336 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Pararosaniline and rosaniline are reddish-blue dyes. 



These compounds on heating with methyl iodide become methylated. 

 The colour becomes bluer. It is still more blue when ethyl iodide is used, 

 and pure blue when phenyl groups are introduced, e.g. aniline blue. 



C 6 H 5 .NH.C 6 H 4 .C 





C 6 H 4 .NH.C 6 H 6 



. C 6 H 



Phenolphthalein is prepared by condensing together phenol and 

 phthalic anhydride : 



C 6 H 4 OH 



CO 



C C 6 H 4 OH 



C.H 4 \/0 + 2C fi H 5 OH = C 6 H 4 \/0 

 CO CO 



Fluorescein is obtained by condensing resorcinol with phthalic an- 

 hydride. 



Eosin is obtained by condensing dibromo-resorcinol with phthalic 

 anhydride. It is a tetrabromofluorescein. 



Erythrosin is obtained by condensing di-iodoresorcinol with phthalic 

 anhydride. It is tetra-iodo-fluorescein. 



These dyes have a magnificent greenish fluorescence and are mostly used 

 for dyeing silk. Eosin is used as red ink, phenolphthalein is the well-known 

 indicator. The phenol acid is colourless, the alkaline salt is coloured. 



Constitution of Triphenylmethane Dyes. The Theory of Colour. 



An examination of the coloured aromatic compounds has shown that 

 they contain particular groupings, e.g. the nitro-group, the azo-group. 

 Quinones are also coloured compounds. The triphenylmethane derivatives 

 are neither nitro-compounds nor azo-compounds ; they are believed to have 

 a quinonoid structure : 



C 6 H 5 -C(OH)-C 6 H 4 -N(CH 3 ) 2 C 6 H 5 -C-C 6 H 4 -N(CH 3 ) 2 



H Cl 



Colourless leuco base. 



HO-C 6 H 4 -C-C 6 H 4 -CO 



C1-N'.(CH 3 ) 2 

 Coloured base. 



HO-C 6 H 4 -C-C 6 H 4 -COONa 



OH 



Colourless anhydride 



of 

 phenolphthalein, 



Coloured so'.lium salt. 



