PROCESS FOR PRODUCING 2,3,5,6-TETRACHLOROPYRIDINE

Abstract: In accordance with the present invention, 2,3,5,6-tetrachloropyridine is advantageously prepared by contacting pentachloropyridine in a solvent selected from alkylnitriles, water, alkylsulfoxides, tetramethylsulfone, C-1 to C-5 alcohols, alkyl carbonates and mixtures thereof, in the presence of an ammonium salt of an organic or inorganic acid with 0.7 to 1.1 g atoms of zinc per mole of pentachloropyridine.

Description: The present invention provides a process for preparing 2,3,5,6-tetrachloropyridine by reacting pentachloropyridine and zinc in a solvent in the presence of an ammonium salt. US-A-3993654 discloses a process for preparing tetrachloropyridine by reacting pentachloropyridine, zinc and HCl, wherein the reaction is carried in an aequous medium at a temperature of at least 110°C and at least autogeneous pressure. From US-A-4259495 a process for producing 2,3,5,6-tetrachloropyridine by dechlorination of pentachloropyridine is known, which process comprises reacting pentachloropyridine in an alkane phosphonic acid dialkyl ester. Subject matter of the present invention is a process for preparing 2,3,5,6-tetrachloropyridine according to claim 1. Preferred embodiments of this process are subject matter of claims 2 to 8. Preferably, the solvent employed is an alkylnitrile, alkylsulfoxide, or a mixture thereof with water. Most preferably, the alkylnitrile solvent is acetonitrile and the alkylsulfoxide solvent is dimethylsulfoxide (DMSO). Preferably, the ammonium salt is ammonium chloride. Also preferred is that the process employs about 1.0 g atom of zinc per mole of pentachloropyridine. The process of the present invention has the advantage of utilizing solvents which are readily separated from the desired product and which lend themselves to conventional recycling procedures. The present process also has the unobvious advantage of utilizing zinc more efficiently than disclosed in previously known methods. Readily recyclable solvents suitable for contacting pentachloropyridine with the requisite ammonium salt and zinc reactants should be those solvents which can readily be separated from the reaction mixture after completion of the reaction. Representative solvents suitable for such contacting include alkylnitriles, water, alkylsulfoxides, tetramethylensulfone, C-1 to C-5 alcohols, and mixtures thereof. Suitable alkylnitriles which can be used according to the invention as solvents are of the formula R-C≡N, wherein R is C-1 to C-5 alkyl. Suitable alkylsulfoxides which can be used according to the invention as solvents are C-1 to C-5 alkylsulfoxides, more preferably C-1 to C-3 alkylsulfoxides and most preferably dimethylsulfoxide (DMSO). Water, at least equivalent in quality to that used for industrial purposes, is a solvent which can be used according to the invention. C-1 to C-5 alcohols which can be used according to the invention as solvents include, for example, methanol, ethanol, n-proponal, t-propanol, n-butanol, iso-butanol and n-pentanol, preferably iso-butanol. The process of the present invention can be conducted using the above-mentioned solvents in about technical grade or better quality. In certain instances, mixtures of the above solvents can be advantageously employed. Preferred mixtures include. acetonitrile and water; and dimethylsulfoxide and water. The amount of solvent to be employed in the process of the present invention is expressed as a ratio of pentachloropyridine to solvent. Generally, the process is conducted using, on a weight percentage basis, a ratio of 1:2 to 1:200 or more (pentachloropyridine:solvent), preferably in a range of 1:4 to 1:20. Most preferably, the process is conducted employing a saturated solution of pentachloropyridine in the solvent, using sufficient solvent to just dissolve all the pentachloropyridine. Within the temperature range of 50° to 175°C, in which the process according to the invention can preferably be performed, temperatures of 78° to 120°C are especially preferred. The quarternary ammonium salts which can be used in the present invention contain, as the cation, the ammonium ion, or the sterically feasible derivatives derived therefrom by partial or complete replacement of the hydrogen atoms by alkyl and/or phenyl groups, wherein the phenyl groups can be unsubstituted or substituted by 1 to 3 simple substituents such as alkyl, alkoxy or halogen. The ammonium salts usable according to the invention contain as anion the radical of any inorganic or organic acid capable of forming ammonium salts, such as a strong inorganic or organic acid with a pka of 2 or less. Advantageously applicable ammonium salts correspond to the formula: in which R₁, R₂, R₃ and R₄ are sterically feasible and can be identical or different and are each hydrogen, alkyl having 1 to 4 carbon atoms, or phenyl or substituted phenyl which is substituted by up to 3 halogens, or by alkyl groups having 1 to 4 carbon atoms or by alkoxy groups having 1 to 4 carbon atoms; X is an anion selected from the group: chloride, bromide, sulfate, hydrogen sulfate, phosphate, hydrogen phosphate, dihydrogen phosphate, acetate, propionate, butyrate, isobutyrate, oxalate, benzoate, benzenesulfonate and alkylsulfonates having 1 to 4 carbon atoms in the alkyl group; and n is 1 to 3 and corresponds to the number of negative charges of the respective anion X. The amount of ammonium salt to be employed in the current process is preferably in the weight ratio of 1:1 to 1:6 (pentachloropyridine:ammonium salt), more preferably from 1:2 to 1:4. Pressures within which the process of the present invention can be conducted range suitably from atmospheric up to the autogenous vapor pressure of the solvent at its boiling point. The zinc to be used according to the invention is finely divided in the form of zinc chips or granules or preferably in the form of zinc dust. The desired 2,3,5,6-tetrachloropyridine can be recovered from the reaction mixture by various procedures, depending in part on the type of solvent employed. Representative procedures include, but are not limited to one or more of distillation, extraction, phase separation, crystallization and filtration, preferably distillation and filtration where acetonitrile is employed as the solvent. The following examples further illustrate typical processes of the present invention. Example 1 - Acetonitrile Solvent and Ammonium Chloride To a flask are added 35 grams (g) (0.139 mole (mol)) of pentachloropyridine (PCP) and 199 g of acetonitrile. The mixture is stirred vigorously and heated to reflux (78 degrees Centigrade (°C)) to dissolve the PCP. To the reactive mixture is added 9.61 g (0.139 mol) of 93.7 percent pure metallic zinc dust, (g atom zinc:mole pentachloropyridine ratio is 1:1). A solution of 15.08 g (0.282 mol) ammonium chloride (NH₄Cl) in 40.2 g water is added drop-wise over a 45 minute (min) period. After agitation for another 2.25 hours (hr), 17.74 g of 12 normal (N) hydrochloric acid is added, a distillation head is attached to the reaction flask, and 164 g of material is distilled out at an overhead temperature of 77-78°C. To the pot is added 137.5 g of 6.25N hydrochloric acid and the mixture is stirred for 1 hr. The chloropyridines precipitate out of solution as a white solid and are removed by vacuum filtration prior to drying to a constant weight in a vacuum oven. Analysis of the chloropyridines by gas chromatography gives a molar selectivity of the pentachloropyridine converted of: 99.35 percent (%) desired product, viz., 2,3,5,6-tetrachloropyridine (28.977 g) 0.22% to 2,3,5-trichloropyridine (0.054 g), 0.01% to 2,3,4,5-tetrachloropyridine (0.003 g), and 0.42% to 2,3,4,6-tetrachloropyridine (0.122 g), at 96.75 percent pentachloropyridine conversion. Of the metallic zinc used in the reaction, 95.61 percent reacts with chloropyridine with 91.71 percent resulting in the formation of 2,3,5,6-tetrachloropyridine. Example 2 - Acetonitrile Solvent and Ammonium Acetate To a flask are added 35 g (0.139 mol) of pentachloropyridine, 21.5 g (0.28 mol) ammonium acetate, 40 g water, and 200 g acetonitrile and the compounds are heated to reflux (78°C) while being agitated vigorously. Subsequently, 9.75 g (0.140 mol) of 93.7 percent zinc dust, (g atom zinc:moles pentachloropyridine is 1:1), is added and the solution is refluxed for 3.58 hrs. After the reaction is complete, a distillation head is attached and 188 g of material is removed overhead. Water and perchloroethylene are added to the flask, partitioning the organic (341.2 g) and aqueous (336.2 g) phases. Analysis for chloropyridines in the organic phase by gas chromatography and for zinc in the aqueous phase by wet methods indicates a molar selectivity of the pentachloropyridine converted of: 96.90% to 2,3,5,6-tetrachloropyridine (27.899 g) 0.19% to 3,5-dichloropyridine (0.037 g), 0.46% to 2,3,5-trichloropyridine (0.111 g), 0.15% to 2,3,6-trichloropyridine (0.036 g), 0.07% to 2,3,4,5-tetrachloropyridine (0.020 g), 2.23% to 2,3,4,6-tetrachloropyridine (0.642 g), and at 95.07 percent pentachloropyridine conversion with 98.13 percent of the pentachloropyridine added accounted for in the final products. Of the metallic zinc used in the reaction, 95.61 percent reacts with chloropyridines with 91.72 percent resulting in the formation of 2,3,5,6-tetrachloropyridine. Example 3 - Water Solvent and Ammonium Chloride A flask containing 115 g (0.458 mol) of pentachloropyridine, 31.61 g (0.453 mol) of 93.7 percent metallic zinc dust, (g atom zinc:moles pentachloropyridine is 1:1), 49 g (0.906 mol) ammonium chloride, and 125 g water is heated to reflux (100°C) for 6.0 hrs while being agitated vigorously. The reaction mixture is quenched by adding 250 g of toluene and 280 g of water. Any solids remaining in the reaction mixture are filtered out (0.03 g) of the solution prior to the addition of 15 g of 12N hydrochloric acid, partitioning the aqueous and organic phases. Analysis for chloropyridines in the organic phase by gas chromatography and analysis for zinc in the aqueous phase by wet methods indicates a molar selectivity of the pentachloropyridine converted of: 96.88% to 2,3,5,6-tetrachloropyridine (90.073 g) 0.12% to 3,5-dichloropyridine (0.076 g), 0.96% to 2,3,5-trichloropyridine (0.751 g), 0.23% to 2,3,6-trichloropyridine (0.180 g), 0.06% to 2,3,4,5-tetrachloropyridine (0.056 g), 1.75% to 2,3,4,6-tetrachloropyridine (1.627 g), and at a 93.60 percent pentachloropyridine conversion with 97.12 percent of the pentachloropyridine added accounted for in the final product. Of the metallic zinc used in the reaction, 95.76 percent reacts with chloropyridine with 91.47 percent resulting in the formation of 2,3,5,6-tetrachloropyridine. Example 4 - Dimethylsulfoxide Solvent and Ammonium Chloride A flask containing 35.01 g (0.139 mol) of pentachloropyridine dissolved in 220 g dimethylsulfoxide is heated to 100°C. To the flask is added 9.62 g (0.139 mol) of 93.7 percent metallic zinc dust, (g atoms zinc:moles pentachloropyridine is 1:1). A solution of 9.73 g (0.182 mol) ammonium chloride in 29.6 g of water is added drop-wise with vigorous agitation over a 40 min period. The reaction mixture is maintained at 100°C for an additional 1.75 hr, filtered to remove any solids (0.2 g) and the chloropyridines are precipitated in the reaction mixture by adding 360 g water and 300 g of 6N hydrochloric acid. The chloropyridines are filtered out of solution and dried in a vacuum oven to constant weight (30.0 g). Analysis of the chloropyridines by gas chromatography indicates a molar selectivity of the pentachloropyridine converted of: 94.96% to 2,3,5,6-tetrachloropyridine (27.585 g) 0.29% to 2,3,5-trichloropyridine (0.071 g), 3.29% 2,3,4,5-tetrachloropyridine (0.956 g), and 1.46% 2,3,4,6-tetrachloropyridine (0.424 g), at 96.18 percent pentachloropyridine conversion with 99.66 percent of the pentachloropyridine added accounted for in the final product. Of the metallic zinc used in the reaction 97.37 percent reacts with chloropyridine with 92.20 percent resulting in the formation of 2,3,5,6-tetrachloropyridine. Example 5 - Isobutanol Solvent and Ammonium Chloride A flask containing 40.0 g (0.159 mol) of pentachloropyridine, 11.03 g (0.159 mol) of 93.7 percent metallic zinc dust (g atom zinc:moles pentachloropyridine is 1:1) and 135 g isobutanol is heated to reflux (92°C) and the reactants are vigorously agitated. A solution of 17.03 g (0.32 mol) ammonium chloride in 48.67 g water is added dropwise to the reaction mixture over 30 min. period. The reaction mixture is refluxed for 4.42 hrs, then quenched with 170 g toluene and 227 g water. Any solids present are filtered out, and the organic phase is separated from the aqueous phase. Analysis of the organic phase by gas chromatography indicates a molar selectivity of the pentachloropyridine converted of: 91.02% to 2,3,5,6-tetrachloropyridine (31.418 g) 0.19% to 3-chloropyridine (0.34 g), 0.54% to 3,5-dichloropyridine (0.127 g), 5.13% to 2,3,5-trichloropyridine (1.490 g), 0.26% to 2,3,6-trichloropyridine (0.075 g), 0.33% to 2,3,4,5-tetrachloropyridine (0.114 g), and 2.54% to 2,3,4,6-tetrachloropyridine (0.877 g), at 92.21 percent pentachloropyridine conversion with 98.94 percent of the pentachloropyridine added accounted for in the final product. Of the metallic zinc used in the reaction, 99.45 percent reacts with chloropyridine with 84.57 percent resulting in the formation of 2,3,5,6-tetrachloropyridine. Example 6 - Tetramethylene Sulfone and Ammonium Chloride A flask containing 35.01 g (0.139 mol) of pentachloropyridine, 9.62 g (0.139 mol) of 93.7 percent metallic zinc dust, (g atoms zinc:moles pentachloropyridine is 1:1), and 249.1 g of tetramethylene sulfone (sulfolane) is heated to 90°C and the reactants are vigorously agitated. A solution of 9.68 g (0.181 mol) ammonium chloride in 29.2 g water is added over a period of 35 min and the temperature of the reaction mixture is maintained at 90°C for an additional 65 min. Any insoluble material is removed from the heated reaction mixture by filtration (0.74 g) (at 76.9 percent Zn and 53.9 percent metallic Zn). The reaction mixture is quenched with 694 g water and 7 g 12N hydrochloric acid, agititated for 1 hour (hr), the precipitated chloropyridines are removed by vacuum filtration and dried in a vacuum oven. Analysis of the chloropyridines by gas chromatography indicates a molar selectivity of the pentachloropyridine converted of: 89.23% to 2,3,5,6-tetrachloropyridine (23.911 g) 5.18% to 2,3,5-trichloropyridine (1.168 g), 0.99% to 2,3,6-trichloropyridine (0.223 g), 0.32% to 2,3,4,5-tetrachloropyridine (0.086 g), and 4.27% to 2,3,4,6-tetrachloropyridine (1.144 g), at 88.70 percent pentachloropyridine conversion with 96.10 percent of the pentachloropyridine added accounted for in the final product. Of the metallic zinc oxidized 99.47 percent reacted with chloropyridine with 83.59 percent resulting in the formation of 2,3,5,6-tetrachloropyridine. Example 7 - Acetonitrile Solvent and Ammonium Chloride at Reduced Zinc:Pentachloropyridine A flask containing 35.2 g (0.140 mol) of pentachloropyridine, 15.0 g (0.28 mol) ammonium chloride, 56.5 g water, and 201.3 g acetonitrile is heated to reflux (78°C) and the reactants are agitated vigorously. To the reaction mixture is added 7.26 g (0.104 mol) of 93.7 percent metallic zinc dust, (g atom zinc:moles pentachloropyridine is 0.74:1) and the reaction mixture is refluxed for 4.40 hrs. The heated reaction mixture is filtered to remove solids (0.17 g of material is recovered, whose analysis indicated 75.0 percent zinc). A distillation head is attached to the reaction flask and 222.1 g of material is removed overhead. Water and toluene are added to the flask and the organic and aqueous phases are separated. Analysis of chloropyridines in the organic phase by gas chromatography and for zinc in the aqueous phase by wet methods indicated a molar selectivity of the pentachloropyridine converted of: 97.27% to 2,3,5,6-tetrachloropyridine (20.868 g) 0.05% to 3,5-dichloropyridine (0.007 g), 0.33% to 2,3,5-trichloropyridine (0.060 g), 0.04% to 2,3,6-trichloropyridine (0.007 g), 0.05% to 2,3,4,5-tetrachloropyridine (0.011 g), and 2.25% to 2,3,4,6-tetrachloropyridine (0.483 g), at 70.63 percent pentachloropyridine conversion with 98.23 percent of the pentachloropyridine added accounted for in the final product. Of the metallic zinc used in the reaction, 97.41 percent reacts with chloropyridine with 94.30 percent resulting in the formation of 2,3,5,6-tetrachloropyridine. Example 8 - Acetonitrile Solvent and Ammonium Chloride at Reaction Temperature of 60°C To a reactor is added 35.0 g (0.139 mol) of pentachloropyridine, 9.62 g (0.139 mol) of 93.7 percent metallic zinc dust (g atom zinc:moles pentachloropyridine is 1:1), 14.9 g (0.278 mol) ammonium chloride, 41.7 g water and 202 g acetonitrile. The reactants are agitated, heated and maintained at a temperature of 60°C for 7.0 hrs. Analysis for chloropyridines by gas chromatography and for zinc by wet methods indicated a molar selectivity of the pentachloropyridine converted of: 99.68% to tetrachloropyridines (30.106 g), and 0.32% to 2,3,5-trichloropyridine (0.081 g), at a pentachloropyridine conversion of 73.4 percent. Example 9 - Acetonitrile Solvent and Ammonium Chloride at a Reaction Temperature of 145°C To a two liter Parr reactor is added 190.09 g (0.760 mol) of pentachloropyridine, 80.97 g (1.52 mol) ammonium chloride, 260 g water, 52.89 g (0.760 mol) of 93.7 percent metallic zinc dust, (g atom zinc:moles pentachloropyridine is 1:1) and 814 g acetonitrile. The reactor is sealed, the reactants agitated, the temperature of the reaction mixture is raised to 145°C within 24 min, and maintained at that temperature for 64 min. The reactor is allowed to cool to room temperature, the contents are transferred to a flask equipped with a distillation apparatus and 964 g of material are removed overhead. Then 500 g toluene is added to the flask and the organic and aqueous layers that form are separated. Analysis of the aqueous phase for zinc by wet methods, and analysis of the organic phase for chloropyridines by gas chromatography indicates a molar selectivity of the pentachloropyridine converted of: 88.67% to 2,3,5,6-tetrachloropyridine (130.541 g), 1.34% to 3,5-dichloropyridine (1.346 g), 0.01% to 2,4,6-trichloropyridine (0.012 g), 3.20% to 2,3,5-trichloropyridine (3.963 g), 1.23% to 2,3,6-trichloropyridine (1.523 g), 0.11% to 2,3,4,5-tetrachloropyridine (0.162 g), 2.31% to 2,3,4,6-tetrachloropyridine (3.401 g), 0.05% to 2-aminotetrachloropyridine (0.079 g), and 3.08 percent 4-aminotetrachloropyridine (4.848 g) at 89.75 percent pentachloropyridine conversion with 95.90 percent of the pentachloropyridine added accounted for in the final product. Of the metallic zinc oxidized, 93.04 percent reacted with chloropyridine with 79.34 percent resulting in the formation of 2,3,5,6-tetrachloropyridine. Upon repeating Examples 1-9 utilizing any of the suitable ammonium salts and solvent cited hereinbefore in place of those utilized in the Examples, substantially the same excellent results are obtained regarding formation of the desired 2,3,5,6-tetrachloropyridine.

Claims: A process for preparing 2,3,5,6-tetrachloropyridine by the reaction of pentachloropyridine and zinc in a solvent and in the presence of an ammonium salt which is characterized in that the solvent is water, a C-1 to C-5 alkylnitrile, a C-1 to C-5 alkylsulfoxide, tetramethylene sulfone, a C-1 to C-5 alcohol, or a mixture thereof and zinc is employed in an effective amount of less than 1.2 gram atoms of zinc per mole of pentachloropyridine. A process according to Claim 1 wherein the solvent is acetonitrile. A process according to Claim 1 wherein the solvent is water. A process according to Claim 1 wherein the solvent is isobutyl alcohol. A process according to any one of Claims 1 to 4 wherein the ammonium salt is ammonium chloride. A process according to any one of Claims 1 to 4 wherein between 0.7 and and 1.1 gram atoms of zinc per mole of pentachloropyridine are employed. A process according to any one of Claims 1 to 4 wherein the temperature is between 50°C and 175°C. A process according to any one of Claims 1 to 4 wherein the weight ratio of pentachloropyridine to ammonium salt is between 1:1 and 1:6.

Inventors: MURPHY, FRANK H.
Applicants: DOWELANCO
Application year: 1987
Main classification: C07D213
IPC codes: C07B61, A01N43, C07D213