230 



tainly differ from its mirror-image, and two enantiomorphously 

 related isomerides would also be possible in such a compound as 



We shall consider such cases afterwards. But if the substitutes R 1 

 do not differ from their enantiomorphous configurations, no isomerism 

 of this kind has ever been observed, and therefore this fact can 

 be used as an argument in proof of the hypothesis of the maximum 

 symmetry of every such substitute in most cases. Van 'tHoff 

 himself undoubtedly felt this : therefore he introduces into his theory 

 the ideas about the special nature of the single, double, and threefold 

 bond between atoms, and he supposes, amongst other things, that 

 the radicals R lt if linked to the carbon-atom by a single tie, can freely 

 rotate round an axis coinciding with the direction of that bond. 

 If R 1 really rotates very quickly in the way just suggested, its proper- 

 ties will indeed appear as though it had a spherical symmetry of 

 its own. x ) 



If now the same hypothesis be applied to all kinds of radicals 

 which may eventually replace the group R^, it is obvious that 

 the rather high degree of symmetry of the arrangement suggested 

 before, cannot be preserved if the four radicals are no longer 

 equal. 



The compound C(R 1 ) 3 R' will have a symmetry which at the 

 greatest could only be that of the group C v z \ and for a compound: 

 C(R l ) 2 (R') 2 at the greatest it could be that of the group C^. 



A compound: C(R 1 ) Z R'R" can at the best have the symmetry 

 of the group 5, while a molecule : CRiR'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 



1) However, as already mentioned, the only exception to this is, when the 

 substitutes R lt which are linked to the central atom, are themselves of a confi- 

 guration, which differs from its mirror-image. In such cases, R l can be brought 

 to coincidence with its mirror-image only by a reflection in a plane, or by an 

 inversion, or most generally: by a rotation round an axis of the second order. The 

 asymmetric substitutes J? x must therefore in all arguments bearing upon configu- 

 rations of molecules in space, be denoted in the molecular formula by the symbols 

 d-and /- (dextro- and /aez/ogyratory respectively), to avoid confusion. Afterwards 

 we shall consider a case, where the necessity of this becomes very evident. 



Cf. on these topics also: W. J. Pope, Pres. Address to the Chem. Sect, of Brit. 

 Assoc. for the Advanc. of Sciences, (1914). 



