Process for producing 2,6-dichloro-3-nitropyridine

A process for producing 2,6-dichloro-3-nitropyridine by reacting 2,6-dichloropyridine with nitric acid in the presence of about 10-65% strength by weight oleum.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process for producing 
2,6-dichloro-3-nitropyridine by an improved nitration process of 
2,6-dichloropyridine. 
2. Description of the Prior Art 
It has been reported that 2,6-dichloro-3-nitropyridine has useful 
herbicidal properties. Furthermore, this compound is useful as a chemical 
intermediate for dyes, pharmaceutical and other agricultural applications. 
It is known that 2,6-dichloropyridine could be nitrated with a mixture of 
HNO.sub.3 and H.sub.2 SO.sub.4 to give 2,6-dichloro-3-nitropyridine. See 
Johnson et al, J. Chem. Soc. (B), 1967, pp. 1204-1210 and U.S. Pat. No. 
3,809,695, issued to Steinmetz et al on May 7, 1974. However, the 
nitration reactions disclosed by these prior art references employed 
relatively high molar ratios of HNO.sub.3 to 2,6-dichloropyridines (i.e., 
both were over 10:1) to effect the nitration. Also, these inventions were 
accompanied by the evolution of hazardous brown nitrogen oxides from the 
reaction mixture. On large commercial scale operations, the employment of 
this relatively high molar ratio and evolution of hazardous fumes are both 
undesirable because of the low productivity associated with the high 
levels of HNO.sub.3 needed and possible environment problems, 
respectively. 
Accordingly, it would be beneficial if the conditions of this nitration 
reaction could be improved so that lower molar ratios of HNO.sub.3 to 
2,6-dichloropyridine could be employed and little or no evolution of 
nitrogen oxides occurred during the reaction. 
BRIEF SUMMARY OF THE INVENTION 
The present invention is, therefore, directed to a process for producing 
2,6-dichloro-3-nitropyridine by reacting 2,6-dichloropyridine with nitric 
acid in the presence of about 10-65% strength oleum by weight. The 
employment of the oleum allows lower molar ratios of HNO.sub.3 to 
2,6-dichloropyridine (e.g., about 1.5:1) to be employed and little or no 
evolution of nitrogen oxides may occur. 
DETAILED DESCRIPTION 
The reaction of the present invention is believed to involve the in-situ 
formation of a 2,6-dichloropyridine:SO.sub.3 complex in the oleum. This 
reaction is illustrated as follows: 
##STR1## 
The reactants of the present invention may be drawn from any commercially 
available source. For example, 2,6-dichloropyridine may be obtained as the 
major by-product from processes which produce 2-chloropyridine. See U.S. 
Pat. Nos. 2,820,791, and 3,153,044, which issued to Shermer and Zaslowsky 
on Jan. 21, 1958, and Oct. 13, 1964, respectively. Preferably, the purity 
of the 2,6-dichloropyridine should be above about 98% by weight when 
pharmaceutical end uses are desired. But, when agricultural uses and the 
like are intended, the purity of the 2,6-dichloropyridine may be lower 
(e.g., above about 85% by weight). When the 2,6-dichloropyridine is 
obtained as a by-product from a process for producing 2-chloropyridine, 
the usual impurities associated with it may include the 2-chloropyridine, 
other isomeric dichloropyridines like 2,5-dichloropyridine, 
2,3-dichloropyridine, 2,4-dichloropyridine and water. 
Nitric acid (HNO.sub.3) is also readily available from many sources. For 
example, suitable sources of this reactant include pure nitric acid (i.e., 
above about 95% by weight) or from a commercial "mixed acids" composed of 
H.sub.2 SO.sub.4 /HNO.sub.3. In these mixed acids, suitable amounts of 
HNO.sub.3 will normally include from about 10% to about 40% by weight 
HNO.sub.3 with the balance being mostly H.sub.2 SO.sub.4. Again, a high 
purity of this reactant is also preferred if the end use will be in 
pharmaceuticals. Thus, in that case, the purity of a suitable mixed acid 
will usually be composed of at least about 98% by weight H.sub.2 SO.sub.4, 
HNO.sub.3 and H.sub.2 O. Also, while this invention is primarily concerned 
with nitric acid being employed as the nitration agent, it may be possible 
in some instances to employ other conventional nitration agents like 
KNO.sub.3 and NaNO.sub.3. 
The oleum employed in the present invention may be also obtained from any 
commercial source. The strength of the oleum should be from about 10% to 
about 65% by weight. The preferred range of oleum strength is from about 
50% to about 65% by weight. The strength of oleum is designated as the 
weight percent of free sulfur trioxide (SO.sub.3) in the acid solution. 
Thus, 10% oleum contains 10% by weight SO.sub.3 and 90% by weight H.sub.2 
SO.sub.4. And, 65% oleum contains 65% by weight SO.sub.3 and 35% by weight 
H.sub.2 SO.sub.4. 
In measuring the strength of the oleum for purposes of this invention, it 
is necessary to know the relative total amounts of SO.sub.3 and H.sub.2 
SO.sub.4 in the reaction mixture. Thus, if a mixed acid system (i.e., 
mixture of HNO.sub.3 /H.sub.2 SO.sub.4) is used as the source of nitric 
acid, this additional H.sub.2 SO.sub.4 must be calculated into the total 
oleum strength. 
These reactants may be combined by any conventional mode of addition 
including those methods shown in the following Examples. Specifically, it 
may be desirable to first add the 2,6-dichloropyridine and the oleum to 
the reactor, followed by slow addition of the nitric acid. Conversely, it 
is also possible to first add the nitric acid and oleum to the reactor, 
followed by the addition of the 2,6-dichloropyridine. Furthermore, the 
present invention also covers the in-situ formation of oleum e.g., liquid 
SO.sub.3 is added to the reaction mixture, then followed by addition of 
2,6-dichloropyridine. Also, the present invention includes the process 
where 2,6-dichloropyridine and nitric acid are combined at a relatively 
low temperature (e.g., from about 0.degree. C. to 20.degree. C.) and then 
oleum is added, followed by the increase in reaction temperature to a 
suitably high level. 
The molar ratio of nitric acid to 2,6-dichloropyridine as reactants is not 
critical to the invention and any suitable ratio may be employed. 
Preferably, it may be suitable to employ a molar ratio of nitric acid to 
2,6-dichloropyridine in the range from about 1:1 to about 20:1; more 
preferably, from about 1.5:1 to about 10:1; and most preferably, from 
about 2:1 to about 6:1. It is advantageous to employ excess HNO.sub.3 
because it will ensure complete reaction of the 2,6-dichloropyridine and 
possibly prevent substantial purification and product contamination 
problems. However, it should be noted suitable molar ratios for the 
process of this invention may be far less than the prior art methods 
discussed above which used only H.sub.2 SO.sub.4, and not oleum. 
Specifically, those prior art methods used molar ratios of at least 10:1 
nitric acid to 2,6-dichloropyridine, whereas the present invention may use 
more commercially advantageous ratios of about 1.5:1 to about 2:1. 
The amount of oleum that may be used in the present process should 
preferably be enough to complex fully with the 2,6-dichloropyridine. Thus, 
it is preferred that at least about 1:1 molar ratio between the SO.sub.3 
and 2,6-dichloropyridine be employed. More preferably, this molar ratio of 
SO.sub.3 to 2,6-dichloropyridine may be in the range of from about 2:1 to 
about 6:1, although larger ratios may also be desirable. 
Any combination of reaction temperatures and times sufficient to convert a 
commercially acceptable amount of 2,6-dichloropyridine to 
2,6-dichloro-3-nitropyridine may be employed. Usually, temperatures in the 
range of about 85.degree. C. to about 150.degree. C. may be employed. 
Times in the range from about 60 minutes to 600 minutes also may be used. 
It should be understood that the reaction conditions, other than the 
reactants and employment of oleum, are not critical to the present 
invention and one having ordinary skill in the art would be able to select 
the optimum reaction temperatures and times. Of course, the optimum 
reaction temperature and time depend on many factors, including equipment 
being employed, molar ratio of reactants, reaction pressure and the like. 
Any suitable method for removing the desired product, 
2,6-dichloro-3-nitropyridine may be utilized. One preferred method is to 
first add water (or vice-versa) to the reaction mixture after completion 
of the reaction to cause precipitation of the desired product. This 
precipitated product is then removed from the resulting solution by 
filtration. The filtrate containing the product is then water-washed to 
remove any occluded nitric and sulfuric acids. The resulting product can 
then be easily dried by any conventional means. It should be noted that 
the addition of water to the reaction mixture may convert any SO.sub.3 in 
the oleum into H.sub.2 SO.sub.4. Thus, this water quenching purification 
removes the SO.sub.3 without the need for an additional separation step. 
Also, it should be appreciated that when the product will be used for 
pharmaceutical or other high-purity applications, further purification 
such as with conventional recrystallization techniques may be employed. 
While this invention is primarily directed toward the nitration of 
2,6-dichloropyridine in the presence of oleum, the inventive concept 
should be effective with other dichloropyridines such as 
2,3-dichloropyridine; 2,4-dichloropyridine; 2,5-dichloropyridine; 
3,4-dichloropyridine; and 3,5-dichloropyridine. Also, the present 
invention may be useful with corresponding dibromopyridines and 
difluoropyridines such as 2,6-difluoropyridine. But, it is believed that 
this oleum nitration technique is not readily useful for the nitration of 
monohalopyridines such as 2-chloropyridines.

The following examples and comparisons are given to further illustrate the 
present invention. All percentages and proportions are by weight unless 
otherwise explicitly indicated. 
EXAMPLE 1 
2,6-Dichloropyridine (0.20 mole; 29.6 grams; VPC assay: 99.9%; H.sub.2 O 
content, 0.24%) was added to 100 grams of 65% oleum (SO.sub.3 content, 
0.80 mole) at 0.degree. C. To this slurry was added 84.0 grams of 30% 
nitric acid/68% sulfuric acid (HNO.sub.3 content, 0.40 mole) over a 15 
minute period (5.degree. to 20.degree. C.). The mixture was heated at 
80.degree.-142.degree. C. over a 5.5 hour period. Virtually no brown fumes 
were evolved during the heat-up period. The additional H.sub.2 SO.sub.4 in 
the mixed acid lowered the oleum strength to about 16% by weight. 
The straw-colored nitration solution (20.degree. C.) was then transferred 
to 800 grams water (0.degree. C.) over a 20 minute period to precipitate 
3-nitro-2,6-dichloropyridine. The filter cake was washed with water to 
remove occluded nitric and sulfuric acid. The pale yellow precipitate was 
air-dried, wt. 28.4 grams (m.p. 54.5.degree.-60.5.degree. C.) and had a % 
H.sub.2 O of 0.56% by weight and a VPC assay as follows: 
______________________________________ 
VPC Moles 
______________________________________ 
2-6-Dichloropyridine 5.7% 0.011 
3-Nitro-2,6-Dichloropyridine 
93.3% 0.137 
______________________________________ 
The uncorrected yield of 3-nitro-2,6-dichloropyridine was 68.5%. Corrected 
for 94.5% conversion of 2,6-dichloropyridine, the adjusted yield is 72.5%. 
Purification of crude 3-nitro-2,6-dichloropyridine (wt. 24.5 grams) can be 
effected by recrystallization from a 50% aqueous i-propylalcohol solution 
(156 grams) to give 21.0 grams of product, m.p. 61.degree.-63.degree. C. 
VPC assay: 97.2% 3-nitro-2,6-dichloropyridine. 
EXAMPLE 2 
The conditions of Example 1 were repeated, except that a more narrow 
temperature range (94.degree.-112.degree. C./6.0 hours instead of 
80.degree.-142.degree. C./5.5 hours) were employed. Conversion of the 
2,6-dichloropyridine was lowered as the following results before 
recrystallization indicate: 
______________________________________ 
% 2,6-Dichloropyridine Conversion 
75.4% 
% 2,6-Dichloro-3-Nitropyridine Yield 
Uncorrected 55.7% 
Corrected 73.9% 
______________________________________ 
Again, virtually no brown fumes were evolved during the heat-up period. 
EXAMPLE 3 
2,6-Dichloropyridine (0.20 mole; 29.6 grams; VPC, 99.9%; H.sub.2 O, 0.24%) 
was added to 160 grams 20% oleum (SO.sub.3 content, 0.40 mole) at 
0.degree. C. To this cream-colored solution was added 63 grams of 30% 
nitric acid/68% sulfuric acid (HNO.sub.3 content, 0.30 mole) over a 20 
minute period (17.5.degree.-28.degree. C.). The mixture was heated at 
88.degree.-144.degree. C. (2 hours). Virtually no brown fumes were evolved 
during the heat-up period. Again, the additional H.sub.2 SO.sub.4 from the 
mixed acid lowered the oleum strength to about 16%. 
The straw-colored solution was processed by transfer to water (0.degree. 
C.), the 2,6-dichloro-3-nitropyridine was filtered and washed free of 
occluded acid and air-dried, wt. 28.7 grams. Assay: 2,6-dichloropyridine 
(22.2%--VPC, 0.043 mole; 78.5% conversion), 2,6-dichloro-3-nitropyridine 
(77.5%--VPC; 0.115 mole; 57.5% uncorrected yield; 73.2% corrected yield). 
EXAMPLE 4 
The conditions of Example 3 were repeated and the following results were 
obtained with the following reactant ratios: 
______________________________________ 
2,6-Dichloropyridine 0.20 mole 
SO.sub.3 in Oleum 0.30 mole 
Nitric Acid 0.22 mole 
______________________________________ 
Conversion of 2,6-dichloropyridine was 59.7%; Yield of 
2,6-dichloro-3-nitropyridine was 46% (uncorrected) and 77% (corrected). 
Again, virtually no brown fumes were evolved. 
EXAMPLE 5 
2,6-Dichloropyridine (0.20 mole; 29.6 grams; VPC, 99.8%; H.sub.2 O, 0.06%) 
was added to 100 grams of 65% oleum (SO.sub.3 content; 0.80 mole) at 
0.degree. C. To this mixture was added 19.4 grams of white fuming nitric 
acid (97.2% assay; 0.30 mole) over a 20 minute period. The nitration 
mixture was heated at 68.degree. to 134.degree. C. (5.5 hours). No brown 
fumes were evolved during this heating period. 
The straw-colored solution was then transferred to 800 grams of water 
(0.degree. C.) to precipitate 3-nitro-2,6-dichloropyridine. The filter 
cake was washed with water to remove occluded nitric acid and sulfuric 
acid. The precipitate was air-dried wt. 29.9 grams; m.p 
51.degree.-55.degree. C.; % H.sub.2 O 0.80%. 
Assay: 2,6-dichloropyridine (22.6%--VPC; 0.045 mole; 77.5% conversion); 
2,6-dichloro-3-nitropyridine (%75.4--VPC; 0.116 mole; 58.0% uncorrected 
yield; 74.8% corrected yield). 
EXAMPLE 6 
2,6-Dichloropyridine (0.20 mole; 29.6 grams; VPC, 99.8%; H.sub.2 O, 0.06%) 
was added to 160 grams 20% oleum (SO.sub.3 content; 0.40 mole) at 
0.degree. C. To this mixture was added 19.4 grams of white fuming nitric 
acid (97.2% assay; 0.30 mole) over a 5 minute period (8.degree. to 
15.degree. C.). The nitration mixture was heated at about 70.degree. to 
about 130.degree. C. (6 hours). No brown fumes were evolved during this 
heating period. 
The straw-colored solution was processed by transfer to 800 grams water 
(0.degree. C.), 2,6-dichloro-3-nitropyridine was filtered and washed free 
of occluded acid and air-dried, wt. 26.7 grams; m.p. 59.degree.-60.degree. 
C.; % H.sub.2 O 0.83%. Assay: 2,6-dichloropyridine (15.6%--VPC; 0.28 mole; 
86% conversion); 2,6-dichloro-3-nitropyridine (%84.3--VPC; 0.116 mole; 
58.0% uncorrected yield; 67.4% corrected yield. 
COMISON I 
2,6-Dichloropyridine (0.20 mole; 29.6 grams; VPC assay, 99.9%; H.sub.2 O 
content, 0.24%) was added to 548.0 grams of 30% nitric acid/68% sulfuric 
acid (HNO.sub.3 content: 2.6 moles) and the straw-colored solution heated 
to reflux (108.degree. C.) for 2.2 hours. Throughout this period, copious 
evolution of brown nitrogen oxides were continuously evolved. 
The nitration solution (20.degree. C.) was then transferred to 800 grams 
H.sub.2 O (0.degree. C.) over a 15 minute period to precipitate 
3-nitro-2,6-dichloropyridine. The filter cake was washed with water to 
remove occluded nitric and sulfuric acids. The yellow precipitate was air 
dried, wt. 30.0 grams (m.p. 61.degree.-64.degree. C.) and had a %H.sub.2 O 
of 1.05% and VPC assay as follows: 
______________________________________ 
VPC Moles 
______________________________________ 
2,6-Dichloropyridine 2.9% 0.006 
3-Nitro-2,6-Dichloropyridine 
97.1% 0.149 
______________________________________ 
The uncorrected yield of 3-nitro-2,6-dichloropyridine was 74.5%. Corrected 
for 97.0% 2,6-dichloropyridine conversion, the adjusted yield is 76.8%. 
COMISON II 
2,6-Dichloropyridine (0.20 mole; 29.6 gms.; VPC assay, 99.8%; H.sub.2 O 
content, 0.06%) was added to 84.0 gms. of 30% nitric acid/68% sulfuric 
acid (HNO.sub.3 content; 0.40 mole) and the straw-colored solution heated 
at 84.degree.-142.degree. C. over a 5.5 hour period. Throughout this 
heating cycle, copious evolution of brown nitrogen oxide fumes were 
continuously evolved. 
The nitration solution (10.degree. C.) was then added to 800 ml. H.sub.2 O 
(0.degree. C.) to precipitate organics. The filter cake was washed with 
500 ml. to remove occluded nitric and sulfuric acids. The white 
precipitate was air dried, wt. 23.0 gms., m.p. 84.degree.-86.degree. C. 
and had %H.sub.2 O of 14.5% and VPC assay as follows: 
______________________________________ 
VPC 
______________________________________ 
2,6-Dichloropyridine 99.7% 
3-Nitro-2,6-Dichloropyridine 
0.3% 
______________________________________ 
This comparison shows that low molar ratios of HNO.sub.3 
:2,6-dichloropyridine (in the absence of oleum) will be unable to 
sufficiently cause conversion to 2,6-dichloro-3-nitropyridine.