W. KAUZMANN 17 



/ 



The only reason they have not been separated in the laboratory is that 

 there is a very low energy barrier between them, so that they change into 

 one another (or 'racemize') much too rapidly for them to be isolated in 

 the two pure forms. The property of the asymmetric carbon atom that 

 makes it so important in giving rise to optical rotatory power is merely 

 that it makes it so difficult for the mirror image forms of a substance to 

 come into equilibrium. In chemical compounds containing asymmetric 

 carbon atoms, chemical bonds have to be broken in order to pass from 

 one form of the molecule to its mirror image form, whereas in a molecule 

 such as butane one has only to rotate the two halves of the molecule about 

 the central carbon-carbon bond. Thus the asymmetric carbon atom is 

 merely a means of stabilizing optical isomers. Organic chemists have found 

 other means of accomplishing this in certain favorable cases (e.g. the use 

 of steric hindrance in ortho-substituted diphenyls) . 



Thus the asymmetric carbon atom provides the geometrical condition 

 necessary for optical rotation but does not itself necessarily interact di- 

 rectly with the light wave. What is it, then, that determines the actual 

 amount of the rotation of the plane of polarization? All of the physical 

 theories agree that the phenomenon arises from the electronic motions that 



,CH, Br 



3 



Fig. 2 

 are induced in the chromophoric groups of the molecules when light shines 

 on them. These electronic motions are the same as those that are responsi- 

 ble for the ultra-violet and visible absorption spectra of molecules; in 

 most cases the largest contribution to the optical rotation comes from 

 the motions that belong to the absorption band whose wave length is 

 closest to the wave length of the light that is used to measure the rotation. 

 For instance, the sign of the optical rotation of visible light by camphor 

 is determined by the electronic motions that give rise to the weak carbonyl 

 absorption band of camphor at 2900 A.-"^ 



Because of the dissymetric environment of the chromophores, the elec- 



^ The electrons on asj-mmetric carbon atoms are almost invariably associated with 

 absorption bands that lie in the far ultra-violet, well below 2000 A. Furthermore, 

 there are usually many other electrons in optically active organic molecules that have 

 similar absorption bands. Therefore the direct contribution of the asymmetric carbon 

 atom itself to the magnitude of the optical rotation is probably almost always small. 



