CARBOHYDRATES 



33 



Stereo-isomerism, or space isomerism, as its name indicates, 

 depends upon the different arrangement of the atoms or groups in 

 the molecule in space, and not upon any difference in the character 

 of the constituent groups. This possibility depends upon the 

 existence in the molecule of the substance in question of one or 

 more asymmetric carbon atoms and manifests itself in differences 

 in the optical activity of the compound.* Thus, in the formula 

 for glucose shown above there appear four asymmetric carbon 

 atoms, namely, those of the four secondary alcohol groups (hi 

 the terminal, or primary alcohol, group, carbon is united to hydro- 

 gen by two bonds, and in the aldehyde group it is united to oxygen 

 by two bonds). Similarly, fructose contains three asymmetric 

 carbon atoms. 



As an example of how the presence of these asymmetric carbon 

 atoms results in the possibility of many different space relation- 

 ships, the following graphic illustrations of the supposed differ- 

 ences between dextro-glucose and levo-glucose, and between 

 dextro- and levo-galactose, may be cited, f 



* It is assumed that the reader, or student, is familiar with the theoretical 

 and experimental evidence in support of the existence of the so-called " asym- 

 metric " carbon atom and its relation to the effect of the compound which 

 contains it, when in solution, hi rotating the plane of polarized light. For 

 purposes of review, or of study of this most interesting and important phe- 

 nomenon, the reader is referred to any standard text-book on Organic Chem- 

 istry. 



t Attention should be called, at this point, to the fact that such formulas 

 as these cannot possibly accurately represent the actual arrangement of the 

 constituent groups of a carbohydrate molecule around an asymmetric carbon 

 atom. The limitations of a plane-surface formula prevent any illustration of 

 the three-dimension relationships in space. Furthermore, there are certain 

 facts in connection with the birotation phenomenon and the relation of the 

 molecular configuration to biochemical properties (which see) that cannot be 



