HEXOSES 55 



The difference between these arrangements resembles that between 

 an image and its reflection in a mirror; the diagrams cannot be super- 

 imposed upon one another so that the corresponding parts will coincide, 

 except by inverting one of the diagrams, and thereby converting it into 

 the other, its mirror-image. Now it would appear that when a carbon 

 atom is united by its valencies to four different masses, either of the 

 above arrangements is possible, the one yielding a dextrorotatory and 

 the other a levorotatory compound. An optically inactive body is 

 produced either by a mixture of equal numbers of the two forms of 

 molecules or by "internal racemization," i. e., by the presence within 

 the molecule of two equally active carbon atoms rotating the plane of 

 polarized light in opposed directions. 



This being the case, the number of possible optical isomers of a 

 substance which contains two asymmetric carbon atoms is four, since 

 either of the two possible varieties, levo- and dextro- of the first asym- 

 metric atom may be combined with either of the two possible varieties 

 of the remaining atoms. Similarly the number of possible optical 

 isomers of a substance which contains three asymmetric carbon atoms 

 is eight, since any of the four possible arrangements about the first 

 two atoms may be combined with either of the two possible arrange- 

 ments about the third atom, and, in general, the number of possible 

 optical isomers of a substance which contains n asymmetric carbon 

 atoms is 2 n . 



THE HEXOSES. 



The relationships which have been described above are very well 

 illustrated among the hexoses. A large number of sugars are known 

 which possess the formula C 6 H]2O 6 . The structural formulae of these 

 sugars have been elucidated by Fischer and others, and it has been 

 shown that a number of these possess a structure 1 which can be repre- 

 sented by the general formula: 



CHO 

 *CHOH 



*CHOH 



I 

 *CHOH 



*CHOH 

 CH 2 OH 



It will be observed that the four carbon atoms which are distin- 

 guished by asterisks are asymmetric, because they are each united 

 with four different masses. For example, take the second carbon atom 

 from the top of the diagrammatic formula. It is united with the 

 following groups: -CHO, -H, -OH and -C 4 H 5 (OH)4. According 

 to the rule which is enunciated above, there must be 2 4 = 16 possible 

 optical isomers of this compound. 



1 Or are readily convertible into substances possessing such a structure, cf. below: 



