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 6 , &c. Monosubstitution derivatives exist only in one form : 



C 6 H 5 .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 : 



XXX 



The following are some of the most important monosubstitution derivatives of 

 benzene : 



Nitrobenzene 



Aniline 



Benzene sulphonic acid 



Phenol 



Toluene 



Benzyl alcohol 



Benzylaldehyde 



Benzoic acid 



C 6 H 5 .N0 2 . 



C 6 H 5 .NH 2 . 



C 6 H 6 .S0 3 H. 



C 6 H 5 .OH. 



C 6 H 5 .CH 3 . 



C 6 H 5 .CH 2 OH. 



C 6 H 5 .CHO. 



C 6 H 5 .COOH. 



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

 The dihydroxybenzenes : 



Pyrocatechin or catechol 

 OH 



OH 



ortho- 



Resorcinol 

 OH 



OH 



meta- 



Hydroquinone 

 OH 



OH 



para- 



/OH 

 Salicylic acid (o-hydroxybenzoic acid) C 6 H 4 ^ *, 



\ 



COOH. 



Tyrosin (parahydroxyphenyl alanine) : 



OH 



CH 2 .CH(NH 2 )COOH, 



Examples of trisubstitution derivatives of benzene are : 



OH 



OH 



Pyrogallol 



OH 



