FATS, PHOSPHATIDS, AND ALLIED SUBSTANCES 63 



The acids listed above are all saturated compounds, that is they 

 contain no double bonds. An acid found in a large number of 

 fats is oleic acid, which has the same number of carbon atoms 

 as stearic acid, but two less hydrogen atoms. It is thus C 18 H 34 O 2 

 and its formula is 



CH 3 (CH 2 ) 7 CH CH(CH 2 ) 7 COOH. 



It is an unsaturated acid, and contains a double bond. Fats 

 containing this acid have a lower melting point than those con- 

 taining the corresponding saturated compound. Some allied 

 compounds contain other alcohols in place of glycerine. Thus 

 cetyl alcohol C 16 H 3 . { OH is found in spermaceti in the head of 

 the sperm whale, and myricil alcohol C 30 H C1 OH in beeswax, etc. 

 Esters of these alcohols usually are called waxes. The follow- 

 ing formula illustrates the structure of a fat. 



O 



CH 2 C R 

 



CH C R 



I 



II 

 CH 2 C R 



R is the rest of an acid molecule. If the fat were tristearin, R 

 would represent a chain of 16 CH 2 groups with a CH 3 group at 

 the far end. Fats are thus tri-atomic esters of glycerine and an 

 organic acid. The three R's may be all the same fatty acid, or 

 they may be different. There is thus the possibility of having a 

 large number of different fats, differing in the kind of fatty 

 acid present. This possibility is realized in nature, and a large 

 number of different fats are known. The naturally occurring 

 fats are rarely made up of a single kind of fat, but usually are 

 mixtures of various kinds such as tripalmitin, tristearin, and 

 triolein, as the fats from these respective acids are called. Oleic 

 acid has a very low melting point, and triolein also melts at a 

 low temperature. The presence of much triolein in a fat lowers 



