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%).