22 II. CHEMISTRY OF FATTY ACIDS AND GLYCEROL 



b. Linolenic Acid. Another polyethenoid acid of especial importance 

 frequently associated with linoleic acid is linolenic acid, which has three 

 unsaturated linkages. According to the systematic nomenclature, this is 

 9,12,15-octadecatrienoic acid, which has the formula, CH3CH2CH:- 

 CHCH2CH : CHCHoCH : CH(CH2)7COOH. 



It would appear that the oils which contain appreciable amounts of 

 linolenic acid are extremely limited, although this acid may be present in 

 small amounts in a number of oils. It is difficult to determine low con- 

 centrations of linolenic acid in the presence of large quantities of linoleic 

 acid. 



The best sources of linolenic acid are the vegetable fats. In the case of 

 linseed oil, values as high as 60% have been reported, although the aver- 

 age is much lower and results as low as 26% have been recorded.^ Ap- 

 preciable amounts (25% or over) have been reported for hempseed and 

 perilla oils. The animal fats are usually practically devoid of linolenic 

 acid, as this substance is another acid which cannot be synthesized by the 

 animal organism. However, as much as 17% of the fat in egg-yolk ob- 

 tained after feeding linseed oil to chickens has been shown to be linolenic 

 acid, while maximum values of 10% were found when hempseed oil was 

 incorporated in the feed of the hens.^" Small amounts of linolenic acid 

 have also been reported in butter fat^^ but the content in this case must be 

 related to the diet of the cow. 



There are eight possibilities for geometrical isomers of linolenic acid but 

 only one form is kno\\Ti to occur naturally. This so-called a-linolenic acid 

 is the all-c/s form (cis-cis-cis). According to McCutcheon, Crawford, and 

 Welch, ^■■^ the spectroscopic pattern in the infrared area indicates that nat- 

 ural linolenic acid contains only cis linkages. All-^rans-linolenic acid 

 (trans-trans-trans) can readily be prepared from natural linolenic acid by 

 treatment with oxides of nitrogen. It is called linolenelaidic acid or elai- 

 dolinolenic acid. As far as it is known, it does not occur naturally. 



Linolenic acid is just as satisfactory as linoleic acid in serving as a source 

 of the essential fatty acid requirement in animal nutrition, but only when 

 sparked by the latter acid.^^a xhe importance in the painting industry as 

 a drying oil has been discussed under linoleic acid. 



c. Elaeostearic Acid. This acid is another member of the triethenoid 

 series which is an isomer of linolenic acid. However, in contradistinc- 

 tion to the latter acid, the double bonds are arranged in alternate (or 

 conjugate) positions, a fact which seems to confer special properties on this 



60 E. M. Cruikshank, Biochem. J., 28, 965-977 (1934). 

 " H. C. Eckstein, /. Biol. Chem., lOS, 135-140 (1933). 



" J. W. McCutcheon, M. F. Crawford, and PI. L. Welch, Oil & Soap, 18, 9-11 (1941). 

 s^" S. M. Greenberg, C. E. Calbert, E. E. Savage, and H. .1. Deuel, Jr., ./. Nulrition, 

 ^/. 473-486 (1950). 



