239 



related isomerides would also be possible in such a compound e.g. 

 as CX^Oa/?'. 



We shall consider such cases afterwards. But if the substitutes /?, 

 do not differ from their enantiomorphous configurations, no isomerism 

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

 be used in most cases as an argument to prove the hypothesis 

 of the maximum symmetry of every such substitute. Van 'tHoff 

 himself undoubtedly felt this: therefore, he introduced into his theory 

 the ideas about the special 

 nature of the single, double, 

 and threefold bond between 

 atoms, and he supposed, 

 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 l really rotates 

 very quickly in the way 



just suggested, its properties will indeed appear as though it had 

 a spherical symmetry of its own. l ) 



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

 which may eventually replace the group R lf it is obvious that the 

 rather high degree of symmetry of the arrangement already suggested, 

 cannot be preserved if the four radicals are no longer equal. 



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

 greatest, could only be that of the group C^\ and for a compound: 

 (^ / ) 2 at the greatest it could be that of the group C* (fig: 163). 



A compound: C(R 1 ) Z R / R // can, at the best, have the symmetry 



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

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

 guration, which differs from its mirror- image. In such cases, R^ 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 R l 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 laevogyr&tory 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, Presid. Address to the Chem. Sect, of the 

 Brit. Assoc. for the Ad vane, of Sciences, (1914). 



