150 Biological Chemistry. 



They will also condense with a phenol 



C 6 H 5 .N 2 .Cl + C 6 H fi (OH) = C 6 H 5 .N 2 .C 6 H 4 (OH) 



or a polyhydroxy phenol 



C 6 H 5 -N 2 -C1 + C 6 H 4 (OH 2 ) = C 6 H 5 N 2 C 6 H 3 (OH) 2 



These reactions are very general. Thus, sulphanilic acid, 

 a benzene derivative containing a sulphonic acid and an 

 amino group, both attached to carbons of a benzene 

 nucleus, can be " diazotized," and the diazonium salt will 

 condense with phenols, aromatic amines, and other sub- 

 stances. The dye " helianthin," for example, can be ob- 

 tained by the condensation of a diazonium derivative of 

 sulphanilic acid and diamethyl aniline 



S0 3 H C 6 H4-N 2 OH + C 6 H 5 -N(CH 3 ) 2 



= S0 8 H AH 4 -N 2 -C 6 H 4 -N(CH 3 ) 2 + H 2 



An enormous variety of coloured substances can be pro- 

 duced in a similar way, all of which belong to the class 

 of azo-dyes, many of which are largely used for tinctorial 

 purposes. 



(e) The diazo-derivatives can be reduced. The re- 

 duction product of phenyl diazonium chloride is of con- 

 siderable importance as a chemical reagent, in that it 

 condenses readily with ketones and aldehydes, a reaction 

 to which attention has been already drawn. The sub- 

 stance in question, phenylhydrazine, which is largely used 

 in biochemical work in the investigation of sugars (which 

 contain aldehydic and ketonic groups), can be prepared 

 from phenyldiazonium chloride by reducing it with stan- 

 nous chloride and hydrochloric acid 



C 6 H 3 -N.NC1 + 2H 2 = C 6 H 6 -NH NH 2 HC1 



In the foregoing pages the chief reactions of benzene 

 and its derivatives have been summarized very shortly. 

 Reasons have been given for the adoption of the so-called 



