14 



PRINCIPLES AND DEFINITIONS 



lieclron with four different radicals situated in its corners 

 as the basis for the explanation of the origin of optical 

 activity. This structure accounts for both the existence 

 as well as the approximate value of optical rotation in the 

 simplest dissymmetric molecules. It should be noticed 

 that a tetrahedric molecule presents a spiral type of dis- 

 tribution of its atoms. In Fig, 2 (a) is represented an 



Fig. 2. Dissymmetric configuration of organic molecules; a) 1-isomer, 

 b) d-isomer. 



irregular tetrahedron in the corners of which are placed 

 four different groups. In the order of diminishing mag- 

 nitude these groups can be arranged in the following- 

 manner: Ri > R2 > R3 > R4. By joining the centers of 

 these groups in the order just given a spiral is obtained. 

 If the largest group (Ri) is placed nearest to a hypotheti- 

 cal observer, the spiral represented in Fig. 2 (a) will 

 appear to rotate counter-clockwise. According to Boys 

 (1934), such a structure would correspond to the left abso- 

 lute configuration of the molecule. If we interchange the 

 groups R2 and Rs, we obtain a figure which is the mirror 

 image of the preceding one ; the spiral twist will now as- 

 sume a clockwise direction and the molecule will possess 

 the right configuration. 



Recently an attempt has been made to adapt the concept 

 of absolute configuration to the definition of the configura- 



