Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

334 ALIZAKIN
6. ALIZARIN
A mixture of 2 g. of potassium chlorate, 30 g. of commercial
sodium hydroxide, 10 g. of finely powdered sodium j8-anthraquinonesulphonate
(" silver salt "), and 40 c.c. of water is heated for twenty
hours at 170° (oil bath) in an autoclave or in an iron tube with
sere wed-on cap. The cooled melt is repeatedly extracted with hot
water and the extracts, after being combined and filtered, are
acidified while hot with excess of hydrochloric acid, which precipitates
the alizarin. When the mixture has cooled the precipitate
is collected at the pump, washed successively with dilute hydrochloric
acid and water, and dried.
For purification the crude product is boiled with glacial acetic
acid (preferably in the extraction apparatus shown in Fig. 27). Fine
red needles ; melting point 289°. Sublimation in a vacuum from
a sausage flask is also to be recommended ; the " sausage " should
be fixed low down and the bulb completely immersed in a nitrate
bath (equal parts of potassium and sodium nitrates). Much poorer
yields of alizarin are obtained by using an open round-bottomed
flask at 189°-190°.
Alizarin or l:2-dihydroxyanthraquinone is one of the most important
dyes. Like indigo, the dye occurs in the plant (the madder
root) as the glucoside of the leuco-compound. The cultivation of the
madder plant, which, chiefly in southern France, extended over large
areas, was brought to an end by the synthesis of the dye from the
anthracene of coal-tar (Graebe and Liebermann, 1869). By distillation
with zinc dust according to the method of Baeyer, these two
chemists had previously obtained anthracene from alizarin.
Anthracene can be oxidised directly to its meso-quinone, anthraquinone,
by means of chromic acid. For almost all its reactions the
middle ring of anthracene provides the point of attack.
Experiment.—Dissolve 1 g. of purest anthracene in just sufficient
good glacial acetic acid at the boiling point; without further heating
add 3 c.c. of concentrated sulphuric acid and, drop by drop, 4 g. of
sodium dichromate dissolved in a quite small amount of water.
(Neglect any turbidity or precipitate which appears after the addition
of the sulphuric acid.) A very vigorous reaction occurs and the
chromic acid is used up almost immediately ; after all the dichromate
has been added boil for five minutes longer. Dilute the solution.
The anthraquinone separates in a flocculent condition. After