DWIGHT FAY MOWERY, JR. Trinity College, Hartford, Connecticut
IT
IS possible to pick up many introductory, or advanced, organic chemistry textbooks and find one of the following statements given as a reason for optical activity or inactivity: "Loss of asymmetry automatically involves the loss of optical activity. The two properties disappear together." Or, "The crux of the theory presented here is that optical activity rests upon molecular asymmetry. The theory has been completely successful in the face of thousands of tests." Or, "The fundamental postulate of the theory is that this isomerism is to be found only when the molecule can exist in two forms which are mirror images of a c h other. Such a molecnle is said to be asymmetric. Opticalactivity isaproperty of anasymmetric molecule." And, finally, "Experience shows that optical activity is only produced when the molecule is completely unsymmetrical in structure. All chemical changes which remove the cause of the asymmetry of the molecule result in a disappearance of the optical activity. One is therefore led to the conclusion that the capacity possessed by many substances of rotating the plane of polarization of light has its origin in unsymmetrical molecular structure." It will be noted that in all cases the connection of optical activity and molecular asymmetry is purely empirical. This would not be so bad in itself if it did not lead to an incorrect picture of optical activity which defies logical explanat~on. The student is led
to believe that an individual molecule of Type I does not rotate the plane of polarized light, whereas an individual molecule of Type I1 does. He finds it justifiably hard to believe that the changing of one atom, or group in an already fairly complex molecule, can cause the difference between optical activity and inactivity. The student also wants to h o w why it is that molecules of Type I, no matter what atoms or groups
are represented by A, B, and C, are always optically inactive, whereas all moleculeS of Type I1 are almys optically active. The answer is very simple, and is found in almost any paper on the theory of optical rotation. In all these theories (1) a summation of the rotations of all the individual molecules, oriented in all possible random positions, is carried out, and in the case of molecules with a plane or center of symmetry, this summation produces a net rotation of exactly zero. The physical picture of this summation is described in a paragraph written by Noller (93) in 1947 and, unfortunately, not incorporated, to the author's knowledge, in any of the newer textbooks. Referring to the compounds pictured, containing a single carbon atom, Noller says: "If two atoms or groups are alike, the random orientation of the molecules will provide each molecule with a mirror image so that any rotatory effects of the individual molecules would cancel. Therefore the only requirement for optical activity is that two like molecules cannot be orientable in such a way that they are mirror images of each other or in other words that the mirror image of a molecule cannot he superimposable." It is evident that if a molecule has a plane, or center, of symmetry, it is identical with its mirror image, and, in any solution of sufficient concentration for optical measurements, there will be, for every molecule oriented in a given way another oriented as a mirror image of the first so that the resultant rotation of the solution will be exactly zero. The rotations produced by the individual molecules will be "externally compensated" in the same manner as for a d,l-mixture. The erroneous impression given by textbooks, of individual molecules which are optically inactive because of a plane or center of symmetry, is probably caused by attempting to explain the conditions for optical activity rather than those for optical inactivity. This is due to the fact that optically inactive compounds were known long before, and in much greater numbers, than optically active ones. If optically active compounds had been discovered first, and then occasionally an optically inactive one had come to light, it is very likely that the correct explanation of optical inactivity would have been arrived a t earlier. This picture of optical inactivity can also be applied to meso-tartaric acid, where it is seen that the inactivity is probably due principally to "external compensation." The "internal compensation" in this case would be limited to exactly "staggered" molecules . or "eclipsed" molecules, which are oriented with their planes of symmetry perpendicular to the path of thelight (3Y.4).
MARCH, 1952
LITERATURE CITED (1) KAUZMANN, W. J., J. E. WALTER,AND H. EYRINQ,Chem. Rev., 26,339407 (1940).
139 (2) NOLLER, C. R., J. CEEM.EDUC.,24,600-4 (1947). (3) WEELAND, G . W., "Advanced Organic Chemistry," John Wiley & Sons, Inc., New York, 1949, p. 191. (4) NOLLER, C. R., Science, 102,508 (1945).
