50 PHYSIOLOGY 



All the hydrogen atoms are of equal value. They may be replaced by other groups, 

 such as OH, Cl, NH 2 , or by more complex groups belonging to the fatty series, e.g. 

 CH 3 , C 2 H 5 , &c. Monosubstitution derivatives, exist only in one form : 



C 6 H 6 .X 



Disubstitution compounds exist in three forms, according to the relative position of the 

 substituted H atoms. These are known as the ortho, meta, and para compounds, and 

 have the formulae : 



H 



The following are some of the most important monosubstitution derivatives of 

 benzene : 



Nitrobenzene C 6 H 5 .N0 2 . 



Aniline C 6 H 5 .NH 2 . 

 Benzene sulphonic acid C 6 H 6 .SO 3 H. 



Phenol C 6 H 5 .OH. 



Toluene C 6 H 5 .CH 3 . . 



Benzyl alcohol C 6 H 6 .CH 2 OH. 



Benzylaldehyde C 6 H 5 .CHO. 



Benzoic acid C 6 H 6 .COOH. 



Of the disubstitution compounds, we need mention only the following : 

 The dihydroxybenzenes : 



Pyrocatechin or catechol Resorcinol Hydroquinone 



OH OH OH 



.....- i 



10H 



ortho- meta- 



OH 



1 \X)OH. 



Salicylic acid (o -hydro xybenzoic acid) C 6 H / 



Tyrosin (parahydroxyphenyl alanino) : 



OH 



CH 2 .CH(NH 2 )COOH. 







Examples of trisubstitution derivatives of benzene are : 



OH 



Pyrogallol 



