PHYSICAL PROPERTIES OF FATS AND OILS 267 



(a) a-Monoglycerides. Fischer el al.^ first observed that 1-monopalmitin 

 and 1-monostearin each occurred in two soUd modifications which were 

 shown later^** to have specific melting points. Rewadikar and Watson'*^ 

 studied the polymorphism of the monoglycerides from 1-monocaprylin 

 through 1-monostearin, without the help of x-ray difTraction data. The 

 classical work of Malkin and el Shurbagy'**® demonstrated the presence of 

 three polymorphic forms: a low-melting type (a), possessing vertically 

 rotating chains, an intermediate-melting (/3') type, and a high-melting 

 (/3) variety, each characterized by tilting chains. The a-forms do not 

 show alternation, but the 13'- and (3- types do exhibit this phenomenon. 



Recently, Lutton and Jackson, ^*^ while confirming most of the earlier 

 data of Malkin and el Shurbagy,''*'^ have come to the conclusion that an 

 additional polymorphic form exists in the case of the 1-monoglycerides. 

 This new form undergoes a reversible transformation to a at about 25° 

 below the a melting point, and therefore has no characteristic melting 

 point of its own. The a-form originates on cooling of the melted mono- 

 glyceride, and it is changed to the ^-modification on heating. The /3- 

 variety is likewise obtained by slow crystallization from the solvent or by 

 transformation from the /3 '-variety, but it does not originate directly from 

 the melted monoglyceride. It is the only thermodynamically stable type. 

 The iS '-polymorph has been obtained only from the solvent. 



All forms of the monoglyceride from the same acid have approximately 

 the same long spacings and appear to be tilted double chain length struc- 

 tures. The short spacings for the several polymorphic forms show marked 

 variations. The sub-a-modification has a single line at 4.15 A., with other 

 medium lines at 3.9, 3.75, and 3.55. In the case of the a-polymorph, there 

 is a single line at 4.15 A., and other weaker lines; the /3'-type has a strong 

 line at 4.15 A., and a medium one at 3.65 A. Finally, in the ;S-mono- 

 glyceride, the strongest line is at 4.55 A. Data on the melting points, 

 long spacings, and the refractive indices for the 1-monoglycerides are in- 

 cluded in Table 35. 



Optically active l- 1-monoglycerides containing the saturated fatty 

 acids from 2 to 18 carbons (except 13, 15, and 17 carbon acids) have been 

 prepared by Baer and H. 0. L. Fischer.^** Only a relatively small optical 

 rotation was observed. This is discussed further in the section on optical 

 activity. 



(6) ^-Monoglycerides. Bergmann and Carter^^ are credited with having 

 prepared the first authentic sample of a /3-monoglyceride. These workers 

 synthesized 2-monopalmitin by catalytic reduction of the 1,3-benzylidene- 



^R. S. Rewadikar and H. E. Watson, J. Indian Inst. Sci., A13, 128-140 (1930); 

 Chem. AbsL, 25, 613 (1931). 



3«6 T. Malkin and M. R. el Shurbagy, J. Chem. Soc, 1936, 1628-1634. 



»' E. S. Lutton and F. L. Jackson, J. Am. Chem. Soc, 70, 2445-2449 (1948). 



»" E. Baer and H. O. L. Fischer, J. Am. Chem. Soc, 67, 2031-2037 (1945). 



