STEREO-CHEMISTRY AND PHYSIOLOGY 451 



Pasteur believed that the optical activit}- of the tartaric acids 

 was undoubtedly due to an asymmetric arrangement of which 

 the right-handed form was the mirror image of the left-handed 

 variety, just as a left-handed glove is a mirror image of a right- 

 handed one. Thirteen years later, in 1873, Wislicenus ^ proved 

 that active and inactive lactic acids had identical structures ; 

 and in his examination of the question, added, " The facts force 

 us to explain the difference between isomeric molecules of 

 similar structure by a different arrangement of atoms in space." 



These suggestions did not lie dormant for any length of time, 

 for in 1874 Le BeP and van't Hoff^ simultaneously and inde- 

 pendently published papers in which the foundations of modern 

 stereo-chemistry were laid. 



Le Bel and van't Hoff pointed out that in every case where 

 optical activity had been observed in a compound, the substance 

 in question contained at least one asyuuuctric carbon atorn^ i.e. 

 an atom whose four valencies are attached to four different 

 radicles. Examples of this are found in lactic acid (i), leucine 

 (ii), malic acid (iii), and amyl alcohol (iv), represented in the 

 formulae below, in each of which the asymmetric atom lies in 

 the centre of the figure : 



In the literature of that time, some cases were recorded which 

 conflicted with this view, but later work has proved them to be 

 due to faulty observation, and the Le Bel-van't Hoff theory has 

 been completely established. 



Now, when we examine the problem in the light of this 

 evidence, we find that it is at once simplified. We need only 

 recur to Pasteur's view that atoms in optically active compounds 

 are situated at the corners of a tetrahedron, and the matter 

 becomes clear. Let us suppose that the asymmetric carbon 

 atom is situated at the centre of a tetrahedon, and that the four 

 groups attached to it lie at the tetrahedron's corners. In the 



' Wislicenus, Annalen der Cketn., 167, 343 (1873), 

 ' Le Bel, Bull. Soc. chim., [2], 22, 377 (1874). 



* van't Hoflf, Voorstell tot uiibreidung der structuur-formules in de ruimte, 

 Utrecht (1874). 



