PHYSICAL PROPERTIES OF FATTY ACIDS 95 



enantiotropism (reversible polymorphism) and monotropism are known to 

 occur \vith the monoesters of the fatty acids. 



b. Polymorphism in the n-Aliphatic Fatty Acid Series. Three poly- 

 morphic forms have been reported for the even-chain fatty acids, while 

 an additional type of polymorph has been noted in the case of the odd- 

 chain fatty acids. The usual designation employed at present for these 

 modifications is by the Greek letters, a, /3, and 7; the fourth form, found 

 only with the odd-chain fatty acids, is referred to as the a'-modification. 

 The polymorphic modifications have different x-ray diffraction patterns, 

 which Piper^^^ has termed A, B, and C for the even-carbon series and A', 

 B', C, and D' for the odd-chain series of fatty acids. Confusion has arisen 

 concerning the two terminologies employed. The a-, /S-, and 7-modifica- 

 tions produce the x-ray diffraction patterns of the C, B, and A types, re- 

 spectively (also C, B', and A' types). The D' diffraction pattern is given 

 by the a'-modification. The different terminologies are summarized in 

 Table 31. 



Table 31 



X-Ray Diffraction Patterns Associated with Several Polymorphic For:ms of 



Fatty Acids 



° S. H. Piper, Trans. Faraday Soc, 25, 348-351 (1929). 



An extended discussion of polymorphism has been given by Francis and 

 pjppj.216,217 ^^^ by J. C. Smith, ^'" Avhich can be only briefly reviewed below. 

 K\\ three modifications are indistinguishable on the basis of the melting 

 point. This is presumably due to the fact that on heating they are rapidly 

 changed to one form which has a constant melting point. On the other 

 hand, all of the polymorphs can be readily differentiated from each other by 

 variations in the x-ray spacings on the long axis. 



The different pol^inorphic forms are usually interconvertible. They 

 tend to revert to the stable modification, i.e., the type which is most 

 stable at ordinary temperature. However, in many instances it is possible 

 to isolate the metastable form, although in some cases the presence of such 

 substances is manifested only by a thermal effect when a transition occurs. 



«6 S. H. Piper, Trans. Faraday Soc, 25, 348-351 (1929). 



'" J. C. Smith, "Fatty Acids and Other Long-Chain Compounds," in Ann. Repts. 

 Prog. Chem., 1938, 35, Chemical Society, London, 1939, pp. 251-268. 



