Rosolic Acid Dyestuffs

These dyestuffs are closely related to those obtained from rosaniline, and may be regarded as rosanilines in which the nitrogen is replaced by a group containing oxygen. 

The actual dyestuffs, like rosaniline salts, are anhydrides of a carbinol. For example, aurin, C19H14O3, is the anhydride of an unknown carbinol, trioxytriphenylcarbinol — 

The compounds of these series have au acid character, and in the free state are yellow, while the salts dissolve in water with a red colour. They cannot be fixed on textile fibres, and are consequently almost useless in dyeing. Some of the colour-lakes are used in paper manufacture and for colouring tapestry. 

Aurin is prepared by heating phenol with oxalic acid and sulphuric acid 6 parts of phenol, 3 parts sulphuric acid, and 4 parts of dried oxalic acid are heated for about twenty-four hours to 120°-130°. 

The melt is extracted with water, and the residue dissolved in hot alcohol, and ammonia gas passed into the solution. The precipitate which forms is boiled with acetic acid or hydrochloric acid [44, 48, 49]. Aurin is also formed by condensation of phenol with formic acid and zinc chloride [45], by boiling the diazo-compound from pararosaniline with water [12], by heating dioxybenzophenone chloride with phenol [46], and by action of salicylic aldehyde on phenol in presence of concentrated sulphuric acid [47]. 

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Reaction XL. (a) Condensation of a Phenol with a Methane Carbon Atom. (A., 194, 123 196, 77 B., 28, R., 743.)—Phenols can be condensed with compounds which can yield a nuclear carbon, to give dyes of the rosolic acid series. The fewco-compound—tri-hydroxytriphenyl car-binol—first formed, is unstable and immediately yields the dyestuff, no oxidising agent being necessary. In this respect these compounds differ from the analogous rosaniline compounds (see p. 382). 

Only a few natural dyestuffs (indigo, alizarin, and purpurin) have been prepared synthetically, but the number of artificial dyestuffs is very large. Some of these, especially the phthalei ns and rosolic acid, seem to approach the natural dyestuffs in constitution, but the greater number belong to special classes, which are without analogue in either the animal or the vegetable kingdom. 

The phthaleins are derivatives of triphenylmethane, and form a group of dyestuffs which are sharply defined from the rosaniline and rosolic acid derivatives already described. In the latter the chromophorous groups are in para positions, while in the former two adjacent positions are taken up. 

Kolbe and R. Schmitt by heating phenol and oxalic acid with sulphuric acid obtained a red dyestuff which they called rosolic acid. Caro and Wanklyn obtained it by diazotising rosaniline and boiling the product with hydrochloric acid. They, and also Graebe and Caro, gave it an incorrect formula. Schorlemmer and R. S. Dale obtained from it a substance called aurine, which Caro and Graebe synthesised from phenol and dihydroxybenzo-phenone. 

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