42 



SCIENCE 



[N. S. Vol. XXXVIII. No. 967 



lay down his plans and draw his designs so 

 that each and every part shall be fitly 

 adapted to its specific use, so the chemist 

 must first in his imagination plan the 

 order and arrangement of the various ele- 

 ments and groupings which are to be com- 

 bined in such a manner as to produce the 

 desired specific results. 



The distinguishing characteristic in the 

 structure of the optically active organic 

 substances is that they contain at least one 

 carbon atom which is combined with four 

 different atoms or groups. If we consider 

 the space distribution of the four different 

 atoms or groups about the central carbon 

 atom, we shall find that two arrangements 

 are possible. The two resulting forms are 

 related to each other in the same manner 

 as an asymmetric object and its mirror 

 image. Such a carbon atom is called an 

 asymmetric carbon atom. We have for 

 each substance containing such an asym- 

 metric carbon atom the possibility of a 

 right-handed structure and a left-handed 

 structure. Corresponding to these theo- 

 retical structures we find that nature has 

 furnished us with dextrorotatory and 

 Isevorotatory isomeric substances, which 

 are closely identical in all of their physical 

 and chemical properties, but differing 

 chiefly in that the one rotates the plane of 

 polarized light as far to the right as the 

 other does to the left. When these two so- 

 called stereoisomerie substances are mixed 

 in equal quantities the resulting product is 

 inactive. So also, when two asymmetric 

 carbon atoms occur within the same mole- 

 cule inactivity may result from internal 

 compensation. It is thus found that 

 among substances of asymmetric structure 

 there are two classes which are optically 

 inactive. The members of the one class — 

 said to be inactive by internal compensa- 

 tion — are not separable into active com- 

 ponents, while the members of the other 



class — said to be inactive by external com- 

 pensation — are separable into dextrorota- 

 tory and Isevorotatory components. 



We have three methods for the separa- 

 tion of the optically active components, all 

 of which are due to the researches of 

 Pasteur.^" 



1. In some instances enantiomorphic 

 crystals may be formed which may he 

 mechanically separated. 



2. By the aid of suitable active sub- 

 stances compounds may be formed which 

 differ in their solubility, thus permitting 

 the two optical isomers to be separated by 

 fractional crystallization. 



3. Through the action of certain micro- 

 organisms one of the optical isomers may 

 be destroyed by fermentation while the 

 other remains unaffected. 



The direct synthesis of optically active 

 substances from inactive material has not 

 been effected, because both of the stereo- 

 isomerie forms are simultaneously pro- 

 duced by synthetic processes, but the same 

 result is accomplished indirectly by first 

 synthesizing tlie inactive mixture, or com- 

 pound, and then separating the compo- 

 nents by one of the methods already men- 

 tioned. 



When, however, we find in nature sub- 

 stances which show optical activity we 

 know that they must contain constituents 

 which are asymmetric in structure. In 

 endeavoring to determine their constitu- 

 tion, the chemist, therefore, gains the dis- 

 tinct advantage of leaving out of consid- 

 eration all that vast array of substances 

 which are symmetrically built, and of be- 

 ing permitted to concentrate his attention 

 and efforts upon the relatively few possi- 

 bilities of asymmetric structure. 



But the chemist is not alone in the ad- 



" ' ' EeseaTches on the Molecular Asymmetry of 

 Natural Organic Products," by Louis Pasteur 

 (1860), Alembic Club Eeprint No. 14. 



