238 SCIENCE PROGRESS 



bringing into action any mechanism whatever. How false 

 this impression is will be realised on referring to the cases 

 discussed below, which are seen to be limited in every 

 direction by special conditions, all of which point to the 

 necessity of some organised system for effecting the inter- 

 change of the various radicles. 



When several asymmetric atoms are present, these may 

 be inverted one at a time : in this case the product is a 

 " stereoisomeride " of the original material, in which the same 

 groups are present, and are linked together in the same 

 order, but differently arranged in space. Since groups which 

 are close together in one modification may be widely separated 

 in the other, the condition of equal stability no longer applies, 

 and when equilibrium is attained the two forms may be present 

 in widely different proportions. 



Finally, attention may be directed to the step-by-step 



inversions discovered by Walden in the case of malic acid, 



HO.CH.COOH 



, which is converted on displacing — OH 

 CH 2 .COOH 



by — Br and again by — OH not into a racemic mixture, but 



into the optical isomeride ; in this case equilibrium between 



isomerides is obviously not established, the inversion of the 



groups evidently taking place under conditions of bromination 



or hydrolysis which do not permit of any reverse action. 



(a) Optical Inversio>i of Acids and of Stigars. — In order to 



produce optical inversion it is only necessary to bring the 



compound into equilibrium with a trace of an optically 



inactive isomeride, since this will revert equally readily to 



the dextro and Icevo forms of the original material and so 



sooner or later reduce it to a racemic mixture. In the case 



of acids containing the group 



R, \ c / H 



R,/ ^COOH 



this can be effected by the simple transference of a hydrogen 

 atom from carbon to oxygen, as set out in the following 

 formulae : 



