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HANDBOOK OF PHOTOGRAPHY 



One or two chlorine or bromine groups may also be substituted in the hydro- 

 quinone structure, yielding 



OH 



and 



CL 



OH 

 OH 



BR 



Chlorohydroquinone (Adurol of Hauff) 



Bromohydroquinone (Adurol of Schering) 



OH 



Another, and by far the largest, group of photographic developers are the "amino- 

 phenols" formed by substituting both hydroxyl (OH) and amino (NH2) radicals into 

 the benzene nucleus. The simplest and one of the oldest developers of this group is 



OH 



4-Aminophenol (p-aminophenol) 



NH2 



Since both hydrogens of the amino group and of the benzene nucleus of the amino- 

 phenols may be further substituted, a large number of derivatives of this basic member 

 of the group are possible, thus 

 OH 



p-Methylaminophenol 



NH— CH., 



is the base for metol which is usually sold in the form of sulphate salt. 



One of the hydrogens of the amino group of p-aminophenoP may be substituted 

 by the carboxymethyl group ( — CH2COOH) with the production of 



1 In organic chemistry the prefix letters o, m, and p, are used respectively for ortho-, meta-, and 

 para- compounds. These prefixes indicate the position of the substituted atoms. The numbers from 

 1 to 6, in clockwise rotation beginning at the top are also used to indicate the position of the substitute 

 atoms, groups, or side chains. Thus, when the substitution products of benzene have the substituent 

 atoms, groups, or chains on adjacent carbon atoms, they are called "ortho-compounds," and the 

 substituent atoms, or groups are said to be in the ortho or 1 : 2 position. When the substituent atoms or 

 groups occupy the 1:3 position, the chain is called a "meta-compound." When the substituent atoms 

 or groups occupy the 1:4 position, the chain is called a "para-compound." The following diagrams 

 give the names, structures, and positions of xylene which was chosen for illustrative purposes. 



m-Xylene p-Xylene 

 CH3 CHs 



5 ^CHa 



