Process for producing N,N'-disubstituted paraphenylene diamine mixtures by sequential reductive alkylation

Mixtures of N,N'-disubstituted paraphenylenediamines are produced in a process wherein a nitrogen-containing compound such as 4-nitrodiphenylamine is reductively alkylated with a plurality of ketones in sequence. Preferably, one ketone is reacted to completion with an excess of the nitrogen-containing compound, then an excess of a second ketone is added and the reaction is continued until the nitrogen-containing compound is consumed. The product is then separated from the volatiles and the unreacted portion of the second ketone is recovered.

This invention relates to a process for preparing a mixture of 
N,N'-disubstituted paraphenylenediamines. 
N,N'-disubstituted paraphenylenediamines are widely used in rubber as 
antidegradants, and are particularly effective in protecting vulcanized 
rubber from ozone attack. A number of different paraphenylenediamine (PPD) 
materials are made and sold commercially for this purpose. 
Blends of two or more PPDs have been advantageously used in rubber, and 
provide certain advantages over the individual PPD materials. Some PPDs 
exhibit melting points which are sufficiently close to room temperature as 
to give handling difficulties. It has been found advantageous to blend two 
or more PPDs for the purpose of obtaining a product which can be handled 
as a liquid under normal temperatures. Blends are also used where the 
particular properties of two or more PPDs are desired in a single product. 
Blends can be produced by physically mixing two or more separately-prepared 
PPDs, but this method requires additional storage and mixing equipment. 
Another method of preparing blends is shown in U.S. Pat. No. 3,542,691, 
wherein a mixture of methyl isobutyl ketone and methyl isoamyl ketone is 
used to reductively alkylate 4-aminodiphenylamine, producing a mixture of 
N,-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine and 
N-(1,4-dimethylamyl)-N'-phenyl-p-phenylenediamine. This method has its 
drawbacks as well, primarily in the recovery of unreacted ketones. The 
reductive alkylation reaction necessarily produces a certain amount of 
by-product alcohols, resulting from hydrogenation of the ketones, and 
these alcohols are extremely difficult to separate from the ketones. The 
mixture of two ketones, their respective alcohol counterparts and water 
presents a serius problem in the recovery of valuable by-products and 
unreacted ketones. 
Thus, the need exists for a method of preparing mixtures of PPDs which 
avoids the effort and expense of separate preparations, yet does not 
entail the problems inherent in the mixed-ketone process. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a process for preparing 
PPD mixtures which avoids the problems associated with the prior art 
processes. 
This and other objects are accomplished by the process of the invention 
which is an improvement in the process for preparing a mixture of two or 
more different, N,N'-disubstitutedd paraphenylenediamines by the reductive 
alkylation of a nitrogen-containing compound selected from 
4-nitrodiphenylamine, 4-aminodiphenylamine, p-nitroaniline and 
paraphenylenediamine with two or more ketones selected from 
##STR1## 
wherein x is an integer of from 2 to 9 and R.sub.1 and R.sub.2 are alkyl 
of 1 to 8 carbon atoms, with the proviso that the total number of carbon 
atoms in R.sub.1 and R.sub.2 together is nine or less, in the presence of 
hydrogen and a catalyst, the improvement comprising charging the ketones 
sequentially and reacting each essentially to completion before charging 
the next. 
The ketones used in the process of the invention include, in addition to 
cyclohexanone, acetone, methyl ethyl ketone, diethyl ketone, methyl propyl 
ketone, methyl isopropyl ketone, ethyl propyl ketone, ethyl isopropyl 
ketone, dipropyl ketone, diisopropyl ketone, methyl butyl ketone, methyl 
isobutyl ketone, methyl sec-butyl ketone, methyl tert-butyl ketone, ethyl 
butyl ketone, ethyl isobutyl ketone, ethyl sec-butyl ketone, ethyl 
tert-butyl ketone, propyl butyl ketone, isopropyl butyl ketone, propyl 
isobutyl ketone, propyl sec-butyl ketone, propyl tert-butyl ketone, 
isopropyl isobutyl ketone, isopropyl sec-butyl ketone, isopropyl 
tert-butyl ketone, dibutyl ketone, diisobutyl ketone, di-sec-butyl ketone, 
di-tert-butyl ketone, butyl isobutyl ketone, butyl sec-butyl ketone, butyl 
tert-butyl ketone, isobutyl sec-butyl ketone, isobutyl tert-butyl ketone, 
sec-butyl tert-butyl ketone, 5-heptanone, 5-methyl-2-hexanone (methyl 
isoamyl ketone) 4-methyl-2-hexanone, 3-methyl-2-hexanone, 
3,4-dimethyl-2-pentanone, 3,3-dimethyl-2-pentanone, 
4,4-dimethyl-2-pentanone, 3-octanone, 4-methyl-3-heptanone, 
5-methyl-3-heptanone, 6-methyl-3-heptanone, 4,4-dimethyl-3-hexanone, 
4,5-dimethyl-3-hexanone, 5,5-dimethyl-3-hexanone, 4-nonanone, 
5-methyl-4-octanone, 6-methyl-4-octanone, 7-methyl-4-octanone, 
5,5-dimethyl-4-heptanone, 5,6-dimethyl-4-heptanone, 
6,6-dimethyl-4-heptanone, and the like. 
Reductive alkylation of 4-nitrodiphenylamine with a ketone produces a 
N-substitued N'-phenyl-p-phenylenediamine by way of a two-step reaction. 
First, the nitro group is hydrogenated to give 4-aminodiphenylamine. Then, 
the ketone adds to the 4-amino group in the second step of the reaction. 
If the starting compound is 4-aminodiphenylamine the reaction can, of 
course, proceed in a single step. If p-nitroaniline or phenylenediamine is 
the nitrogen containing compound, successive ketone additions produce a 
product which is a mixture of symmetrical and unsymmetrical PPDs. 
By reacting "essentially to completion" is meant reacting to the extent 
that very little, if any of the unreacted ketone is left in the reactor. 
Ideally, none will be left, however, trace amounts can remain, as a 
practical matter, and will not negate the value of the process. 
The process of the invention is preferably performed at superatmospheric 
temperatures and pressures, more preferably at temperatures of from 
50.degree. to 240.degree. C. and pressures of from about 1.5 to 15 MPa, 
although higher pressures, up to 30 MPa can be used if desired. The 
reaction vessel used must be capable of withstanding the pressures used, 
so the use of extremely high pressures should be avoided, since they 
require prohibitively expensive equipment. 
A variety of catalysts are known to be effective in reductive alkylation or 
hydrogenation reactions. Among these catalysts are nickel and/or platinum 
compounds, and cobalt or copper chromite. Preferred in the process of the 
invention is platinum, and most preferably, platinum on carbon together 
with the acidic carbon co-catalyst described in Summers U.S. Pat. No. 
3,414,616. 
By definition, the process of the invention involves the reaction of two or 
more ketones in sequence. There is no theoretical limit to the number of 
ketones which could be used, although as a practical matter the use of 
more than three ketones is unlikely, and most processes will use only two. 
No solvent need be used in the process of the invention, since the 
nitrogen-containing compounds are fluid at the temperature employed, and 
the ketones act as solvents or diluents in the reaction zone. If desired, 
however, a compatible solvent could be used, and if relatively inert to 
the reactants, catalyst and product, would not interfere with the process. 
At the recommended temperatures and pressures the time of the reaction is 
sufficiently short as to be economically acceptable, yet not so short as 
to be difficult to control. Ideally, the reaction can be completed in 
several hours from initial charge to completion. If a nitrogen-containing 
compound is 4-nitrodiphenylamine, the nitro reduction phase of the process 
can be completed in from ten to sixty minutes, preferably from 14 to 30 
minutes, and is signalled by a sharp drop in temperature. The first and 
subsequent reductive alkylation reactions can be completed in from 30 to 
150 minutes each. 
Recovery of the product merely requires removal of the catalyst and 
volatiles therefrom, and yields can run from 80 to 99+%. It is generally 
economical to recover and recycle excess ketone, if present, and usually 
preferably to separate from the ketone as much as is practicable of the 
impurities, comprising water and alcohols. The separation can be performed 
by distillation, with separation of aqueous portions of the azeotropes 
encountered, or with separation of the aqueous layer followed by 
distillation of the ketone layer. 
A number of advantages are realized by the process of the invention, as 
compared with the known method of charging a mixture of ketones together 
with a nitrogen-containing compound: 
First, it is possible, in the process of the invention, to recover excess 
ketone which is a single ketone, rather than a mixture of two or more 
ketones. In recycling the excess ketone a difficult and costly separation 
step can thus be avoided. 
Second, an initial charge of all the nitrogen-containing compound plus only 
one ketone results in a substantial excess of the nitrogen-containing 
compound being present during the initial stage of the reaction, thus 
providing a driving force for the reaction. Then, the charge of the last 
ketone can be in large excess, if desired, so as to provide not only a 
driving force for the reaction but also a solvent for the reaction mass. 
Third, the process of the invention gives a precise method of achieving a 
desired ratio of paraphenylenediamines in the product. The initial ketone 
charge can be reacted until no free ketone is found in the reaction zone, 
and the final step can be carried out until no nitrogen-containing 
compound remains, assuring precise control. In contrast, the reaction of 
the mixture of ketones is subject to their inherently differing reaction 
rates, which will be further changed by differing concentration and 
temperatures as the reaction proceeds. 
Finally, the process of the invention permits a lower concentration of 
ketones in the reaction zone at any one time, thus reducing the amounts of 
ketone which are hydrogenated to alcohols, an undesirable side reaction 
which not only consumes a reactant but produces a by-product which is 
difficult to separate from the recovered unreacted ketone. 
In summary, the process of the invention provides an accurate, cost-saving 
method of producing mixtures of alkyl (or cycloalkyl) substituted 
paraphenylene diamines.

DETAILED DESCRIPTION 
A better understanding of the invention may be obtained by reference to the 
following examples, in which all parts are by weight unless otherwise 
indicated. 
EXAMPLE I 
To a Parr autoclave equipped with an agitator, coil for heating or cooling, 
thermowell, vents, rupture discs, appropriate sampling vents and stainless 
steel filter are charged 214.3 parts by weight (1.0 mole) of 
4-nitrodiphenylamine (4-NDPA), 56.4 parts by weight (0.494 mole) of methyl 
isoamyl ketone (MIAK), 6.0 parts by weight of 1% platinum on carbon (63% 
water) and 6.0 parts by weight of acidic carbon co-catalyst (Summers U.S. 
Pat. No. 3,414,616). 
The autoclave is purged twice with nitrogen and twice with hydrogen, and 
the reactor contents heated to 115.degree..+-.5.degree. C. Hydrogen is fed 
into the system to a pressure of from 2.0 to 2.8 MPa. After 20 minutes 
reaction a drop in temperature signals the end of the nitro reduction. The 
autoclave contents are then raised to 150.degree..+-.5.degree. C. and 
2.8-3.5 MPa hydrogen. After 113 minutes at the higher pressure and 
temperature, sampling reveals that all of the MIAK is reacted. 
The autoclave is then cooled and charged with 120 parts by weight (1.20 
moles) of methyl isobutyl ketone (MIBK). The temperature of the reactor 
contents is then raised to 150.degree..+-.5.degree. C. and the hydrogen 
pressure is adjusted to 2.8-3.5 MPa for 95 minutes. At the end of this 
time no 4-NDPA is detectable and the crude product is filtered. 
The MIBK-water azeotrope is distilled from the filtrate and the aqueous 
layer of distillate returned to the distillation flask until only a 
single-phase aqueous distillate is collected. 
The residue (product) is then heated to 150.degree. C. at a pressure of 40 
mm mercury (5.3 Pa) for 30 minutes, to remove residual volatiles. 270 
grams of product are recovered. Analysis by gas-chromatography indicates 
the final product to be 50.8% 
N-(1,4-dimethylamyl)-N'-phenyl-p-phenylenediamine and 48.7% 
N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine. 
EXAMPLE II 
The process of Example I is repeated several times, except that the molar 
ratio of MIAK:MIBK is changed from about 1:1 to about 2:1. The reductions 
proceed as described in Example I, with the nitro reduction completed in 
15-30 minutes as signalled by a temperature drop. The reductive alkylation 
of MIAK is completed in 75-115 minutes, as indicated by GC analysis of the 
reactor contents. The reductive alkylation of MIBK is then completed in 
75-90 minutes, monitored again by GC analysis. Analyses of the product 
gave 62.2-65.0% N-1,4-(dimethylamyl)-N'-phenyl-p-phenylenediamine and 
32.6-34.7% N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine. 
EXAMPLE III 
In order to evaluate the process of the invention in producing di-alkyl 
paraphenylenediamines, the method of Example I was repeated, except that 
paranitroaniline was substituted for 4-NDPA. The following were charged to 
the Parr autoclave: 
110.4 g. (0.80 mole) paranitroaniline 
91.3 g. (0.80 mole) MIAK 
12.0 g. catalyst (1% Pt on carbon, 63% water) 
6.0 g. carbon co-catalyst. 
The autoclave was purged twice with nitrogen, then twice with hydrogen. A 
pressure of 2.1 MPa hydrogen was placed on the autoclave and the 
exothermic reaction started at 20.degree. C. The temperature was allowed 
to increase to 120.degree. C. (over a fifteen-minute period) and the 
hydrogen pressure was maintained at 2.0-2.8 MPa. After twenty minutes the 
hydrogen pressure stayed constant and the temperature began to decrease, 
indicating completion of the nitro reduction. The autoclave contents were 
then heated to 150.degree. C..+-.5.degree. C. and the hydrogen pressure 
kept at 2.8-3.5 MPa for three hours. 
The autoclave was then cooled, and 184.4 g. (1.84 moles) MIBK were charged. 
Purging was again performed with nitrogen, then with hydrogen and the 
reactor contents were then heated to 150.degree..+-.5.degree. C. at 
2.8-3.5 MPa hydrogen pressure for three hours. 
The crude product was filtered, as before, and distilled. After removing 
residual volatiles the product was analyzed and found to contain: 
24.5% N,N'-di(1,3-dimethylbutyl)-p-phenylenediamine 
54.0% N-(1,3-dimethylbutyl)-N'-(1,4-dimethylamyl-p-phenylenediamine 
19.1% N,N'-di(1,4-dimethylamyl)-p-phenylenediamine. 
The non-aqueous portion of the recovery stream had the following analysis: 
______________________________________ 
MIBK - 96.6% 
methyl isobutyl carbinol 
(MIBC) - 1.8% 
MIAK - 1.1% 
methyl isoamyl carbinol 
(MIAC) - 0.5% 
______________________________________ 
In this instance, not all of the MIAK reacted, hence a small portion (about 
1% of the charge) remained in the crude product and was distilled off. 
EXAMPLE IV 
In order to compare the process of the invention with the method shown in 
U.S. Pat. No. 3,542,691, an autoclave as in Example I above was charged 
with the following materials: 
______________________________________ 
4-Aminodiphenylamine 
92.0 g (0.5 mole) 
methyl isoamyl ketone 
162.6 g (1.43 mole) 
methyl isobutyl ketone 
99.8 g (1.0 mole) 
______________________________________ 
Equivalent amounts of the platinum-on-carbon catalyst and the acidic carbon 
co-catalyst were also charged, the reactor was purged and pressured to 650 
p.s.i. (4.5 MPa) with hydrogen. The temperature of the contents of the 
autoclave was increased to 175.degree. C., and held at about that 
temperature for 2.5 hours, until the reaction was completed. The following 
materials were recovered, in the amounts indicated: 
______________________________________ 
N--(1,3-dimethylbutyl)-N'--phenyl-p-phenylene- 
diamine - 30.9 g (.115 m) 
N--(1,4-dimethylamyl)-N'--phenyl-p-phenylene- 
diamine - 108.6 g (.385 m) 
______________________________________ 
The crude distillate contained the following mixture: 
______________________________________ 
methyl isobutyl ketone - 87.5 g 
(.875 m) 
methyl isoamyl ketone - 117.8 g 
(1.034 m) 
methyl isobutyl carbinol - 1.0 g 
(.010 m) 
methyl isoamyl carbinol - 1.3 g 
(.0114 m) 
water - about 9.0 g. 
______________________________________ 
The water was separated from the mixture, but the resulting mixture of 
ketones and alcohols was not practical to separate, because of the 
closeness of the boiling ranges of the materials, as shown: 
______________________________________ 
Boiling Point, .degree.C. 
______________________________________ 
methyl isobutyl ketone 
117-119 
methyl isobutyl carbinol 
139-140 
methyl isoamyl ketone 
143-144 
methyl isoamyl carbinol 
148-150 
______________________________________ 
The difficulties inherent in the method of U.S. Pat. No. 3,542,691 are 
several. First, since a large excess of ketones is used, the desired ratio 
of the product paraphenylenediamines is difficult to maintain. Also, the 
large excess of ketone limits the payload of the reactor. Second, 
recycling of the unreacted ketones would cause a build-up of by-product 
alcohols in the system, further reducing the payload. (Recycling would be 
necessary, since 78% of the ketones charged would be otherwise lost.) And, 
as has been pointed out, separation of the alcohols would be prohibitive, 
requiring a column having in excess of 100 theoretical plates. 
The products of the process of the invention find use as antidegradants for 
polymers, and especially as antiozonants for diene rubber. Their inclusion 
in rubber compounds in the amount of from 0.5 to 5 parts by weight per 100 
parts by weight of rubber gives excellent protection from the degrading 
effects of ozone, especially in tire sidewall applications. 
The products also find use in synthetic rubber as a stabilizer during the 
recovery, drying and storage of the rubber. They are also useful as 
inhibitors of polymerization for monomeric materials such as unsaturated 
carboxy acids and their esters. 
Although the invention has been illustrated by typical examples, it is not 
limited thereto. Changes and modifications of the examples of the 
invention herein chosen for purposes of disclosure can be made which do 
not constitute departure from the spirit and scope of the invention.