266 III. CHEMISTRY OF NEUTRAL FATS 



been prepared by Sidhu and Daubert.'*^ These exist in double chain 

 length as postulated by Malkin.^^- Three different polymorphic modifica- 

 tions are indicated by the transition temperature. Table 33 lists the re- 

 cent data on the symmetrical diacid diglycerides. 



(6) Unsymmetrical Diglycerides. Because of the ready shift of the 

 acyl group from the 2 to the 3 position, the unsymmetrical (1,2) isomers 

 are more difficult to prepare than are the symmetrical (1,3) compounds. 

 Several 1,2-diglycerides composed of the same and of different acids which 

 have been synthesized are listed in Table 34, together with their melting 

 points. Sowden and Fischer^^* have prepared the optically active forms 

 of the 1,2-dipalmitin and 1,2-distearin. 



Table 3-4 

 Melting Points of Some Unsymmetkical Diglycerides 



Unsymmetrical Melting I^Jd 



diglyceride point, °C. (chloroform) 



1,2-Dimyristin° 59 .0 — 



1,2-Dipalniitin° 64.0 — 



1,2-Dipalmitin'' 70-73 — 



1,2-Distearin'' 68 . 5-69 — 



l-Palmityl-2-stearin' 60 . 5-61 — 



l-Stearo-2-palmitin* 73 . 4 — 



l-Stearo-2-palmitin'^ 68 . 5-69 . 5 — 



D-1,2-Dipalmitin'' 67-67.5 -2.3° 



D-1,2-Distearin'' 74.5-75 -2.7 



--0 



° B. F. Daubert and C. G. King, J. Am. Chem. Soc, 61, 3328-3330 (1939). 

 '• P. Golendeev, /. Gen. Chem. U. S. S. R., 6, 1841-1846 (1936); Chem. AbsL, 31, 4274 

 (1937). 



<= P. E. Verkade, W. D. Cohen, and A. K. Vioege, Rec. trav. chim., 59, 1123-1140 

 (1940). 

 -^ J. C. Sowden and H. O. L. Fischer, J. Ain. Chem. Soc, 63, 3244-3248 (1941). 



The melting points of the same compounds prepared by the several in- 

 \'estigators do not show close agreement. In one case'^ this may have been 

 due to confusion with the 1,3-isomer, since precautions to prevent the shift 

 of the acyl group were not taken. 



f. Monoglycerides. Two types of monoglycerides have been investi- 

 gated, namely, the symmetrical (j8 or 2) and the unsymmetrical (a or 1). 

 The unsymmetrical form is optically active, since the /3-carbon of glycerol 

 is asymmetrical. On the other hand, the symmetrical (/3) monoglyceride 

 is optically inactive, since no asymmetric carbon exists in the glycerol 

 moiety. Although the a-monoglycerides are readily prepared by the 

 method of Fischer, Bergmann, and Barwind,^ the j8-form is quite difficult 

 to synthesize. 



=»» S. S. Sidhu and B. F. Daubert, J. Am! Chem. Soc, 68, 2603-2605 (1946). 



M* J. C. Sowden and H. O. L. Fischer, J. Am. Chem. Soc, 63, 3244-3248 (1941). 



