Stereoisomerism. 1 7 1 



The instances already mentioned which exhibit optical 

 activity are those of amyl alcohol, lactic acid, and tartaric 

 acid. The chief isomeride of the amyl alcohols formed as 

 a by-product in the alcoholic fermentation processes has 

 the formula 



CH 3 v 



>H-CH 2 OH 

 CH 3 -CH/ 



and has been stated to be optically active. It will be 

 noticed that this substance contains an asymmetric carbon 

 atom (the symbol of which is surrounded by a ring). 

 Lactic acid has the formula 



CH 3 .H(OH).COOH 



Again, the asymmetric carbon atom is in evidence. 



If an optically active substance is submitted to such 

 reactions that the asymmetry of the carbon atom disap- 

 pears, the product obtained will be inactive. Thus, lactic 

 acid on reduction with hydriodic acid yields propionic 

 acid, CH 3 -CH 2 -COOH, which contains no asymmetric 

 carbon atom ; an optically active lactic acid cannot yield, 

 therefore, anything but an optically inactive propionic acid 

 on reduction, as, according to the Le Bel-Van 't Hoff hypo- 

 thesis, propionic acid cannot yield optical isomerides owing 

 to the absence of the asymmetric carbon atom. On the other 

 hand, if one of the groups attached to the asymmetric 

 carbon atom is replaced by a group or element differing 

 from the three groups or elements already attached to it, 

 an asymmetric derivative is possible. Thus, if active amyl 

 alcohol is treated with phosphorus and iodine, so as to 

 replace the hydroxyl group with the latter element, the 



CH 

 substance ;>CH CH 2 I is obtained, which is 



optically active. 



