OPTICAL ACTIVITY XNL) THE P w m - C T OF ASYMMETRY. ~BY J. WALLACE T.T'ALKEK.
In a recent communication' Bose and Il-illers have attempted to show that there is a possibility of determining a constant of optical activity for each atom or radical attached to an asymmetric carbon atom. The constant for a given radical is of such a nature that it will express the optical effect of such atom or radical no matter what the cornpound be into which it is introduced, and from these constants the>- endeavor to show that the molecular rotation of any compound containing these radicals may be calculated by Crum Brown's equation for the product of asymmetry, ziz., M [ a l = (C, - C,)(C,- C,)(c, - C,) (C, - CJ (C,- C,) (C, - C4), where C,, C,, C, and C, are the activity constants of the four atoms or radicals. The authors, after discussing the various aspects of the case, decide that it is better to choose the activity of pure liquids instead of solutions for comparison and they of course limit themselves to such as are likely t o be free, or nearly so, from polymerization. For this reason they have selected the esters of the following acids bromopropionic, chloropropionic, valeric, lactic, glyceric, bromosuccinic, chlorosuccinic, malic, rnethoxypropionic, ethoxypropionic, methoxysuccinic, and ethoxysuccinic acids. In all, 42 substances are thus available and these contain 17 different radicals, for which they deduce the constants: -~ ~-
Constant
Radical
1
Constant
Radical .
-H
I -CH,.COOC3H7
I2
Pu
PU
-C,H, -COOCH, -COOC,H, -COOC,H, -COOC,H, -CH,.COOCH, -CH,.COOC,H, -
-
o 053
9 5 9 34 9 21 9 I3
1 --Br
1
-
23 o _702
24.0 24.2
9.0
8.75 11.81
2E2.0H
1 -OCH,
I -OC,H, -
22 2
Zeit. phys. Cheni., 65,
/-C1
rnnru
-
(1909)
20.35 1 2 .j 12.8
-
Optical Activity nizd the Product
0)
Asymt~zetry
j75
Vsing these values, they proceed to calculate the molecular rotations of the various 42 substances and find on the whole a fairly close agreement with the experimental results. calculated The higher limit of the ratio is 1.466 and its lon.er observed
~~
~
-
~~
Radical -
~~
Constant
~~
'
~
~
Radical
I
~~~
~~
I
-OH -H -CH,OH -CH3 -COOCH3
IIO IO0
92
1
90 65
' -CH,.COOCH,
-COOC,H, --CH,.COOC,H, -C2H, -c1
Constant __
61 60
56 52
49
The values calculated for the molecular rotations of the following substances by means of these constants are given
J . Wallace Walker
576
in the second column of the following table, the observed values in the third. The fourth column contains the ratio calculated _____ and the fifth column the corresponding ratio for observed Bose and Willer’s values. ~
!Calculated
Substance
~
1 Methyl lactate Ethyl lactate Methyl chloropropionate Ethyl chloropropionate 1 Methyl malate 1 Ethyl malate Methyl glycerate 1 Ethyl glycerate Methyl valerate Ethyl valerate ~
796 1216 2888 2j76 1115 2025 576 1230 I750
’
I1
I,
788 1200
2927 2760 1203 1901 612 922
, I
Calculated Observed ~~~
Walker
0.990 0.987 1.013 1.071 I . 130 0.939 1.063 0.750
I
I .064 1.001
~
I 1
’
~
~
1 ~
BG:lc: I . 280 0.963 1.170 I .006 1.329 0.970 1.466 0.791 0.875 1.002
calculated observed ’ obtained by means of these constants, show in the main a slightly closer approach to unity than those from Bose and Willers’ numbers. I n one important respect they have an advantage over the latter, in that they show a diminution of rotation on ascending the series of valeric esters in conformity with observed facts, whereas their numbers show an increase. When however the rotations of the chlorosuccinic esters were calculated the divergence was much greater, the values obtained being for methyl chlorosuccinate 5346 instead of M [ a ] = 7650 and for ethyl chlorosuccinate 2 7 6 5 instead of M [ a ] = 5730. I n the latter case the discrepancy is over 50 percent and that in the wrong direction from what a knowledge of the experimental conditions would lead us t o anticipate, since the observed value may easily be too low through racemization having occurred during the action of phosphorus pentachloride on the malic ester. It is far less
It is evident from this table that the ratios
Optical Activity and the Product of Asymmetry
577
likely that it is too high. This result indicated that it was necessary either t o reject the values selected for the constants altogether and to look for another series of numbers-a task which seemed endless-or to devise a crucial method of determining if it may not be impossible t o assign rotation equivalents t o all radicals from which the molecular rotations of the substances containing them may be calculated by Crum Brown’s equation. The method finally evolved was as follows: It is possible t o select eight distinct optically active substances, whose molecular rotations we may call M [a],, M [ a ] , , * * * * * N [ a ] , ,all possessing one common radical, whose rotation equivalent is C,, and whose other radicals may be chosen from a group of six with the rotation equivalents C,, C, * * * C7. Now, forming the products of asymmetry for each pair of substances which have three radicals in common, equating t o their molecular rotations and dividing the one equation by the other, we obtain : for the radicals
c,,c,, c,,
c 4
M [a11 = (C,-C J (C,-C,)(C,-C4) (C,- c,>
c,,c,, c,,c,
(C,-C4) (C,-C4) 2 = (el- C,)(C1C,)(C,-C5)(C,- c,> (C,-C,)(C,-C,)
I.[
M
Now, if we further divide equation ( I ) by equation (2) and equation (3) by equation (4), we find that, if the radicals possess such a property as that of an ‘optical activity con-
31[cz] at 20"
J . Wallace Walker
580
as disproving the existence of a fixed coefficient of optical activity for each radical in the sense expressed by Crum Brown's equation, as applied by Bose and Willers. EXPERIMENTAL
Part I, with D r . Samuel Smiles The esters of mandelic and phenylchloroacetic acid were prepared by us some' years ago, using every precaution to insure purity and avoid racemization. The mandelic acid, obtained from amygdalin, was repeatedly recrystallized until it melted sharply at 132.2'. Its specific rotation in z percent aqueous solution a t 17' was found t o be [ a ] , = - 154.2'. The esters were made from it by means of the silver salt and alkyl halides, and they were very carefully crystallized from a mixture of benzene and light petroleum until they also showed very sharp melting points. After their physical constants had been determined, they were then transformed into the corresponding phenylchloroacetic esters by the action of phosphorus pentachloride upon their solutions in dry chloroform. The phenylchloroacetic esters were finally purified by fractional distillation under reduced pressure. Their densities were very carefully determined and their rotations in the liquid condition even a t temperatures much below the points of solidification of the mandelates. These results are tabulated below. hl ANDELATES ~
~ ~
~. ~~
~~
E t h y l ester, M. P. 30° -_ _ _ _ _ _ _ ~ Density
1848 I756 1660 1.1568 I . I472 I 1380 I 1284
IO0
I
20
1 I
30 40 50 60 50
-28630' 27950 26760 25960 25170 24450 23820
1 I I
I383 1290
I200 IIOj I IOIj I 0920 I
I
0830
[a1 -23300' 2 2 570 2 1920 21300
20750 20220
19710
Optical Activity and the Product of Asymmetry n-Propyl ester, M. P.
Temperature
~
IO0 20
I .I100
30 40 50 60 70
I 0915
%-Butyl ester, M . P . 31'
24O
[M]a
Density
-21130~ 2 0 340 19980 19400 18860 18320 17770
I .IOOj
,0827 I 0740 I 0645 I ojj6 I
Methyl ester, B. P. 124' a t 8 mm Temperature
Den sit y
IO0
I .217j 1,2080 I . 1980 I . 1886
581
Wal
Density
'
I
.os00
I .0 7 2 0 I . 0632
1,0545 ,0460 I . 0370 I . 0283 I
-2
I990 20380 19780 19220 18740 -
Ethyl ester, 1 3 3 O a t
IZ
mm
-I 20
30 40 50 60 70
I . I790 I . 1692 I . 1600
f7417O
Temperature
Density
IO0 20
30 40 50 60 70
I . 1390 I . 1290 I . 1195 I . I100 1.IO10 I .0913 I ,0817
1678
I . 1580
6596 6301 6135 6033 5978
I.
n-Propyl ester, B. P. 140' a t r g xnm
__-__
I.
7002
I490
1 . I393
I .
1297
I . I202 I.1110
$6818' 6622 6416 6200 5993 5777 5571
%-Butylester, 163-4O at
20
mm
M[al
-k 5905' 5736 5565 5368 52IO
5041 4863
1130 1040 I .09jo I ,0860 1.0773 I .0680 I .oj9j I. I.
Several of these substances had already been examined by U-alden'. His observations made at 20' are subjoined for comparison. Zeit. phys. Chem., 17,Soj (1895)
!
584
J . Wallace Walker
The molecular rotations of the lactates and chloropropionates are found to be somewhat lower, and those of the bromopropionates somewhat higher than the values given formerly. The lack of concord between the results obtained by different investigators in these series of substances may be mainly due t o the comparative ease with which racemization occurs in replacing the hydroxyl group by chlorine, and may be considered to detract from the reliability of the conclusion drawn in the theoretical part of this communication. Further, the lowest members of any homologous series are those which show greatest divergences from the generalizations which have been drawn regarding other physical constants, for example, the boiling points. For this reason, it has been considered advisable to select other substances for an examination in which it is hoped that both of these disadvantages may be avoided. The series selected is that of the esters of the asynrmetrical aliphatic acids and the work has been for some time in progress.
