STEREOISOMERISM AND OPTICAL ACTIVITY 241 



[Supplementary Note 



On p. 235 it has been stated that certain relations hold good 

 in open-chain-compounds of all types, except one. The exception 

 is of such an extraordinary character that it deserves some 

 attention in this place. It may be represented by the following 

 general formula : l 



Rd \ c / R1 



Rl/ ^Rd 



where Rd and Rl represent two enantiomorphous radicals. If a 

 model of such a formula be constructed, it will be found that 

 the structure as a whole is devoid of any plane of symmetry, if 

 the symmetry of the radicals also is taken into consideration. 

 This is shown in the following figure : 



If P is a plane that passes through the central carbon so as to 

 make 3 and 4 lie symmetrically on either side, it does cut 1 and 2 

 asymmetrically, as in each case the black ball is opposed by the 

 dotted ball, the white being supposed to lie in the plane itself; 

 and the same will be found to be the case with every plane. 



But this configuration is identical with its mirror-image. 



If the two pairs of radicals are, however, joined up to form 

 two rings, so that the central carbon is a member of both the 

 rings, we get a structure like this : 



XHC 



/(CHsK /(CH 2 ) n x 

 NCH,)/ X (CH 2 ) n < 



which is enantiomorphous. This again brings out the distinction 

 between open-chain- and ring-compounds.] 



16 



1 Mohr,/. Pr. Chem. [2] 68, 369 (1903). 



