CARBOHYDRATES 53 



whole is optically inaclive by internal compensation. Such substances 

 are called meso-isomers. They occur as symmetrical molecules having 

 more than one asymmetric carbon atom. In other words, the mole- 

 cule as a whole is symmetrical whereas certain of the individual car- 

 bon atoms are not. 



Most compounds ^vith more than one asymmetric carbon have dif- 

 ferent groups on the halves of the molecule and cannot exhibit the 

 symmetry of meso-isomers. They may, of course, occur as racemic mix- 

 tures corresponding to an equal mixture of (+)- and (— )-tartaric 

 acids. Moreover, the synthesis of such compounds normally leads to 

 these optically inactive mixtiues. 



However, when a compoimd possessing an asymmetric center under- 

 goes reaction to form a second center of asymmetry close by, there is a 

 directive influence exerted by the first center. In other words, an 

 asymmetric carbon atom in a molecule tends to control the configiua- 

 tion of the second center of asymmetry introduced later. Naturally, 

 the use of a racemic mixture in the reaction will yield a racemic 

 product because both the original isomers possess equal and opposite 

 directive effects. However, when a single isomer is placed in the reac- 

 tion, a preponderance of one form results in these asymmetric syn- 

 theses. The general result may be illustrated by the reaction shown, 

 where one product is more abundant than the other. 



H H Y H XY 



I II II 



CH3— C— C— R + Y2 -> CH3— C— C— R + CH3— C— C— R 



I II II II 



HO X HO XY HO Y 



As might be expected, the relative quantities of the isomers depend 

 upon the original asymmetric structure, the nature of the reaction, and 

 the proximity of the second center of asymmetry to the first. When 

 the two are far apart there is little or no effect. On the other hand, 

 the presence of several centers of asymmetry is believed to enhance the 

 control of configuration. Moreover, the intervention of an optically 

 active catalyst can control the configuration about centers of asym- 

 metry. Both of these last effects in combination arc believed to bring 

 about the high degree of optical specificity found in biological mate- 

 rials. 



In the foregoing illustrative case, the formation of mirror images is 

 impossible, a fact believed to account at least in part for such asym- 

 metric synthesis. This situation arises from the fact that a single 

 isomer was used in the reaction and the configuration about this 



