230 OPTICALLY ACTIVE FERMENTATION PRODUCTS. 



Particular interest attaches to the special instance wherein the 

 two semi-molecules are equal, and which therefore comprises the 



n 



compounds expressed by the formula b c C * Tartaric 



tr \? 



acid, 



COOH 



COOH COOH CHOH 



H-^C C H 

 HO/ \)H CHOH 



COOH 



is the type of this group. 



In this case we have D = d and L = I. The four above-named 

 general expressions for the rotatory power of the individual active 

 forms are, in this special case, resolved into 



2D 2L 



The modifications 2!) and 2L correspond to the dextro- and 

 levo-tartaric acids; and, in place of the two faintly rotatory 

 modifications, we have a single optically inactive form, which, in 

 the tartaric acid group, is named meso-tartaric acid. 



This last-named acid is inactive as a result of intramolecular 

 compensation, and is therefore distinct from the modification 

 produced by the coalescence of the two optically active forms 



I 2L }' wn i c k double molecule is also optically inactive, and in 

 the tartaric acid group is named racemic acid. 



The existence of two fundamentally different groups of optically 

 inactive compounds containing asymmetric carbon atoms is thus 

 theoretically possible, viz. : 



1. Monomolecular, indivisible, optically inactive in con- 

 sequence of intramolecular compensation of their optically 

 active atom groups. Type :D-^ = oorL-Z = o. Example : 

 meso-tartaric acid. 



2. Bimolecular, optically inactive, appearing only as 

 double molecules, one of which belongs to the dextro- and 

 the other to the levo- modification of the compound. Type : 

 2D - 2L = 0. The inactive double molecule can be split up 

 into two optically active components, 2D and 2L. Example : 

 racemic acid (Fr. adde racemique, Ger. Traubensdure). 



The first of these two modifications, viz., the monomolecular, 

 indivisible kind, is termed the anti-combination ; the second, or 

 bimolecular modification, being known as the para- form, or (from 

 the best known example) racemic modification. 



