52 II. CHEMISTRY OF FATTY ACIDS AND GLYCEROL 



acid. The melting points and freezing points, which usually approximate 

 each other, are summarized in Table 12. 



The irregularity in the melting points for the even- and odd-chain acids 

 is not noted in the boiling points as indicated by the results of Genseke"--^^ 

 obtained under conditions of vacuum-steam distillation (Fig. 1) or of Pool 

 and Ralston, ^^'' who determined the boiling points over a range of pressures 

 from 1 to 760 mm. and computed the corresponding vapor pressure- 

 temperature curves given in Figures 1 and 2. 



b. Melting Point as Related to Number of Double Bonds. Generally 

 the introduction of one unsaturated linkage results in a considerable de- 

 crease in the melting point of an acid, while the presence of additional 

 double bonds further lowers the melting point. This is illustrated by 

 data on the commonly occurring acids given in Table 13. 



Table 13 

 Effect of Degree of Unsatitr.\tiok on Melting Point of Some cis Forms of Fatty 



Acids" 



" Adapted from K. S. Marklej^ Fatty Acids, Interscience, New York, 1947, p. 37 ff. 

 * a and i3 forms, respectively. 



c. Melting Point as Related to the Position of the Double Bonds. 



There is a progressive decrease in the melting points of the cis-octade- 

 cenoic acids as the distance between the double bond and the carboxyl 

 group is increased. Thus, when the unsaturated linkage is present in the 2 

 position, the melting point is 59°C., while in the acid in which the double 

 bond is at the 12 position, the substance is liquid at room temperature, 

 with a melting point of about 10°C. However, in the case of the trans 



21^ W. Genseke, in G. Hefter and H. Schonfeld, eds., Chemie und Technologie der Fette 

 und Fettprodukte, Vol. II, Springer, Vienna, 1937, p. 508. 



21" W. O. Pool and A. W. Ralston, Ind. Eng. Chem., 34, 1104-1105 (1942). 



