THE ISOMERIC TARTARIC ACIDS. 229 



138. The Isomerie Tartarie Acids. 



Of the compounds containing two asymmetric carbon atoms in 

 the molecule we will refer to only that group in which these two 

 atoms are connected by a single bond, as is represented in the 

 subjoined typical formula 







~? 



As regards the positional arrangement of the two groups of 

 substitutes (a, b, c, or a, /?, y) in the molecule, four possibilities 

 are conceivable: (i) both grouped in the dextro- position; (2) 

 both in the levo- position; (3) the one dextro- and the other 

 levo- ; (4) or, finally, vice versa. 



a \ 



Assuming the group ^0, to possess a (numerically) greater 

 c' 



/" 



optical rotatory power (D or L) than C ft which may be 



7 



expressed in letters thus 



D > d L > I, 



Then the rotatory power of the entire molecule will have the 

 following values 



D + d, L + l, 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. Con- 

 sequently there result four stereoisomeric active modifications : 

 the first strongly dextro-rotatory; the second strongly levo-rota- 

 tory; 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-Z) modifications, can coalesce to 

 form inactive double molecules 



/ D + d \ j / D - d) 



{ L + I } and { L - I [ 



The total number of all conceivable stereoisomeric modifications 



"\ /" 



of the compound b C C 3 is therefore six. 



