A Publication
of Reliable Methods
for the Preparation
of Organic Compounds
Annual Volume
Org. Synth. 1942, 22, 28
DOI: 10.15227/orgsyn.022.0028
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.
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 air9 and by potassium permanganate.10 The condensation of nitrobenzene and aniline acetate also yields azobenzene.11

References and Notes
  1. Nobel, Ann., 98, 253 (1856).
  2. Werigo, Ann., 135, 176 (1865).
  3. Lucius and Bruning, Ger. pat. 216,246 [C. A., 4, 813 (1910)].
  4. Greisheim, Ger. pat. 225,245 [C. A., 5, 592 (1911)].
  5. Opolonick, Ind. Eng. Chem., 27, 1045 (1935).
  6. Treed and Signaigo, U. S. pat. 2,344,244 [C. A., 38, 3663 (1944)]; Henke and Brown, J. Phys. Chem., 26, 324, 631 (1922).
  7. Alexejew, Z. Chem., 4, 497 (1868).
  8. Bozoslavski, J. Gen. Chem. U.S.S.R., 16, 193 (1946).
  9. Alekseevskii and Golbrakht, Russ. pat. 32,499 [C. A., 28, 3425 (1934)]; Brown and Triske, J. Phys. & Colloid Chem., 51, 1394 (1947).
  10. Glaser, Ann., 142, 364 (1867).
  11. Baeyer, Ber., 7, 1638 (1874).

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