Alizarin, or 1·2 Dioxyanthraquinone,
a vegetable dyestuff formerly prepared from madder root (Rubia tinctorum) which contains a glucoside ruberythric acid (C26H28O14). This glucoside is readily hydrolysed by acids or ferments, breaking up into alizarin and glucose:
Alizarin was known to the ancients, and until 1868 was obtained entirely from madder root. The first step in the synthetical production of alizarin was the discovery in 1868 of C. Graebe and C. Lieberman that on heating with zinc dust, alizarin was converted into anthracene. In order to synthesize alizarin, they converted anthracene into anthraquinone and then brominated the quinone. The dibrominated product so obtained was then fused with caustic potash, the melt dissolved in water, and on the addition of hydrochloric acid to the solution, alizarin was precipitated. This process, owing to its expensive nature, was not in use very long, being superseded by another, discovered simultaneously by the above-named chemists and by Sir W. H. Perkin; the method being to sulphonate anthraquinone, and then to convert the sulphonic acid into its sodium salt and fuse this with caustic soda.
In practice, the crude anthracene is purified by solution in the higher pyridine bases, after which treatment it is frequently sublimed. It is then oxidized to anthraquinone by means of sodium dichromate and sulphuric acid in leaden vats, steam heated so that the mixture can be brought to the boil. When oxidation is complete the crude anthraquinone is separated in filter presses and heated with an excess of commercial oil of vitriol to 120° C., the various impurities present in the crude material being sulphonated and rendered soluble in water, whilst the anthraquinone is unaffected; it is then washed, to remove impurities, and dried. The anthraquinone so obtained is then heated for some hours at about 150-160° C. with fuming sulphuric acid (containing about 40-50 % SO3), and by this treatment is converted into anthraquinone-β-monosulphonic acid. The solution is poured into water and sodium carbonate is added to neutralize the excess of acid, when the sodium salt of the monosulphonic acid (known as silver salt) separates out. This is filtered, washed, and then fused with caustic soda, when the sulpho-group is replaced by a hydroxyl group, and a second hydroxyl group is simultaneously formed; in order to render the formation of this second group easier, a little potassium chlorate or sodium nitrate is added to the reaction mixture. The melt is dissolved in water and the dyestuff is liberated from the sodium salt by hydrochloric or sulphuric acid, or is converted into the calcium salt by digestion with hot milk of lime, then filtered and the calcium salt decomposed by acid. The precipitated alizarin is then well washed and made into a paste with water, in which form it is put on to the market.
K. Lagodzinski (Berichte, 1895, 28, p. 1427) has synthesized alizarin by condensing hemipinic acid [(CH3O)2C6H2(COOH)2] with benzene in the presence of aluminium chloride. The product on acidification gives a compound C15H12O5·H2O which is probably an oxy-methoxy-benzoyl benzoic acid. This is dissolved in cold concentrated sulphuric acid, in which it forms a yellowish red solution, but on heating to 100° C. the colour changes to red and violet, and on pouring out upon ice, the monomethyl ether of alizarin is precipitated. This compound is hydrolysed by hydriodic acid and alizarin is obtained. It can also be synthesized by heating catechol with phthalic anhydride and sulphuric acid at 150° C.
Pure alizarin crystallizes in red prisms melting at 290° C. It is insoluble in water, and not very soluble in alcohol. It dissolves readily in caustic alkalis on account of its phenolic character, and it forms a yellow-coloured di-acetate. Its value as a dyestuff depends on its power of forming insoluble compounds (lakes) with metallic oxides. It has no affinity for vegetable fibres, and consequently cotton goods must be mordanted before dyeing with it (see Dyeing.)
Numerous derivatives of alizarin are known. On solution in glacial acetic acid and addition of nitric acid, β-nitroalizarin (alizarin orange)
is produced, and this on heating with sulphuric acid and glycerin is converted into alizarin blue.
The trioxyanthraquinones—purpurin, anthrapurpurin, anthragallol and flavopurpurin—are also very valuable dyestuffs. These compounds may be represented by the following formulae:
Purpurin (1⋅2⋅4 trioxyanthraquinone) is found with alizarin in madder root; it is now prepared synthetically by oxidizing alizarin with manganese dioxide and sulphuric acid. After the separation of the silver salt (see above) obtained on sulphonating anthraquinone, the remaining acid liquid gives on treatment with calcium carbonate the calcium salt of anthraquinone 2⋅6 disulphonic acid (anthraquinone-α-disulphonic acid). This is converted into the sodium salt by means of sodium carbonate, and on alkali fusion yields flavopurpurin. In a similar manner anthrapurpurin is prepared by alkali fusion of anthraquinone 2⋅8 disulphonic acid. Anthragallol is synthetically prepared by the condensation of benzoic and gallic acids with sulphuric acid
or from pyrogallol and phthalic anhydride in the presence of sulphuric acid or zinc chloride.
A. Baeyer in 1890, by heating alizarin with fuming sulphuric acid for 24-48 hours at 35-40° C., obtained a product, which after treatment with caustic soda gave a sulphuric acid ester of quinalizarin, and this after acidification and boiling was converted into quinalizarin (Alizarin Bordeaux) or 1⋅2⋅6⋅9 tetra-oxyanthraquinone. Penta-oxyanthraquinones have been obtained from purpurin and anthrapurpurin, while a hexa- oxyanthraquinone has been obtained from 1⋅5 dinitro- anthraquinone.