OPTICAL ACTIVITY AND SALT EFFECT* BY P. A. LEVENE AND ALEXANDRE ROTHEN
The value of the optical rotation of a substance in the homogeneous state is, for a given wave length, a function only of the temperature, but when the rotation is measured in a solvent, then the value of the rotation depends not, only upon the temperature but also on the concentration and on the nature of the solvent. The mechanism of the action of the external conditions is as yet not fully understood, notwithstanding the numerous investigations on the subject. X convenient system for such investigations is one which permits the greatest number of variations, as is an aqueous solution of an optically active substance to which an inactive electrolyte is added. I t must be remembered, however, that many of the water-soluble optically active substances are by themselves electrolytes and therefore are capable of existence in solution in at least two states, namely, the ionized and nonionized. Furthermore, it was stated by Landolt and by Oudemans that the ions and the undissociated molecules each have their own individual rotation, unaffected by concentration and that the optical rotation of an electrolyte is a linear function of the degree of dissociation. However, there never was unanimous agreement as to the validity of this relationship. Disagreements were often due to differences in the properties of the optically active electrolytes chosen. Only a few instances will be mentioned, as the literature on the subject is too voluminous to be reviewed in this place. Walker,’ analyzing the behavior of mandelic acid in aqueous solution, found complete agreement with the law. Noyes* compared the rotations of 0.033 N solutions of H, Li, Na, T1, K, Zn, Be and Ba salts of a-bromocamphoric acid and found the rotations to be almost identical, although the conductivity ratio would indicate that the acid, the monovalent and the divalent salts each have a different proportion of non-ionized molecules. Noyes then concluded : “If there were no other evidence to the contrary, t’hese facts would almost warrant the conclusion that the salts are completely ionized up to the concentration in question.” I-ery recently \T7alden,3in a study of a number of cyclic compounds, failed to observe an agreement with Landolt-Oudemans’ law and was inclined t o explain the discrepancy on the assumption that “in a salt not only t,he free ions but also the latent ions which are held together through a heteropolar linkage have an identical rotation.” In this laboratory Landolt-Oudemans’ law has been made the basis of a method to determine the dissociation constants of amino acids, peptides, pyrimidine bases, and nucleosides, particularly in ranges of pH where electrometric titrations cannot be relied upon.A * From the Laboratories of The Rockefeller Institute for Medical Research, New York.
2 j68
P. A . LEVEKE AKD ALEXANDRE ROTHEX
In recent years attempts were made to correlate the deviations from the law on the assumption of salt effect. The introduction of this concept was of great value for the understanding of the behavior of optically active electrolytes generally, and particularly for the interpretation of the deviations from Landolt and Oudemans' law, One of the first investigations which emphasized the influence of ions was that of Stubbsj on the rotation of 1-malic acid and that of Clough'j on the rotation of d-tartaric acid. These investigations brought out the fact that the salt effect has to be attributed to the action of free ions and that the magnitude of the effect is a function of valence and of atomic weight or atomic radius. A more explicit interpretation of the salt effect was given by the investigation of Mallemann' who studied the influence of neutral salts on the rotation and dispersion of tartaric and malic acids and by that of Darmoiss on the rotation of tartarates. These investigations emphasized the importance of the concept of deformation of the optically active electrolyte by the added ions. However, in order to give a complete explanation of the rotatory behavior of tartaric acid they had to assume the existence, in aqueous solution, of two isomeric forms. This assumption was shown to be superfluous by Bruhat and Legris.9 The program of the present investigation was planned with a view of avoiding the complicating factors present in the systems previously studied. The optically active electrolytes chosen by the earlier workers had two disadvantages. Some of these electrolytes belonged to the group of cyclic compounds and it was shown by LeveneIo that in the cyclic compounds, because of the restricted deformability, the values of the rotations of the ions and of the undissociated molecules are so close as to preclude any possibility of accurately measuring the degree of dissociation by the optical method. On the other hand, substances such as tartaric and malic acids complicate the system by possessing two ionizable groups. We therefore have chosen mandelic acid for our investigation. This substance offers two advantages. On one hand, it is a monobasic acid, having one ionizable group; on the other, it has a high rotation and conveniently shows a large difference between the numerical values of the rotations of the ion and of the undissociated acid. I n discussing the effect of the electrolytes, previous workers took account principally of the cation, omitting from consideration that cation and anion might have antagonistic actions and that the quantitative effect might be the algebraic sum of the effects of both. Such was the effect observed by Fajans" in his studies on the refractivity of electrolytes. It was found by us that the salt effect on optical rotation can best be interpreted on the assumption that the anion and the cation have opposite effects on the deformation of the optically active electrolyte and therefore opposite effects on the optical rotation. Similar to the effect of electrolytes is that of nonionizable polar molecules, those containing positive groups, such as nitroethane and acetic acid, acting as cations and those, like alcohol, with a strong negative group acting as anions.
OPTICAL ACTIVITY AND SALT EFFECT
2 569
In the case of mandelic acid, the cation produces an increase in the numerical value of the rotation. This effect is in the same direction as that produced by the addition of one H+ to the ion. However, we must call attention here to the fact that the numerical value of the change is not as important as the direction of the change. Thus, the effect of a cation on the rotation of 1-malic acid which has a low levorotation is to produce a change to the right. The numerical value drops until the curve passes the zero point and then this value continually increases. The effect of concentration of the optically active electrolyte on its rotation may be explained on the basis of the mutual effect of its own molecules, so that the effect of temperature should, a priori, be opposite to that of concentration; this is actually the case. I t must be emphasized that the numerical value of the change in rotation produced by monovalent electrolytes is of a very low order of magnitude as compared with the change produced by ionizat'ion. However, the effect of the divalent alkaline-earth metals is considerable. Hence, in calculating the degree of dissociation by the optical method, the determination being made in dilute solution, the effect of the monovalent electrolyte may be disregarded without vitiating the results, whereas the effect of the divalent alkaline earths has to be taken into consideration and the necessary correction introduced. In both cases the results of the measurements by the optical method are in agreement with the theory of complete dissociation of the salts of t,he weak electrolytes. The action of neutral salts on a-methylglucoside and sucrose was also studied. In the cases of a-methylglucoside and of sucrose the effect of electrolytes on their rotation was less marked than in the case of mandelic acid; the difference was very striking for the divalent ions.
Experimental The following procedure was adopted for the measurement of rotations of mandelic acid. A stock solution of 0.025 mol. of mandelic acid (levo or dextro) plus the required volume of water was made up to 2 5 0 cc. and its density determined. This solution was always kept at low temperature ( j " ) . Portions of this solution were pipetted and weighed to prepare the different samples. As most of the rotations were taken at a concentration of 0.0j s the stock solution was good for ten runs. Each new stock solution, though carefully prepared, was always checked with a blank. This was found necessary as the moisture content of the solid mandelic acid varied with the time. I n five months the moisture dropped from 1.00 per cent to O . ; I per cent. The variation of the specific rotation was found to agree with this change of moisture. The densities of all the samples were taken as all the solutions were prepared by weight. The densities were determined in a pycnometer of n capacity of 33 cc. The stem of the pycnometer was graduated in 0.01cc., this permitted
2570
P . A , LEVENE AND ALEXANDRE ROTHEN
a reading of the volume within ko.001 cc. The densities were measured a t The usual correction was applied for the reduction a temperature of 25.0'. of the weights to vacuo. The rotations were measured a t 25.0' in a Schmidt and Haensch polariscope reading to 0.001~.The source of light was a ribbon filament lamp; monochromatic light was obtained with a calibrated prism and adequate filters and odiaphragms. T i e measurements were made for the wave lengths X = 5892 A and X = 5461 A. All measurements were made in a one-meter water-jacketed tube. h series of about ten readings was made for each solution and the mean value taken. As a rule, the difference between the mean values of two series of ten readings was less than 0.003'. The zero reading was taken for each solution, after the rotation had been measured. This was done in the following way: The solution was rinsed out and the tube carefully washed without unscrewing the caps and finally filled with water. X series of ten readings was again made. The error in the values of the molecular rotations was on the average 3 : I O O O O ~ as in most instances the observed angles were in the neighborhood of ten degrees. The sucrose employed was Xerck's C. P. product recrystallized from water and alcohol. Pure mandelic acid at its maximum rotation was used.12 A11 salts used were of the purest grade obtainable. Some of them were recrystallized and most of them were analyzed to confirm their purity. The purity of the LiOH and Ca(OH)*was checked by conductivity titrations. The 0.1s HC1 solution used was prepared from the constant boiling hydrochloric acid-water mixture. The 0.1 N NaOH solution used was standardized against the acid. Determination o j the Rotation of the Undissociated Mande l i c A c i d and the Mandelate Ion.-The difference in rotation of mandelic acid and mandelate ion was used a long time ago by Walker' for the purpose of determining the degree of dissociation of the acid under the influence of external conditions. His experimental results agree approximately with the theoretical values. The observed discrepancy corresponds exactly to the salt effect as calculated on the basis of our data and cannot therefore be explained by the presence of non-ionized molecules of sodium mandelate (Walker's discrepancy 0.08"; correction due to the salt effect, 0.08"). The molecular rotations of the acid and its ion, calculated by the relation
*
hl
[AI1 = [a1Io0 where [>\.I]
= [a] = RI =
Molecular rotation Specific rotation 1Iolecular weight.
were determined in the most dilute solutions possible to reduce the salt effect to its minimum. Three series of experiments were made at concentrations of mandelic acid of 0.01K, 0.02 N and 0 . o j S .
OPTICAL ACTIVITY AND SALT EFFECT
2571
The rotation of the ion was determined by adding one equivalent or more of sodium hydroxide. The values were found to vary slightly and linearly with the concentration of KaOH; it was then easy to extrapolate for a zero concentration. The rotation of the undissociated molecule was found by adding various amounts of hydrochloric acid. The results are summarized in Table I. The coefficient of dissociation 6 given in column 8 was calculated from the law of mass action using 4.3 IO-^ as the constant. The activity of hydrochloric acid was calculated with the coefficients given by Scatchard. Replacing the activity of hydrochloric acid by the concentration of HCl does not affect the value of the molecular rotation more by than 0.03per cent when the mandelic acid is less than I per cent dissociated. I t was then further assumed for the calculation of the molecular rotation of the undissociated molecule that the small salt effect of hydrochloric acid was proportional to the concentration of hydrochloric acid, an assumption which mas found to be perfectly valid.
TABLE I Data for the Calculation of the Molecular Rotation of Mandelic Acid and Mandelate Ion Concentration of Run Mandelic S O . Acid in mol per liter a t 25' >
0.010080
Equiva. lent of NaOH HC1 = NaOH - =HC1
+
+0.993
IhLI~ Observed 25 Mol. Rotan~ tion
1.788 1.778 1.779 8.j36
6 7 28
o . o ~ o o ; ~+ 1 . 2 4 2 0.01006~ + 1 . 4 9 0
32
8.888 8.871 o.o.+gg;o + 3 . 0 0 0 0.04942~ + 1 . 0 2 0 8.746 0.01006? -0.994 2.362 0.010068 -1.990 2.374
33 34 8 9 17
' 9 30
3'
o.oqg38:
+I.OZO
o.ojoo8~ +2.ooo
1jj.40
-0.999
4,723
[MI:
0
-
176.7:
-
176.88
-
1772.6
-
177.60 176.97
-
234.70 235.80
W I :
degree Calc. Calc. Mol. [ M 1;46r of Mol. Rotation 25 Observed disso- Rotation of Cndis- a 5161 hlo. Rociation of Ion sociated tation Acid 2.149 213.2
176.7:
23j.95 - 1 , 0 3 0 1 1 . 8 4 3 236.9, 0.0499;g 0.049930 - z . o o o 1 1 . 8 6 0 2 3 7 . j 2 0.049813 - 3 . 0 0 0 1 1 . 8 j i 237.91 0,020017
c&,
-
0.0437
176.70 176.j o
-
2.139
212.6
-
2,150
213.5
176.70 176 . i o
-
10,499
176.70 176.;0 - 237.29
2 1 2 . j ~
-
-
-
-
-
-
2,851 2 8 3 , 3
0.0233
-
23j.1:
0.0233
-
237.23
2.861
-
-
0.01oo o.ooj3; 0.0036,
-
237.26
-
-
-
23; . Z G
-
-
-
237
. 26
-
The following equation viae applied: 6[AI-] $- ( I - 6) x [AI] = [AI,] 6 = coefficient of dissociation [AI-] = rotation of mandelate ion [MI = rotation of mandelic acid [1\1,] = molecular rotation observed e = salt effect due to HC1 CIIci = concentration of €IC1
ZCIlC,
284.2
-
2572
P. A. LEVENE AND ALEXANDRE ROTHEN
As seen in Table I (runs 2 8 , 32, 33, 34) the rotation of mandelate ion is 131-1; = I 76. io f o.ogjo. We found for the molecular 'rotation of the undissociated acid the value [?*I]: = 237.26' by solving the above equation using runs 2 9 and 3 0 . Exactly the same value was obtained by using the figures of runs 29 and 3 I or 30 and 3 I z is equal to 0.30' for a concentration of o . o j N HC1. z' (salt effect of sodium hydroxide) is equal to 0.030' for a concentration of 0 . 0 ; N SaOH. The measurements made for the wave length 5461 are less accurate but the values found by the same method for the rotation of the acid and the ion
[MI,:, = 285.7' AI-]^^^ = 213.5' show that the dispersion for both forms is identical.
Calculation of the Dissociation Coiistnnt of Mande l i c Acid.-The rotations of the ion and of the molecule being known, the dissociation constant can be determined for solutions of mandelic acid without any further data. The most accurate determination would be obtained when the acid is half dissociated; unfortunately, the dilution would be too great and the observed angle too small. I t should be emphasized that the rotation measures the concentration (the correction for the salt effect being made) and not the activity of the species. In Table I1 are given observed rotations at different concentrations and the calculated constant I
