SPACE-ISOMERISM 



79 



instead of drawing the actual tetrahedral perspective, to 

 write a projection of the formula on the plane of the paper 

 (Fig. 21), the asymmetric carbon atom being distinguished by 

 a circle round it or by heavy type. Thus the dextro and 

 Isevo lactic acid can be written as follows : 



CH< 



HO 



C0 2 H C0 2 H 

 Fio. 21. 



OH 



or in projection looked at from above : 



CI^ CHg 



HOCH HCOH 



The case of the tartaric acids is somewhat more complex, as 

 there are here two asymmetric carbon atoms in the molecules ; 

 the following configurations are therefore possible : 



(1) 

 COOH 



(2) 

 COOH 



(3) 

 COOH 



H-C-OH HO-C-H H C-OH 



I i I 



HO C-H H C-OH H-C-OH 



I I I 



COOH COOH COOH 



