydride or lactide form is that these are present in the quickly concentrated solution in far less than the equilibrium amounts. On standing the anhydride and lactide forms appear primarily a t the expense of the ethylene oxide form. Normal O=C-OH
I I
HOCH
Ethylene oxide HO-C-OH
-+
Anhydride CH3
+
I
HC-0-C=O
I
HO-C=O
CHI
I
HOCH
I
\
CH 3 Lactide
I
CH
0 = C-0-CH
I
CHa Of these reactions the second course seems much more probable since the first requires the splitting off of water in the concentrated solution. It appears, then, that the ethylene oxide form loses water as the solution is concentrated and is then capable of condensation directly into the familiar lactide, thus replacing a moderate dextrorotation with an enormous levorotation. This is the explanation of the rotation change of the most concentrated solution and is supported by the fact that when first examined polarimetrically, titration of this solution showed about 0.5% present as reacidification while after standing for eighteen months and reaching equilibrium, titration showed 2 3 % of the total alkali required was required to neutralize the reacidification. The
OPTICAL ROTATION OF LACTIC ACID
2525
titration of a two cc sample weighing 2.304 g. required 18.74 cc S NaOH and the reacidification required 5.66 cc S NaOH. If one should adopt the Wislicenus method of calculation it shows 0.916 g anhydride, 1.17s g lactic acid. This is equivalent to 51% lactic acid, 39.7% anhydride, and 9.3% water. The defects of this method of calculation have already been pointed out and it cannot be taken to prove that there is not present some of the lactide form also. It is emphasized merely that the process of coming to equilibrium is accompanied by a very large increase in the fraction of the lactic acid which is not immediately titratable by alkali in the cold. In order to show these I
I
I
I
1
FIG.2 Time-Rotation of 1 (+) Lactic Acid Ordinates are specific rotations Specific Rotations 41 dayso 1.5 years -4 -19 5 5 -22.00
B
+2
88'
+ 2.88"
data comparably, the specific rotations of these mixtures of anhydride and lactic acid have been shown in Fig. 2 . The explanation of the rotations of the more dilute solutions follows the same line. The original concentrated lactic acid contains an amount of the ethylene oxide form which is above the equilibrium value for any more dilute solution so that dilution will always be accompanied by a rapid decrease in the proportion of this form. The amount of the anhydride or lactide forms present in the original concentrated solution is far below the equilibrium value for such a concentration but may be above the equilibrium concentration for a more dilute solution. T h e n the original solution is first diluted the ethylene oxide form a t equilibrium mlth the new Concentration is more than sufficient t o overcome the small amount of the levorotatory anhydride forms so that the solution is dextrorotatory. The anhydride forms are contained in larger than the equilibrium amounts and hydrate slowly, the disappearance of these levorotatory forms in favor of the dextrorotatory ethylene oxide form giving rise to the increased dextrorotation of the first dilution solution.
2526
WILDER D. BANCROFT AND HERBERT L. DAVIS
Exactly the same explanation applies to the second dilution solution except that here the very low equilibrium value of the ethylene oxide form is unable to overcome the levorotation of the anhydride forms even though t,he latter be low in actual amount. The anhydride forms hydrate (at a faster rate than in the first dilution) and once more the dextrorotation of the ethylene oxide forms is the strongest rotation. I t should be noted that so little time elapsed that diluting the first dilution was practically the same as diluting the original solution I :4 all at once, Such a phenomenon as that just reported was noted by Wislicenus and he says, p. 328: “A paralactic acid, which contains only a few percent of the esteranhydride rotates the plane of polarized light to the left. When I did not know as yet about the spontaneous esterification and had already concluded that the specific rotation of paralactic acid is about +3.5’, I made this observatidn first with not’ exactly happy astonishment. The preparation was a thin syrup made by evaporation on the water bath which first showed this phenomenon. Addition of water and long st,anding depressed the optical activity to zero, some time later positive rotation appeared with growing power.” Wislicenus had reported that a solution of lactic acid (0.4 g per cc) gradually became more strongly dextrorotatory with time. If the solution were diluted somewhat, its specific rotation fell sharply; on standing, this diluted solution again showed increasing dextrorotation and so for as many as three dilutions. These solutions were thus more dilute than the original solution here reported and in addition to this Wislicenus avoided the discovery of this phenomenon for in general his solutions were not examined until about two weeks after their preparation. As has been shown in the case of the second dilution solution, the increase in dextrorotation is merely the continuation of the same process going on in the levorotatory solution. The continuity of the curve indicates that even in the dextrorotatory solution we have decreasing amounts of the anhydride form. The wide variation in the rotatory power of solutions of sarcolactic acid is now apparent. A rapidly concentrated solution has strong dextrorotation which on standing may change to an even stronger levorotation. If the original solution be diluted, it may show levorotation which will then gradually change to dextrorotation. There are, of course, some intermediate stages of dilution for which the change in rotation will be small. If one considers sarcolactic acid solutions a t equilibrium, increase in concentration will be accompanied by an increase in dextrorotatory power up to a maximum; beyond this the dextrorotation will decrease and finally change to levorotation as the formation of the anhydride and lactide becomes important. Infinitely dilute solutions of sarcolactic acid will also be levorotatory but are not susceptible of observation ordinarily. The ionization constant shows that the 0.5 N solution of lactic acid is 0.0164 ionized. Thus rotation observations end about where dissociation begins. In conclusion we may add that the supporting evidence for the existence and properties of such an ethylene oxide form of lactic acid is quite analogous
OPTICAL ROTATIOS O F LACTIC ACID
2527
to that given in the case of malic acid.’ I t was shown that if the hydroxyl hydrogen were replaced by less labile groups compounds were obtained whose rotations were remarkably constant when compared to the malic acid. The same is true of the acids analogous t o lactic acid. Many of theee acids and their derivatives have been made by Purdie and his students2 They report data for methoxy and ethoxypropionic acids and their salts. The acids are found to be of nearly a constant rotatory power over rather wide concentration ranges while the behavior of the salts is to be explained by ionization differences, with greater approach to constancy in the substituted acids. Thus we find, p. 877: “The rotation of calcium lactate is not only much less than strontium and barium lactate, but on dilution shows no sign of approaching the common maximum nearly reached by these and the alkali salts. This anomaly it will be seen disappears in the calcium alkoxypropionates. The behavior of calcium ethoxypropionate is quite similar to that of the barium and sodium salts, taking into account its probably smaller degree of dissocintion, and the same may be said of the propoxy salt.” A more recent indication of the influence of the labile hydrogen atom has been pointed “It was suggested by Wood, Such and Scarf (1923) that the complex rotatory dispersion of the esters of lactic acid is due (a) to their persistent low rotatory powers, (b) to the influence of the hydroxyl group attached.to the asymmetric carbon atom. The rotatory power of ethyl d-alpha-p-toluenesulphonoxy-propionate was determined over a large range of temperature and for light of several wave-lengths. Under the experimental conditions tried, it exhibited simple rotatory dispersion in marked contrast to the complex rotatory dispersive power shown by ethyl ‘d’ lactate. I t is remarkable that this complexity of rotatory dispersion of ethyl ‘d’ lactate should disappear when the p-toluenesulphonyl group is substituted for the hydrogen atom of the hydroxyl group attached to the asymmetric carbon atom.” Such a decrease in abnormdlity would be predicted on the basis of the present paper. Summary I. Sarcolactic acid, commonly called dextrolactic acid, should be designated as l(+)lactic acid to show that, although its dilute aqueous solutions happen to be dextrorotatory, the levorotations of its salts and esters show it to be essentially the levo form of lactic acid. 2. This problem is directly amlogqus to that previously encountered in the case of 1-malic acid and the solution is similar. It is shown that as with malic acid, so also in the case of sarcolactic acid there is an additional tautomeric modification to be taken into account. Again the evidence supports a dextrorotatory ethylene oxide form.
Bancroft and Davis: J. Phys. Chem., 34, 897 (1930). .Soc., 73, 862 (1898); Purdie and Irvine: 75, 483 (1899). Kenyon, Phillips and Turley: J. Chem. SOC.,127, 409 ( 1 9 2 j ) .
* Purdie and Lander: J. Chem
WILDER D. BANCROFT AND HERBERT L. DAVIS
2528
3 . The literature of lactic acid includes the description of three forms. The lactide form possesses a very large specific rotation of about -300' and exists in very concentrated solutions and as a solid. The anhydride has a moderately large rotation which is of the order of -4j' but has never been isolated. The normal form of lactic acid corresponding to the formula as ordinarily written is also levorotatory with a power which is not far different from - 14' in molecular rotation. 4. Since the three forms of sarcolactic acid which have been studied are all levorotatory it follows that some other explanation must be offered for the dextrorotation of its aqueous solutions.
5. The full picture of a sarcolactic acid solution will include equilibria between the four forms. Normal CH3.CHOH.COOH
a
Anhydride
Lactide
CH3.CHOH.CO
CH 3.CH .CO
I I
*
0
'\I
CHSCH .COOH Ethylene Oxide
OH
CH 3.CHC(
I / 0 I 1
0
OC
. CH.CH,
rJ
OH 6. T h e equilibria involving the anhydride or lactide forms are reached very slowly, while that between the normal form and the ethylene oxide form is reached very rapidly. Thus a rapidly concentrated solution will contain an equilibrium amount of the ethylene oxide form but far below the equilibrium amounts of the anhydride and ld>.tideforms. This solution may, on standing, change from a rotation of 15' to - z 'I as a result of formation of the anhydride forms from the ethylene oxide form.
+
7 . If a freshly prepared, rapidly concentrated, sarcolactic acid solution be diluted sufficiently, it will show levorotation instead of the dextrorotation of the concentrated solution. This is evidence for the existence in the solution of sufficient of the strongly levorotatory anhydride forms to overcome the dextrorotation of the small amount of the ethylene oxide form in the diluted solution. On standing, this diluted solution becomes increasingly dextrorotatory and the anhydride forms slowly hydrate. 8. Solutions of sarcolactic acid which are in equilibrium will show, with increasing concentration, increasing dextrorotations as the proportion of the
OPTICAL ROTATION OF LACTIC ACID
2529
ethylene oxide form increases. Then the formation of the anhydride forms becomes important; the dextrorotation passes through a maximum and then becomes increasingly levorotatory. 9. The supporting evidence for the ethylene oxide form of sarcolactic acid is quite similar to that advanced in the case of malic acid. When the alcoholic hydrogen, whose migration to the ketonic oxygen of the adjacent carboxyl group is responsible for the formation of the oppositely rotatory substance with different rotatory dispersion and hence for the abnormalities in the lactic acid systems, is replaced by less mobile groups, the resultant compounds possess very different properties with the disappearance of the abnormalities in the dispersion and in the rotation-dilution phenomena.
Cornell L h v e r s z t y
