Org. Synth. 1942, 22, 28
DOI: 10.15227/orgsyn.022.0028
AZOBENZENE
Submitted by H. E. Bigelow and D. B. Robinson.
Checked by W. E. Bachmann and W. S. Struve.
1. Procedure
A 5-l. three-necked round-bottomed flask, fitted with a mercury-sealed stirrer and a reflux condenser, is placed on a steam cone. In the flask are placed 250 g. (208 ml., 2 moles) of nitrobenzene, 2.5 l. of methanol, and a solution of 325 g. (8.1 moles) of sodium hydroxide (Note 1) in 750 ml. of distilled water. To the mixture is added 265 g. (4.1 moles) of zinc dust (Note 2), the stirrer is started, and the mixture is refluxed for 10 hours (Note 3). The mixture is filtered while hot, and the precipitate of sodium zincate is washed on the filter with a little warm methanol. All the methanol is distilled from the filtrate, the residue is chilled, and the crystalline azobenzene is filtered.
In order to remove zinc salts from the crude azobenzene, the latter is added to 500 ml. of 2% hydrochloric acid, the mixture is warmed to about 70° in order to melt the azobenzene and is stirred rapidly for about 5 minutes. Stirring is continued while the mixture is chilled to solidify the azobenzene. The product is filtered, washed well with water, and recrystallized from a mixture of 720 ml. of 95% ethanol and 60 ml. of water. The yield of azobenzene melting at 66–67.5° is 156–160 g. (84–86%).
2. Notes
1.
This amount assumes 100% purity. The checkers used
342 g. of 95% sodium hydroxide.
2.
This amount assumes 100% purity. The checkers used
288 g. of 92% zinc dust.
3.
At the end of this time, the reddish mixture should be free from the odor of
nitrobenzene. If it is not, refluxing is continued for 2–3 hours longer.
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3. Discussion
Azobenzene has been prepared by many different methods, of which the following are representative. It may be obtained by the reduction of
nitrobenzene with
iron and
acetic acid;
1 with
sodium amalgam;
2 with alkali sulfides;
3 with cellulose,
4 molasses,
5 or
dextrose5 in alkaline solution; and by catalytic reduction.
6 The reduction with
zinc and
sodium hydroxide described here is a modification of Alexejew's method.
7 Azobenzene also results from the reduction of diazotized
aniline with cuprous salts.
8 Aniline has been oxidized to
azobenzene by air
9 and by
potassium permanganate.
10 The condensation of
nitrobenzene and
aniline acetate also yields
azobenzene.
11
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
sodium zincate
ethanol (64-17-5)
hydrochloric acid (7647-01-0)
acetic acid (64-19-7)
methanol (67-56-1)
aniline (62-53-3)
sodium hydroxide (1310-73-2)
iron (7439-89-6)
potassium permanganate (7722-64-7)
zinc (7440-66-6)
sodium (13966-32-0)
Nitrobenzene (98-95-3)
dextrose (492-62-6)
Azobenzene (103-33-3)
aniline acetate
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