A Publication
of Reliable Methods
for the Preparation
of Organic Compounds
Annual Volume
Org. Synth. 1931, 11, 16
DOI: 10.15227/orgsyn.011.0016
Submitted by H. E. Bigelow and Albert Palmer.
Checked by Henry Gilman and H. J. Harwood.
1. Procedure
Sodium arsenite is prepared by dissolving 226 g. (1.1 moles) of powdered arsenious oxide, made into a paste with a little water, in a solution of 275 g. (6.9 moles) of sodium hydroxide dissolved in 600 cc. of water. This solution, diluted with 600 cc. of water, is poured into a 2-l. three-necked flask provided with a reflux condenser and a mechanical stirrer, and 150 g. (125 cc., 1.2 moles) of freshly distilled nitrobenzene is added (Note 1).
The mixture is refluxed on an oil bath for eight hours with constant and vigorous stirring (Note 2). After removing the oil bath, the reaction mixture is allowed to cool to about 80°, while stirring is continued, and is then transferred to a separatory funnel previously heated to about the same temperature in an oven (Note 3).
The upper layer of oil is separated (Note 4), run at once into an open vessel, and washed with water to which a little hydrochloric acid has been added. Yellow crystals form at once (Note 5), and the yield of azoxybenzene melting at 35.5–36.5° is 102 g. (85 per cent of the theoretical amount) (Note 6) and (Note 7).
2. Notes
1. The excess of sodium arsenite and the eight-hour period of heating ensure the complete utilization of nitrobenzene. This makes it unnecessary to use steam distillation or other processes for the removal of unreacted nitrobenzene.
Crude nitrobenzene may be used, but a good grade is recommended. When crude nitrobenzene is used, a darker product having a slightly lower melting point is usually obtained.
2. The internal temperature should be about 104°, and the temperature of the bath should not greatly exceed 115°. A smaller flask might be used were it not for the danger of foaming which would result from accidental stopping of the stirrer. If the reaction is interrupted, the oil bath must be removed even though the stirring has been stopped; otherwise, on resumption of stirring, the superheated material may be ejected through the condenser. With these precautions in mind it is unnecessary to have a continuous eight-hour period of heating.
Contrary to the general statement found in the literature, azoxybenzene is somewhat volatile with steam. Therefore, the presence of oil drops in the condenser at the end of the eight-hour period of refluxing is no criterion of unaltered nitrobenzene. Azoxybenzene is easily volatile with steam at 140–150°.
3. In this way the separation is effected at about 60°, and the danger of the solution's cooling to a point where sodium arsenate separates is avoided. Should the sodium arsenate separate because of undue cooling, the mixture is heated again, with stirring, until the arsenate redissolves. On dilution with sufficient water to keep the arsenate in solution, the oil settles to the bottom and does not separate readily from the liquid. Furthermore, the volume of solution is so large with such dilution that it is less easily handled.
4. The solution from which the original oil separated will yield, when diluted with an equal volume of water, a small additional quantity of azoxybenzene. This may be recovered by extraction with benzene, but the quantity of compound so obtained does not justify this extra procedure.
5. The presence of hydrochloric acid accelerates crystallization. Should crystallization be retarded, it is recommended that the oil be seeded with a crystal of azoxybenzene.
6. Recrystallization from 50 cc. of hot 95 per cent alcohol gives 72 g. of azoxybenzene. The recrystallized product melts at the same temperature, 35.5–36.5°, as the crude material but is distinctly lighter in color.
7. The following alternative procedure for preparing azoxybenzene is convenient.
In a 1-l. three-necked flask fitted with a reflux condenser and an efficient stirrer (p. 117) there are placed 60 g. of sodium hydroxide, 200 cc. of water, and 41 g. (34.2 cc., 0.33 mole) of nitrobenzene. The flask is immersed in a water bath kept at 55–60°, and 45 g. (0.23 mole) of dextrose is introduced in portions, with continuous stirring, in the course of one hour. The temperature of the bath is then raised to 100° and kept there for two hours. The hot mixture is poured into a 2-l. long-necked flask and steam-distilled to removed nitrobenzene and aniline. This requires some twenty minutes, during which time about 2 l. of distillate passes over. When the distillate is clear, the residue is poured into a beaker and cooled well in an ice bath. The azoxybenzene, which solidifies, is collected, the lumps are ground in a mortar, and the product is washed with water and dried. The yield of material melting at 34–35° is 26–27 g. (79–82 per cent of the theoretical amount). Crystallization from 15 cc. of methyl alcohol gives material melting at 35–35.5° with 90 per cent recovery. (Nicholas Opolonick, private communication. Checked by Louis F. Fieser and M. Fieser.)
3. Discussion
Azoxybenzene has been prepared by reduction of nitrobenzene with alcoholic potassium hydroxide,1 with sodium amalgam,2 with hydrogen in the presence of lead oxide,3 with methyl alcohol and sodium hydroxide,4 with sodium methoxide and methyl alcohol,5 with lead suboxide in alkaline suspension,6 with dextrose in alkaline suspension (see (Note 7), above), and electrolytically;7 by oxidation of azobenzene with chromic anhydride;8 by treatment of β-phenylhydroxylamine with alkaline potassium permanganate,9 with nitrobenzene,10 with mineral acids,11 and with mercury acetamide;12 and by oxidation of aniline with hydrogen peroxide,13 and with acid permanganate solution in the presence of formaldehyde.14 The procedure described is a slight modification of one in the literature.15

References and Notes
  1. Zinin, J. prakt. Chem. (1) 36, 98 (1845).
  2. Alexeyeff, Bull. soc. chim. (1) 1, 325 (1864).
  3. Brown and Henke, U. S. pat. 1,451,489 [C. A. 17, 1969 (1923)].
  4. Lachman, J. Am. Chem. Soc. 24, 1180 (1902).
  5. Brühl, Ber. 37, 2076 (1904).
  6. Deutsche Gold- und Silber-Scheideanstalt vorm. Roessler, Ger. pat. 486,598 [C. A. 24, 1389 (1930)].
  7. Löb, Ber. 33, 2332 (1900); Ger. pat. 116,467 (Chem. Zentr. 1901, I, 149).
  8. Wreden, Ber. 6, 557 (1873).
  9. Reissert, ibid. 29, 641 (1896).
  10. Bamberger and Renauld, ibid. 30, 2278 (1897).
  11. Bamberger and Lagutt, ibid. 31, 1501 (1898).
  12. Forster, J. Chem. Soc. 73, 786 (1898).
  13. Prud'homme, Bull. soc. chim. (3) 7, 622 (1892).
  14. Bamberger and Tschirner, Ber. 32,342 (1899).
  15. Loesner, J. prakt. Chem. (2) 50, 564 (1894).

Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)

arsenious oxide


lead suboxide

alcohol (64-17-5)

hydrochloric acid (7647-01-0)

Benzene (71-43-2)

methyl alcohol (67-56-1)

aniline (62-53-3)

hydrogen (1333-74-0)

sodium hydroxide (1310-73-2)

formaldehyde (630-08-0)

potassium permanganate (7722-64-7)

lead oxide

sodium arsenite

sodium methoxide (124-41-4)

potassium hydroxide (1310-58-3)

sodium (13966-32-0)

Nitrobenzene (98-95-3)

hydrogen peroxide (7722-84-1)

dextrose (492-62-6)

sodium arsenate

chromic anhydride

Azoxybenzene (495-48-7)

Azobenzene (103-33-3)

mercury acetamide