150 



THE CARBOHYDRATES. 



[CH. V. 



Now make two exactly similar models in each of which 

 the carbon atom is represented as being united to four 

 different groups. Being exactly similar the two models 

 are naturally superposable. Now change the position of 

 any two matches in one model only. The two models thus 

 formed cannot be superposed. On examination it will 

 be found that the two models have a relationship to one 

 another similar to that of the right to the left hand, or of 

 one object to its image in a mirror. This is represented 

 in fig. 26. 



If now still another model be constructed it will be 

 found that it can be superposed on one or other of the two 

 previous ones. That is, there exist two modifications 

 and two only, of compounds of the type Cabxy. 



If the model of the type Ca 2 bx be examined it will be 

 seen that it can be divided into two symmetrical halves. 

 The plane of symmetry and method of division is indicated 

 in fig. 27. It must be understood that though a plane of 

 symmetry exists the atoms or groups are not actually split 

 into halves by it. An examination of the figures shewn 



in fig. 26 will reveal the 

 fact that they do not 

 possess a plane of sym- 

 metry. It has been ascer- 

 tained that all compounds 

 of the type Cabxy exist 

 in two modifications. The 

 solutions of one of these 

 rotates the plane of polari- 

 sation to the right : that 

 of the other exactly the 

 same degree to the left. 

 The former is the "dextro- 

 rotatory" or the ^-com- 

 pound : the latter is the 

 "laevo-rotatory" or /-com- 

 pound. These are sometimes known as enantiomorphs. 

 Since the compound has no plane of symmetry a carbon 



Fig. 27. 



Plane of symmetry of model 

 shewn in Fig. 24. 



