24 



Biological Stains 



The double bonds in benzene are believed not to be fixed but to 

 oscillate between adjacent carbon atoms. 



The importance of benzene in organic chemistry is because of 

 the infinite number of ways in which it can combine with other 

 radicals and elements, forming compounds of extreme complexity. 

 If the elements in such compounds are combined in certain ways 

 the substance is colored, and such colored compounds, after slight 

 additional changes in the molecule, become dyes. In this respect 

 the colored benzene derivatives are unlike the colored pigments of 

 simpler composition; many of the latter may show intense color, 

 but do not act as dyes. 



One type of substitution in the benzene ring is specially impor- 

 tant in dye chemistry. Two atoms or groups having two' valency 

 bonds instead of one may also replace two hydrogen atoms, pro- 

 vided the replacement takes place simultaneously and the hydro- 

 gen atoms replaced are situated either in the ortho or in the para 

 position to each other. Thus two oxygen atoms (which are bi- 

 valent) may replace two hydrogen atoms (which are monovalent) 

 forming the compound known as quinone 0611400, the formula for 

 which is 



o 



It 



/\ 



HC CH 



I 



11 

 O 





or as commonly written 



In printed formulae, such as those that follow in this book, the 

 quinone ring is often abbreviated still further by omitting the 

 double bonds within the ring. The substituent atoms or groups 

 may or may not be alike, so long as both have two valency bonds 

 entering into the combination. This type of substitution involves 

 a rearranging of the double valency bonds in the benzene ring; and 

 in compounds of this type, called quinoid compounds, the double 

 bonds are supposed to be fixed, not mobile as in benzene. This 



