.291 



Fischer, make the supposition, that during the reaction instable 

 intermediate compounds must necessarily be formed, in order to 

 preserve a continuous asymmetry of the molecule while the 

 substitution takes place; and Fischer points out, that precisely 

 the substitutes NH 2 , OH, Cl, etc., with which the Wai den-in ver- 

 sion ordinarily occurs, are those which have a "residual affinity". 

 This enables them to form such intermediate compounds, which 

 are then later decomposed, and the place vacated by the old 

 substitute is occupied by the new one, or by one of the three 

 already present. 



Werner makes a suggestion to account for the phenomenon 

 by supposing that the other substitutes have a guiding influence 

 which tries, by means of attraction, to bring the new substitute 

 into the range of the three others; the mechanism, as proposed 

 by this author, does not involve any reference to the fact that 

 one group would take the place formerly held by another. 



The most recent investigations of Debye *) on the molecular 

 structure of the graphite-crystal, made by means of R on t gen- 

 rays, also throw some new light on the problem under conside- 

 ration. Debye was able to demonstrate that the carbon-atom 

 not only acts with four equal valencies which are directed in 

 space as supposed by Van 't Hoff, but that this atom can 

 also act with three valencies of equal strength, while the fourth 

 is considerably reduced in comparison with the others, acting in 

 a direction perpendicular to the plane of the last, and with only a 

 very slight intensity. If this be true, we might suppose that during 

 the chemical process a passing detachment of the radical takes 

 place, and that as soon as the new substitute arrives, the resti- 

 tution of this fourth reduced valency into the original state of 

 four equal valencies can occur towards the one or towards the 

 other side of the plane of the three others, both these events 

 finally leading to two enantiomorphous tetrahedron-configurations. 



Evidently the view exposed here has some analogy with 

 H o 1 1 e m a n ' s attempt to give an explanation of the W a 1 d e n- 

 inversion from the standpoint of the electron-theory 2 ). 



J ) P. Debye, Phys. Zeits., 18, 291, 483, (1917). 



A review of the numerous attempts to explain the mechanism oi the 

 W a 1 d e n-in version is published by P. Walden in his monography: Optische 

 Umkehr-Erscheinungen, Braunschweig, (1919), page 112159. 



2) A. F. Holleman, Receuil des Trav. d. Chim. d. Pays-Bas, 32, 125, (1913). 



