54 II. CHEMISTRY OF FATTY ACIDS AXD GLYCEROL 



Not only does the position of a single double bond influence the melting- 

 point, but variations in the melting points of triethenoid acids occur with 

 alterations in the relative positions of the double l^onds. Whenever the 

 bonds are in conjugate position, i.e., on alternating carbon atoms, the re- 

 sulting compounds melt at a considerably higher temperature than do the 

 corresponding non-conjugated acids. This is illustrated by several well- 

 known examples in Table 15. 



d. Melting Point as Related to Geometrical Configuration. In estab- 

 lishing the melting point of unsaturated fatty acids, in addition to the 

 number of bonds and their position in the molecule, a standard of equal 

 importance is the factor of geometrical form. The change from a cis to a 

 trans form is always accompanied by a marked rise in melting point, pro- 

 vided other features of the structure remain unchanged. This alteration 

 occurs not only in the case of the acids themselves but also with their 

 hy droxy-deri vati ves . 



Table 1G 

 Melting Points of cis and trans Isomers of Mono- and Polyethenoid Acids" 



" Most of the data are from K. S. Markley, Fatty Acids, Interscienee, New York, 1947. 



'' a- and /3-forms, respectively. 



" Contains hydroxy] grouj) at position 12. 



e. Melting Point as Related to Position and Number of Methyl Side 

 Chains. The introduction of methyl side chains is always accompanied 

 by the lowering of the melting point (Table 17). The extent of the de- 

 crease in melting point reaches a maximum when the methyl group is pres- 

 ent near the center of the molecule. Thus, the 10-methyl-substituted 

 octadecenoic acid has a markedly lower melting point than does the normal 

 acid, while in the case of the 2-methylated acid the melting point is altered 

 much less (Table 18). The introduction of two methyl groups into the 

 position adjacent to the carboxyl (position 2) has an augmented effect in 

 lowering the melting point over that of the monomethyl-substituted deriva- 

 tive (Table 19). 



