McLeod—The Walden Inversion—A Critical Review. 503 
THE WALDEN INVERSION—A CRITICAL REVIEW. 
A. F. McLeod. 
In 1891, Walden 1 asserted that by means of a simple cycle, he 
could pass directly from one optically active acid to its anti¬ 
pode. By treatment of natural 1 malic acid, for example, with 
phosphorus pentachloride, he obtained a laevorotatory chlorsuc- 
cinie acid, in which, by subsequent treatment with moist silver 
oxide, he was able to substitute hydroxyl (OH) for chlorine 
(Cl), and recover the hydroxy acid used as starting material. 
To his great surprise, however, the malic acid thus obtained was 
strongly dextrorotatory. When Walden used, in place of moist 
silver oxide, stronger bases, e. g. potassium or sodium hydrox¬ 
ide, he recovered the original 1 malic acid. An inversion in 
optical activity must have occurred, therefore, in his first cycle 
either with phorphorns pentachloride or with silver oxide. 
This work was ridiculed by all of the leading chemists at that 
time, since it was entirely out of harmony with the ideas ad¬ 
vanced by Vant’ Hoff and Le Bel. These men had pointed out 
that all of the optically active bodies studied by Pasteur had at 
least one asymmetric carbon atom (meaning by ‘asymmetric,’ 
that all of the atoms or groups attached to such a C atom were 
different each from the other). This idea had served to put the 
chemistry of the optically active bodies on a substantial and ra¬ 
tional basis. Earlier chemists 2 had been forced to assign dif¬ 
ferent structural formulae to optical insomers—for a long time 
the formula CH 2 OH-CH 2 -COOH was given to ‘fleishmilch- 
1 Ber, 28, 1293. 
2 Wislicenus: Ann. 166, 6; Moldenhauer: Ann. 131, 323. 
