Mat 15, 1908] 



SCIENCE 



769 



carried out a dozen years ago, two com- 

 pounds were obtained, each of which melted 

 at 245° to 247° ; both had the same com- 

 position and they were at first supposed to 

 be identical; but mixtures of the two 

 melted 30° to 40° lower than either sub- 

 stance alone, and this gave us our very 

 first warning that we were dealing with 

 isomers. Again, in a more recent investi- 

 gation on a new class of nitrogen stereo- 

 isomers, the syn and anti chlorimido com- 

 pounds, 



R — C— E. H — C — H 



II and II . 



cm NCI 



we were on the point of abandoning the 

 search for nitrogen stereoisomers other than 

 the long-known oximes and hydrazones, 

 when the simple observation was made that 

 a crude chlorimido ester, melting at 65°, 

 gave analytical data representing a per- 

 fectly pure substance and yet could be re- 

 solved into two distinct compounds, each 

 giving the same analytical results, but each 

 of which melted some twenty degrees 

 higher than the original substance, a mix- 

 ture of which, however, melted again at 

 65°. Then we knew that we had at last 

 stumbled on the long-sought stereoisomers, 

 the simplest representatives of the whole 

 type. Structural identity being proved, 

 the rigorous proof of stereoisomerism 

 versus physical or crystal isomerism was 

 brought in part with the aid of the low 

 melting point of the mixture of the two 

 substances. 



The physico-chemical theory of the 

 separation of stereoisomers of the asym- 

 metric carbon type, for instance, of the 

 separation of d and I tartaric acids, by the 

 crystallization of their salts with optically 

 active bases, has led to extremely important 

 and useful developments in recent years ; 

 the theory is, in brief, that whereas two 

 opposite forms, -j- A and — A, must have 

 the same physical and chemical properties. 



except in the rotation of the plane of 

 polarized light, the compounds 



f +A 



and 



— A 

 + B 



obtained when the two forms are combined 

 with a single active component, say -j- B, 

 are no longer optical images and will have 

 different physical and chemical properties. 

 The reeog-nition of this simple fact has led 

 to the extension of the method of separa- 

 tion to other substances than acids and 

 bases, namely to esters, amides, hydrazones 

 and so forth. Most important, though, is 

 the fact recognized first by Marckwald that 

 for the same reason the chemical activity 

 of two asymmetric substances, -|- A and 

 —A towards an asymmetric compound -[-B 

 may be different and that consequently, 

 for reactions that are not instantaneous 

 like salt formation, but involve time, like 

 esterifieation, the velocity of formation of 

 the compound, + A + B, say an ester, 

 may be quite different from the velocity of 

 formation of — A + B. This must make 

 possible a separation by chemical means, 

 rather than by physical. But, vice versa, 

 the velocity of decomposition of such 

 esters when once formed may likewise be 

 different. All these conclusions were con- 

 firmed by experiment and the result is of 

 greatest importance to science, for it gives 

 us a direct explanation of the long-known 

 fact that enzymes will attack only one of 

 a pair of stereoisomers and not the other, 

 will cause, for instance, d-glucose to fer- 

 ment, but not ^-glucose ; for in the enzymes 

 we have asymmetrical substances (Fischer) 

 corresponding in every way to what we 

 have symbolized as + B. 



Again, the much-discussed, once hotly 

 fought question of the true structure of 

 so-called tautomeric- compounds has found 

 its rational solution with the aid of phys- 

 ical chemistry. "Whether acid amides have 

 the structure, ECO.NH^ or RC(OH) :NH, 



