CHAPTER VII 



THE XANTHENE DYES 



THE group of colored compounds known as xanthese dyes com- 

 prises a number of basic and acid dyes and quite a series of 

 indicators. In fact, the most valuable indicators known to the 

 chemist fall in this group. They are derivatives of the compound 



xanthene : 



O 



C 



/ \ 

 H H 



1. THE PYRONINS 



The pyronins are methylated di-amino derivatives of xanthene. 

 They are closely related to the diphenyl methanes and are some- 

 times classed with them, as they have a carbon atom attached to 

 two benzene rings, and show the same tendency toward quinone 

 structure. Their formula, on the other hand is like that of the 

 oxazins except that the nitrogen of the central ring is replaced by 

 a methenyl (CH) radical. Like the oxazins, the atomic grouping 

 may be assumed to be in either the paraquinoid or the ortho- 

 quinoid form, thus: 



H 



/ 

 H,N_/\_0_/\_N— H 



CI 



H.N 



or 



NH. 



Another arrangement of the atoms is possible in which no quinoid 

 ring exists, namely: 



H.N 



CI 



I 



o 



NH. 



"C" 



I 

 H 



This latter form might also be assumed for the oxazins and thiazins 

 as well, and this type or formula is frequently used for the azins; 

 but the xanthene dyes are more often represented in this form. If 

 this formula is adopted the quinoid ring cannot be accepted as 



75 



