Submitted by L. A. Bigelow and H. H. Reynolds.
Checked by Roger Adams and G. S. Hiers.
1. Procedure
One hundred fifteen grams (0.9 mole) of p-chlorophenol,
300 g. (2.0 moles) of phthalic anhydride,
50 g. of crystallized boric acid, and 2 kg. (1090 cc.) of 95 per cent sulfuric acid are thoroughly mixed in a
1.5-l. round-bottomed flask. The flask is then immersed to the neck in an
oil bath and the temperature gradually raised to 200° during the course of one-half to three-quarters of an hour (Note
1). It is then kept constant at 200° for three and one-half hours. Owing to the tendency of the
phthalic anhydride to sublime, the mouth of the flask is covered with a piece of
porous plate. When cool, the colored melt is poured slowly into 5 l. of cold water with continuous stirring, and filtered. The precipitate is boiled with 10 l. of water and filtered hot to remove the excess of
phthalic anhydride which goes into solution. The residue is then suspended in 10 l. of boiling water, and to this suspension is added a sufficient quantity of
10 N potassium hydroxide solution to produce a purple color (about 30 cc.), followed by 300 cc. more (Note
2). The alkaline solution is filtered hot, and the residue (about 5–10 g.) is washed with
200 cc. of hot 2 per cent potassium hydroxide solution and discarded.
The purple
quinizarin solution is next saturated with
carbon dioxide and the precipitated
quinizarin again filtered. The product is then boiled with
5 l. of a 10 per cent solution of sodium carbonate until it appears black (probably the
monosodium salt) to dissolve the last traces of
purpurin. The mixture is cooled to room temperature and filtered, and the precipitate is boiled with
5 l. of 5 per cent hydrochloric acid to liberate the
quinizarin. The mixture is again cooled to room temperature, and the final product is filtered, washed with cold water, and dried at 100°.
The yield is
147–160 g. (
68–74 per cent of the theoretical amount). It sinters at
190–191° and melts at
199–200° (corr.). A sample twice recrystallized from glacial
acetic acid melted at
200–202° (corr.) (Note
3). The crystal form of this product compares very favorably with that of
quinizarin of the highest purity, as observed under the
microscope.
2. Notes
1. An
enameled bucket placed on a
Fletcher radial burner serves well for this purpose.
2. The "purple color" end-point is not sharp, but as soon as the red hue of the mixture has distinctly changed toward purple, this point is considered reached. A variation of 5–10 cc. either way has little effect, but a large excess gives a large alkali-insoluble residue.
3. Purified
quinizarin is reported as melting at
194–195°.
1 A melting point of
191–193°, probably uncorrected, is also given.
2
3. Discussion
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