II, Clll^MlS'l-K'V 5 



Sodium />-:nniii()l)tMiZ()at(' exhibits inaxiinuin ahsoipl ion at 'JCiCt lu^i with 

 an cxlinctioii ('(u^lHcitMit of 11,700 (o 14,!»00.'^- =** 



'Vhv hy(h-oclilori(le of PABA has an extinction coefiicient of only !)70 at 

 the \vav(>len,iitli of maximum alisorption, 270 mju'-'- In this eonncetion may 

 be mentioned the rule proposed by Kumler and Strait'" that an aromatic- 

 amiiu\ in a sohition suflicieiitly acid to piotkice tlie sah, will have an ab- 

 sorjition spectrum similar to the eompovuid wiiich one would ha\-e if the 

 amino (or alkylamino) fin)ui) were replaced by hydrogen (oi- an alkyl fi;roup). 

 Kumler and Strait found that tlie spectrum of PAllV hydiochloride was 

 remarkably similar to that of IxMizoic acid-* which exhibits an abs()ri)tion 

 maximum at 273 m/i with an extinction coefhcient of 980. 



Riegel and Buchwald-" proposed that the high degree of absorption 

 which is characteristic of neutral PABA-type molecules (e = ca. 15,000) 

 can be attributed to the hemiquinoid form in which such compounds can 

 exist. 



OH 

 / 



OH 



The low degree of absorption of aniline in the vicinity of 270 mn was 

 accounted for on the basis that this substance cannot assume such a struc- 

 ture. 



Kumler,-^ howe^'er, demonstrated that the absorption of p-dimethyl- 

 aminobenzoic acid is actually greater than that of PABA but occurs in a 

 region of longer wavelength (e = 25,400 at 308 mju). Kumler was able to 

 show that, with every increase in basicity of the amino group, peak ab- 

 sorption is shifted to a longer wavelength, and the extinction coefficient 

 becomes greater. These phenomena were explained-^ • -" on the basis of the 

 resonant forms of this type of molecule, a high degree of absorption being 

 attributable to a structure such as 



OH 



HzN-*^ 



0- 



or, in the case of the dimethylamino derivative, 



CHs OH 



N+==< >=C 



CH, O- 



=9 \V. 1). Kumler and L. A. .Slniit, ./. Am. Chew. Soc. 65, 2349 (1043). 



