Preparation of N,N'-dialkylphenylenediamines

N,N'-dialkylphenylenediamines are prepared by the reductive alkylation of a nitroaniline and a ketone in the presence of hydrogen and a hydrogenation catalyst. The desired product may be obtained in a more economical way when effecting the process in a continuous manner by utilizing an organic solvent comprising an ether compound such as a monoether of a dihydric alcohol, a diether of a dihydric alcohol, or a cyclic diether.

BACKGROUND OF THE INVENTION 
N,N'-dialkylphenylenediamines which are prepared by the reductive 
alkylation of a nitroaniline and a ketone in the presence of hydrogen and 
a suitable hydrogenation catalyst are utilized as additives for petroleum 
products such as gasoline in which the compounds act as an antioxidant and 
an inhibitor sweetening agent. The reaction for preparing the desired 
diamines may be effected under high pressure when utilizing either a batch 
or continuous type of operation. For example, when utilizing a batch type 
process the solid nitroaniline such as p-nitroaniline is poured into the 
reactor along with the catalyst, following which the ketone and the 
solvent are then added, the reactor is sealed, pressured with hydrogen, 
and heated to the desired operating temperature while subjecting the 
mixture to continuous agitation such as by stirring. The resultant 
product, which may comprise either a liquid or a low melting solid, is 
filtered free from the catalyst following which the solvent may then be 
removed by distillation. 
When effecting the reductive alkylation process in a continuous manner, the 
reactants must, of necessity, be in solution in order that they may be 
continuously pumped into a reactor containing the catalyst and heated 
under hydrogen pressure. However, some difficulty is encountered when 
utilizing a continuous process inasmuch as p-nitroaniline has a poor 
solubility in most organic solvents. For example, it is almost insoluble 
in aliphatic hydrocarbons, only slightly soluble in aromatic hydrocarbons, 
and has a limited solubility in esters, alcohols and ketones. When 
utilizing a high molecular weight ketone as the alkylating agent, it has 
been found that excess ketone must be used as both the alkylating agent 
and solvent. However, the ketones which are used possess a particular 
disadvantage of being reduced to the corresponding alcohols and therefore 
it is necessary to dehydrogenate the alcohols back to the ketone. 
Likewise, when using low molecular weight ketones as alkylating agents, 
the ketones cannot be used in excess as a solvent due to the fact that 
they possess the facility of replacing both amine hydrogens of an aromatic 
amine, thereby forming undesirable overalkylated products such as 
trialkylated and tetraalkylated compounds. 
The preparation of N,N'-diisopropyl-p-phenylenediamine, a particularly 
effective gasoline antioxidant and inhibitor sweetening agent, may be ued 
as an illustration of the problems which are encountered when preparing an 
N,N'-dialkyl-p-phenylenediamine. This compound is prepared by the 
reductive alkylation of p-nitroaniline with acetone. Inasmuch as, as 
hereinbefore set forth, p-nitroaniline as limited solubility in ketones, 
it is necessary to use an 8:1 mole ratio of acetone to p-nitroaniline in 
order to obtain a solution in which the p-nitroaniline is sufficiently 
solubilized so that the solution is capable of being pumped into a 
continuous plant. However, stoichiometrically, only a 2:1 mole ratio is 
required. The use of such an excess of acetone will cause further reaction 
with the dialkyl product to form unacceptable amounts of tri-N-alkylated 
by-products and will result in the loss of both acetone and hydrogen in 
the reduction of acetone to alcohol. In addition, the excess acetone will 
also interfere with the separation of water in the solvent recovery system 
of the plant. 
Some prior art references have shown reductive alkylation processes. For 
example, Canadian Patent No. 862,797 discloses a reductive alkylation 
process in which a sulfided platinum catalyst is used for the reductive 
alkylation of an organic compound containing an amino and/or a nitro 
substituent. Likewise, U.S. Pat. No. 2,969,394 relates to a novel 
combination process which includes the reductive alkylation of an aromatic 
amino or nitro compound with a ketone during which an alcohol is formed 
from the ketone and converted back to the ketone for further use within 
the process. The patent describes an integrated continuous process for the 
manufacture of N,N'-di-sec-butyl-p-phenylenediamine in which the 
by-product, namely, 2-butanol, which results from the reduction of an 
excess of methyl ethyl ketone, which is employed to dissolve the 
p-nitroaniline, is isolated and converted back to the ketone for recycle. 
As will be hereinafter shown in greater detail, in the process of the 
present invention the use of excess ketone is avoided by solubilizing the 
nitroaniline in a solvent of the type set forth and therefore utilizing 
only enough ketone to satisfy the stoichiometry of the reductive 
alkylation reaction. 
Another prior art patent, namely, U.S. Pat. No. 3,522,309 teaches the use 
of polar solvents such as a 5-20% amount of lower alcohols or hydrogenated 
hydrocarbons to increase the rate of the reductive alkylation reaction. 
However, the solvents which were utilized in this reference are not 
particularly effective as solvents for the nitroanilines. 
As hereinafter set forth in greater detail, it has now been discovered that 
by utilizing certain organic solvents of the ether type it is possible to 
reductively alkylate a nitroaniline with ketones, and particularly low 
molecular weight ketones, in which the alkylating agent is present in a 
relatively low mole ratio, the ethers acting as a solvent for the 
nitroanilines. 
SPECIFICATION 
The invention relates to a process for the reductive alkylation of 
nitroanilines with a ketone. More specifically, the invention is concerned 
with an improvement in the reductive alkylation process for preparing 
N,N'-diaklylphenylenediamines whereby said process may be effected using 
relatively low mole ratios of alkylating agent to nitroaniline. 
As hereinbefore set forth the product of the process of the present 
invention, namely, N,N'-dialkylphenylenediamines may be utilized as 
additives for petroleum products such as gasoline, fuel oil, jet fuel, 
heating oil, etc., whereby oxidation of the petroleum product with the 
corresponding formation of undesired gums and tars will be prevented or 
retarded. In addition, these compounds will also act as inhibitor 
sweetening agents. 
It is therefore an object of this invention to provide an improved process 
for the preparation of N,N'-dialkylphenylenediamines. 
A further object of this invention is to provide an improvement in the 
process for the reductive alkylation of a nitroaniline with a ketone 
utilizing a solvent which possesses a particular configuration. 
In one aspect an embodiment of this invention resides in a process for the 
preparation of an N,N'-dialkylphenylenediamine which consists in the 
reductive alkylation of a nitroaniline and a ketone in the presence of 
hydrogen and a hydrogenation catalyst at reaction conditions and 
recovering the resultant N,N'-dialkylphenylenediamine, the improvement 
which consists in effecting said process in the presence of an organic 
solvent comprising an ether selected from the group consisting of the 
monoethers of dihydric alcohols, the diethers of dihydric alcohols and 
cyclic diethers. 
A specific embodiment of this invention is found in a process for 
prepearing an N,N'-dialkyl-p-phenylenediamine which comprises reductively 
alkylating p-nitroaniline with acetone in the presence of hydrogen and a 
hydrogenation catalyst at a temperature in the range of from about 
80.degree. to about 240.degree. C. and a pressure in the range of from 
about 2 to about 2000 pounds per square inch (psi), said process being 
effected in the presence of an organic solvent comprising 
2-methoxyethanol. 
Other objects and embodiments will be found in the following further 
detailed description of the present invention. 
As hereinbefore set forth the present invention is concerned with an 
improvement in a process for the reductive alkylation of nitroanilines 
with a ketone, the improvement which comprises utilizing a particular type 
of solvent for the nitroaniline thereby permitting the reaction to be 
effected in the presence of a lesser amount of ketone. One advantage of 
utilizing this particular type of solvent for the reductive alkylation 
reaction is that p-nitroaniline is sufficiently soluble in the ethers so 
as to provide an economical plant throughput. In addition, the rate of 
reaction will be favorably increased as well as the amount of ketone which 
is utilized in the reaction can be reduced to slightly over the 
stoichiometric amounts, thus avoiding the formation of large quantities of 
alcohols with a concomitant dehydrogenation to form the desired ketone 
again. Furthermore, by utilizing the near stoichiometric amounts of 
ketones, it is possible to prevent the formation of over-alkylated 
by-products, especially in reactions where low molecular weight ketones 
such as acetone, methyl ethyl ketone, cyclohexanone, etc., are employed. 
Examples of ether compounds which may be employed as solvents for the 
reaction of the present invention will include monoethers of dihydric 
alcohols, diethers of dihydric alcohols, and cyclic diethers. Some 
representative examples of these ethers which may be employed will include 
monoethers of dihydric alcohols such as 2-methoxyethanol, 2-ethoxyethanol, 
2-propoxyethanol, 1-methoxy-2-propanol, 1-ethoxy-2-propanol, 
1-propoxy-2-propanol, 1-methoxy-2-butanol, 1-ethoxy-2-butanol, 
1-propoxy-2-butanol, etc.; the diethers of dihydric alcohols such as 
ethyleneglycol dimethyl ether, ethyleneglycol diethyl ether, 
ethyleneglycol dipropyl ether, diethyleneglycol dimethyl ether, 
diethyleneglycol diethyl ether, diethyleneglycol dipropyl ether, 
propyleneglycol dimethyl ether, propyleneglycol diethyl ether, 
propyleneglycol dipropyl ether, dipropyleneglycol dimethyl ether, 
dipropyleneglycol diethyl ether, dipropyleneglycol dipropyl ether, etc.; 
cyclic ethers such as 1,3-dioxane, 1,4-dioxane, etc. It is to be 
understood that the aforementioned ethers are only representative of the 
class of compounds which may be employed as solvents and that the present 
invention is not necessarily limited thereto. 
The reductive alkylation of a nitroaniline with a ketone is effected by 
charging the nitroaniline such as o-nitroaniline or p-nitroaniline along 
with a ketone and the solvent to an appropriate apparatus along with the 
hydrogenation catalyst. The hydrogenation catalyst which is employed in 
the reductive alkylation can be chosen from any of those well known in the 
art such as nickel, platinum composited on a solid support such as 
alumina, palladium composited on alumina, or any other solid support such 
as carbon, diatomaceous earths, etc. The apparatus is sealed and pressured 
to the desired operating pressure with hydrogen, said pressure ranging 
from about 2 to about 2000 psi. Following this, the reaction mixture is 
thoroughly admixed by mechanical means such as stirrers and heated to the 
desired operating temperature which is in a range of from about 80.degree. 
to about 240.degree. C. Inasmuch as the reaction is exothermic in nature, 
due to the rapid reduction of the nitro groups, hydrogen will be rapidly 
consumed. Therefore it is necessary to maintain the desired operating 
pressure by an additional amount of hydrogen. At the end of the 
predetermined residence time, heating may be discontinued and the reaction 
mixture recovered from the apparatus after the apparatus has been returned 
to room temperature and the excess pressure vented. The desired product 
comprising an N,N'-dialkylphenylenediamine is separated from any unreacted 
starting materials and by-products by conventional means such as washing, 
drying, fractional distillation, etc., and recovered. 
Examples of ketones which may be employed as alkylating agents in the 
reductive alkylation process of the present invention will include 
aliphatic ketones such as acetone, methyl ethyl ketone, diethyl ketone, 
methyl propyl ketone, methyl butyl ketone, methyl amyl ketone, methyl 
hexyl ketone, methyl heptyl ketone, methyl octyl ketone, methyl decyl 
ketone, ethyl propyl ketone, ethyl butyl ketone, ethyl amyl ketone, ethyl 
hexyl ketone, ethyl heptyl ketone, ethyl octyl ketone, ethyl nonyl ketone, 
dipropyl ketone, propyl butyl ketone, propyl amyl ketone, propyl hexyl 
ketone, propyl heptyl ketone, dibutyl ketone, the alkyl chains being 
either straight or branched chained in configuration, etc.; cycloaliphatic 
ketones such a cyclobutanone, cyclopentanone, cyclohexanone, 
cycloheptanone, cyclooctanone, etc. 
It is also contemplated within the scope of this invention that the 
N,N'-dialkylphenylenediamine may also be prepared in a continuous manner 
of operation. When this type of operation is employed, the reactants 
comprising the nitroaniline dissolved in an appropriate solvent of the 
type hereinbefore set forth in greater detail and the ketone are 
continuously charged to a reactor which is maintained at the proper 
operating conditions of temperature and pressure and which contains a 
hydrogenation catalyst. In addition, hydrogen is also continuously charged 
to the vessel. Upon completion of the desired residence time in the 
reactor, the effluent is continuously withdrawn and subjected to 
conventional means of separation whereby the N,N'-dialkylphenylenediamine 
is separated from any unreacted starting materials and recovered, the 
unreacted starting materials being recycled to the reactor to form a 
portion of the feed stock. 
Examples of N,N'-dialkylphenylenediamines which may be prepared according 
to the process of this invention will include 
N,N'-diisopropyl-p-phenylenediamine, N,N'-di-sec-butyl-p-phenylenediamine, 
N,N'-di-sec-amyl-p-phenylenediamine, N,N'-di-sec-hexyl-p-phenylenediamine, 
N,N'-di-sec-heptyl-p-phenylenediamine, 
N,N'-di-sec-octyl-p-phenylenediamine, 
N,N'-di-sec-nonyl-p-phenylenediamine, N,N'-di-sec-decyl-p-phenylenediamine 
, N,N'-dicyclopentyl-p-phenylenediamine, 
N,N'-dicyclohexyl-p-phenylenediamine, 
N,N'-dicycloheptyl-p-phenylenediamine, 
N,N'-diisopropyl-o-phenylenediamine, N,N'-di-sec-butyl-o-phenylenediamine, 
N,N'-di-sec-amyl-o-phenylenediamine, N,N'-di-sec-hexyl-o-phenylenediamine, 
N,N'-di-sec-heptyl-o-phenylenediamine, 
N,N'-di-sec-octyl-o-phenylenediamine, 
N,N'-di-sec-nonyl-o-phenylenediamine, N,N'-di-sec-decyl-o-phenylenediamine 
, N,N'-dicyclopentyl-o-phenylenediamine, 
N,N'-dicyclohexyl-o-phenylenediamine, 
N,N'-dicycloheptyl-o-phenylenediamine, etc.

The following examples are given for purposes of illustrating the process 
of this invention utilizing solvents comprising ethers. However, it is to 
be understood that these examples are given merely for purposes of 
illustration and that the present invention is not necessarily limited 
thereto. 
EXAMPLE I 
A charge consisting of 55.2 grams (0.4 mole) of p-nitroaniline, 61.5 grams 
(1.06 mole) of acetone and 280 grams of 2-methoxyethanol was added to a 
magnetically stirred 1-liter, stainless steel autoclave equipped with a 
hydrogen charging system, a sampling di-leg, a heater, a thermocouple 
well, and a water cooling coil. The autoclave also contained 12.7 grams of 
a hydrogenation catalyst comprising platinum on alumina. The autoclave was 
charged with the starting materials, sealed, flushed with hydrogen and 
thereafter pressured to 900 pounds per square inch gauge (psig) with 
hydrogen. The mixture was stirred and heated to a temperature of 
100.degree. C. Generally, an exothermic reaction took place as the 
temperature increased, this reaction indicating a rapid reduction of the 
nitro groups. Hydrogen was replaced in increments in order to maintain a 
pressure of 900 psig. The reaction was effected for a period of 4.8 hours, 
samples of the mixture being removed periodically through a dip-leg and 
analyzed by means of gasliquid chromatography. This analysis indicated 
that at the end of the period the substituted p-phenylenediamines 
recovered consisted of 97% N,N'-diisopropyl-p-phenylenediamine with 2% of 
the triisopropyl product and 1% of the monoisopropyl product. 
EXAMPLE II 
In this example a charge consisting of 55.2 grams of p-nitroaniline, 61.5 
grams of acetone and 280 grams of 1-methoxy-2-propanol was placed in an 
autoclave similar to nature to that set forth in Example I above, said 
autoclave also containing 12.7 grams of a platinum on alumina 
hydrogenation catalyst. As in the above example, the autoclave was sealed, 
flushed with hydrogen and thereafter pressured to 900 psig. Stirring was 
commenced and the reaction mixture was heated to a temperature of 
100.degree. C. The autoclave and contents thereof were maintained at this 
temperature for a period of 5 hours, hydrogen being periodically in order 
to maintain the desired operating pressure. Analysis of the product by 
means of gas-liquid chromatography indicated that there was a 97% 
production of N,N'-diisopropyl-p-phenylenediamine with a 1% production of 
the triisopropyl product and 2% of the monoisopropyl product also being 
prepared. 
EXAMPLE III 
In a manner similar to that set forth in the above examples, the reductive 
alkylation of p-nitroaniline with acetone was effected in the presence of 
280 grams of tetrahydrofuran. The reaction was effected in the presence of 
a platinum on alumina hydrogenation catalyst at a temperature of 
100.degree. C., a hydrogen pressure of 900 psig for a period of 5.2 hours. 
The reductively alkylated product which was obtained in this experiment 
consisted of 93% of N,N'-diisopropyl-p-phenylenediamine, 1% of the 
triisopropyl product and 6% of the monoisopropyl product. 
EXAMPLE IV 
To illustrate the use of a diether of a dihydric alcohol, the above 
experiments were repeated utilizing 280 grams of diethyleneglycol dimethyl 
ether as the solvent. The reaction conditions employed as well as the 
charge stock were similar to those in the above examples, the reaction 
being effected for a period of 4.5 hours. At the end of the reaction time, 
analysis of the product by means of gas-liquid chromatography determined 
that there had been a 93% production of 
N,N'-diisopropyl-p-phenylenediamien, a 4% production of 
N,N,N'-triisopropyl-p-phenylenediamine, and a 2% production of 
N-isopropyl-p-phenylenediamine. 
it is therefore noted from the above examples that by utilizing a solvent 
of the type hereinbefore set forth, that is, an ether compound, it is 
possible to effect the reductive alkylation of p-nitroaniline with acetone 
using only an amount of acetone which is slightly above the stoichiometric 
amount in order to obtain the desired N,N'-diisopropyl-p-phenylenediamine 
product. The ability of the solvent to dissolve th p-nitroaniline so that 
such a relatively small amount of acetone could be used was a contributing 
factor to the obtention of the desired dialkyl product. 
In addition, the ability of these solvents to remove water from the 
reaction mixture was also a contributing factor to obtaining the desired 
product in a relatively easy method. For example, 2-methoxyethanol was 
found to be soluble in and forming a binary azeotrope with water which 
consists of 85% water and 15% 2-methoxyethanol. Likewise, the 
diethyleneglycol diethyl ether formed a binary azeotrope which consisted 
of 78% water and 22% ether, while the other two solvents showed a binary 
mixture consisting of 49% water and 51% 2-methoxy-2-propanol and 5% water 
and 95% tetrahydrofuran respectively. 
EXAMPLE V 
To illustrate the applicability of the present process in a continuous 
manner of operation, a charge stock consisting of 15.1% acetone, 68.9% 
2-methoxyethanol and 16% p-nitroaniline, the acetone/p-nitroaniline mole 
ratio being 2.3:1, was charged to a high pressure tubular reactor which 
was heated by a jacket at a LHSV of 0.5. In this reactor which was 
maintained at a temperature of 125.degree. C., and a hydrogen pressure of 
1000 psig, the reaction was effected for a period of 4 hours. The effluent 
was withdrawn to a liquid/vapor separator, passed from this separator to a 
liquid level controller and from there to a receiver. The effluent was 
analyzed by means of a gas-liquid chromatograph and the product was found 
to consist of 96% of N,N'-diisopropyl-p-phenylenediamine and 4% of 
N,N,N'-triisopropyl-p-phenylenediamine. 
When the above run was repeated for a second 4-hour period utilizing a 
temperature of 150.degree. C., and a LHSV of 1.0, the product was analyzed 
and found to consist of 96% N,N'-diisopropyl-p-phenylenediamine, 3% 
N,N,N'-triisopropyl-p-phenylenediamine and 1% 
N-isopropyl-p-phenylenediamine. A third run was effected for an additional 
4-hour period utilizing a temperature of 170.degree. C., and a LHSV of 
1.5. Analysis of the product which was recovered from this run disclosed 
that the dialkyl product was present in a 95% amount, the trialkyl product 
was present in a 3% amount and the monoalkyl product was present in a 2% 
amount. 
EXAMPLE VI 
In this example another series of runs was effected in a continuous manner 
in an apparatus similar to that set forth in Example V above. The feed 
stock in this series of runs consisted of 18.1% acetone, 65.6% 
2-methoxyethanol and 16.3% of p-nitroaniline, the acetone/p-nitroaniline 
mole ratio being 2.7:1. In the first 4-hour period, the reaction was 
effected at a temperature of 125.degree. C., and a hydrogen pressure of 
1000 psig, the feed stock being charged to the reactor at a LHSV of 1.0. 
The product which was recovered was analyzed and found to consist of 96% 
N,N'-diisopropyl-p-phenylenediamine and 4% 
N,N,N'-triisopropyl-p-phenylenediamine. 
When the run above was repeated using a temperature of 150.degree. C., and 
a LHSV of 1.5, the product consisted of 94% of the dialkyl product and 6% 
of the trialkyl product. A third run which was effected for a period of 4 
hours at a temperature of 175.degree. C., and a LHSV of 2.0 resulted in a 
product distribution which comprised 94% 
N,N'-diisopropyl-p-phenylenediamine and 6% of the trialkylated product, 
namely, N,N,N'-triisopropyl-p-phenylenediamine. 
EXAMPLE VII 
In this example a charge consisting of 55.2 grams (0.4 mole) of 
o-nitroaniline, 61.5 grams (1.06 mole) of acetone, and 280 grams of 
2-methoxyethanol may be added to a magnetically stirred stainless steel 
autoclave which is provided with a hydrogen charging system, a sampling 
dip-leg, a heater, a thermocouple well, and a water cooling coil. The 
catalyst which may be employed for this process comprises 12.7 grams of 
platinum on alumina. After charging the autoclave with the starting 
materials it may then be sealed, flushed with hydrogen, and pressured to 
about 1000 psig with hydrogen. Thereafter the mixture may be stirred and 
heated to a temperature of 100.degree. C. being maintined for a period of 
about 5 hours while maintaining the pressure of 900 psig by the addition 
of incremental portions of hydrogen. At the end of the 5 hour period 
heating may be discontinued, the excess pressure discharged, and after 
return to room temperature the desired product comprising 
N,N'-diisopropyl-o-phenylenediamine may be recovered from the autoclave. 
EXAMPLE VIII 
The use of other ketones such as methyl ethyl ketone, diethyl ketone and 
cyclohexanone which are relatively low molecular weight compounds as 
alkylating agents in the reductive alkylation of o-nitroaniline or 
p-nitroaniline using solvents of the type hereinbefore set forth may 
permit the use of slightly over stoichiometric amounts of the ketones due 
to the ability of these solvents to dissolve the nitroanilines.