Purification of N,N'-disubstituted p-phenylenediamine antiozonants and oil stabilizers

Process for purifying N,N'-disubstituted p-phenylenediamine antiozonants and oil stabilizers utilizes a steam-distillation step to remove Schiff's base derivatives of p-phenylenediamine and N-monosubstituted p-phenylenediamine as well as aldehyde and ketone contaminants and corresponding alcohols thereof.

This invention relates to a process for preparing substantially pure N,N' 
-dialkyl, and N-alkyl-N' aryl disubstituted p-phenylenediamine 
antiozonants and oil stabilizers, and more particularly relates to a 
method for quality upgrading of such products by economical removal of 
contaminants. 
The p-phenylenediamine-type compounds are widely used in the rubber 
industry as age resistors and for the protection of natural and synthetic 
elastomers against ozone, especially under dynamic conditions. These 
compounds are also useful as additives for oil and gasoline wherein they 
act as stabilizers. 
A typical commerical process for the preparation of N,N'-disubstituted 
p-phenylenediamines is the reductive alkylation of p-phenylenediamine or 
N-monosubstituted p-phenylenediamines with excess of aldehydes or ketones. 
Examples of the foregoing preparations using a platinum type catalyst are 
shown in U.S. Pat. No. 3,336,386 and with a nickle type catalytic system 
are described in U.S. Pat. No. 3,366,684. 
Although the alkylation actually comprises two chemical reactions, the 
first being the formation of a Schiff's base and the second the 
hydrogenation of the Schiff's base, for economical reasons the reaction is 
performed as a "one step" process without isolating the intermediates and 
without removing all byproducts or residual amounts of reactants. That is 
along with the N,N' 1-disubstituted p-phenylenediamine product, there are 
present small amounts of unreacted p-phenylenediamine or N-monosubstituted 
p-phenylenediamine, aldehyde or ketone whichever is the case, mono- and 
di- Schiff's base and alcohol. The final product is usually isolated as 
distillation residue of the reaction mixture by stripping off the water 
and alcohol formed as byproducts and the excess ketone or aldehyde. 
Extensive removal of contaminants, especially those of moderately high 
boiling points, such as the Schiff's bases, by vacuum distillation is 
necessarily quite expensive. That is, vacuum distillation would result in 
the removal of large amounts of useful product at a relatively slow rate 
of distillation. In contradistinction, a distillation at atmospheric 
pressure, which is more favorable for separation of products having close 
boiling points, would require higher temperatures and hence produce still 
additional contamination by cracking. Purification by crystallization is 
entirely out of the question for all practical purposes because of the 
excessive costs involved. 
In the utilization of the N,N' -disubstituted p-phenylenediamines, a 
substantially pure product is frequently not only desirable but necessary 
as when further halding steps are required as when used as an oil additive 
or as an antiozonant. In the case of the use of these materials in a 
liquid state as oil additives, contaminants can cause relatively rapid 
change in oil viscosities. In the case of solid antiozonants which are to 
be compounded into nervy rubber elastomers on a mill or internal mixer, it 
is preferable that such materials be in a free-flowing flake form so as to 
facilitate economical and safe handling. Contamination in N,N' 
-disubstituted p-phenylenediamine materials having a melting points 
approximately twenty to forty degrees above ambient temperature, for 
example N-(1,3 dimethylbutyl)-N'-phenyl-p-phenylenediamine (m.p. 50- 
52.degree. C, b.p. 380.degree. C), so depresses the melting point of these 
products that a semisolid mushy consistency is effected at ordinary 
temperatures, thereby presenting difficulty in economical weighing and 
application accuracy and in avoiding material losses. As a consequence, 
the caked mushy masses must be warmed or kept warm to a point that they 
can be utilized in a liquid state as an antiozonant. In the case of the 
use of these materials in a liquid state as oil additives, contamination 
can cause relatively rapid change in oil viscosity. That is, Schiff's 
bases are quite sensitive to hydrolysis under normal storage conditions 
and thus may be the source of relatively low boiling carbonyl compounds. 
The latter could for example cause not only undesirable changes in oil 
viscosities but also contribute to the formation of bubbles during the 
curing procedure of elastomers. Furthermore, in the case of solid 
antiozonants which are to be compounded into rubberlike elastomers on a 
mill or in an internal mixer, it is preferable that these additives be in 
a free-flowing flake form to facilitate handling. 
It is therefore an object of this invention to provide an improved method 
for making substantially pure, N,N' -disubstituted p-phenylenediamine from 
contaminated products thereof containing Schiff's base derivatives of 
p-phenylenediamine and N-monosubstituted p-phenylenediamine. 
Other objects of this invention are to provide an improved method of the 
character described which is easily and economically accomplished and both 
highly efficient and effective in operation. 
The reactions which produce the material to which the present invention 
relates may be represented by equations A and B: 
##STR1## 
where R.sub.1 is an aliphatic or aromatic group, R.sub.2 is an aliphatic 
group or hydrogen and R.sub.3 is an aliphatic group. 
No attempt is made herein to list all of the specific examples of operative 
reactants i.e. p-phenylenediamine (Ia) or N'-monosubstituted 
p-phenylenediamine (I) and aldehydes and ketones (II) of from about 2 to 
18 carbons or of the N,N' -disubstituted p-phenylenediamine products (IV) 
or (IVa) which result. It is sufficient to indicate that excess aldehyde 
or ketone (II) is utilized in the reaction and that mono-Schiff's bases 
(III) or (IIIa) or di-Schiff's bases (IIIb) are the product contaminants 
which the process of the present invention encompasses. 
The Schiff's base (III, III-a or III-b, see equations A and B) can be 
present in the final reaction mixture not only by virtue of incomplete 
hydrogenation at the time the hydrogenation is discontinued but also and 
especially due to a subsequent Schiff's base formation under the 
relatively favorable conditions of the last phase of the process. In the 
"one step" process for manufacturing of IV or IV-a, the water formed is 
not removed from the reaction mixture until the hydrogen pressure is 
relieved, thus until the hydrogenation is discontinued. The presence of 
water, however, is detrimental to the exhaustive conversion of I or I-a to 
Schiff's bases III, III-a or III-b. Thus some unreacted I (or I-a) is 
present in the reaction mixture subsequently subjected to the stripping 
operation at elevated temperature to distill off the water and alcohol 
formed as well as the excessive carbonyl reactant II, R.sub.2 R.sub.3 
C(O). This operation facilitates an additional Schiff's base formation by 
the reaction of thusfar unreacted I or I-a with the carbonyl compound II 
which is used in excess. The Schiff's base formed at this stage of the 
process remains in the distillation residue representing the crude product 
.

The following examples are given to illustrate the invention: 
EXAMPLE I 
A mixture of 500g. (5 moles) 4-methyl-2-pentanone and 184 g. (1 mole) 
N-phenyl-p-phenylenediamine containing 10 g. copper chromite catalyst was 
reacted at 150.degree.-160.degree. C and 1000 psi hydrogen pressure until 
the hydrogen uptake became very slow. After filtering off the catalyst and 
topping off the filtrate at 100 mm. and 100.degree. C, 208 g. still pot 
residue of crude product 
N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine was obtained, while the 
excess 4-methyl-2-pentanone, and the water and 4-methyl-2-pentanol 
byproducts were stripped off; no attempt was made to isolate the Schiff's 
base III [R.sub.1 .dbd.C.sub.6 H.sub.5, R.sub.2 .dbd.CH.sub.3, R.sub.3 
.dbd.--CH.sub.2 CH(CH.sub.3).sub.2 ]. The antiozonant product 
N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (IV) [R.sub.1 
.dbd.C.sub.6 H.sub.5, R.sub.2 .dbd.CH.sub.3, R.sub.3 .dbd.CH.sub.2 
CH(CH.sub.3).sub.2 ] was soft and mushy, melting at 43.degree.-50.degree. 
C; when a sample of this product was kept at 46.degree.-47.degree. C. for 
one hour, most of the sample liquefied, and when held for 23 hours at 
40.degree.-42.degree. C. or for 3.3 hours at 44.degree.-45.degree. C, much 
of it melted. 
EXAMPLE 2 
A 150 g. portion of the product of Example I was subjected to 
steam-distillation by way of superheated (180.degree. C) steam-stripping. 
The still pot temperature was 102.degree. C. at the start of the 
steam-stripping and 175.degree. C. when terminated. A total of 161 g. 
aqueous distillate containing 4-methyl-2-pentanone was collected. The 
product residue weighed 147.6 g. (1.6% removed) and the melting range was 
48.degree.-50.degree. C. This product is easily flaked and the flakes 
retained their form for 23 hours at 40.degree.-42.degree. C., or for at 
least 3.3 hours at 44.degree.-45.degree. C. or for at least 1 hour at 
46.degree.-47.degree. C. 
EXAMPLE 3 
Another 150 g. sample of the product of Example 1 was subjected to 
180.degree. C. superheated steam-stripping as in Example 2 until a total 
of 500 g. of aqueous distillate was collected. The distillate contained 4 
g. of an oil layer, part of which was identified by infra-red analysis as 
4-methyl-2-pentanone. The still pot residue product weighed 145.8 g. (2.8% 
removed) and the melting range thereof was 48.degree.-50.degree. C. The 
steam-stripped product residue was easily flaked, and exhibited 
substantially the same aging characteristics as the product residue under 
Example 2. 
EXAMPLE 4 
Still another 150 g. sample of the product obtained by way of Example 1 was 
subjected to superheated steam-stripping at 180.degree. C., as described 
in Example 2, until 864 g. distillate containing 4-methyl-2-pentanone was 
collected. The still pot residue product weighed 140.3g. (6.5% removed), 
and the melting range of the product was 48.degree.-50.degree. C. The 
product was easily flaked and exhibited the same aging characteristics as 
that of Example 3. 
A summary of the physical characteristics after aging of 
N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine obtained commercially 
in crude form as product IV[R.sub.1 .dbd.C.sub.6 H.sub.5, R.sub.2 
.dbd.CH.sub.3, R.sub.3 .dbd.CH.sub.2 CH(CH.sub.3).sub.2 ] compared with 
purified flaked product IV following steam-stripping in accordance with 
this invention is shown in Table I below: 
TABLE I 
__________________________________________________________________________ 
Aging of Crude Commercial and Purified Flaked IV[R.sub.1 = C.sub.6 
H.sub.5, R.sub.2 = CH.sub.3, 
R.sub.3 = CH.sub.2 CH(CH.sub.3).sub.2 ] 
1.6% 2.8% 6.5% 
Temperature 
Time 
Commercial IV 
Steam-Stripped 
Steam-Stripped 
Steam-Stripped 
.degree. C. 
Hours 
m.p. 45-50.degree. C. 
Flaked IV 
Flaked IV 
Flaked IV 
__________________________________________________________________________ 
23.0 
Partly melted, 
Free Flowing, 
Free Flowing, 
Free Flowing, 
40-42 partly caked. 
slightly caked. 
slightly caked. 
slightly caked. 
3.0 Some melting Free Flowing. 
and caking 
-- no caking. -- 
44-45 3.3 Partly melted, 
Free Flowing, 
Free flowing, 
Free Flowing, 
partly caked. 
slightly caked. 
slightly caked. 
slightly caked. 
46-47 1.0 Mostly melted, 
Free flowing, 
Free flowing, 
Free flowing, 
the remainder 
slightly caked. 
slightly caked. 
slightly caked. 
caked. 
__________________________________________________________________________ 
When a sample of the N-(1,3-dimethylbutyl)-N-phenyl-p-phenylenediamine 
(product IV) purified in accordance with Example 3 was contaminated with 
5% by weight of Schiff's base III, i.e. 
N-(1,3-dimethylbutylidene)-N-phenyl-p-phenylenediamine the resulting 
mixture had a m.p. 46.degree.-49.degree. C. After storage for seven days 
at room temperature (20.degree.-25.degree. C.) and high humidity, the m.p. 
range of the contaminated product widened to 42.degree.-47.degree. C. When 
a sample of the foregoing purified product IV was contaminated with 5% by 
weight of Schiff's base as above, the mixture caked-up after 3 hours at 
40.degree.-42.degree. C. As may be seen from Table II below, this behavior 
is characteristic of the unpurified form of commercially available 
material IV [R.sub.1 .dbd.C.sub.6 H.sub.5, R.sub.2 .dbd.CH.sub.3, R.sub.3 
.dbd.CH.sub.2 CH(CH.sub.3).sub.2 ]. A mixture of 95% of purified product 
IV, prepared in accordance with Example 3, to which 5% of 
N-phenyl-p-phenylenediamine [(I) R.sub.1 .dbd.C.sub. 6 H.sub.5 ] had been 
added, had a m.p. 48.degree.-50.degree. C. which did not change 
appreciably after storage for seven days at a temperature of 
20.degree.-25.degree. C. and high humidity. 
The foregoing results indicate that contamination of the product with 
Schiff's base is undesirable in solid antiozonants. 
TABLE II 
______________________________________ 
Aging at 40-42.degree. C. of Blend IV (2.8% steam-stripped) with III 
[R.sub.1 = C.sub.6 H.sub.5, R.sub.2 = CH.sub.3, R.sub.3 = CH.sub.2 
CH(CH.sub.3).sub.2 ] 
Blend of 95% 
of 2.8% Steam 
Time 2.8% Steam Stripped IV with 
in hrs. 
Commercial IV 
Stripped IV 5% of III 
at (melting range, 
(Melting range 
(Melting range, 
40-42.degree. C 
43-50.degree. C) 
49-52.degree. C.) 
47-50.degree. C) 
______________________________________ 
Not caked, 
2 Moderately caked, 
free flowing, 
Moderately caked; 
ketone odor No ketone odor 
ketone odor 
Not caked, 
3 Moderately caked, 
free flowing 
Extensively caked; 
ketone odor. 
No ketone odor. 
ketone odor. 
______________________________________ 
EXAMPLE 5-N,N' -bis(1-methylheptyl)-p-phenylenediamine 
A mixture of 1024 g. (8 moles) of 2-octanone and 108 g. (1 mole) 
p-phenylenediamine was reacted and worked up in the same manner as set 
forth generally in Example 1 to yield 333 g. (100%) crude N, 
N'-bis(1-methylheptyl)-p-phenylenediamine product [IVa, R.sub.2 
.dbd.CH.sub.3, R.sub.3 .dbd.CH.sub.2 (CH.sub.2).sub.4 CH.sub.3 ]. 
A 150 g. portion of the product was steam-stripped with superheated steam 
at 150.degree. C. in the manner of that of Example 3 leaving a liquid 
still pot residue of 145.5 g. (3% removed). The small oil layer of the 
distillate contained 2-octanone. The complete removal of the Schiff's base 
can be accomplished by continuing the steam distillation until no more 
2-octanone co-distills. 
EXAMPLE 6-N,N' -bis(1-ethyl-3-methylpentyl-p-phenylenediamine 
A mixture of 768 g. (6 moles) 5-methyl-3-heptanone and 108 g. (1 mole) 
p-phenylenediamine was reacted and worked up in the same manner as that of 
Example 1 to give 330 g. (99%) residue of crude 
N,N'-bis(1-ethyl-3-methylpentyl)-p-phenylenediamine product [IVa, R.sub.2 
.dbd.C.sub.2 H.sub.5, R.sub.3 .dbd.CH.sub.2 CH(CH.sub.3)C.sub.2 H.sub.5 ] 
containing some mono-Schiff's base III-c and some di-Schiff's base III-d. 
A 150 g. portion of the product was stripped with superheated steam 
(130.degree. C) in the manner of that of Example 3 to leave a still pot 
residue of 144.2 g. liquid (3.9% removed). The small oil layer of the 
distillate contained 5-methyl-3-heptanone. Continuing the steam 
distillation until 5-methyl-3-heptanone no longer co-distills assures 
complete removal of the Schiff's base from the product residue. 
EXAMPLE 7 
A 50 g. sample of N,N' -di(1-ethyl-3-methylpentylidene)-p-phenylenediamine 
IIId, equivalent to the Schiff's base IIIb prepared by the reaction of 
5-methyl-3-heptanone with p-phenylenediamine was stripped with superheated 
steam (160.degree. C.) as described in Example 2 until a 446 g. distillate 
was collected. The distillate contained 4.7 g. of an oil layer which was 
found to be primarily 5-methyl-3-heptanone. 
It can be shown that the well known oil additive and elastomer antiozonant, 
N,N'-bis(1-ethyl-3-methylpentyl)-p-phenylenediamine contains varying 
amounts of mono-Schiff's base IIIc and di-Schiff's base IIId respectively. 
##STR2## 
Example 7 demonstrated that when pure IIId is treated with steam, the 
N,N'-di(1-ethyl-3-methylpentylidene)-p-phenylenediamine hydrolizes to form 
5-methyl-3-heptanone which codistills with the steam. Similarly, the steam 
distillation of Example 6 until no further 5-methyl-3-heptanone is 
co-distilled would provide a product residue which is free of not only the 
mono- and di-Schiff's bases, IIIc and IIId, respectively, but also of the 
ketone. Accordingly, the purified form of the N,N' disubstituted 
p-phenylenediamine oil additive would not be subject to degradation at 
ambient temperature and humidity since there would be no Schiff's base, 
ketone nor alcohol derivatives of the ketone present to lower the 
viscosity of the oil per se. 
Although this invention has been described in considerable detail, such 
description is intended as being illustrative rather than limiting, since 
the invention may be variously embodied, and the scope of the invention is 
to be determined as claimed.