MIXED COMPOUNDS 71 



as represented by the groups present or the order of their ar- 

 rangement. 



Asymmetric Carbon. — On examining the structural formula 

 for the lactic acids, viz. a-hydroxy-propionic acid, we find a 

 distinctive fact. 



H 



CH3 - C - COOH Lactic add 



OH 



We see that the second carbon atom differs from the other two 

 in this, that it has united to it four diferent elements or groups, 

 viz., CH3, H, OH and COOH. Now it has been found that all 

 compounds, which when in solution exhibit optical activity 

 toward polarized light, contain at least one carbon atom which 

 is thus united to four different elements or groups. The union 

 of four different groups to one carbon would give to that car- 

 bon in its space relations an unsymmetrical arrangement. Such 

 arrangement would make possible an unsymmetrical action 

 such as exhibited toward polarized light in that one form is 

 right-handed or dextro-rotatory and the other form left-handed 

 or levo-rotatory. Such an unsymmetrical arrangement also 

 would be like the unsymmetrical nature of the right and left 

 hand. This complete unsymmetrical nature would be lost 

 whenever any two, or all four, groups united to a carbon atom 

 were the same. Furthermore it was found by Pasteur that 

 certain compounds, e.g. the salts of tartaric acid, crystallized 

 in unsymmetrical or right-and left-handed forms and that these 

 right- and left-handed crystal forms when put into solution 

 exhibit right- and left-handed action toward polarized light. 



van't Hoff - Le Bel. — This theory as we have outlined it, 

 which connects optical activity and the existence of structurally 

 identical isomers with the presence in such compounds of an 

 unsymmetrical carbon atom, was advanced simultaneously 

 by two chemists, van't Hoff, a Dutchman, and Le Bel, a French- 

 man. It is known as the theory of the asymmetric {zmsymmetrical) 



