ELECTROCHEMICAL SYKTHESIS O F PHENYLHUDROXYLAhIINE BY F. M. FREDERIKSEN
Snowdonl has pointed out that the chemical and electrochemical methods of reducing nitrobenzene are by no means similar though, in general, they should be. ,4t the suggestion of Professor Bancroft, I have made a few experiments t o fill one gap in the case of phenylhydroxylamine. Bamberger' prepared phenylhydroxylamine chemically by heating nitrobenzene for a few minutes with zinc dust and water. The yield varies very much-from o to j7 percent of the theoretical. Bamberger calls attention to the fact that different samples of zinc dust give very different yields even though the content of metallic zinc be the same and even though all other conditions are kept the same. Thus, one sample of zinc dust gave aniline and no phenylhydroxylamine. Bamberger considers that these enor~nousdifferences may be due, in part, to the fact that one sample of zinc dust gives chiefly molecular and, therefore, inactive hydrogen. Another way of wording the same thing is to say that different impurities in the zinc affect the ovx-voltage differently. The best yield was obtained by adding ten grams of nitrobenzene t o half a liter of boiling water, throwing in 7 j g zinc dust (about 67 percent metallic zinc) all a t once, boiling for three-quarters of an hour, cooling rapidly and saturating with sodium chloride. The phenylhydroxylamine mas shaken out with ether, the ether distilled off, and the crj-stallized oil washed with ligroin. The yield was 57 percent of the theoretical. Other products are azoxybenzene, azobenzene, and aniline. The question of yield is also discussed by U7ohl3 who had discovered the reaction independently of Bamberger. At first, nitrobenzene was heated for twenty minutes with Jour. Phys. Chem., 15,797 (1911). Ber. deutsch. chern. Ges., 27, 1348, 1548 (1894). Ihid., 27, 1432 (1894).
ten times its volume of water and an excess of zinc dust. 143th jo percent of zinc dust a yield of about I z percent phenylhydroxylamine was obtained. When three times the theoretical amount of zinc was taken, the yield increased to about 20 percent. It is not feasible to heat for a long period because azoxybenzene, azobenzene, and aniline are then formed. It was found that salts like calcium chloride, magnesium chloride, and zinc chloride accelerate the reaction between zinc dust and nitrobenzene. According to TT-ohlj this is because they form insoluble salts with zinc hydroxide. To obtain the best yield, z j o cc 60 percent aqueous alcohol were heated in a flask with reverse cooler. To the flask were added 60 g nitrobenzene and then 6 g anhydrous calcium chloride, after which 7 j g commercial zinc dust were added in the course of fifteen minutes. The contents of the flask were heated for about five minutes after the last of the zinc dust was added, and were then cooled and filtered. The alcohol was distilled rapidly from the filtrate until an oil layer appeared which solidified on cooling. About 70-75 percent of the theoretical yield of phenylhydroxylamine was obtained in this way.] TVislicenus and Kaufmann' found that a 38 percent yield of phenylhydroxylamine could be obtained by reducing . nitrobenzene in 90 percent alcohol with amalgamated aluminum. I n a later paper, Wislicenus3 points out that better results are obtained with an ether solution. Xitrobenzene is dissolved in a t least ten times its volume of ether, freshly prepared amalgamated aluminum is added and then water, a little at a time. The flask is equipped with a reverse cooler and is placed in ice to prevent the ether from foaming too much. When one starts with 2-30 g nitrobenzene the yield of phenylhydroxylamine is said to be quantitative. Bamberger and Knecht4 obtained an 85 percent yield Bamberger states that he has never been able t o average more than 3 5 5 yield in this way and that the maximum observed was jo';; Ber. deutsch. chem Ges , 29, 561 (1896) 2 Ber. deutsch. chem. Ges., 28, 1326, 1983 (189j). Ibid., 29, 494 (1896). Ibid , 29, 863 (1896).
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of phenylhydroxylamine (3.8 g) by dissolving j g nitrobenzene in 50 cc 90 percent alcohol, and adding first a solution of 12 g aluminum sulphate in IOO g water, and then 250 g of a j percent zinc amalgam. The mixture was shaken vigorously for two hours, the temperature being kept below j '. I repeated Wohl's work heating 250 cc 60 percent alcohol with 60 g nitrobenzene and 6 g CaCl? (ammonium chloride is just as good) on a water bath in a flask equipped with a return condenser. After bringing the solution just to boiling, 95 g zinc dust were added through an interval of fifteen minutes, the temperature of the water bath being regulated so that the liquid in the flask boiled gently while the reaction was taking place. Five minutes after all the zinc dust had been added, the reaction mixture was cooled, first with running water and then with ice. The mixture was filtered by suction and the zinc oxide washed with about j o cc alcohol. The alcohol was boiled off until the temperature of the distilling vapor reached 86", this being about the point at which the solution clouds, owing to the separation of an oil. When the solution is cooled in a freezing mixture, the phenylhydroxylamine separates in crystals contaminated with some azobenzene. The crystalline mass, colored a bright orange by the azobenzene, was filtered by suction and then washed with ligroin. The washing must be done with care because phenylhydroxylamine is somewhat soluble in ligroin. The azobenzene is removed practically completely, leaving the phenylhydroxylamine almost pure and white. The yields varied between 30 and 40 percent, which is in accord with the results of Bamberger. An attempt was now made to duplicate the reaction electrolytically by using zinc electrodes in an ammonium chloride solution. Of course a zinc anode is not essential theoretically, but it obviates the use of a diaphragm and cuts down the voltage considerably. Since it would be difficult to use electrodes having a surface equal to, say, IOO g zinc dust, the reaction must take longer. I n order to prevent the decomposiBer. deutsch. chem. Ges., 29, 864 (1896).
Electrochentical S y d z e s i s o j Plzeiz),lhi~drox~lanzil.ie 699 tion of the phenylhydroxylamine which would probably occur if the solution were kept heated a long time, it was deemed advisable to run a t a low temperature. The solution was placed in a 350 cc beaker and stirred diring electrolysis by means of a spiral stirring rod run by a motor. After the reduction had gone on as long as desired, the solution was filtered through a Buchner by suction and the alcohol then distilled off as in the chemical reduction. On account of the smaller amounts of substance taken, the solution was not cooled and filtered but was saturated with sodium chloride and extracted with ether. The ethereal extract contained the phenylhydroxylamine and azobenzene with only a trace of salt or of ammonium chloride. The ether was driven off and the residue was crystallized from a small amount of benzene to which ligroin was added after solution and filtration. Phenylhydroxylamine is very soluble in hot benzene and only slightly soluble in cold benzene. Addition to the benzene of an approximately equal volume of ligroin cuts down the solubility still more. The azobenzene remains in solution. Theoretically i t requires 107.2 ampere hours (4 X 26.8) to reduce one molecular weight of nitrobenzene (123 g) to phenylhydroxylamine. Since nitrobenzene has a specific gravity of 1 . 2 , I O cc nitrobenzene is approximately 1 2 g and calls for 10.46 ampere hours, which should give 10.73 g phenylhydroxylamine. The following results were obtained : Run I . 250 cc 60 percent alcohol, 12.5 g ammonium chloride, I O cc nitrobenzene. Zinc electrodes. Cell packed in ice. Average temperature 12'. One ampere for 9.5 hours. S o stirring for the first hour. Yield, 28 percent. Run 2 . 250 cc 60 percent alcohol, I O g ammonium chloride, j cc nitrobenzene. Zinc electrodes. Cell cooled in running water. Average temperature 14'. 0.7 5 amperes for 6.7 hours. Yield, 2 1 percent. Run 3. 250 cc 60 percent alcohol, I O g ammonium chloride, 5 cc nitrobenzene. Zinc electrodes. Cell cooled in
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freezing mixture. Average temperature -5 '. 0.7j amperes for 6.7 hours. Yield, 20 percent. Run 4. 2 5 0 cc 60 percent alcohol, I O g ammonium chloride, 5 cc nitrobenzene. Zinc electrodes. Cell cooled with running water. Average temperature 19 '. 2 amperes for 2.67 hours. Yield, 29 percent. Run 5 . 2 j o cc 60 percent alcohol, I O g ammonium chloride, j cc nitrobenzene. Zinc electrodes. Cell not cooled. Average temperature, 36'. 4.8 amperes for 1 . 1 1 hours. Yield, 2 2 percent. These yields are all lower than those for the chemical reduction; but part of this difference is undoubtedly due to the fact that the losses are greater when 6 g nitrobenzene are used instead of 60 g in the same amount of solution. Also, we do not know what the reduction efficiency of the zinc dust is, though that could be determined; but we know that it is necessary to use more than the theoretical amount of zinc. The time at my disposal did not permit me to make runs to see what effect an excess of ampere hours would have. It is possible to tell something about the results during the run. If much hydrogen is evolved at the cathode or if much zinc is precipitated there, the efficiency is necessarily low. A deep red color to the solution shows a considerable formation of azobenzene, an orange color a much less formation, and a light yellow color hardly any formation of azobenzene. The general results of this paper are: I . It is possible to reduce nitrobenzene to phenylhydroxylamine electrochemically without the use of a diaphragm by using a zinc anode. 2 . The chemical process of reducing nitrobenzene to phenylhydroxylamine with zinc dust can be duplicated electrochemically. 3. The yield from the electrochemical process is less at present than the yield from the chemical process. Part of this difference is due to the fact that the percentage losses are larger the smaller the amount of nitrobenzene taken.
Electrochewtical S y d i e s i s o j Phe7tylhydroxylamine
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4. The efficiency of reduction and the conversion to azobenzene both decrease with falling temperatre. One has t o strike a mean therefore between slight conversion to azobenzene but low reduction efficiency and high reduction efficiency with high conversion to azobenzene. The best results so far have been obtained a t about room temperature. I take this opportunity to express my appreciation of the active interest Professor Bancroft has taken in the progress of the work. Cornell Cnpdersity
