CHEMICAL PROPERTIES OF FATTY ACIDS AND RELATED COMPOUNDS 153 



Hilditeh and Vidyarthi,^^^ over 90% of methyl linoleate or methyl linole- 

 nate is transformed into the monoethenoid compound before any saturated 

 esters are formed. In the case of the various monoethenoid acids, it has 

 been shown that hydrogenation occurs most readily in the 9,10-position and 

 with most difficulty when it is adjacent to the carboxyl group, i.e., in 2,3- 

 position.'^^'^ This is similar to the ease with which halogens are added 

 when the unsaturated linkage occupies the different positions.^" 



The mechanism whereby catalysts effect the addition of hydrogen is not 

 known. However, it is supposed that the catalyst first reacts with the 

 double bond to form an adsorption complex at the surface of the molecule: 



H H 

 ~CH2CH2C=C— CH^CHs" 



-Ni-Ni- 

 Fatty acid-nickel complex 



This compound then reacts with hydrogen to regenerate the catalyst with 

 the evolution of heat and with the simultaneous production of the com- 

 pound containing the two added hydrogen atoms. 



b. Halogenation. Chlorine, bromine, iodine, and fluorine combine 

 readily with unsaturated acids and their esters, including the triglycer- 

 ides, to yield saturated compounds. The reaction between fluorine and 

 the saturated hydrocarbons may proceed with explosive violence, with the 

 production of hydrogen fluoride or carbon fluorides. ^^^ The reaction will 

 occur even when inert carbonaceous material such as lampblack is exposed 

 to fluorine. ^^^ However, when oleic acid is fluorinated, two difiuoro- 

 stearic acids, which melt at 81° and 95°C., respectively,^ are produced by a 

 normal reaction. Elaidic acid under similar conditions gives rise to a 

 difluorostearic acid^ having a melting point of 84-85 °C. Lead tetra- 

 fluoride serves as an excellent fluorinating agent.^^° 



An example of simple halogenation of unsaturated acids is afforded by the 

 determination of the iodine number. Since free iodine reacts with fat very 

 slowly, it is usually combined with other substances to facilitate the reac- 

 tion. One of the earliest reagents for this use was the so-called Hiibl solu- 

 tion, which contains an alcoholic solution of iodine in the presence of mer- 

 curic chloride. ^^^ Later modifications were the Wijs reagent, ^^^-^^^ in 



«35 T. P. Hilditeh and N. R. Vidyarthi, Proc. Roy. Soc. London, A122, 563-570 (1929). 

 636 G. V. Pigulevskli and P. A. Artamonov, /. Gen. Chem. U. S. S. R., 12, 510-517, 

 (1942); Chem. Abst., 37, 2716 (1943). 



6" G. Ponzio and C. Gastaldi, Gazz. chim. ital., A2, 11, 92-95 (1912). 



638 B. Humiston, J. Phys. Chem., 23, 572-577 (1919). 



639 H. Moissan, Compt. rend., 110, 276-279 (1890). 



6« O. Dimroth and W. Bockemiiller, Ber., 64, 516-522 (1931). 

 6^' Baron Hiibl, Dinglers Pohjtech. J., 253, 281-295 (1884). 

 6^2 J. J. A. Wijs, Ber., 31, 750-752 (1898). 

 6" J. J. A. Wijs, Z. Untemnch. Nahr. GenunRm., 1, 561 (1898). 



