P-Naphthol orange

By condensation of diazotised sulphanilic acid with p-naphthol in the presence of sodium hydroxide, the useful dyestuff Orange II (p-sulphobenzene-azo-3-naphthol) is obtained ... 

It is interesting to note that azo dyestuffs may be conveniently reduced either by a solution of stannous chloride in hydrochloric acid or by sodium hyposulphite. Thus phenyl-azo-p-naphthol 3delds both aniline and a-amino-p-naphthol (see formula above), and methyl orange gives p-aminodimethylaniline and sulphanilic acid ... 

Place 20 g. of Orange II (Section IV,79) in a 600 ml. beaker and dissolve it in 250 ml. of water at 40-50°. Add, with stirring, 24-25 g. of sodium hyposulphite (Na SjO ) this discharges the colour and yields a pink or cream-coloured, finely-divided precipitate of a-amino-p-naphthol (compare Section IV,76). Heat the mixture nearly to boiling until it commences to froth considerably, then cool to 25° in ice, filter on a... 

Many dyes used in the clothing industry contain sulfonic acid groups which impart water solubility and help to make the dye become fast to the fabric. The dye becomes fast by attaching itself to polar sites in the fibres of, for instance, cotton, wool or silk. Many azo dyes, like Congo Red (see Introduction, p. 5), contain one or more sulfonic acid groups. Another example is Orange II (7), synthesised by coupling p-naphthol (8) with diazotised sulfanilic acid (9) (Scheme 2). 

Water as the solvent. Water may be used to dissolve tropeolin 00, methyl orange, p-nitrophenol, sodium alizarine sulfonate, propyl-a-naphthol orange, nile blue, alizarine yellow, diazo violet, and tropeolin 0. 

Among the azo dyes we may include o-methyl red, naphthol orange, curcumin, o-p-dioxyazo-p-nitrobenzene, and tropeolin 0 (sodium sal) of resorcin-azo-benzenesulfonic acid). 

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