THE SMALL SCALE PREPARATION OF AZOBENZENE AND OF HYDRAZOBENZENE A. I. VOGEL, A. WATLING, and 1. WATLING Woolwich Polytechnic, London, England N o TEXTBOOK of elementary practical organic chemistry contains a satisfactory preparation of azobenzene directly from nitrobenzene' and many give the preparation of hydrazobenzene from nitrohenzene2 by reduction with zinc dust (powder) and sodium hydroxide solution. We have found that the reduction with zinc dust and sodium hydroxide is erratic and is dependent upon the quality of the zinc powder: activation of the commercial zinc dust by treatment with dilute hydrochloric acid3 is not always effective and the reduction mav occnuv as long as 24 hours. Such a ureuaration cannot therefore b e recommended for -elementary students. An eminently satisfactory procedure for the preparation of azobenzene consists in the reduction of pure nitrobenzene with magnesium turnings and anhydrous methanol4:
A large excess of magnesium must be avoided since this will produce hydrazobenzene (see below). Hydrazobenzene may be conveniently prepared by reduction of nitrobenzene either with Devarda's alloy and sodium hydroxide solution or with excess of magnesium turnings and anhydrous methanol:
Experimental details follow. AZOBENZENE
In a 250-ml. wide-mouthed flask equipped with an efficient reflux condenser (4-in. jacket), place 6.2 g. (5.2 ml.) of C.P. nitrobenzene, 110 ml. of commercial ' Cf. 'Organic Syntheses," Collective Vol. 3, John Wiley &
Sons, Ine., New York, 1955, p. 103; VOGEL,A. I., "A TextBook of Practical Oresnie Chemistrv Ineludine Qualitative Organic Analysis," 3rd e:., Longmans, l re en & Co., Inc., London & New York, 1955, p. 631. 'VOOEL, A. I., lot. ut., p. 632; CASON, J., AND H. RAPAPORT, "Laboratory Text in Organic Chemistry, Prentiee-Hall, Inc., R., AND R. J . JOHNSON, "LabNew York, 1950, p. 148; ADAMS, oratory Experiments in Organic Chemistry," 4th ed., The Macmillan Co.. New York. 1949.359. ' ALAMS; R., AND J. R. JOHNSON, IOC. tit., p. 359. 4 Z ~L., AND~ P. ROM, ~ Ann., ~ 468, ~129 (1929) ~ ~
-
v
absolute methanol, and a small crystal of iodine. Add 6.0 g. of magnesium turnings in two portions. When most of the magnesium has reacted, heat on a steam bath for 30 minutes. Pour the cooled reaction mixture into 200 ml. of cold water. Add glacial acetic acid cautiously, with stirring, until acid to litmus and cool in an ice bath. Collect the azobenzene by suction filtration and recrystallize it from 90% ethanol. The yield of azobenzene, m.p. 68', is 3.1 g.
Reduction with Magnesium. In a 500-ml. widemouthed flask equipped with an efficient reflux condenser (4-in. jacket), place 10.0 g. (8.4 ml.) of C.P. nitrobenzene, 200 ml. of commercial absolute methanol, and a small crystal of iodine. Introduce 15.0 g. of magnesium turnings, in ca. 3 g. portions, over a period of about 1 hour; moderate the vigor of the reaction, if necessary, by momentary immersion of the flask in an ice water bath. Finally heat on a steam bath until the reaction mixture is colorless. Prepare a Biichner funnel containing a thin layer of filter aid supported upon filter paper. Filter the hot reaction mixture through the preheated Biichner funnel; rinse the flask with 20 ml. of hot methanol. Add about 100 ml. of water to the filtrate and allow to cool slowly in a corked flask until crystallization is complete. Filter rapidly and avoid excessive passage of air through the solid. Recrystallize the crude hydrazobenzene from alcohol containing a little dissolved sulfur dioxide with minimum exposure to air; dry in a desiccator previously filled with nitrogen. The yield of hydrazobenzene, m.p. 126", is 4.5 g. It is a white solid; exposure to air and light turns it yellow hecause of oxidation to azobenzene. Reduetion with Devarda's Alloy. Place a solution of 10 g. of sodium hydroxide in 10 ml. of water, 100 ml. of ethanol, and 10.0 g. (8.4 ml.) of C.P. nitrobenzene in a 250-ml. wide-mouthed flask equipped with an efficient reflux condenser. Introduce 12.0 g. of Devarda's alloy powder in 2 g. portions: start the reaction by warming gently on a steam bath. Isolate the hydrazobenzene as above. If the product is colored, dissolve it in ether, filter off the colored material, and evaporate the ether. of white hvdrazobenzene, m..p 125", is 5.2 g. ~The yield ,
JOURNAL O F CHEMICAL EDUCATION
