THE CHEMICAL CONSTITUTION OF THE UNITS 147 



If we picture a carbon atom with its four valencies A, B, C, D, as a 

 sphere (i) from which the four groups project, then a reacting molecule 

 may attach itself at any vacant place between them (2). If the reaction 

 take place between B and X, at the moment of reaction B will become 



B B v B v YA A 



1 lA JAY XAI I 



4 c A *c Aye B y c * 4 



(D (2) (3) (4) (5) 



FIG. 6. 



loosened (3). At this moment A or C or D may, on account of the re- 

 action, alter their position and take the place of B. The new group 

 must then enter the position of A, or C, or D. The compound (5) 

 will result, if the shifting take place in every molecule. 



The same hypothesis explains the reactions which occur with un- 

 saturated compounds, but the deductions are more difficult to follow. 

 It also will explain substitution in compounds with several asymmetric 

 carbon atoms ; if a homogeneous product result, then there is normal 

 substitution or complete inversion ; if a mixture of products which are 

 not optical antipodes, then there is partial inversion or complete 

 racemisation of the asymmetric carbon atom affected. 



3. The Correlation of the Configurations of the Amino Acids. 



Since in the interconversion of the amino-, chloro- or bromo-, and 

 hydro xy-acids a "Walden inversion" of the groups attached to the 

 asymmetric carbon atom occurs it is not yet possible to correlate the 

 configuration of the amino acids together nor with that of other op- 

 tically active compounds, especially the tartaric acids and glucose. 



The configuration of d-malic acid is known with reference to d- 

 tartaric acid and d-glucose. 



Since no " Walden inversion " occurs when amino acids are con- 

 verted into the corresponding hydroxy acid by the action of nitrous 

 acid, the configuration of d-aspartic acid, which gives d-malic acid, may 

 be said to be established. 



A glyceric acid results by the action of nitrous acid on serine. 

 Neuberg and Silbermann believed that they obtained 1-glyceric acid on 

 account of the relationship of this acid 'to 1-tartaric acid, but, a little 

 later, Neuberg stated that the observation required confirmation. 



10 



