138 Biological Stains 



It is next theoretically possible to attach amino groups to the ben- 

 zene rings. Thus in the case of di-phenyl carbinol it is possible to 

 obtain di-amino di-phenyl carbinol: 



H 



H2N_/~V_ C_/~\_NH2 



OH 



Now this latter compound contains the necessary auxochrome 

 groups; but it is not yet a dye. No carbinol is a dye, because it 

 lacks a chromophore group. The carbinols are important in dye 

 chemistry, however, because upon dehydration a rearrangement of 

 the bonds in the molecule takes place giving the quinoid benzene 

 ring, which as we have seen is a powerful chromophore. Thus: 



H 



HaN_/^\_C_/~V_NH, -^ H.N_/— \_C_/~\^NH+H,0 



OH H 



This latter compound is colored and is the anhydride of a true dye 

 base. Upon hydration it should theoretically become: 



H H 



I I 



H2N_/~\_C_/—\_N_0H 



\_/ \_/ I 



H 



Such a compound could exist only in aqueous solution. It is 

 known only by its salts, the true dyes, as: 



H 



I 

 H2N _ /-\ C_ 



Although the theoretical compound given above is the true dye 

 base, the carbinols are often known as carbinol bases or color bases 

 of the phenyl methane dyes. As stated above, they lack the 

 chromophore group, and hence are colorless. 



1. DI-PHENYL METHANE DERIVATIVES 



The di-phenyl methanes are seldom of biological significance. 

 Only one deserves mention here. 



