Of the 155 distinct chemical substances found in coal-tar, only nine are used in making coal tar colors. These are benzol, toluol, xylol, phe nol, napthalene, anthracene, methyl anthracene, phenanthrene and carbazol. Nearly all of the commercial coal-tar dyes are made from benzal, naphthalene and anthracene.
From the nine derivatives, or °crudes,0 are made about 270 substances called °intermedi ates° which are not dyes, but from which the dyes are made. The latter numbered recently over 1,500, entirely distinct, and each requiring a separate process of manufacture. It is worthy of note in passing that the list of 270 inter mediates does not by any means exhaust the possibilities of the nine coal-tar crudes. On the contrary, it is estimated that there are probably more than 2,000 of these intermediate substances, with millions of individual dyes yet to be discovered. In Germany alone upward of &000 patents have been granted for as many different coal-tar dyes and distinct processes of malang them. Billow calculated that theo retically 3,160,000 azo-dyes alone may be pro duced.
Classification.-- Several different classifica tions of the coal-tar colors have been proposed, some from the chemist's standpoint, others from the dyer's point of view. The classifica tion here given is according to Wahl, and is based upon the chromophore dominant in each group, the factor of greatest moment to the manufacturer of coal-tar dyeituffs.
I. Nitro-dyestuffs.
II. Nitroso-dyestuffs or quinone-osin.es.
III. Aso-dyestuffs.
IV. Hydrazones.
V. Stilbene dyestuffs.
VI. Diphenylinethane dyestuffs.
VII. Tnphenylinethane dyestuffs.
VIII. Xanthene dyestuffs.Viii. Xanthene dyestuffs.
IX. Acridine dyestuffs.
X. Anthracene dyestuffs.
XI. Ouinone-trnide dyestuffs (indamines. indophenols, =mines, thiazines. azines).
XII. Indigo and indwoid dyestuth...
XIII. Thiazol dyestuffs.Xiii. Thiazol dyestuffs.
XIV. Sulphur dyestuffs.
XV. Anihne black.
Nitro-dyestuffs.— The dyes in this group contain the chromophore NO, which gives an acid character to the molecules of which it is a dominant constituent. This acid reaction is carefully preserved in making combinations to produce new colors. The majority of the nitro dyes are derived from the phenols or naphthols, or their sulphonic acids.
As examples of this group may be mentioned picric acid, aurantia, martius yellow, victoria orange and naphthol yellow S. Picric acid was the first of the artificial dyestuffs, having been made in the 18th century by. the action of nitric acid on hides, and aloes, and also on natural. indigo. As its color (yellow) is not fast either to light or to washing, it is rarely used except in small admixtures with other dyes. Aurantia dyes wool and silk a shade of orange. It has been superseded by some of the azo-dycs, Martius yellow dyes wool and silk a bright yel low which fades with heat, and is not altogether fast as to washing. Victoria orange dyes wool and silk yellow; and napthol yellow S., also laiown as acid yellow, dyes wool and silk a brilliant yellow.
The nitrosophenols are obtained either by the action of nascent nitrous acid on phenol, or by the action of hydroxyla mine on the quinones ;— hence the alternate title quinone-oximes. Their dyeing properties depend on their combination with metallic mordants, the one most commonly used being ferric hydroxide, which yields intense shades of green. The most important dyes of this group are fast green or Russian green— an olive shade, gambin G and gambin R and naphthol green, all produced on an iron mordant.
group is founded on the chromophore .N :N. An intermediate step is the production of diazo-compounds —by treating the salt of a primary aromatic amine with nascent nitrous acid. While these diazo compounds are, for the most part, colorless, they combine with amines and phenols to form dyes of great intensity. Further, azo-dyestuffs con taining an amido-group may be diazotised to yield another series containing two .N :N. chromophores: the.se are called diazo-com pounds. Similarly other series are produced — trisazo-compounds, with three azo-chromo phores, and tetrazo-cotnpounds, with four azo chromophores.
The production of azo-compounds requires, first, the diazotisation of the primary amine; second, the °coupling° of the diazo-compound thus formed with an amine or a phenol; third, the precipitation of the dyestuff from the solution.