Submitted by A. F. Holleman
Checked by H. T. Clarke and C. R. Noller.
1. Procedure
An
amalgam prepared from 9.0 g. (0.39 atom) of sodium and 750 g. (55 cc.) of mercury (Note
1) is warmed to about 50° (Note
2) in a
500-cc. Pyrex round-bottomed flask (Note
3). To it is then added a cold suspension of
25 g. (0.13 mole) of xanthone (p. 552) in
175 cc. of 95 per cent ethyl alcohol. The flask is at once stoppered, held in a cloth, and vigorously shaken, the
stopper being raised from time to time to release any pressure. The temperature rises rapidly to 60–70°. The solid
xanthone rapidly goes into solution, a very faint and transient blue color being developed. At the end of about five minutes the alcoholic solution is clear and practically colorless.
After a further ten-minute shaking the
mercury is separated and washed with
10–15 cc. of alcohol. The alcoholic solution is filtered while warm and slowly poured with stirring into 2 l. of cold distilled water. The precipitated
xanthydrol is filtered with suction, washed with water until free of alkali, and dried at 40–50° to constant weight. The crystalline product, which weighs
23–24 g. (
91–95 per cent of the theoretical amount), melts at
121–123° and is practically pure (Note
4) and (Note
5).
2. Notes
1. The
amalgam may be conveniently prepared by placing the
sodium in the
500-cc. flask, covering it with
15–20 cc. of dry toluene, and cautiously melting the metal over a free flame. The flame is then extinguished and the
mercury added in drops, with occasional shaking. The reaction is extremely vigorous at first, but the rate of addition may be rapidly increased after the first 2–3 cc. of
mercury has been added. The addition is so regulated that the
toluene boils continuously, and it is finally allowed to volatilize completely. The
toluene vapor remaining above the
amalgam tends to protect it from the action of the air.
2. The
amalgam is semi-solid at room temperature, but is completely melted at
50°. If made as in (Note
1), it is merely allowed to cool to 50° before the alcoholic
xanthone is added.
3. A
thick-walled flask is necessary; although no great pressure is developed at any time, the impact of the
mercury during shaking might break
thin glass.
4.
Xanthydrol is employed as a reagent for the determination of
urea, with which it forms an insoluble condensation product. The material obtained is entirely satisfactory for this purpose; if desired, however, it may be recrystallized from alcohol, whereby the melting point is raised by about 1°.
5. Because of the marked instability of
xanthydrol, melting-point determination may afford little assurance of purity. It has been suggested that
xanthydrol be prepared directly prior to use or else kept in an alcohol solution in which it is much more stable than in the solid state (A. M. Ward,
1 private communication).
3. Discussion
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