THE PREPARATION" OF ‘
pa AMINOPHENOL‘ 1
FROM NITROBENZENE .

I Thesis 'fcrthc Degkefi'of M. S.
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THE PREPARATION OF
p-AMINOPHENOL
FROM

NITROBENZENE

A Thesis Submitted to the Faculty
of
Michigan State College

In Partial Fulfillment of the
Requirements for the Degree
of
Master of Science

Department of Chemistry

by y‘

fl

xy
Ernest P? Jensen

June, 1935

CONTENTS

Introduction

Apparatus and Materials

Procedure

Experimental Data

Discussion of Data

Summary

98884

Page

Page

Page

Page

Page

Page

10

16

19

23

1.

INTRODUCTION

In the electrolytic reduction of nitrobenzene
in acid solution, the following compounds are formed
in order: nitrosobensene, beta-phenylhydroxylamine
and aniline (Brockman - Eleotro-Organio Chemistry -
John Wiley — (1936) page 303). Nitrobensene is a
poor depolarizer in acid solution but as soon as the
reduction is started, nitrosobensene is formed. This
is a good depolariser and is readily reduced to beta-
phenylhydroxylamine. Further reduction produces
aniline but if the reduction is stOpped at this point,
beta-phenylhydroxylamine undergoes an intramoleoular
rearrangement to form p-aminophenol. This rearrange-
ment takes place readily in sulphuric acid. The
rearrangement, however, is not a simple interchanging
of the hydrogen atom in the para position of the ring
with the hydroxyl on the nitrogen atom. Instead,
beta-arylhydroxylamine (HN06H4HOH) 1- formed and this
rearranges, the hydrogen attached to the same carbon
as the hydroxyl going to the nitrogen to form an

amino group.

The methods of preparation of p-aminophenol

may be divided into two general classes; chemical and

3.

electrochemical.

Para-nitrophenol may be reduced to p-amino-
phenol by the action of tin and hydrochloric acid
or by iron and acetic acid. In alkaline solution
it is reduced by sodium hyposulphite. p-Nitroso-
phenol may be reduced by sodium sulphide.

A process that has been used by manufacturers
in this country is to diasotise aniline and couple
it with phenol to form p-hydroxyasobensene (Warner
and Brown - J. Phys. Chem. 31, 653-73 (1937)). This
is then reduced to p-aminophenol and aniline. The
aniline may then be distilled off and recovered
while the p-aminophenol is crystallised out of the

remaining solution.

Nitrobenzene may be reduced to p-aminophenol

by zinc dust and sulphuric acid at 50°-80°C.

tomatsu and Amatatsu (C. A. §§, 500 (1931)),
in an investigation of the catalytic hydrogenation
of various aromatic nitro compounds, found that
p-nitrophenol shaken in an autoclave with a reduced
nickel catalyst in an atmosphere of hydrogen under
a pressure of 50 to 100 atmospheres and temperature

of 100°C yielded p-aminophenol quantitatively.

3.

Among the electrochemical methods of prepara-
tion is the"oxidation of aniline in concentrated
sulphuric acid at a platinum anode (Brockman - Electro-
Organic Chemistry - John Wiley and Sons, Inc. (1926)).
However, the conductivity of the solution is very
poor and too much energy is required. The presence
of concentrated sulphuric acid protects the p-amino-
phenol from further oxidation but if water is present
the aminophenol reacts with the excess aniline to
form p-aminohydroxydiphenylamine which is further
oxidized to aniline black.

J. Warner and 0. Brown (J. Phys. Chem. 31,
652-73 (1927)) succeeded in reducing p-nitrophenol
in a solution of caustic soda at a lead cathode

with very high efficiencies.

McDaniel, Schneider and Ballard (Trans. of
Am. Electrochem. Soc. g2, 441 (1921)) attempted'to
prepare p-aminophenol by the reduction of nitrobenzene
in concentrated sulphuric acid at a platinum cathode.
This was done during the World War as an emergency
measure and was found to be too costly to compete
with chemical methods. The current yield was around
25% while the chemical yield was only 40-50%. The

cost of the platinum electrodes was not the prohibitive

cost since they could be used almost indefinitely
but rather the cost of the sulphuric acid which
could not be recovered was excessive. The temper-
ature of the solution had to be kept at 30°C to'
prevent the formation of the sulphonic acid
derivative which is of no value as a photographic

developer.

lhen nitrobensene is reduced in sulphuric
acid, the reaction goes as far as the formation of
phenylhydroxylamine, when an intramolecular re-
arrangement takes place to form p-aminophenol. This
rearrangement is a well known phenomenon taking
place in mineral acids (Cohen - Theoretical Organic

Chemistry - Macuillan (1928)).

Practically all the work on the reduction of.
nitrobenzene in sulphuric acid has been done using
the concentrated acid. This is due to the fact
that the necessary rearrangement of phenylhydroxylamine
takes place much faster in the concentrated than
the dilute acid. Also, nitrobenzens is insoluble
in acid less than 88% 83804 by weight which gives
rise to the problem of keeping the nitrobensene in
a stable emulsion in more dilute acid. This problem

is well met by the use of wood-workers glue as an

v.

emulsifying agent.

An investigation of the reduction of
nitrobensene in dilute sulphuric acid of the same
specific gravity as nitrobenzene was made by
Adam F. Schuch (The Electrolytic Preparation of
p-Aminophenol (1931)) in this laboratory. He
used platinum and lead sheets as anodes and
platinum sheet, monel metal, nickel and copper
gauzes as cathodes. He found that an anode of
lead sheet and cathode of copper gauze gave the
best yields.

This work has been done to carry out the
reduction on a commercial scale and to improve
on the yield. Runs were made varying the temper-
ature, current density, the percent of theoretical
current passed and the amount of nitrobenzene in

the emulsion.

The product was identified by melting
point tests and a sample dried in an electric oven
at 100°C for 24 hours was found to have a melting
point of 307°C. Before drying in the oven, the
compound decomposed at about 260°C. The Inter-
national Critical Tables (Vol. I, page 200) give

6.

the melting point of p—aminophenol hydrochloride
as 306°C at which point the compound decomposes.
It is soluble 14 g/lOOc.c. while the free base
is soluble 1.1 g/100c.c. (Handbook of Chemistry
and Physics, 17th Ed., page 380) of water at 0°C.

The photographs found in this work were
developed with a solution made from some of the
p-aminophenol hydrochloride produced in the
experiments. The formula used is as follows

(Jones - Photographic Seneitometry - Eastman Kodak
Company):

Sodium Sulfite (anhydrous) 50g.
Sodium Carbonate (anhydrous) 50g.
p-Aminophenol hydrochloride 7.25g.

Dissolve in order and add a small amount of
sodium hydroxide to dissolve the p-aminophenol.
Add water to make 1000c.c.

7.

APPARATUS AND MATERIALS

The cell used for all the runs made is
shown in the drawings, figure 1 and figure 2. It
consisted of a lead tank 7 1/8” x 9 5/8“ x 14 1/4'
made in two parts.he1d together by bolts. The
joint thus formed between the two sides was kept
from leaking by using a rubber gasket. A diaphragm
of plywood was used for most of the runs and this
and this was sealed into the joint, where the two
sections of the cell bolted together. by pouring
sulphur around the edges. This served to hold the
diaphragm in place as well as to prevent leakage
from one compartment to the other. The plywood
was found to work very well if the right kind was
used. Three-ply wood, three sixteenths of an inch
thick with a basswood center, was found to be very
good. This did not warp enough to cause any trouble
and stood up for more than 125 hours of actual use

when runs were being made.

Figure 1 shows a side view of the cell as
it looked when it was assembled. The top of the
cell was fitted.with a wooden top with a framework
which supported a water motor (7). The water motor

was connected directly to a stirrer (6) which was

 

 

 

 

 

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made of a brass rod covered with lead. A sheath
was made by wrapping a strip of lead around the
brass rod and welding the joint. The stirrer
extended down into the cathode compartment (5)
and kept the catholyte stirred thus maintaining

a good emulsion throughout the run. The cathode
was made of copper gauze and had an area of 5.8
sq. dm. This was placed between the stirrer and
the diaphragm (3). The anode compartment (4)
contained sulphuric acid of the same specific
gravity as nitrobenzene (sp. gr. 1.2). The ter-
minals for the anode and cathode (2) were brought
out at the top and extended a little above the
wooden cover. The temperature within the cell was
controlled by running water or steam through coils

(1) which extended down into each compartment.

The materials used were as follows:
Nitrobenzsne — commercial.
Sulphuric acid - c.p. sp. gr. 1.84
Sodium chloride - commercial.
Sodium bisulphite.

Hydrochloric acid - C.p.
Ibcdworkers glue.

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Electrodes:
cathode - copper gauze.

anode - lead sheet.

9.

10.

PROCEDURE

Sulphuric acid was diluted with distilled
water to about 30% 32304 by weight and the acid
cooled to room temperature. This acid is a little
more concentrated than is desired but as soon as
the solution was cooled, a 100c.c. sample was taken.
A few cubic centimeters of nitrobensene was added
to this sample and shaken. Water was added to the
sample until the droplets of nitrobensene just hung
in the liquid and did not settle out at the top or
bottom. Then an amount of water proportional to
the number of cubic centimeters required to adjust
1000.c. was added to the main portion. After a
thorough mixing another test sample was taken and
the specific gravity tested again in the same
manner. Further adjustments were made if necessary.
This method was found to be superior to using a

hydrometer.

The cathode solution was prepared by placing
the proper amount of dilute sulphuric acid in the
cathode compartment. Then about 50c.c. of wood-
workers glue was added and stirred until dissolved.
The required amount of nitrobenzene was then added

and this was beaten into a creamy emulsion by

11.

fi’)
fi

vigorous agitation for about minutes. During this
time the temperature was adjusted to the prOper

value by running steam through the coils.

The anode solution consisted of sulphuric
acid of the same specific gravity as used in the
catholyte. This was placed in the anode compart-
ment at the same time the cathode compartment was

filled.

The current was then turned on and the run
started. When the runs were made at 40°C, trouble
was encountered due to foaming at the start but
this was overcome by heating the solution to about
60°C for about twenty minutes after which the

temperature was readjusted to its proper value.

When the run was finished, the catholyte
and anolyte were siphoned into separate jars. The
catholyte was purified and the p-aminophenol extract-
ed while the lead sulphate formed in the anolyte
was allowed to settle out. The anolyte was used
over again. The cell was cleaned and rinsed out

with distilled water after each run.

Several methods of purification were tried.

The solution was neutralized with sodium hydroxide

12.

and the free base filtered out on a suction filter.
The free base was then boiled with hydrochloric
acid to give the hydrochloride salt. This method
was undesirable because of the cost and also because
a slight excess of sodium hydroxide redissolved the
free base forming sodium para-aminophenolate,

NGOCSH4N33.

The product was evaporated to one-fourth its
volume on the steam bath and hydrochloric acid added.
This was not only slow but uncertain and sulphonic

acids were apt to be formed.

The catholyte was neutralized with sodium
carbonate to give the free base. In this case as
well as with sodium hydroxide, tarry substances were
formed which made the purification of the free base
difficult. The free base was not redissolved when

sodium carbonate was used.

The best method, however, was to use sodium
chloride (Kirkhgof and.Suirkin - C. A. 36, 1957
(1932)). The cathode solution was heated almost to
boiling in a four liter beaker and sodium chloride
added. Two hundred grams of sodium chloride were

added for each liter of solution. The solution was

13.

then covered with an evaporating dish and boiled

for two hours. In boiling, the sodium chloride
reacted with the sulphuric acid to form hydrochloric
acid. When the solution was cooled in ice water,
the crystals of p-aminophenol hydrochloride separat-
ed out along with some sodium chloride which did
not react. During the boiling, the volume of the
solution was decreased to about two-thirds the

original volume.

The crude product which was always black
in color, was filtered out on a suction filter and
the filtrate discarded. On further boiling with
more sodium chloride, no more p-aminophenol hydro-
chloride came out. Sodium chloride throws out
p-aminophenol hydrochloride quantitatively as was
proved by tests made on a sample of the purified
product. When a saturated salt solution was added
to a solution saturated with p-aminophenol hydro-
chloride, all the p-aminophenol hydrochloride was

thrown out immediately.

The crude product was then dissolved in the‘
least amount of hot water and lOOc.c. of c.p.
hydrochloric acid added. Sodium bisulphite was then
added to the extent of 50 grams per liter. This

14.

was to prevent the oxidation and to decrease the
solubility of p—aminophenol hydrochloride. The
solution was then boiled slowly with charcoal
(Norite A) for twenty minutes to deoolorize the
solution. About thirty grams of Norite A was
usually sufficient although more was required in
some cases. After twenty minutes of slow boiling,
the solution was filtered hot. The filtrate should
be clear or light yellow. If the solution was
light yellow (caused by the oxidation of p-amino-
phenol) it could be cleared up at this point
simply by adding a little more sodium bisulphite.
Hydrochloric acid c.p. was then added to the hot
solution until the precipitate which formed just
dissolved. About lOOc.c. per liter was all that,
was required. The clear solution was set in a
pan of ice water to crystallize. Three or four
hours were all that were required for the crystal-
lization. The crystals of p-aminophenol hydro—
chloride were filtered, dried in an evaporating dish
on a steam bath and weighed.

Brown and Warner (Trans. Am. Electrochem.
Soc. 4;, 225 (1922)) give a method of titrating

for the amount of p-aminophenol. The solution is

acidified with hydrochloric acid and the amine is
diazotized with standard sodium nitrite according
to the reaction:
H006H4NH2 Nanoz 2301 =
H0C6H4N3N01 2H30 Na01
Sodium nitrite was added until a blue color was

indicated on starch iodide paper.

This, however, failed to indicate an ends
point because of phenylhydroxylamine in the sample.
According to Cohen (Theoretical Organic Chemistry -
MacMillan (1928)), phenylhydroxylamine separates
iodine from potassium iodide and dilute sulphuric
acid. This would give the characteristic blue color
with starch and in all trials, the test showed blue
before and during the time sodium nitrite was added.

No endpoint could be observed.

Run

ODUIIFOJNH

7*.

8..

9*.
10"
11
12

Series A '

EXPERIMENTAL DATA

1200g. nitrobenzene in 5 liters H2604 sp. gr. 1.2

Theoretical yield of p-aminophenol hydrochloride

=1420g.

Theoretical number of ampere hours

Amp.
hrs.

1150
1150
1150
1150
1150
1150
1120
1550
1060
1070
1250
1074

Amp.
hrs.

110%
110%
110%
110%
110%
110%
107%
148%
102%
102%
1201.
103%

EMF
volts

2.5
3.5
3.6
3.4
3.4
3.5
3.5
3.1
4.0
3.2
3.3
3.3

COD.

SEE;

dmz
7.4

11
11
11
11
11

9.65

8.6
15.9
17.6
17.2
15.2

Temp.
70°C

80°
60°
40°
50°
75°
40°
40°
40°
70°
70°
85°

Yield
0g.

190
276
439
552
562
537
843
638
378*
305‘
336*

1046

Chem.

eff.
0%
13.4
19.4
31
38.9
39.6
37.8
59.5
45
53.2
43
46

C. sf

(1%
12.2
17.5
28.2
35.4
36.
34.4
40.2
44.
52.2
35.8
44.5

' Only half of the total catholyte was purified.

*’ Cell lined with sulphur.

Therefore multiply these weights by two.

f.

5

d1 -e o: (n e» cs a: rd

Series B

17.

1800g. nitrobenzene in 4.5 liters H2804 sp. gr. 1.2

Theoretical yield of p-aminophenol hydrochloride

=2l30g.

Theoretical number of ampere hours a 1569

Amp.
hrs.

1724
1726
1584
2354
1726
1726
1520
2110

%
2:2:
110%
110%
101%
150%
110%
110%

97%
135%

EMF
volts

~3.7
3.8
3.8
4.3
4.2
3.1
2.7
3.1

G.D.
4221
dmz
15.5
15.9
16.5

16.5

9.65

10.3
10.7
9.5

Temp.

40°c
50°
80°
40°
400
60°
80°
40°

Chem.

Yield"I eff. 0. eff.
262.5g. 24.6% 22.4%
425 39.9 36.3

415 39 38.6

298 28 18.7

436 41 37.2

386 36.2 32.9

453 42.6 43.9

475 44.6 33.

' Only half of the total catholyte was purified.

Therefore multiply these weights by two.

E’

(DKIOIUIIPOINH

Series C

18.

600g. nitrobenzene in 5.5 liters H2504 sp. gr. 1.2

Theoretical yield of p-aminophenol hydrochloride

= 710g.

Theoretical number of ampere hours = 523

Amp.
hrs.

576
575
575
785
575
575
575
785

%
Amp.
hrs.

110%
110%
110%
150%
110%
110%
110%
150%

EMF
volts

3.8
3.5
3.4
4.4
6.

2.8
3.4
3.5

0.0.
Egg;
dmz Temp.
15.5 4000
16.2 60°
17.9 80°
16.7 40°
8. 40°
10.3 60°
9.5 80°
9.15 40°

Yield'
57g.

186
170
0
78
162
171
169

Chem.

eff.

16.1%

52.5
47.9
0
22.
45.6
48.2
47.6

eff.
14.6%
47.7
43.5

0
20.
41.5
43.8
31.8

* Only half of the total catholyte was purified.

Therefore multiply these weights by two.

DISCUSSION OF DATA

The first six runs of series A were made
with a diaphragm of asbestos paper using a perfor-
ated lead sheet for a support. This, however, was
found to be too porous for the cathode and anode
portions diffused into each other. Then too, the
metal partition acted as an electrode which made
an anode in the cathode compartment. A diaphragm
of three—sixteenths inch plywood with a basswood
center was then used and the results found to be
quite satisfactory. One of these diaphragms
lasted more than 125 hours of actual working time
and the two solutions did not mix. This type of

diaphragm was used for the remainder of the runs.

Runs 7 to 10 inclusive were made with the
cell lined with sulphur. This lining was made by
pouring molten sulphur over the inside surfaces
of the cell. The reason for doing this was to
determine whether or not the sides of the cell
were acting as electrodes. The fact that the yields
did not increase proved that this belief was false.
The sulphur was therefore removed from the cell
and the remainder of the runs were made without

any lining.

20.

Series B was made using a larger amount of
nitrobenzene in the solution and varying the temper-
ature, current density, and the amount of current
passed. This series of runs was not very satisfac-
tory due to the fact that it was difficult to
maintain a good emulsion. A good deal of unreduced
nitrobenzene was left in the solution and the product

was very oily and difficult to purify.

Series C was made using a smaller amount of
nitrobenzene in the emulsion and varying the temper-
ature, current density and the amount of current
passed. In the case of run number 4, the catholyte
contained a large amount of an unidentified green
oily substance. No p-aminophenol hydrochloride was
obtained from this run. In all other cases, however,
the emulsion held very well and these runs were the
easiest of any to purify. Runs 1 and 5 also contained
a good deal of the green oily substance but not

enough to interfere with the purification.

In most cases, the runs started out with a
grey color. This soon turned to a light purple which

became darker and finally turned black.

The runs at 80°C always foamed as soon as the

theoretical amount of current had been passed. This
could be stopped for a short time by adding a few
grams of nitrobenzene or a little glue. However,
the foaming started again within a short time and

it was impossible to run the cell any longer. The
froth was very stiff and would stand up out of the

cell several inches.

The anolyte could be used for six to eight
runs before it became dark colored. The darkening
did not take place until the diaphragm was almost

worn out.

At low temperatures, a lower current density
seems to be more favorable. In run number 8 where
the yield was 59.5%, the temperature was 40°C and
the current density was 8.6 amperes per sq. dm.

At higher temperatures, a higher current density
works better (see runs 10 series A and 2 series 0).
This is probably because the speed of the rearrange-
ment of phenylhydroxylamine is faster at the higher

temperature.

The most favorable temperatures for the

operation of the cell seem to be from 40° to 70°C.

The highest current efficiency was obtained

in run 10 series A. In this trial, only 2% excess
current was passed and the current efficiency was

52.2%.

It is believed that the yield of p-amino-
phenol obtained by reduction in dilute acid is
bound to be low because of the fact that the speed
of the rearrangement of phenylhydroxylamine is too
slow to prevent the reduction of some of this
compound to aniline. The formation of aniline is
confirmed by the fact that large amounts of aniline
hydrochloride were found in the solution after the

p-aminophenol had been extracted.

SUMIARY

1. p-Aminophenol may be produced electrolytically
on a commercial scale using an emulsion of nitrobenzene
in dilute sulphuric acid of the same specific gravity
as nitrobenzene.

2. A cathode of copper gauze and an anode of
sheet lead are suitable electrodes.

3. The most favorable temperature 40° to 70°C.

4. No great excess of current need be passed.

5. The efficiencies are high - 59%.

6. A diaphragm of ply-wood, three-sixteenths
of an inch thick with a basswood center is suitable.

7. The operating voltage with such a diaphragm
is low - 3 to 4 volts.

8. The method of purification with sodium
chloride is fast, cheap and gives the highest yields.

 

J542

Jensen

 

 

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