ELECTROLYTIC REDUCTION O F NITROBENZENE WITHOUT A DIAPHRAGM BY
E. F. FARNAU
Up to a few years ago, save for a single series of experiments by Dieffenbach,’ and some patents by Meister Lucius and Bruning, no electrolytic reduction of nitrobenzene has been carried out without the use of a diaphragm. In Dieffenbach’s experiments nitrobenzene in a neutral solution was reduced a t the cathode to p-phenylhydroxylamine, and this in turn was oxidized a t the anode to nitrosobenzene. The electrodes used were non-attackable. Meister Lucius & Briining claim to make azobenzene and azoxybenzene by electrolyzing in an alkaline solution without a diaphragm, using a small anode and a large cathode. Recently some work without a diaphragm, and with soluble anodes has been done in this laboratory by Snowdon,s on nitrobenzene suspended in aqueous solutions. The advantages of a one-solution electrolyte are obvious : The diaphragm-generally a porous cup-increases the resistance of the cell, and much of the reduction product is lost due to adsorption in the pores of the cup. Further, the soluble anode prevents oxidation of the reduction product, and lowers the decomposition voltage in the cell. Pinally, a suspension of nitrobenzene in aqueous s o h tion eliminates the use of organic solvents which may introduce complications. Allen4 in this laboratory has determined the nature and yield of the various reduction products obtained from nitrobenzene by the action of sodium hydroxide and ferrous solutions, and has found that aniline is obtained in good yield __
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Chem. Centralblatt, [51 2, I, 911 (1908). Zeit. Elektrochemie, 8, 217 (1902). Jour. Phys. Chem., 15, 797 (1911). Ibid., 16,89 (1912).
E. F . Farnaw with neutral solutions a t room temperature. It was of interest to seek an electrolytic analogue of this reaction. Electrolysis of a nitrobenzene suspension in sodium sulphate solution between iron electrodes offered such an example. In order to simplify the apparatus as much as possible, an 8l/,” length of 2 * / ; iron pipe capped a t one end served a t once as container and as cathode; and a stirrer with a shaft of ‘/: tool steel and paddles of twisted iron wire was made anode. The shaft fitted into a bearing-metal journal set in a cork stopper closing the vessel. The stirrer was actuated by a D. C. motor, the speed being from 1000-1500 r. p. m. The cell was placed in series with a lamp bank resistance and an ammeter, and in parallel with a voltmeter. The I I O volt D.C.line supplied the current. The electrolyte consisted of 250 cc I O percent sodium sulphate and I O cc (equal to 1 2 grams) nitrobenzene. Little care was taken to keep the temperature constant, the runs usually beginning a t 2 5 O and ending a t 35’. After completion of the run, the solution with its finely divided suspension of orange-red iron oxide and oily drops of aniline and unaltered nitrobenzene was steam-distilled until the drops condensing were clear. The distillate, having been made strongly acid with conc. hydrochloric acid, was diluted to exactly 250 cc, and allowed to settle; I O cc portions after addition of excess of I O percent potassium bromide, were titrated with %/IO potassium bromate, using starch-iodide solution as external indicator. Some of the tribromaniline precipitated in one of these determinations was dried and its melting point found to be 1 1 7 ~ - 1 1 8 ~agreeing , closely with that given by Rritzsche, I I 7 O. The nitrobenzene settling out of the acid solution was decanted into a graduated test tube, in which its volume was determined. The stirrer was weighed before and after each run. The results are given in Tables I and 11. Vaubel: Phys. Chem. Meth. quant. Best. org. Verbdg. Bd., 11, 168.
* Liebig’s Ann., 44,
291 (1842).
Reduction of Nitrobenzene without a Diaphragm
25 I
TABLEI I
No. of run
Stirrer, r. p. m. Temperature Anode loss Corrosion efficiency Length of run, rnin. Current (amperes) Amp/dmz (anode) Amp/dmz cathode Voltage Nitrobenzene taken Nitrobenzene reduced Nitrobenzene recovered Chemical efficiency Current eff. calc. I Current eff. calc. 2 Current eff. calc. 3
I200
2 5 "35
-
I11
I1
O
84 3.5 1.7 1.5 I O . 5-8. o I 2 g. 6 . 1 6 g. 4 . 4 4 g. 88 %
242 % 162% 81%
1500 25O-35O 8 . 6 g. 98% 170 3.0
I200
25O-34O 7 . 8 g. 104% I35 3.0 1.5 1.3 3.5-1 . .j I 2 g. 6 . 4 1 g. 4 . 0 1 g. 86% 247% 165%
1.5
1.3 6.0-4.9 I 2 g. 7.03 g. 3.60g. 89% 216% 144% 72 %
82%
Calc. I is on the basis that only ferrous hydroxide acts as reducing agent. Calc. 2 is on the basis that only cathodic reduction occurs. Calc. 3 is on the basis that both ferrous hydroxide and cathodic hydrogen act as reducing agents. The low voltage in Exp. I11 was due t o improvement in the brush contact for the stirrer shaft. Exp. IV, Table .II,was made in order to determine the amount of purely chemical reduction of nitrobenzene by iron in the presence of sodium sulphate. In Exp. V ferrous sulphate in equivalent concentration was substituted for the sodium sulphate.
TABLEI1 IV
v
I 500
I 500
25 O 0 . 8 g.
26' 0 . 3 g.
No. of run
Stirrer, r. p. m. Temperature Anode loss Length of run, rnin. Nitrobenzene taken Nitrobenzene reduced Reduction, g./hr.
2
160
g. 1.67g. 0.039 I2
190
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g. 0.44g. 0.14 I2
252
'
E . F . Farnau
Conclusions ( I ) Electrolytic reduction of nitrobenzene to aniline without the use of a diaphragm is feasible; indeed high current and chemical efficiencies are obtained, and a t a comparatively low vo1tage.l ( 2 ) Both the ferrous hydroxide and cathodic hydrogen take part in the reduction, the former being oxidized to ferric hydroxide. (3) Practically no other reduction product than aniline is obtained a t room temperature. This research was suggested by Professor Bancroft, and carried out under his supervision; I wish to express my great appreciation of his kindly advice and criticism. Cornell University, April 18, I ~ I I
Compare the current and voltage in experiments where non-attackable anodes and porous cups were employed: 9-15 amps., I O volts. Elbs and Silbermann: Zeit. Elektrochemie, 7, 589 (1901); 2-21/2 amps., 3*/* volts. Elbs: Zeit. Elektrochemie, 4, 472 (1895); 2 amps., 5 volts. Lob: &it. Elektrochemie, 4, 431 (1897).
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