Qfi II. CHEMISTRY OF FATTY ACIDS AND GLYCEROL 



The ease with which polymorphic forms may be changed is markedly in- 

 fluenced by the presence of extremely small amounts of impurities, as has 

 been recently pointed out by Ralston.'' Thus, when small amounts of 

 octadecane are present in hexadecane, two crystalline forms of the latter 

 regularly occur. *^^ On the other hand, the presence of a homologue stabi- 

 lizes the metastable modifications of the higher alcohols ^'^^ and of the 

 nitriles.^^° In some cases the conversion of one modification into the 

 higher melting and more stable form cannot be realized if impurities are 

 present. ^^''~*^- However, a small amount of impurity is apparently neces- 

 sary in the case of such secondary amines as dihexylamine, dioctylamine, 

 and didecylamine.^ 



The Q!-modifi cation of the fatty acids is a metastable form which is 

 generally obtained on cooling the melted acid or when a solution of the acid 

 is rapidly evaporated. In the case of the even-chain fatty acids, the a- 

 f orm is converted to the more stable |S-modification at a certain temperature 

 which is specific for the fatty acid involved. Dupre la Tour'^^'^ found this 

 transition temperature to be the following: lauric acid, 6.5°C.; myristic 

 acid, 24°C.; palmitic acid, 40°C.; stearic acid, 54°C.; and cerotic acid, 

 80°C'. The reverse change does not occur when the /3-form is cooled. ^^^ 

 In the case of the odd-chain fatty acids, the a-modification formed on 

 rapid solidification of the melted acid changes to the /3-form without change 

 in temperature. In this case the a-form of the odd-carbon acids can be re- 

 tained only when very rapid cooling is employed. ^^^ When fatty acids 

 are crystallized from highly polar soh^ents such as hot glacial acetic acid, 

 even at a slow rate, the a-polymorph results. The x-ray diffraction pat- 

 terns in the case of the a-form have been shown to be of the C type, in which 

 the spacings on the long axis are shorter than in the B spacing^ (see Table 

 32). 



The /3-form of fatty acid crystals is usually produced immediately when 

 solutions of the fatty acid in non-polar solvents are allowed to evaporate 

 slowly. Benzene is an excellent solvent from which to prepare the /3- 

 polymorphic form. n-Eicosanoic acid is an exception to the above rule, 

 since both a- and j8-forms result when crystallization takes place from hot 

 glacial acetic acid. The molecular arrangements of the i3-forms show the 

 B spacings. The long axis in this case has a considerably greater length 

 than in the a-form. 



328 J. C. Smith, /. Chem. Soc, 1933, 737-741. 



329 E. J. Hoffman, C. W. Hoerr, and A. W. Ralston, /. Am. Chem. Soc, 67, 1542- 

 1545 (1945). 



330 C. W. Hoerr, H. J. Harwood, and A. W. Ralston, /. Org. Chem., 11, 199-206 (1946). 



331 M. C. Bloom and M. J. Buerger, Z. KrisL, 96, 365-375 (1937). 



332 R. H. Ferguson and E. S. Lutton, Chem. Revs., 29, 355-384 (1941). 



333 F. Dupre la Tour, Ann. phys. [10], 18, 199-284 (1932). 



33* J. Thibaud and F. Dupr^ la Tour, /. rhim. phys., 29, 153-167 (1932). 

 335 G. M. de Boer, Nature, 119, 634-635 (1927). 



