130 II. CHEMISTRY OF FATTY ACIDS AND GLYCEROL 



without decomposition, but those composed of the long-chain fatty acids are 

 unstable to heat and cannot be distilled even at low barometric pressures. 

 The melting points and boiling points of some of the commoner anhydrides 

 are included in Table 46. 



The densities (di) of the symmetrical anhydrides of the saturated acids 

 are as follows^: C4, 0.994620; C5, 0.9292"; Cg, 0.9279ii; C7, 0.93220; Cg, 

 0.9065l7•^• Cio, 0.8596^0. q^^^ 0.8552^o. q^^^ 0.8502^0; Ci., 0.847^o. q^^^ 

 0.8443^0. The refractive indices (n^) are as follows^: C4, 1.4143i»; C7, 

 1.431220; Cg, 1.43581^- ^• Cio, 1. 4234^0. ^12, 1. 4292^0; Ch, 1.4335^0. Cig, 

 1.4357^0- and Cis, 1. 4379^0 T^g lower dicarboxyhc acids form inner an- 

 hydrides which are monomeric rings. Adipic acid apparently forms both a 

 monomeric anhydride melting at 20°C. and a linear polymer melting at 

 80-85°C. The latter cannot be distilled, but it breaks down to the mono- 

 meric form. On heating to 100°C., or on standing, it can be reconverted to 

 the linear polymer. 



(f) Fatty Acid Amides. The amides are closely related to the fatty 

 acids in that the hydroxyl radical of the carboxyl group has been replaced 

 by an amino group. In some cases, one or both hydrogens of the amino 

 group are replaced by alkyl, aryl, or other components which give rise to a 

 large variety of substituted amides. The general formula for the fatty 

 acid amide is R-C0NH2. 



a'. Methods of Preparation of Acid Amides: The ammonium salt 

 method is one of the oldest procedures and one which is still widely used. 

 This method involves the removal of water by heating the ammonium salt 

 of the fatty acid as follows : 



RC-ONH« ^ ^ RC-NHg+HgO 



Ammonium Amide 



salt 



Hofmann^^^ prepared stearamide by this reaction while Menschutkin^^^ 

 synthesized the amides of a number of fatty acids by heating their sodium 

 salts with ammonium chloride. A somewhat similar method involves heat- 

 ing the ethyl ester with an alcoholic solution of ammonia, '^^^ but the latter 

 reaction has the disadvantage of being reversible. ^^* Since the amides can 

 lose an additional molecule of water to form the nitrile, the conditions of de- 

 hydration must be satisfactorily adjusted. 



Another early method for the synthesis of the amide involves the reac- 

 tion between the acid chloride and dry ammonia, or an amine, according 

 to the following reaction: 



«s A. W. Hofmann, Ber., 15, 977-984 (1882). 

 49« N. Menschutkin, Ber., 17, 846-848 (1884). 

 «7 H. Cadet, Bull soc. chim. [1], 1, 73-77 (1859). 

 «8 L. Meyer, Ber., 22, 24-27 (1889). 



