112 II. CHEMISTRY OF FATTY ACIDS AND GLYCEROL 



In addition to the chemical reactions which are concerned only with 

 the carboxyl group, the unsaturated acids readily add halogens or hydrogen 

 to form saturated compounds. Furthermore, they are oxidizable at their 

 unsaturated linkages to yield hydroxy-acids, oxides, or peroxides. Undei- 

 conditions of strenuous oxidation, such oxidation products may be split 

 between the carbons which constituted the original double bond, to form 

 short-chain acids or aldehyde acids. 



(1) Reactions Involving Carboxyl Group 



a. Ionic Reaction with Production of Soaps. The fatty acids readily 

 react to form salts wherein the hydrogen of the carboxyl is replaced by a 

 metal. Such metallic salts of the higher fatty acids are referred to as 

 soaps. All metallic salts of the fatty acid are included in this category, ir- 

 respective of whether the resulting compounds are soluble or insoluble in 

 water. The soaps most widely known are the sodium and potassium 

 forms; however, there is a wide variety of other metallic soaps which are of 

 importance in industry and in our everyday life. 



Soaps are prepared by any one of several methods. The simplest is by 

 the reaction of a base on the free fatty acid. A second method involves 

 double decomposition by mixing solutions of soluble soap and soluble 

 metallic salt, with the resultant precipitation of the insoluble soap. The 

 third procedure commonly employed involves the heating of the tri- 

 glyceride or other ester with a suitable amount of the metallic oxide or hy- 

 droxide. Caustic soda (sodium hydroxide) or potash (potassium hy- 

 droxide) are the alkalies most frequently used; for the preparation of 

 special products such as calcium or barium soaps, calcium or barium hy- 

 droxide is substituted for the commoner alkalies. 



There are various special soaps which have specific uses. The soaps 

 which are used for driers in paints, varnishes, enamels, and other protective 

 coverings are chiefly the lead, manganese, cobalt, zinc, or calcium com- 

 pounds. These can be made by the reaction of the oxides or salts on lin- 

 seed oil with the application of heat or, preferably, by precipitation of the 

 soluble soaps of linseed oil. 



A second application of the special soaps is in the manufactiu'e of greases. 

 The usual greases consist of a liquid phase of mineral oil or vegetable oil 

 which is mixed with a dispersed or gel phase, generally consisting of a soap. 

 Calcium stearate or oleate is frequently used in cup greases, while aluminum 

 oleate, lead salts of oleic, stearic, iso-oleic or erucic acids, zinc stearate or 

 oleate, and barium, magnesium, chromium, iron, cobalt, or nickel soaps 

 find special application in certain lubricants. Lithium stearate has found 

 some use in lubricants for aircraft engines. An extensive treatise on the 

 manufacture of greases has been written by Klemgard.^''" 



'" E. N. Klemgard, Lubricating Greases: Their Manufacture and Use, Reinhold, New 

 York, 1937. 



