ELECTROLYTIC PREPARATION OF PARA-AMIDO- PHENOL In

ELECTROLYTIC PREPARATION O F PARA-AMIDOPHENOL BY J .

c.

WARNER' AND 0.

w.

BROWN

I n previous papers,2 the results of a study of the various factors influencing the electrolytic preparation of orthoamidophenol and its recovery from solution have been given. The purpose of the present investigation was to make a similar study of. the factors influencing the electrolytic preparation of para-amidophenol. I n the past, para-amidophenol has been prepared by a great variety of methods. Paul3 prepared it by reducing para-nitrophenol with tin and hydrochloric acid. Grandmougin, in a similar. manner accomplished the reduction by the use of iron and acetic acid. He also reduced paranitrophenol to para-amidophenol in an alkaline solution by means of sodium hyposulphite. Para-nitrosophenol, reduced by means of sodium sulphide] likewise yields para-amidophenol. Grandmougin5 has prepared para-amidophenol in the following manner :-Aniline is diazotized and coupled to phenol forming para-hydroxyazobenzene. This compound is then reduced forming para-amidophenol and aniline. The aniline is distilled from the mixture and the para-amidophenol is crystallized from the mother liquor. This process has been used by manufacturers in this country. Nitrobenzene is directly reduced to para-amidophenol by means of zinc dust6 and sulphuric acid a t 50" to 80" C 1 This paper is constructed from a dissertation presented by J. C. Warner to the Faculty of the Graduate School of Indiana University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. in Chemistry. 2 Trans. Am. Electrochem. SOC., 41, 143 (1922). SZeit. angew. Chem., 9, 594 (1896). Jour. prakt. Chem., 76, 135 (1907). Ibid., 76, 126 (1907). 6 German Pat., 96,853 (1898).

Electrolytic Preparation of Para-Amido-Phenol

653

Noyes and Clement, Gattermanq2 S t r ~ j i and , ~ Darmstader4 have accomplished this direct reduction by the electrolysis of a solution of nitrobenzene in strong (80-90 percent) sulphuric acid. In recent years, McDaniel, Schneider, and Ballard of the Eastman Kodak Company5 attempted to develop this method into a commercial process for the preparation of paraamidophenol. It has, however, proven too costly for successful operation. Mignonaq6 Brochet,’ and Brown and Carrick* have prepared para-amidophenol by the catalytic reduction of paranitrophenol. The vapor pressure of para-nitrophenol, however, is so low at the temperature at which the reaction is best carried out, that the process must of necessity be very slow. E l b ~ ,in . ~ an article entitled, “Quantitative Research on Processes of Reduction,”’ gives the results of some experimental work upon the electrolytic reduction of para-nitrophenol to para-amidophenol. In this work, he based his calculation of yields upon the quantity of hydrogen evolved a t the cathode, assuming that all of the hydrogen not evolved was used in reducing para-nitrophenol to the corresponding amine. Later, in his book entitled, “Electrolytic Preparations” (page 88), Elbs has given directions by which he states that para-nitrophenol may be reduced to para-amidophenol with a material yield of 80 percent. A systematic study of the various factors influencing this reduction, however, has not been recorded. I n our investigation, the influence of various factors upon

7

Ber. deutsth. chem. Ges., 26, 990 (1893). Ibid., 26, 1846 (1893). Jour. Soc. Chem. Jnd., 37, 4598 (1918). German Patents, 150,800 (1904); 154,086 (1904). Trans. Am. Electrochem. SOC.,39, 319 (1921). Bull. Soc. chim. France, 7, 270 (1910). Eng. Pat., 16936 (1913); French Pat., 458,033, 1912; U.S. Pat., 1,247,629. Jour. Am. Chem. Soc., 41, 436 (1919). Jour. prakt. Chem., 43, 39 (1891).

654

J . C. Warner and 0. W . Brown

the reduction of para-nitrophenol to para-amidophenol a t copper and lead cathodes has been studied and a satisfactory method for the recovery of para-amidophenol from solutions has been developed.

Experimental The apparatus used was similiar to that described in our previous papers. In all experiments, the cathode liquor consisted of 400 cc of caustic soda or sodium carbonate solution in which was dissolved various quantities of para-nitrophenol. The container for the cathode solution was an 800 cc pyrex beaker. The anode solution, consisting of about 100 cc of 15 percent caustic soda was placed in a porous cup 10 cms high and ,5 cms in diameter. The porous cup was suspended in the cathode liquor. The anode, in all experiments, was an iron wire gauze having a surface of 1.2 square decimeters (counting both sides). The temperature of the cathode liquor was regulated by means of an electric hot plate beneath the beaker and glass cooling coils in the cathode compartment through which water could be circulated. A glass stirrer, driven at about 800 r.p.m. by means of a small electric motor, was placed in the cathode compartment between the cathode and the porous cup. At the end of each experiment, the cathode, stirrer, porous cup, and cooling coils were washed off into the cathode liquor and the latter transferred to a 1000 cc graduated flask and diluted to the mark. The quantity of para-amidophenol produced was determined by titration with standard sodium nitrite solutions as described in’ a previous article. Commercial para-nitrophenol, without purification, was used throughout this investigation. When reductions were carried out using para-nitrophenol which had been purified by recrystallization from water, the same yields were obtained as when the commercial product was reduced under like conditions. These experiments led us t o the conclusion that the commercial para-nitrophenol was quite pure.

Electrolytic Preparatiotz of Para-Anzido-Phenol

655

Reduction at a Copper Cathode I n this part of the investigation, the cathode was an 18 mesh copper wire gauze. The diameter of the wires in the gauze was 0.041 cm. The area of the gauze was one square decimeter (counting both sides). Sodium Hydroxide Electrolyte The influence of current density at the cathode upon ,the efficiency of the reduction process is shown by the results of experiments given in Table I. I n all of these experiments the theoretical number of ampere hours was used. Hence, material yield and current efficiency in percent of theory have the same numerical values.

TABLE I Influence of Current Density Cathode solution; 400 cc 5 percent NaOH 5 grams para-nitrophenol Temperature; 62" * 2" C Ampere hours used; 5.75 (theory). Cathodic C . D.-Amps. per sq. dcm.

roltage drop througl cell 5 min. after start

Material yield and current efficiency. Percent

1 1 2 2 2 4 4 B G 10 10 10 10

2.0 2.1 3.5 2.7 3.0 3.8 4.3 6.2 5.3 7.1 7.3 6.4 7.1

92.7 91.2 92.0 91.1 92.0 90.6 89.4 88.8 87.1 82.8 s2.2 83.0 53.5

%v. material yield and current efficiency. Percent

91.9 91.7 90.0 87.9 82.8

The data given in Table I are shown graphically in Figure 1. From the figure, i t is evident that there is a gradual decrease in current efficiency and material yield as the current density is increased.

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656

The influence of concentration of caustic soda in the cathode liquor was next studied. The results of these experiments are recorded in Table 11.

2

8

4 Fig. 1

TABLE I1 Influence of Concentration of Caustic Soda Cathode solution ; 400 cc NaOH solution. 5 grams para-nitrophenol. Cathodic current density; 4.0 amps. per sq. dcm. Temperature; 62" * 2' C. Ampere hours used; 5.78 (theory). Conc. NaOH in cathode liquor. Percent by weight

Voltage drop through cell, 5 min. after start

Current efficiency and material yield. Percent

Av. current effiiency and material yield. Percent

3 3 5 5

4.4 4.1 3.5 4.3 3.6 3.4 3.4 4.8 4.7 4.6 3.5

58.4 88.8 90.6 89.4 92.1 91.8 90.9 91.2 91.2 88.3 87.9

58.6

8

'

8 10 10 10 15 15

90.0 91.9

91.1 88.1

Electrolytic Prepayation of Para-Amido-Phenol

657

The results shown in Table I1 are shown graphically in Figure 2. Current efficiency and material yield increase with increased concentrations of caustic soda until a concentration of eight percent is reached. Thereafter, there is a regular decline as the concentration increases. The increase in yields at first is probably due to the fact that the solution becomes more conducting as the concentration of caustic increases. The decrease in current efficiency and material yield when more concentrated solutions of caustic soda are used

Fig. 2

may be due to the decreased solubility of the sodium salt of para-nitrophenol in the stronger caustic or possibly because the para-amidophenol is less stable in the more concentrated alkali. The relation between concentration of para-nitrophenol in the cathode solution and current efficiency is shown by the experiments recorded in Table 111. The results given in Table I11 are shown graphically in Figure 3. Current efficiency and material yield increase as the quantity of paralnitrophenol in the cathode liquor


658

is increased until a concentration of ten grams in 400 cc is reached. E'urther increases in concentration of para-nitro-

Fig. 3

TABLE I11 Influence of Concentration of Para-nitrophenol Cathode solution; 400 cc 5 percent NaOH para-nitrophenol Cathodic current density; 4.0 amps. per sq. dcm. Tern xature: 62' * 2" C Gms p-nitro Voltage drop through cell, phenol in 5 min. 400 cc cathode after start solution

3 3 .i 5 S 8 10 10 15 15 20 20

'

6.2 5.4 4.9 5.3 5 . 1. 5.3 6.5 6.3 6.2 5.4 5.2 5.3

Ampere hours used

3.47 3 -47 5.7s 5.78 9.25 9.25 11.56 11.56 17.30 17.30 23.12 23.12

Current effiAv. current efficiency and ciency and material yield. material yield. Percent Percent

85.4 S4.0 S8.2 8G.2 90.4 91.2 92.3 91.9 90.4 90.7 59.4 50.2

84.7 86.2

00.8 92.1 90.5 89.3


659

phenol cause a decrease in current efficiency. Since the quantity of para-nitrophenol remaining in solution when hydrogen begins to be evolved at the cathode is constant, one would expect current efficiency to increase as the concentration of para-nitrophenol was increased because the unchanged nitrophenol would be a smaller percentage of the total quantity to be reduced. The decline of the later portion of the curve is undoubtedly due to the fact that the beneficial influence of

Fig. 4

increased concentrations of para-nitrophenol is more than counterbalanced by losses. These losses are probably due to increased decomposition of the para-amidophenol when longer periods are required for the reduction and to increased loss by diffusion into the porous cup. The diffusion of paraamidophenol into the cup would be dependent upon the difference in concentrations of the amine outside and inside the cup and upon the time for diffusion.

J . C. Warner axd 0. W . Brown

660

The results of experiments which show the influence of temperature are recorded in Tables IV and V.

TABLE IV Influence of Temperature at 10.0 Amperes per Square Decimeter Cathode solution; 400 cc 5 percent NaOH 5 grams para-nitrophenol Ampere hours used; 5.78 (theory) Temperature Degrees Centrigrade

45 45 62 62 72 82 05

Voltage drop through cell 5 min. after start


8.7 9.3 6.4 7.1 7..0 7.2 7.1

80.1 80.1 83.0 83.5 85.2 87.9 88.6

TABLE V Influence of Temperature a t 4.0 Amperes per Square Decimeter Cathode solution; 400 cc 5 percent NaOH 5 grams para-nitrophenol Ampere hours used; 5.78 (theory) Temperature Degrees Centigrade

35 35 62 62 68 74 81 93

I



5.4 5.1 3.s 4.3 4.7 4.0 4.1 3.7

85.8 85.1 90.6 89.4 92.2 92.7 92.2 92.2

The results given in Tables IV and V are shown graphically in Figure 4: With a current density of 10.0 amperes per sq. dcm., the current efficiency and material yield increase as the temperature is increased (Curve I) until a temperature


GG1

of about 80" C is reached. Thereafter, the beneficial influence of higher temperatures is partly counterbalanced by losses and the increase in efficiency is no longer as great. This decrease in the beneficial influence of temperature might possibly be due to a lowering of the cathodic overvoltage. This, however, is hardly a likely cause, since in a similar study of

TABLE VI Current Efficiency and Material Yield with Current less than and in Excess of Theory Cathode solution; 400 cc 8 percent NaOH 10 grams para-nitrophenol Cathodic current density; 2.0 amps. per sq. dcm. Temperature; 72 O - 74 C Ampere hrs. Percent of theory

Voltage drop :hrough cel min. afte: start

Current efficiency. Percent of theory

Av. current

Material yield. Percent

Av. material yield. Percent

25 25 51 51 75 75 90 90 100 100 110 110 120 120

3.1 3.6 3.2 3.4 3.2 3.4 3.3 3.4 3.3 3.9 3.2 3.6 3.0 3.4.

99.8 100.8 100.4 99.2 98.5 99.0 97.4 97.4 93.1 92.2 85.5 55.3 78.0 78.2

100.3

23.0 23.0 51.0 51 .o 73.9 74.2 87.7 87.7 92.2 93.1 94.1' 93.9 93.6 94.2

25.0

efficiency. Percent

99.8 98.75 97.4 '

92.7

85.4 78.1

51.0 74.1 87.7 92.7 94.0 93.9

the effect of temperature upon the preparation of orthoamidophenol, material yield and current efficiency were found to increase directly with the temperature. It seems reasonable t o suppose t h a t the explanation of the difference in these two cases is to be found in the difference in the stability of the ortho and para-amidophenols. At a current density of 4.0 amperes per sq. dcm. (Curve II), material yield and current efficiency increase with an

'

662


increase in temperature up to about 70" C. Thereafter, an increase in temperature causes no further increase in yield. Since, in all the preceding experiments, the theoretical number of ampere hours was used, some experiments were carried out in which the relationship between current efficiency and material yield was studied when various numbers of amperebours, greater and less than theory, were used. The results of these experiments are given in Table VI. The results shown in Table VI are plotted in Figure 5 . The current efficiency curve remains practically horizontal until about 65 percent of the theoretical number of ampere

Fig. 5

hours have been passed. Thereafter, there is a downward trend of the efficiency curve. From these data, i t is evident that under the conditions of these experiments, theoretical current efficiency is obtained as long as the concentration of para-nitrophenol does not drop below about 0.9 gram in 100 cc of cathode solution. Material yield increases with the number of ampere hours until current in an amount equal to 110 percent of theory has been passed, but under these conditions the use of current in excess of this amount does not cause an increase in material yield.

Electrolytic Preparatiout of Para-Amido-Phenol

Cathodic C. D. amps. per sq. dcm.


2 2 4 4 6 10 10

... ... 5.8 6.5 5.2 8.0 9.0

663

Current efficiency Av. current effiand material yield. ciency and material Percent yield. Percent

86.3 86.9 52.8 82.8

;75.0 E:

86.6

1

52.8 79.8 75.0

The data given in Table VI1 are shown graphically in Figure 1. From the curve, it can be seen that material yield and current efficiency decrease as the current density is increased. The yields at all current densities are lower than the yields obtained at corresponding current densities when NaOH is used as solvent for the para-nitrophenol. The voltage drop through the cell in all of these experiments was greater than in corresponding experiments with NaOH. The influence of the concentration of sodium carbonate in the cathode liquor upon current efficiency and material


664

yield is given by the results of experiments recorded in Table VIII.

TABLE VI11 Influence of Concentration of Sodium Carbonate Anode liquor; 15 percent NaOH Cathode liquor; 400 cc sodium carbonate solution 5 grams para-nitrophenol Cathodic current density; 4.0 amps. per sq. dcm. Temperature; 62" * 2" C Ampere hours used: 5.78 (theory) Conc. NazCOs. lOHlO in cathode liq. Percent by wgt



Av. current efficiency and material yield. Percent

5 5 10 15 15 20 20 25 25

5.8 6.5 5.0 3.8 4.6 4.1 4.1 4.0 4.6

82.8 82.8 84.2 85.6 85.6 86.3 86.9 86.3 86.9

82.8 84.2 85.6 S6.6 86.6

The results recorded in Table VI11 are plotted in Figure 2. Current efficiency and material yield increase with increased concentration of sodium carbonate in the cathode liquor until a concentration of 20 percent Na2CO3.10H20is reached. Thereafter, increase in the concentration causes no further increase in yield. The limit of solubility of sodium carbonate in water prevented the use of stronger solutions of the carbonate. Para-nitrophenol is more readily soluble in sodium carbonate solutions than in solutions of sodium hydroxide. Solution in the former is always accompanied by the evolution of carbon dioxide. Although the course of the reduction in sodium carbonate solutions has not been as thoroughly investigated as the reduction in solutions of caustic soda, i t seems that sufficient data have been obtained t o prove that sodium carbonate is

Electrolytic P.reparation of Para-Amido-Phenol

665

inferior to sodium hydroxide for use in the electrolytic reduction of para-nitrophenol to the corresponding amine.

Sodium Bicarbonate and Sulphuric Acid Electrolytes Although the reduction in these electrolytes has not been thoroughly investigated, experiments conducted in this laboratory by Mr. Levi H. Wellman, show that sodium bicarbonate and sulphuric acid are both inferior to sodium hydroxide and sodium carbonate solutions as electrolytes in carrying out this reduction. Reduction at a Lead Cathode The reduction at a lead cathode was studied only in a sodium hydroxide electrolyte. From experience with the electrolytic preparation of ortho-amidophenol, one would expect lead to be a very good electrode material to use in the reduction of para-nitrophenol to the corresponding amine. The results of the following experiments show that this expectation is fulfilled. The apparatus used and the conditions maintained in these experiments were the same as in the previous experiments except that the copper cathode was replaced by a cathode covered with spongy lead. The sponge lead cathode was prepared by plating lead on an 18 mesh copper wire gauze, having a surface of one sq. dcm. (counting both sides), from a bath of 15 percent NaOH solution in which litharge had been dissolved. Sheet lead anodes were used and a current of 10.0 amperes was passed for a period of ten minutes. The cathodes prepared in this manner were kept under water until used. The results of experiments showing the influence of cathodic current density upon current efficiency and material yield are recorded in Table IX. The results given in Table IX are plotted in Figure 1. As the current density is increased, the current efficiency increases a t first until a current density of 4.0 amperes per sq. dcm. is reached. Thereafter, there is a gradual decrease


666

. TABLE IX Influence of Current Density Cathode solution; 400 cc 5 percent NaOH 5 grams para-nitrophenol Temperature; 62" =t2" C Ampere hours used; 5.78 (theory) Cathodic C: D. amps. per sq. dcm.

Voltage drop through cell 5 rnin. after start


1 1 2 2 4 4 G 6 10 10

1.7 1.8 2.0 2.8 2.5 3.5 3.9 4.8

88.7 89.4 92.8 92.8 94.5 93.8 92.8 92.8 91.4 91.4

Conc. NaOH in cathode liquor. Percent by weight

Voltage drop through cell 5 rnin. after start



3 . 3 5 ' 5 8 8 10 10 15 15

2.1 3.1 2.5 3.5 2.5 3.3 2.7 3.4 3.2 4.0

92.6 92.4 94.5 93.8 94.8 94.8 94.5 94.2 93.5 93.5

92.5

-

-


89.1 92.8 94.2 92.8 91.4

94.2 94.8 94.3 93.5


667

in yield as the current density is increased. The shape of the current density curve with a lead cathode is similiar to the corresponding curve obtained in the study of the reduction of ortho-nitrophenol at a lead cathode. The influence of the concentration of caustic soda in the cathode liquor is shown by the data recorded in Table X. The results from Table X are shown graphically in Figure 2. As the concentration of caustic soda in the cathode liquor is increased, there is a rather rapid increase in the current efficiency until a concentration of eight percent is reached. A further increase in the concentration of caustic soda causes a gradual decrease in current efficiency. The data given in Table XI show the relation between the temperature of the cathode liquor and the current efficiency and material yield.

TABLE XI Influence of Temperature Cathode solution; 400 cc 8 percent NaOH 5 grams para-nitrophenol Cathodic current density; 4.0 amperes per sq. dcm. Ampere hours used: 5.78 (theory) Temperature degrees Centigrade

33 33 49 53 62 62 73 78 93 95

Voltage' drop through cell 6 min. after start

3.5 4.5 2.8 3.0 2.5 3.3 2.5 2.7 2.5 3.0


92.7 92.5 93.5 04.1 94.8 94.8 94.1 94.5 92.5 91.8

The data given in Table XI are graphically represented in Figure 4 (Curve 111). There is a gradual increase in yield its the temperature is increased until a temperature of 65' to 70" C is reached. As the temperature is increased above

J . C. Warn.er and 0. W . Brown

668

this point there is a decrease .in yield. The counteracting influences which cause yields to decrease as the temperature is raised above 70" C may be an increase in the rate of decomposition of the para-amidophenol, a lowering of the cathodic overvoltage of lead, or the influence of temperature upon the nature of the lead which is redeposited upon the cathode. At high temperatures (90-95 C), the redeposited lead seems t o be much more crystalline and hence would afford a much smaller active cathode surface. The influence of the concentration of para-nitrophenol in the cathode liquor upon current efficiency is shown b y the results of experiments recorded in Table XII.

TABLE XI1 Influence of Concentration of Paranitrophenol Cathode solution; 400 cc 8 percent NaOH Para-nitrophenol Cathodic current density; 4.00 amperes per sq. dcm. Temperature; 62". * 2" C Gms. p-nitro phenol in 400 cc of cathode solution

3 3 5 5 8 8

10 10 15 15


3.5 5.0 2.5 3.3 3.5 3.5 3.5 3.5 3.5 3.3

Ampere hours used

3.47 3.47 5.78 5.78 9.25 9.25 11.56 11.56 17.30 17.30

.

Current efficiency and naterial yield. Percent

Av. current efficiency , and material yield. Percent

91.9 92.2 94.8 04.8 94.7 94.7 94.5 93.8 91.6 81.2

92.0 94.8 94.7 94.2 91.4

The results given in Table XI1 are plotted in Figure 3. It is evident from the curve that current efficiency increases with an increase in the concentration of para-nitrophenol in the cathode liquor until a concentration of about 6 to 7 grams of para-nitrophenol in 400 cc of cathode solution is

Elecdrolj'tic Pveqaration

0.f

Para-Amido-Phewol

669

reached, A further increase in concentration causes a decrease in yield. The changes in the direction of the curve may be explained in the same way as the changes of the similar curve obtained from a study of the reduction a t a copper cathode. The difference in the maximum points on the two curves may be due to the different concentrations of NaOH used as electrolyte.

Recovery of Para-Amidophenol from Solution As has been mentioned in a previous paper, ortho-amidophenol can very conveniently ,be liberated from its alkaline solutions by saturating a solution of the amine with carbon dioxide, whereupon the free base separates out as small platelike crystals. By filtering, washing, and drying, ortho-amidophenol may be obtained which ranges in purity from 97-99 percent. When an attempt was made to recover para-amidophenol from its alkaline solutions by the same method, very unsatisfactory yields of only fairly pure material were obtained. The best sample obtained by this procedure was only 90 percent pure and possessed a dark brown color. By use of a different method, however, it has been possible to recover para-amidophenol of high purity from alkaline solutions of the amine with very good yields. The method which has been used successfully involves the liberation of the amine from alkaline solutions (to which a small amount of sodium bisulphite has been added) by means of the addition of (1: 1) sulphuric acid. A number of experiments were carried out to study to some extent the effect of the quantity of sodium bisulphite added. The alkaline solutions of para-amidophenol were in all cases obtained by electrolytically reducing para-nitrophenol in alkaline solutions at a lead cathode under like conditions. Each solution contained 10.9 grams of para-amidophenol and was allowed to stand at room temperature for 16 hours, was then placed in a bath of running water while the para-amidophenol was liberated by the dropwise addition

J . C. Wawzer and 0. W . Brown

670

of (1 : 1) sulphuric’ acid until a phenolphthalein end-point was reached. The liquor was then filtered by means of suction and the para-amidophenol washed twice with cold distilled water, sucked as dry as possible on the Beuchner filter, and then dried over night in a vacuum desiccator. The dry para-amidophenol was weighed and the purity determined. The results of these experiments are given in Table XIII.

TABLE XI11

I

Influence of NaHS03 upon the Recovery of Para-amidophenol

Gm~~3c~.””8 1 ~ $ o o p D h ~ ~ ~1 l of solution

0.0 5.0 10.0

T;gt

recovered

1

9.2 9.3 9.5

1

96.4 98.7 98.1

1

1

Gms pure p-amidophenol

8.87 9.18 9.38

1

Percent recovery

81.3 84.2 85.5

From this data, i t seems evident that there is no advantage in the use of more than small amounts of sodium bisulphite in the recovery. If the para-amidophenol is liberated immediately after being prepared, gdod yields of very pure material can be obtained without the addition of the bisulphite. When para-amidophenol was liberated immediately after the reduction from a solution containing 10.9 grams of the amine, 9.0 grams of para-amidophenol, 95.4 percent pure, were obtained. The mother liquor, after filtering off the precipitated para-amidophenol, still contained 2.06 grams of the amine. The para-amidophenol remaining in the mother liquor may be almost completely recovered by making slightly acid, evaporating to dryness on the steam bath, powdering the residue, extracting i t with small quantities of boiling alcohol, and evaporating the alcohol under vacuum. When this procedure was followed in the recovery of para-amidophenol from 400 cc of an alkaline solution containing 10.9 grams of the amine and 5 grams of sodium bisulphite, the following results were obtained.

Electrolytic Preparation of Para-Amido-Pheizol Weight ppt. p-amidophenol-gms Purity-percent Weight pure p-amidophenol-gms

9.4 99.2

Weight p-amidophenol from residue-gms Purity-percent Weight pure p-amidophenol-gms Total pure p-amidophenol-gms,

1.6 57.7

Loss-gms

Loss-percent

671

9.32

1.40 10.72 0.18 1.65

The para-amidophenol obtained by extracting the residue from the evaporation of the mother liquor might be further purified by recrystallization from a suitable solvent. The ash content of the samples of the amine from residues was about 4.0 percent. The ash content of the samples of precipitated para-amidophenol was in all cases less than one percent. This procedure has also proven itself superior to the carbon dioxide method for the liberation of ortho-amidophenol from its alkaline solutions. By this method, orthoamidophenol ranging in purity from 99.5 to 99.7 percent may be obtained. As the best procedure for the laboratory preparation of para-amidophenol, the following is recommended. Prepare a lead cathode by plating sponge lead on a copper gauze from a solution of litharge in 15 percent caustic soda at a current density of 10.0 amperes per sq. dcm. Start the reduction with a cathode solution of 8 percent NaOH containing about 3 t o 4 grams of para-nitrophenol per 100 cc. Use as anode an iron wire gauze and as anode liquor a 15 percent solution of NaOH. Maintain the cathode liquor at a temperature of 65-75" C. Start the reduction with a current density of 10.0 amperes per sq. dcm. When one half the nitrophenol has been reduced, add one half the original amount of the nitrophenol. Continue the reduction at a current density of 10.0 amperes per sq. dcm. until 95 percent of the theoretical number of ampere hours has been

672

J . C. Warner and 0. W..Brown

passed. Finish the reduction with a current density of 4.0 amperes per sq. dcm., passing a ten percent excess of current. At the end of the reduction, wash off the porous cup, cathode, and stirrer into the cathode liquor with as small a quantity of water as possible. Add about 1 gram of sodium bisulphite for each 100 cc of cathode liquor and cool to room temperature. Liberate the amine by the dropwise addition of (1: 1) sulphuric acid to a phenolphthalein end-point. Filter on a Beuchner funnel with suction, wash the residue twice with cold distilled water, suck as dry as possible on the filter bed, and dry in a vacuum dessicator. Make the mother liquor slightly acid with sulphuric acid and evaporate t o dryness on a steam bath, powder the dry residue, and extract several times with small quantities of boiling absolute alcohol. Evaporate the alcohol from the amidophenol in a vacuum desiccator. A 90 to 95 percent yield should be obtained in the reduction and practically all of this can be recovered. About nine tenths of the amine should be obtained from the precipitation as a product ranging in purity from 98.5 to 99.5 percent. Onetenth can be obtained by extraction of the residue from the mother liquor as a product ranging in purity from 85 to 90 percent. Summary and Conclusions 1. Lead is superior to copper for use as a cathode in the reduction of para-nitrophenol to para-amidophenol, Lead cathodes also permit the use of higher current densities during the reduction. 2. An electrolyte containing NaOH gives higher current efficiencies and material yields of para-amidophenol than electrolytes containing sodium carbonate, sodium bicarbonate or sulphuric acid. 3. With a copper or lead cathode, the highest current efficiencies and material yields are obtained with an electrolyte containing about eight percent NaOH. 4. At a copper cathode, the highest current efficiencies

Electvolytic Preflaralion of Para-Amido-Phenol

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and material yields are obtained when a current density of 1 to 2 amperes per sq. dcm. is used. 5. The most favorable concentration of para-nitrophenol in the cathode liquor is 2.0 to 3.0 grams of the nitrophenol in 100 cc. 6. With a current density of 4.0 amperes per sq. dcm., the most favorable temperature is 65" to 75" C. With a current density of 10.0 amperes per sq. dcm., the reduction is best carried out near the boiling point. 7. When solutions of 10 grams of para-nitrophenol in 400 cc of 8 percent caustic soda were reduced a t a copper cathode with a current density of 2.0 amperes per sq. dcm., and at a temperature of 72"-74" C, the follovqing results were obtained. a. Theoretical current efficiency was obtained as long as the concentration of para-nitrophenol did not drop below about 1.0 gram in 100 cc. b. Material yields increased as the number of ampere hours was increased until ten percent in excess of the theoretical amount of current was used. The use of a larger excess of current causes no further increase in material yield. 8. Para-amidophenol of high purity (99.0-99.5 percent) can be obtained from alkaline solutions of the amine by neutralization with sulphuric acid in the presence of sodium bisulphite, filtering, washing with water, and drying under vacuum. 9. The para-amidophenol remaining in the mother liquor after precipitation with sulphuric acid can almost all be recovered by making the mother liquor slightly acid, evaporating to dryness on a steam bath, and extracting the dry residue with small portions of boiling absolute alcohol. The alcohol should then be evaporated from the para-amidophenol under vacuum. 10. On standing, alkaline para-amidophenol solutions decompose quite rapidly. If small quantities of sodium bisulphite are added, this decomposition is prevented. Laboratory of Physical Chemistry Indiana University Bloomington, Indiana

ELECTROLYTIC PREPARATION OF PARA-AMIDO- PHENOL In

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