228 



that the velocity of racemisation in these cases appears to be a con- 

 stitutive property.- Thus for instance the active normal butyl-methyl- 

 benzyl-phenyl-ammonium-iodide appeared to racemise in chloroform- 

 solution much more slowly than the corresponding isobutyl-derivative 

 did under similar circumstances ; and analogous results were obtained, 

 if bromoform was used as a solvent. The reaction appeared to be 

 mono-molecular, with a rather high temperature-coefficient. 



14. The general conclusions to which Pasteur finally arrived 

 may be briefly summarised as follows: 



1) If the atoms of a chemical molecule be "dissymmetrically" 

 arranged, this molecular "dissymmetry" implies the possibility of 

 the existence of two opposed configurations of the molecule. Both 

 isomerides have the same chemical properties, but they are endowed 

 always with equal, but oppositely directed rotatory power. The 

 presence of molecular dissymmetry therefore reveals itself by this 

 rotatory power of the molecules and is wholly determined by their 

 chemical nature. 



2) When the atoms of a chemical molecule are dissymmetrically 

 arranged, it is at the same time betrayed by the occurrence of non- 

 superposable crystalline forms with both isomerides. 



Let us see now, in how far these general conclusions, which are 

 generally considered as constituting Pasteur's law, are really 

 justified and confirmed by experience. 



In the first instance we may ask : when has a chemical molecule 

 such a "dissymmetrical" arrangement of its atoms as to make the 

 existence of 'two enantiomorphously related isomerides possible? 



Pasteur himself does not speak very definitely about the special 

 conditions under which this occurs, and he only postulates that 

 the atomic configuration shall be "non-superposable" with its mir- 

 ror-image. 



The first attempt to give a more concrete explanation of these 

 conditions was successfully made in 1874 by Van 't Hoff 1 ) and Le 

 Bel 2 ), who independently of one another introduced into chemical 

 science the conception of the so-called "asymmetric atoms", and 

 thus became the very founders of modern stereochemistry. For of 



1) J. H. Van 'tHoff, Voorstel tot uitbreiding der tegenwoordig in de schei- 

 kunde gebruikte struktuurformules in de ruimte, Utrecht, (1874); La Chimie 

 dans 1'Espace (1874); Die Lagerung der Atome im Raume, (1877). 



2) J. A. Le Bel, Bull, de la Soc. Chim. Paris (2). 22. 377. (1874). 



