154 FATS, OILS AND LIPINES. [cH. VI. 



An ester is a compound formed by the condensation of 

 an alcohol with an acid. Glycerol, being a trivalent 

 alcohol, can condense with three molecules of a fatty acid. 



CH 2 OH HOOC. X CH 2 .OOC. X 



CH.OH + HOOC. Y = CH.OOC. Y + 3 H 2 O 



CH 2 .OH HOOC. Z CH 2 .OOC. Z 



Glycerol. Fatty acids. Neutral fat or 



fatty oil. 



The three radicles, X, Y and Z may be the same, or they 

 may differ. 



The fatty acids most commonly found in the composi- 

 tion of these substances are 



Palmitic acid, C 15 H 31 .COOH 



or CH 3 .(CH 2 ) 14 .COOH. 



Stearic acid, C 17 H 35 .COOH 



or CH 3 (CH 2 ) 16 .COOH. 



Oleic acid, C 17 H33.COOH 



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



As can be seen from the formulae, the first two are 

 saturated acids of the acetic acid series, whilst oleic acid 

 is unsaturated and belongs to the acrylic acid series. 

 Occasionally other acids, more unsaturated than oleic acid, 

 are found, such as linoleic acid. 



Palmitic acid melts at 62-6C. ; stearic acid at 69'3C ; 

 oleic at i4C., solidifying at 4C. They are all insoluble in 

 water, and only slightly soluble in cold alcohol, with the 

 exception of oleic acid, which dissolves readily. They are 

 all freely soluble in ether. The sodium and potassium 

 salts of these acids are known as soaps, which are readily 

 soluble in water. If these salts be diluted with water they 

 are hydrolytically dissociated. 



NaA + HOH = NaOH + HA 

 Soap. Fatty acid. 



