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A. I. KINGEK AND EMIL J. BAUMAOT 



the form isolated by Pasteur from racemic acid rotates the plane of 

 polarized light to the left ; racemic acid, optically inactive, is in reality a 

 mixture of both the dextrorotatory and the levorotatory. 



Here are two substances having the same empirical formula and the 

 same chemical groups similarly arranged, but their physical properties 

 their crystalline form and behavior toward polarized light are markedly 

 different. It will likewise be found that their chemical properties are 

 different. These are not due to differences in chemical composition, but 

 to differences in molecular form. More than a quarter of a century 

 later, Le Bel and Van't Hoff independently formulated the hypothesis of 

 the asymmetric carbon atom, on the basis of Pasteur's fundamental dis- 

 covery. Only such compounds of carbon as have so-called asymmetric 



Fig. 1. Illustrating two carbon atoms with their four valences taken up by 

 four different radicles arranged in such a way that the space relationship of the 

 two is like that of a mirror image. 



carbon atoms can exist in stereoisomeric forms. An asymmetric carbon 

 atom is one that has four different atoms or atomic groups attached 

 to it. 



If the carbon atom is pictured as lying at the center of a tetrahedron 

 with the four atoms attached to it at the apices, it is possible to arrange 

 these in two ways, one of which is the mirror image or antipode of the 

 other (Fig. 1). 



Molecular asymmetry of this type is most readily recognized by means 

 of the action of such substances on polarized light. Compounds having one 

 or more asymmetric carbon atoms usually have the power of rotating the 

 plane of polarized light except when one asymmetric carbon atom is 

 neutralized by one or more other asymmetric atoms. However, one does 

 not meet such substances very often. One of the first cases known in 

 which one asymmetric carbon atom neutralizes another is mesotartaric 

 acid, discovered by Pasteur. The various tartaric acids may be repre- 

 sented thus: 



