McLeod—The Walden Inversion — A Critical Review. 505 
That such an inversion actually takes place is wonderful in¬ 
deed and, in my opinion, still remains to be proved, in spite of 
the apparently absolute demonstration by Walden, Purdie, 
Pisher, McKenzie and many others. Some recent experimental 
work which I have done, indicates that the acid product ob¬ 
tained by the action of silver oxide in water solution on dl brom- 
propionic acid (a change entirely analagous to the one given 
above) has properties entirely different from those of ordinary 
dl lactic acid. This evidence will be discussed at greater length 
in the experimental part of this paper. (Since some confirma¬ 
tory evidence in the absolute proof to the contrary is still lack¬ 
ing, let us assume here for simplicity, that the Walden inver¬ 
sion is an established fact. 
For a long time Walden was misled by the fact that his chlor- 
succinic acids gave an actual rotation opposite to that indicated 
by their sign i. e. d chlorsuccinic rotated laevo and vice versa. 
But he decided finally * 4 that the action of phosphorus penta- 
chloride on malic acid was normal, a conclusion which he de¬ 
duced largely from theoretical and physico-chemical considera¬ 
tions. Having determined this, it followed as a matter of 
course that the action of potassium hydroxide was normal, 
while that of moist silver oxide was abnormal. The question 
as to whether the action of nitrous acid on asparaginic acid to 
give malic was normal or abnormal, Walden left open, since he 
was unable to determine. Having contributed a splendid and 
practically complete demonstration of the chemistry of the opti¬ 
cally active malic acid series, Walden published the last of his 
five papers in 1899, and left this field of work. 
With the exception of a small but very significant contribu¬ 
tion by Purdie and Williamson, 5 nothing of vital importance 
was done on the Walden inversion from 1899 until March, 1907, 
when Emil Fisher published his first article, “Zur Kenntnis der 
Waldensche TJmkehrung.” 6 In synthesizing various optically 
active polypeptides, Fisher was forced to determine absolutely 
sBerichte 32, 1855. 
4 Ber, 32, 1833 and 1855. 
s Trans. L. Ch. Soc. [1896] p. 838. 
e Ber, 40, 1051. 
