56 



II. CHEMISTRY OF FATTY ACIDS AND GLYCEROL 



Table 19 

 Effect of Position of Side Chain Methyl Group on Melting Points of Some 



OCTADECANOIC AciDS 



" Stearic acid. 



>> F. Francis and S. H. Piper, J. Am. Chem. Soc, 61, 577-581 (1939). 

 ' A. Gascard, Ann. chim. [9], 15, 332-382 (1921). 

 '' Nonadecanoic acid. 



' J. Cason, /. Am. Chem. Soc, 64, 1106-1110 (1942). 

 / J. Cason and F. S. Prout, J. Am. Chem. Soc, 66, 46-50 (1944). 

 » J. Cason, C. E. Adams, L. L. Bennett, and U. D. Register, J. Am. Chem. Soc, 66, 

 1764-1767(1944). 

 '' H. W. Ralston, Fatty Acids and Their Derivatives, Wiley, New York, 1948, p. 43. 

 ' M. A. Spielman, /. Biol. Chem., 106, 87-96 (1934). 

 '■ A. K. Schneider and M. A. Spielman, J. Biol. Chem., H2, 345-354 (1942). 



The least effect of substitution obtains when it occurs on the penultimate 

 carbon (next to the last carbon). The melting point of 17-methyloctadec- 

 anoic acid is practically identical with that of the straight-chain C19 acid 

 or with that of the parent non-methylated Cig (stearic) acid. Similar re- 

 sults are reported for a series of iso-acids synthesized by Fordyce and John- 

 son. ^^^ These authors found that isomyristic (12-methyltridecanoic) acid 

 melted at 50.5-51°C. compared with a value of 54.4°C. for myristic acid-'^ 

 or 41.5°C. for the n-Ci3 acid"*; isopalmitic (14-methylpentadecanoic) acid 

 melted at 61.8-62.4°C., compared with a melting point of 62.8°G. for 

 palmitic acid,-'^ or one of 52.1°C. for pentadecanoic acid^^^; isostearic (16- 

 methylheptadecanoic) acid was found to melt at 67.6-68.2°C., while stearic 

 acid2i6.2i9 melted at 69.6°C. and heptadecanoic acid^'^ melted at 61.3°C. 



Thus a number of factors influence the melting point of fatty acids. A 

 shortening of the chain length and the introduction of one or more double 

 bonds (especially at a distance from the carboxyl group) are factors which 

 tend to lower the melting points of the acids. On the other hand, the ar- 



216 C. R. Fordyce and J. R. Johnson, J. Am. Chem. Soc, 55, 3368-3372 (1933). 



2'« F. Francis and S. H. Piper, J. Am. Chem.. Soc, 61, 577-581 (1939). 



2'^ F. Francis, F. J. E. Collins, and S. H. Piper, Proc Roy. Soc London, A 158, 691-718 

 (1937). 



2'8 F. Francis, S. H. Piper, and T. Malkin, Proc Roy. Soc London, A 128, 214-252 

 (1930). 



219 A. Gascard, Ann. chim.. [9], 15, 332-382 (1921). 



