1921.] Recent Advances in Stereochemistry. 227 
change of configuration is brought about. Now it does not 
follow that a change of sign of rotation represents an abnormal 
reaction (i.e. a reaction in which a change of configuration 
takes place), for there are numerous cases where such a change 
occurs without assuming change of configuration. For 
example, the introduction of acyl groups into malic and tar- 
taric esters produces derivatives of opposite rotation from 
Since as we have seen it is the reagent which brings about 
a normal or abnormal reaction by replacement several chem- 
points out that substituents such as NH,, OH, Cl, etc., with 
which the Walden Inversion ordinarily occurs, are those which 
possess “ residual affinity.” This enables them to form inter- 
mediate compounds which are later decomposed and the 
place vacated by the old substituent is occupied by the new 
one (in which case no change of configuration occurs) or by 
one of the three already present. 
Werner supposes that the nature of the groups already pre- 
sent has a guiding influence, by which the entering group may 
contiguous to the group X_ to be displaced, in which case no 
inversion occurs, or the entering group may be attracted to 
the face remote from X, in which case inversion will occur. 
The mechanism as proposed by Werner does not involve any 
reference to the fact that one group would take the place 
formerly held by the other. 
The most recent investigations of Debye on the molecular 
structure of the Graphite crystal, made by means of X-rays, 
throw some new light also on the problem under consideration. 
Dehye showed that under certain conditions the carbon atom 
can act with three valencies of equal strength, while the fourth 
valency is considerably reduced in comparison with the others, 
with only a slight intensity. It is supposed that during the 
chemical process, a passing detachment of the radical takes 
place, and as soon as the new substituent arrives, the restitu- 
tion of this fourth reduced valency into the original state of four 
equal valencies can occur towards one or towards the other side 
of the plane of the other three, both these events leading to two 
enantiomorphous configurations. But it still remains unex- 
plained why such a change of place does not always lead 
