54 PHYSIOLOGY 



AMINES. These may be regarded as formed from ammonia NH 8 by 

 replacing one or more of the H atoms by an organic radical. Thus we may 

 have: 



/CH 3 /CH 3 /CH 3 



N(-H Ne-CH 3 N^CH 3 



\H \H X CH 3 



(methylamine) (dimethylamine) (trimethylamine) 



Under the action of living organisms primary amines may be formed from 

 a-amino-acids by a process of decarboxylation. Thus : 



CH 3 CH 3 



I I 



CH.NH 2 - CO 2 - 



COOH 



(a-amino-propionic acid) (ethylamine) 



AROMATIC COMPOUNDS 



These all contain a nucleus, made up of six carbon atoms, which is extremely 

 stable, so that processes of oxidation, reduction, &c., can be carried out in the 

 compound without destruction of the nucleus. The simplest aromatic com- 

 pound is benzene C 6 H 6 . It behaves as a saturated compound. It is represented 

 as a hexagon with a hydrogen atom at each angle. 



H I ) H 



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 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 : 



H \^J H H i x y X H k^y H 



H H X 



ortho- meta- para- 



The following are some of the most important monosubstitution derivatives 

 of benzene : 



Nitrobenzene C 6 H 5 .NO 2 . 



Aniline C 6 H$.NH 2 . 



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



Phenol C 6 H 5 .OH. 



Toluene C 6 H 6 . CH 8 . 



