no PRACTICAL ORGANIC AND BIO-CHEMISTRY 



COMPOUNDS CONTAINING THREE CARBON ATOMS. 



The number of compounds which can be derived from propane are 

 more numerous than those from ethane, since the extra carbon atom 

 introduces further possible combinations and permutations. The 

 principal compounds are : 



The first two compounds are isomers ; in their nomenclature the 

 position of the hydroxyl group is indicated by the Greek letters a and fi y 

 the lettering or numbering being commenced at the carbon atom next 

 to the COOH group, which stands at the end of the chain (cf. p. 100). 



Lactic Acid. 



Lactic is formed by the fermentation of sugar by lactic acid 

 bacteria ; hence its presence in milk when it turns sour. Lactic acid 

 is contained in muscle, especially after activity, and other organs of 

 the animal body. 



Preparation. 



Lactic acid has been prepared synthetically by several methods, 

 but its usual method of preparation is by fermentation, i.e. by bio- 

 logical means. 



To a solution of 50 gm. of cane sugar in 500 cc. of water 20 gm. 

 of chalk or zinc carbonate and 20-30 c.c. of sour milk (which con- 

 tains lactic acid bacteria) are added and the mixture is kept in a warm 

 place, or better in an incubator at 37, for 6-8 days and occasionally 

 shaken. The chalk or zinc carbonate is added to neutralise the lactic 

 acid which hinders the growth of the bacteria. Calcium or zinc lactate 

 is formed. 



The solution is boiled to kill bacteria, filtered, and evaporated 

 on the water-bath till crystallisation commences and allowed to cool. 

 The lactate is filtered off, pressed between sheets of filter paper and 

 recrystallised from hot water. The acid is obtained from the salt by 

 decomposition with sulphuric acid, extraction of the liquid with ether, 

 and removal of the ether by distillation. 



