DEVELOPERS AND THEORY OF DEVELOPMENT 



321 



By removing some of the hydrogen atoms, chemical radicals and groups can be 

 substituted into the benezene nucleus. Thus phenol represents the substitution of 

 one hydrogen in the ring by means of a hj^droxyl (OH) group: • 



OH 



/\ 



Phenol 



This is also known as hydroxybenzene. Aniline represents the substitution of an 

 amino (NHa) group in the ring forming: 



NH, 



Aminobenzene 



All the organic developing agents in use today are derivatives of one of the above two 

 basic agents. 



The substitution of two hydrogen atoms by means of the hydroxyl group produces 



1 : 4-Dihydroxybenzene (hydroquinone) 



and the substitution of three hydrogen atoms of benzene by hydroxjd group produces 

 OH 



OH 

 OH 



1:2: 3-Trihydroxybenzene (pyrogallol) 



The relative positioning of the substituent groups and the kind of substituent has 

 a decided influence on the relative activity of the agent as a photographic developer. 

 Thus 



OH 



OH 



1 : 3-Dihydroxybenzene (resorcinol) 



is not a developer, while 

 OH 

 AoH 



1 : 2-Dihydroxybenzene (pyrocatechol or catechin) 



is an energetic developer. 



Developing agents belonging to the class in which the hydrogens of the benzene 

 nucleus have been substituted by hydroxyl groups are known as "polyphenols." 



