Structure of Simpler Organic Compounds. 117 



This reaction would not prove the constitution of lactic 

 acid unless the constitution of the bromine derivative were 

 also established, for the compound, CH 2 Br CHj COOH, 

 is also capable of existence. (6) The following synthesis, 

 however, places the constitution beyond doubt. Acet- 

 aldehyde forms, as already stated (p. 81), an addition 

 product with hydrocyanic acid 



OH 

 CH S CH 



On hydrolysis the (CN) group undergoes the usual change, 

 the product being lactic acid 



/OH /OH 



CH 3 CH< + 2H 2 = CH 3 CH< 



X CN X COOH + NH 3 



The constitution of lactic acid can also be well estab- 

 lished by other synthetical methods. An isomeride of the 

 constitution CH 2 (OH) - CH 2 COOH, or ft * hydroxy pro- 

 pionic acid (lactic acid is a hydroxy propionic acid), is 

 also known, but is of little importance. 



Lactic acid is a thick syrupy liquid, which is miscible 

 with water, alcohol, and ether in all proportions. It cannot 

 be distilled, and as it has no definite boiling or melting 

 point, it is best identified by means of its salts. Of these 

 the most suitable are the calcium and the zinc salts 



(C 8 H 6 8 ) 2 Ca.5H 2 and (C s H 6 3 ) 3 Zn.3H 2 



Some hydroxyl derivatives of polybasic acids are of 

 interest. 



Malic acid 



CH(OH)-COOH 



CH 2 - 



COOH 



lich is found native in apples and other fruits, is a 



* For the significance of the symbols a and /S, pee p. 77. 



