THE CHEMICAL CONSTITUTION OF THE UNITS 143 



2. The Walden Inversion. 



It was first observed by Walden, in 1896, that a change of con- 

 figuration took place in the conversion of the malic acids into the 

 chlorosuccinic acids and vice versa. His results were collected to- 

 gether in 1897 and expressed in the following scheme, in which was 

 included Tilden and Marshall's observations on aspartic acid : 



PCI, 



1-aspartic acid 



l-chlorosuccinic acid 



1-malic acid 



KOH, NH 3 



PCI, 



d-malic acid 



d-chlorosuccinic acid 



KOH, NH 3 



Walden concluded that potassium hydrate and phosphorus penta- 

 chloride acted optically normally, i.e., without alteration of the con- 

 figuration, but that silver oxide, and therefore also nitrous acid and 

 nitrosyl chloride, acted optically abnormally, but which of these 

 reactions was really the normal one he was not able to decide. The 

 conclusion was remarkable, since the action of silver oxide takes place 

 in aqueous solutions at a low temperature and the effect of potash in 

 producing racemisation is well known. Still more curious is the sup- 

 position that nitrous acid and nitrosyl chloride act optically abnor- 

 mally. 



A similar change in rotation was observed in 1905 by Fischer and 

 Warburg in the conversion of alanine into the corresponding halogen 

 fatty acid by nitrosyl bromide and in the reconversion of this com- 

 pound into alanine by the action of ammonia : 



d-alanine < 



NOBr 



NH, 



1-bromopropionic acid 



NH, 



d-bromopropionic acid 



t 

 NOBr 



1-alanine 



This was termed the " Walden inversion " by Fischer in 1907. 



