>9i4.] NITROGEN IN AMMONIUM SALTS. 19 



An amino acid may, theoretically, assume in the aqueous solu- 



.COoH 

 tion the following forms: (a) the free acid, R^ ; (b) a cyclic 



.CO. CO— O 



salt, R<^ yO, vr according to Werner, R<^ : ; (c) a 



^NH.'^ ^NH,— H 



bimolecular or polymolecular salt formed by the union of two or 



.CO,— H3N 

 more molecules, R<^ /^j (^) the ions of the acid group, 



^NH3— OX^ 

 CO; CO.H 



R< and H^ (c) the ions of the base, r/ and OH"; (/) 



.co; 



the double, amphoteric ion, R<^ .^ 



NH3- 

 The " inner salt " structure was first proposed by Erlenmeyer 

 and SiegeP in 1875. Ten years later Ostwald* noticed that solutions 

 of glycocoll, CHoNHoCOoH, have a very low molecular conductivity 

 and that this is only slightly increased by dilution. He states that in 

 its behavior it is more like a neutral salt than an acid. In 1891 

 Marckwald^ called attention to the fact that amino acids of the 

 aliphatic series react only slowly with the mustard oils, while other 

 primary amines react quite readily. Since the amino acids react 

 easily in alkaline solutions, he held that the acids are, in reality, 

 inner salts. Sakurai® attempted to substantiate the " inner salt " 

 structure on the preparation from halogen derivatives of the acids 

 and on the resistance which amino acids ofifer to the formation of 

 acid chlorides. Walker" points out that conductivity determinations 

 tell us very little about the structure of glycocoll but that since the 

 conductivity of phenylglycocoll, CcH-NHCH„CO„H, is greater than 

 that of acetic acid it must contain a carboxyl group which ionizes. 

 Tilden and Forster^ showed that the amino group of amino acids 



2 " Zwitterion." 



^Ann., 176, 349 (1875). 



4/. prakt. Chcm., 32, 369 (1885). 



^ Ber., 24, 3278 (1891). 



^ Proc. Chem. Soc, 10, No. 138 (1894). 



"^ Proc. Chem. Soc, 10, No. 139 (1895). 



^ Chem. News, 71, 239 (1895). 



