COMPLEX AROMATIC COMPOUNDS 335 



DYES. 



Most of the dyes in common use are derivatives of the complex aromatic 

 compounds. They may be divided into the following groups : 



(1) Nitro compounds. (3) Azo compounds. 



(2) Triphenylmethane compounds : (4) Phenols. 



(a) basic. (5) Indigo (p. 345). 



(b] acidic. 



Nitro Compounds. 



The nitro compounds are yelkxw dyes, mainly used for dyeing silk and 

 wool, and on this account, since cotton is the chief material which requires 

 dyeing, are few in number. 



Picric acid dyes silk and wool, but not cotton, as can be easily verified 

 by dipping silk or wool and cotton into picric acid solution, removing and 

 washing. The silk or wool is dyed, the cotton is not dyed. 



Martius yellow, dinitro-a-naphthol, is the chief nitro compound used 

 for dyeing silk and wool. The commercial substance is the sodium salt. 



Naphthol yellow is the sulphonic acid of Martius yellow. The 

 potassium salt is the commercial dye. 



Triphenyl Methane Compounds. 



Malachite green is prepared by heating a mixture of benzaldehyde 

 and dimethylaniline with zinc chloride : 



X C 6 H 5 N(CH 3 ) 2 



C 6 H 5 . CHO + 2 C 6 H 5 . N(CH 3 ) 2 = C 6 H 5 . CH< + 2 H 2 O. 



\C 6 H 5 N(CH 3 ) 2 



It dyes silk and wool a bluish-greqn ; but cotton only after mordanting 



(P- 337> 



Brilliant green is prepared in the same way using diethylaniline : 



/C.H.NfC.H,), 



C 6 H 5 . CHO + 2 C 6 H 5 N(C 2 H 5 ) 2 = C 6 H 5 . CH< + 2 H 2 O. 



\C 6 H 5 N(C 2 H S ) 2 



Acid green is prepared from benzaldehyde and ethylbenzylaniline : 



.C.H.NCC.H.^H, 



C 6 H 5 CHO + C 6 H 5 N(C 2 H 5 )C 7 H 7 = C 6 H 5 . CH( + 2 H 2 O. 



\C 6 H 5 N(C 2 H 5 )C 7 H 7 



It dyes silk and wool in an acid solution, but is not used for dyeing cotton. 

 Pararosaniline is prepared by oxidising a mixture of /-toluidine and 

 aniline with arsenic acid or nitrobenzene. Probably the/-toluidine is first 

 oxidised to the aldehyde : 



X C 6 H 4 .NH 2 

 H 2 N . C 6 H 4 . CHO + 2 C 6 H 5 NH 2 = H 2 N . C 6 H 4 . CH( + 2 H 2 O. 



\C 6 H 4 .NH 2 

 On further oxidation it gives the carbinol : 



X C 6 H 4 .NH 2 

 H a N . C 6 H 4 . C^ 



|\C 6 H 4 .NH a 

 OH 



Rosaniline, Fuchsine or Magenta. 



This compound is prepared by oxidising a mixture of /-toluidine, 

 0-toluidine and aniline as above : 



H 2 N.C 6 H 4 .CHO + HN * = H 2 N.C.H 4 .CH 



\ 



X C 6 H 4 NH 2 

 H 8 N.C 6 H 4 .C<( 



| \C 6 H 3 (CH 3 )NH 2 

 OH 



