98 



II. CHEMISTRY OF FATTY ACIDS AND GLYCEROL 



Although the a- and j8-forms are the types of crystals most frequently 

 encountered in both the odd and even type of fatty acids, Piper et al.,^^^ in 

 1926, were able to demonstrate a third form, the 7-modification (A spacing), 

 which possesses an even longer spacing on the long axis than does the ^- 

 form. This variety of palmitic or of stearic acid was formed when the 

 acid crystals were pressed on a glass plate. It was found that the 7-form 

 can co-exist with the a-form but not with the jS-form, although the a- and 

 /3-types can occur together. The 7-modification has not been demon- 

 strated for any odd- or even-chain fatty acids with more than 18 carbon 

 atoms. ^" 



15 20 25 



NUMBER OF C ATOMS 



Fig. 22. Crystal spacings of saturated acids. ^ 



The odd-chain acids exhibit certain peculiarities. One of these, to which 

 reference has already been made, is the almost spontaneous transformation 

 from the a- to the /3-form after crystallization of the melted acid has taken 

 place. Moreover, the /3-form of crystal always results with the odd-chain 

 acids when they are crystallized from solvents, even those of the highly polar 

 type. The odd-carbon fatty acids, however, do exhibit a reversibility of 

 the transition a — » /3 in the case of undecanoic, nonanoic, heptoic, and the 

 lower acids, but not for acids containing 13 or more carbon atoms. *'^ 

 pjpgj.326 discovered the modification having the D pattern, but little is 



"6 S. H. Piper, T. Malkin, and H. E. Austin, J. Chem. Soc, 1926, 2310-2318. 

 "7 W. E. Garner and A. M. Iving, J. Chem. Soc, 1929, 1849-1861. 



