452 



SCIENCE PROGRESS 



case of lactic acid, we shall find that two arrangements are 

 possible, each being the mirror image of the other : 



COOH 



COOH 



The two arrangements are unsuperposable, being related to one 



another as a man's right hand is to his left. Compounds which 



owe their difference to an arrangement of atoms in space are said 



to be stereo-isomeric. 



We must now consider the effect of introducing a second 



asymmetric carbon atom into the molecule. This was first 



observed by Pasteur in the researches on the tartaric acids 



which we have already mentioned. When we examine the 



structural formula of tartaric acid, we see that it contains two 



asymmetric carbon atoms, which are marked by heavy type : 



H 



I 

 HO-C-COOH 



HO-C-COOH 



H 

 If we divide the molecule into two halves of identical structure, 

 — CH(OH)COOH, we shall see that we have now three possible 

 arrangements of atoms in space instead of two, as in the case of 

 lactic acid. Instead of the second group — CH(OH)COOH, let 

 us write X for convenience; then, as before, we have the two 

 arrangements : 



^°0" gOOH 



