COMPLEX AROMATIC COMPOUNDS 337 



Malachite green, as prepared above, is a colourless crystalline solid and 

 is known as the leuco base ; on oxidation it is converted into the green 



compound 



/C 6 H 4 . N(CH 3 ) 2 /C fi H 4 . N(CH 3 ) 2 



C 6 H 4 . CH + O = C 6 H 4 . C . OH 



\C 6 H 4 .N(CH 3 ) 2 \C 6 H 4 .N(CH 3 ) 2 



which is known as the colour base. 



The colour bases of the rosanilines are formed directly ; on reduction they 

 give the leuco base. 



These colour bases form salts with acids, e.g. 



C 23 H 26 N 2 + HC1 = C 23 H 25 N 2 C1 + H 2 O. 



Azodyes. 



Diazonium salts are prepared by treating aniline and other aromatic 

 amines with nitrous acid. Besides yielding phenol on boiling and giving 

 various derivatives by the Sandmeyer reaction these diazonium salts have the 

 property of combining with amines and phenols : 



C 6 H 5 . N : N . OH + C 6 H 5 . NH = C 6 H g . N : N C 6 H 4 . NH 2 + H a O 

 C 6 H 5 . N : N . OH + C 6 H 5 . OH " = C a H 5 . N : N C 6 H 4 . OH + H 2 O. 



By means of this reaction an enormous number of dyes can be prepared 

 which are either basic (aniline component) or acidic (phenol component). 

 The following are examples : 



Helianthin from diazotised sulphanilic acid and dimethylaniline. 



Chrysoidin from diazotised aniline and w-phenylenediamine. 



Bismarck brown from diazotised w-phenylenediamine and w-phenyl- 

 enediamine. 



Resorcin yellow from diazotised sulphanilic acid and resorcinol. 



Congo red from diazotised benzidine and naphthionic acid (naphthyl- 

 amine sulphonic acid). 



The free acid is blue in colour, the salts are red. 



The Process of Dyeing. 



There are many coloured substances amongst the aromatic compounds 

 which are not dyes, e.g. dinitrobenzene is yellow, azobenzene is red, but the 

 chief essential that a coloured substance should be a dye is that it should 

 form an insoluble compound, which cannot be washed out by water, upon 

 the fabric or material to be dyed. It will be noticed that the dyes above 

 mentioned are either basic or acidic substances and are thus capable of form- 

 ing salts with alkalies, or with acids. 



Silk and wool are proteins and are compounds of the nature of amino 

 acids, i.e. they are both basic and acidic in their properties. Most dyes 

 combine with them and give insoluble salts. Cotton is a carbohydrate and 

 forms no compounds with acids and bases. Cotton can, however, be dyed by 

 mordanting, i.e. impregnating the fabric with an acid such as tannic acid, 

 or a base such as alumina, ferric oxide, etc. Basic dyes form insoluble 

 tannates, acidic dyes form insoluble salts or lakes. In calico printing the 

 pattern is marked out with the mordant in a thick solution, such as gum, to 

 prevent it from spreading. 



The formation of insoluble salts upon the fabric is the chief method of 

 producing insoluble deposits upon the fibre, but most probably the following 

 method occurs at the same time. The dyes are all compounds of high mole- 

 cular weight and form colloidal solutions (suspensions of the finest particles). 

 The fabric, whether it consists of silk, wool or cotton, is a colloid. The pro- 

 cess of precipitation of colloids from solution by means of electrolytes, especi- 

 ally those with trivalent ions, and the process of mutual precipitation of two 

 colloids will also be concerned in the fixing of the dye upon the fabric, 



