240 



of the group S, while a molecule: CR^R'R'R" has ordinarily 

 no other symmetry than that of group C x (= A)\ i.e., it does not 

 possess any symmetry-properties at all. Such a molecule can, 

 therefore, exist in two enantiomorphously related configurations, 

 because it does not possess any symmetry-property of the second 

 order whatsoever. 



Van 'tHoff calls a carbon-atom, the four valencies of which 

 are saturated by four different univalent atoms or radicals, an 

 asymmetric carbon-atom. He was able to indicate the presence of 

 at least one such asymmetric carbon-atom in all compounds which 

 in the liquid or gaseous state exhibit the power of deviating the 



plane of vibration 

 of the polarised 

 light. Since then, 

 this hypothesis 

 has proved valid 

 in a great num- 

 ber of cases; and 

 it was by this 

 conception of the 



plurivalent . asymmetric atoms, that a rational definition of the 

 circumstances under which Pasteur's "molecular dissymmetry" 

 occurred, seemed now to be given, thus enabling the science of 

 stereochemistry to be founded, as we know it to-day. 



It should be remarked, that by substituting for R l other radicals 

 R f , R", R'", etc., a gradual distortion of the molecule takes place: 

 not only are the distances of the radicals R^, R', R" , R" f , etc. from 

 the central carbon-atom undoubtedly different, but as a consequence 

 of the mutual attractions and repulsions of the substitutes, the 

 forces issuing from the carbon-atom will certainly deviate from 

 those supposed in the case of fig. 162, provided always, that we 

 accept the rather rough mechanical image proposed by Van 't Hof f. 

 In the case of a compound: CR^R'R'R"', the "regular" tetrahedral 

 arrangement of fig. 162 will have quite disappeared ; it will be replaced 

 by a wholly zVregular one. This must be firmly kept in mind, even 

 though it should appear equally true, that the two imaginary arrange- 

 ments a and b in fig. 164. would again seem to be enantiomorphously 

 related, if we had the power to replace the radicals R by different 

 groups without thereby producing a distortion of the original molecule 

 Such a symmetrical arrangement of different groups round 



