16 PRINCIPLES AND DEFINITIONS 



erally admitted (Lowry, 1935) that the sign of the rotation 

 will be reversed, i.e., that the molecules 



CH, X C,H, X 



\ / \ / 



\ / \ / 



C and C 



/ \ / \ 



/ \ / \ 



C,H, Y C,H, Y 



will have opposite rotations, although the position of the 

 univalent radical C2H5CXY is identical, and although there 

 has been only a substitution of one chemical group in the 

 molecule by another. Such possibilities render illusory 

 any conclusion as to the configuration of a substance on 

 the basis of the direction of its rotation. 



To clarify this situation, Fischer (1894) proposed to 

 take as a prototype of configuration that of a specific 

 isomer of some definite substance and compare to this pro- 

 totype the optical isomers of other substances. In this 

 manner a series of optical isomers of different substances 

 can be established, all the members of this series possess- 

 ing the same relative configuration. Wohl and Freuden- 

 berg (1923) suggested that the members of one such series 

 be designated by the letter d and their antipodes by the 

 letter /, while the sign of their optical rotation w^ould be 

 indicated by (+) for a rotation to the right and by (-) for 

 a rotation to the left. The decision as to which one of the 

 two series should be marked by the letter d is, of course, 

 arbitrary, the absolute configuration of the substance being- 

 unknown. According to this system, a substance belong- 

 ing, for example, to the left steric series, but rotating the 

 plane of polarized light to the right will be marked by I (+). 

 Fischer, furthermore, suggested to take as a standard of 

 comparison dextrorotatory glucose, conventionally taking 

 it as a r/-form. He proposed that, in writing the formulas, 

 the aldehydic or ketonic group of sugars and the carbonyl 

 group of monobasic acids be put on top and the chain of 

 carbon atoms in a downward direction, the hydroxyl of the 

 fifth carbon atom being to the right. If one figures out. 



