250 in. CHEMISTRY OF NEUTRAL FATS 



triglyceride are prol:)Hbly in general alignment. Clarkson and Malkin^^' 

 are of the opinion that the single diffuse band in the x-raj^ pattern (at 4.2 

 angstrom units) indicates that the long hydrocarbon chains are fixed at 

 random degrees of rotation aroinid the chain axis. The diffuseness of the 

 band also suggests that the distances between the hydrocarbon chains are 

 not very exactly defined. The lack of the long spacings in this polj^morph 

 probably indicates that the methyl and glyceryl groups at the ends of the 

 long chains do not determine the planes. Clarkson and Malkin*^- have 

 identified the 7-form with the glass polymorph. 



In the a-form the hydrocarbon chains are also parallel and "frozen" 

 around a central axis, although they are free to rotate within fi.xed limits. 

 In contradistinction to the glass derivative, the sharpness of the single line 

 which is believed to represent the side spacings indicates that the hydro- 

 carbon chains are separated by uniform distances. These chains deter- 

 mine the planes, since the long spacings are present; apparently the hy- 

 drocarbon chains are perpendicular to the end-group plane because the 

 long spacings are of considerable magnitude. Presumably, the structural 

 unit of the crystal usually exists in a double chain length. 



In the jS-form of the triglycerides, the hydrocarbon chains are supposedly 

 fixed in their position as to their relative degree of rotation around the cen- 

 tral axis. Such a deduction is arrived at on the basis of the multiplicity of 

 the short spacings. The other features, such as the fact that the end 

 groups determine the plane and that the structural unit has a double plane 

 length, are similar to those of the a-modification. However, because of the 

 shorter long spaces in the jS-series, it is believed that the hydrocarbon 

 chains are tilted at an angle of about 65° to the end-group planes, in con- 

 tradistinction to the 90° angle in the a-form. Malkin believes that this is 

 the chief characteristic of the /3-polymorph. This may well be the feature 

 which causes the /3-form to be the most stable. Malkin is of the opinion 

 that an alternative structure, in which the three chains of a given molecule 

 lie side by side, is probably incorrect, for mechanical reasons which appear 

 in a three-dimensional mechanical model. 



The jS'-form which has been repeatedly observed with the mixed tri- 

 glycerides resembles the 7- and a-forms in being thermodynamically un- 

 stable. The melting points are intermediate between the a- and the j3- 

 modifications. The hydrocarbon chains are usually intermediate between 

 the 90° tilt of the a-polymorph and the 65° angle of the stable /J-form, in- 

 sofar as the relation to the end-group planes is concerned. A summary of 

 these differences exhibited by the 7-, a-, and j3-forms is included in Table 

 26. 



Bailey and collaborators^^^ and Lutton^^' have come to the conclusion 



^ A. E. Bailey, M. E. Jefferson, F. B. Kreeger, and S. T. Bauer, Oil & Soap, 22, 10-13 

 (1945). 



