General Nature of Dyes 31 



LEUCO COMPOUxNDS 



The different chromophores differ considerably from one another, 

 but they all have one property in common. In the language of 

 chemistry, they all have unsatisfied affinities for hydrogen; or in 

 other words, they are all easily reducible, for combining with 

 hydrogen is the opposite of oxidation and is, therefore, reduction. 

 The nitro group may be reduced to an amino group; in the azin 

 group the bond between the nitrogen atoms may break and two 

 hydrogen atoms be taken on; while in the various chromophores 

 with double bonds (such as the quinoid ring) the double bond may 

 break and hydrogen atoms become attached to the valencies thus 

 freed. 



Now in every case this reduction destroys the chromophore 

 group, and as a result the compound loses its color. In other words 

 a dye retains its color only as long as its affinities for hydrogen are 

 not completely satisfied. These colorless compounds are known 

 as leuco compounds; thus fuchsin yields leuco-fuchsin on reduc- 

 tion, and methylene blue reduces to leuco-methylene-blue. For 

 example: 



H.N / \ CH, 



H3N 



NH. 



fuchsin leuco-fuchsin 



Ordinarily this reaction is reversible under conditions favoring 

 oxidation. It is of especial significance to the bacteriologist, as 

 dyes can often be used as indicators of reduction. 



Certain dyes form a still different type of leuco compound, often 

 called a "leuco-base." We have seen that the basic dyes ordinarily 

 occur as salts of some colorless acid; now, in the case of certain 

 dyes, notably the tri-phenyl methanes and xanthenes (Chapters 

 VII and VIII), as soon as the acid radical is removed, the com- 

 pound becomes colorless. This is because a rearrangement of the 

 atoms in the molecule takes place upon neutralization so as to 

 give, not the true dye base, but a compound known as a carbinol 

 (see p. 137) in which the chromophore does not occur. Thus the 

 theoretical base of fuchsin which should be obtained upon removal 

 of the acid radical is: 



CH3 



\ 

 H2N_/— \ 



HoN 



C^/~\^NH20H 



./ 



