1903.] on Recent Advances in Stereochemistry. 303 



involved the introduction of the idea that the chemical individuality 

 of a substance is dependent upon its molecular constitution as well 

 as upon its molecular composition. A third great development in 

 the atomic theory had yet to take place. 



Whilst the theoretical views which culminated in Kekule's consti- 

 tutional formulae were at first found sufficient to explain numerous 

 observed cases of isomerism, instances soon began to accumulate of 

 substances which exist in so many isomeric forms that the Kekule 

 method of representation is incapable of accounting for them all. At 

 an early date, Pasteur showed clearly that substances exist which 

 have the same molecular composition and the same molecular consti- 

 tution, but which nevertheless differ in important respects. A crisis 

 was ultimately reached when, in 1870, Wislicenus demonstrated the 

 existence of three isomeric lactic acids, all having the molecular 

 composition, CaHgOa, and the molecular constitution — 



H 0-H O 



I I II 



H— C— C— C 



I 1 ^0-H 

 H H 



and contended that he had amply proved the insufficiency of Kekule's 

 method of writing constitutional formulse. 



The step needed to rid the atomic theory of these apparent 

 anomalies was indicated by van't Hoff and Le Bel in 1874 ; they 

 pointed out that the weakness of the Kekule method lies in the tacit 

 assumption that the molecule is spread out upon a plane surface : that 

 by throwing this assumption aside and taking a rational view of the 

 way in which the molecule is extended in space, all difficulties 

 immediately vanish. The considerations put forward by van't Hoff 

 and Le Bel form the basis of the subject now known as Stereochemistry ^ 

 the branch of science which deals with the manner in which the 

 atoms are distributed within the molecule in three-dimensional space ; 

 they deal, in the first place, with the arrangement of the constituent 

 atoms in the simple organic compound, methane, the molecule of 

 which has the composition, CH^, or consists of one carbon atom and 

 four hydrogen atoms. The Kekule constitutional formula pictures 

 the component atoms of the methane molecule as if joined together 

 in one plane (Fig. 1), whilst according to the new view, the four 

 hydrogen atoms are imagined situated at the four apices of a regular 

 tetrahedron of which the carbon atom occupies the centre (Fig. 2). 

 This is conveniently illustrated with the aid of a few cardboard 

 models. 



Consider now the result of replacing three of the four hydrogen 

 atoms present in the methane molecule by three different groups of 

 atoms — the three groups, CH3, OH, and CO2H, for example. One 

 of the most striking results which has accrued from the chemical 

 investigation of the past century has been the demonstration of the 



