II. CHEMISTRY 273 



contain less than 50 % linoleic acid. It was packaged in an ordinary screw- 

 cap bottle and thus had access to oxygen of the air. The essential fatty 

 acids and their compounds are readily attacked by oxygen, and any 

 preparation which is not stored in an ampule or sealed under nitrogen or 

 under vacuum should not be accepted as pure. To the writer's knowledge, 

 the only source of authentic bromination-debromination linoleic and lino- 

 lenic acids and their esters is the Hormel Foundation of Austin, Minnesota. 

 Arachidonate concentrate containing a maximum of 70 % arachidonic acid 

 was once available from the Nutritional Biochemicals Corporation. 



D. CHEMICAL PROPERTIES 



The polyunsaturated acids belonging to the essential fatty acid group are 

 similar in their properties. Chemically their reactivity is restricted to re- 

 actions of double bonds and of carboxyl groups, and segregation of these 

 acids on the basis of chemical reaction is difficult and only partial. For prac- 

 tical purposes, there is negligible difference between the carboxyl reactivi- 

 ties of the various members of the group, and all separative procedures 

 based upon chemical reactivity utilize the differences in the number of 

 double bonds of the various members of the group. Thus, the preparation 

 of linoleate is based upon the insolubility of tetrabromostearic acid in pe- 

 troleum ether, and the preparation of linolenic acid is based upon the in- 

 solubility of hexabromostearic acid and the solubility of tetrabromostearic 

 acid in ethyl ether. The more highly unsaturated acids form polybromides 

 which are all virtually insoluble in ether, and thus separation of the higher 

 members of the series is impossible by this method. For example, arachi- 

 donic acid prepared by bromination-debromination always contains a con- 

 siderable amount of pentaenoic acid. 



All the essential fatty acids add halogens, and this reaction is the basis 

 of the commonest method for determination of total unsaturation. Various 

 halogenating reagents have been employed for the addition reaction, but 

 the results are expressed as grams of iodine absorbed per 100 g. of fat, and 

 the value is called the iodine value. 



Thiocyanogen also adds to double bonds, but not to all double bonds, pre- 

 sumably because of steric hindrance. Thus oleic acid adds one mole of 

 thiocyanogen, linoleic acid adds one, and linolenic acid adds two. The thio- 

 cyanogen value, combined with the iodine value, can be used to calculate 

 the linoleic acid content of fats. 



The unsaturated linkages in the essential fatty acids are readily hydro- 

 genated, and in hydrogenated vegetable oils the essential fatty acid content 

 is reduced. Partial hydrogenation disproportionately reduces the essential 

 fatty acid content of fats, for the polyimsaturated acids are preferentially 

 hydrogenated. Although hydrogenation of essential fatty acids is the basis 



