ASYMMETRIC CARBON ATOM 105 



of the method (p. 98) depending on relative distances in the 

 molecule. The fact is that in methane derivatives of the type 



in which, therefore, the carbon is combined with four dif- 



ferent atoms or groups, such as lactic acid, just mentioned, 



C(CH 3 )H(OH)COOH, 



two (oppositely optically active) isomers regularly occur. 

 The difference of the groups or atoms is essential, since, 

 e. g., if in the above the hydroxyl group be replaced by 

 hydrogen, both isomeric lactic acids lead to the same 

 (optically inactive) propionic acid, 



C(CH 3 )H 2 COOH. 



A configuration of methane, such as is shown in Figs. 14 

 and 15. with the four groups R in one plane and uniformly 

 arranged, would not agree with the facts, for then the type 



with only two different groups should lead to isomerism. as 

 shown by the difference between Figs. 14 and 15. But the 



R, - C R 2 Ra C - R 2 



J, 



Fig. 14. Fig 15. 



arrangement at the angles of a regular tetrahedron excludes 

 isomerism in this and other cases, unless all four groups are 



Fig. 16. Fig. 17. 



different, as is indicated by the difference between Figs. 16 

 and 17 ; this difference would obviously vanish if only R. 



