176 OPTICALLY ACTIVE FERMENTATION PRODUCTS 



Then the rotatory power of the entire molegule will have the following values 

 I) + d, L + I, D - d, or L -I, 



according to which of the four possible compounds is present. 



The optical effect of the one semi-molecule will therefore be strengthened 

 by that of the other semi-molecule in the first two instances, or weakened 

 thereby in the other two cases. Consequently there result four stereoisomeric 

 active modifications : the first strongly dextro-rotatory ; the second strongly 

 levo-rotatory ; the third faintly dextro-rotatory ; and the fourth faintly levo- 

 rotatory. Furthermore, one molecule apiece of the two strongly rotatory 

 (D + d and L + Z), as also one apiece of the two faintly rotatory (D - d and L - I) 

 modifications, can coalesce to form inactive double molecules 



The total number of all conceivable stereoisomeric modifications of the com- 



\ /" 



pound b C C 13 is therefore six. 



c y 



Particular interest attaches to the special instance wherein the two semi- 

 molecules are equal, and which therefore comprises the compounds expressed by 



the formula b C C b. Tartaric acid, 



c c 



COOH 



COOH COOH CHOH 

 ! C H 



\) 



H C 



\ 



CHOH 



COOH 



is the type of this group. 



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

 expressions for the rotatory power of the individual active forms are, in this 

 special case, resolved into 



2D zL 



The modifications zD 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 which 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. : 



i. Monomolecular, indivisible, optically inactive in consequence of 

 intramolecular compensation of their optically active atom groups. Type : 

 D-cJ = oorL-Z = o. Example: meso-tartaric acid. 



