METHOD OF SELECTIVELY CHLORINATING 2-CHLORO-5-(TRICHLOROMETHYL) PYRIDINE IN THE 3-POSITION

Abstract: The selective chlorination of 2-chloro-5-­ (trichloromethyl)pyridine in the 3-position is achieved by reacting 2-chloro-5-(trichloromethyl)pyridine in the liquid phase with an effective amount of Cl₂ at an elevated pressure and elevated temperature in the absence of an added catalyst.

Description: This invention relates to a method of selectively chlorinating the 3-position of 2-chloro-5-­ (trichloromethyl)pyridine by a non-catalyzed, liquid phase chlorination reaction resulting in the formation of 2,3-dichloro-5-(trichloromethyl)pyridine. 2,3-Dichloro-5-(trichloromethyl)pyridine is a well known compound useful as an intermediate in the preparation of agricultural chemicals, such as, for example, 2-(4-((3-chloro-5-trifluoromethyl)-2-pyridinyl)­ oxy)phenoxy propionic acid and derivatives thereof. Known methods of preparing 2,3-dichloro-5-(trichloro­ methyl)pyridine usually involve the use of a catalyst in a liquid phase chlorination reaction. See U.S. Patent 4,331,811 and EP-A 0 060 462, respectively, which teach and claim the use of tungsten, molybdenum and ruthenium catalysts in this reaction. From Chemical Abstracts, Vol.98, No.1, abstract no.143281b a process for the production of 3-chloro-5-(trichloromethyl)­ pyridine compounds is known, wherein practically equimolar amounts of FeCl₃ are employed as a catalyst. In accordance with the present invention, 2-chloro-5-(trichloromethyl)pyridine is contacted with chlorine (Cl₂) in the liquid phase in the absence of a catalyst under certain elevated temperature and pressure conditions to selectively chlorinate the 3-position of the pyridine ring. The present process can be conducted in a continuous, cyclical operation or in batch operation to produce 2,3-dichloro-­ 5-(trichloromethyl)pyridine in a commercially viable yield and, preferably, in a ratio to 2,6-dichloro-­ 3-(trichloromethyl)pyridine of at least 2.8:1. One advantage of the present invention is that no catalyst need be employed, thus eliminating the cost of the catalyst and the additional catalyst removal step. It has been unexpectedly found that reacting 2-chloro-­ 5-(trichloromethyl)pyridine in the liquid phase with Cl₂ under certain elevated temperature and pressure conditions in the absence of a catalyst selectively chlorinates the 3-position of the pyridine ring yielding 2,3-dichloro-­ 5-(trichloromethyl)pyridine. Additionally, the absence of a catalyst reduces tar formation in the reaction vessel. Further, it has been found that when employing the particular temperature and pressure conditions of this invention, iron catalysts, such as FeCl₃, may be employed to increase the reaction rate without destroying the desired selectivity. In conducting the method of the present inven­ tion, gaseous chlorine (Cl₂) is passed into liquid 2-chloro-5-(trichloromethyl)pyridine at an elevated temperature and an elevated pressure. A solvent, such as chlorinated hydrocarbons, may optionally be employed but it is preferred to conduct the present reaction neat. An amount of chlorine (Cl₂) gas reactant is employed that is sufficient to selectively chlorinate the 3-position of the pyridine starting material. Usually at least about an equimolar amount of chlorine, based on the molar amount of 2-chloro-5-(trichloromethyl)pyridine, is employed. An excess of up to 10 molar proportions or more of chlorine per mole of pyridine starting material is desirably employed. The most suitable rate at which the chlorine gas is fed into the reaction will vary with such factors as reaction temperature, pressure, reaction mixture volume, etc. and is readily determinable to one skilled in the art. Certain elevated temperature and pressure are critical elements of the present invention. Temperatures in the range of from 60 to 180°C are required while it is advantageous to conduct the present reaction at a tempera­ ture of from 120 to 170°C. A preferred temperature range in which to carry out the present reaction is from 140°C to 160°C and most preferably at about 150°C. The present reaction is advantageously run at superatmospheric pressures of from 50 psig to 1,000 psig (446 to 6996 kPa). Higher pressures may be employed but, from an economic standpoint, are usually cost prohibitive. A preferred pressure range is from 100 psig to 300 psig (791 to 2170 kPa). A particularly preferred pressure to run the present reaction is about 200 psig (1480 kPa). It has further been found that improved conver­ sions are obtained when HCl is continuously added to the reaction mixture. In carrying out the present invention, 2-chloro-5-(trichloromethyl)pyridine, usually in the liquid form (about 53°C), is preferably added to a reac­ tion vessel, heated to between 60° and 180°C and chlorine flow is commenced, usually at a sufficient rate to pres­ surize the reaction vessel to about 200 kPa (15 psig) or more. The reaction is continued until sufficient amounts of 2,3-dichloro-5-(trichloromethyl)pyridine are obtained. Samples from the reaction vessel and vent gases are periodically taken and analyzed, employing known methods, to monitor the course of the reaction. The reaction is terminated by stopping the heating of the reaction vessel and the flow of chlorine thereto and allowing the reaction vessel pressure to drop to atmospheric pressure. The desired 2,3-dichloro-5-(trichloromethyl)pyridine is then recovered employing known separatory or purification techniques such as distillation, crystal refining or recrystallization, from a solvent. It is readily apparent to one skilled in the art that the reaction be run for an amount of time which maximizes the yield of 2,3-dichloro-5-(trichloromethyl)­ pyridine. The optimum reaction time will depend on a variety of factors, such as, for example, specific starting materials employed, pressure, temperature, amounts of reactants employed, and rate of the chlorine feed to name a few. Each operation of the present invention is monitored as described above to determine the optimum reaction time for that particular operation. In one embodiment of the present invention, substantially pure 2-chloro-5-(trichloromethyl)pyridine is reacted with chlorine at a temperature of about 150°C and at a pressure of about 200 psig (1480 kPa). The 2,3-­ dichloro-5-(trichloromethyl)pyridine product is distilled from the reaction mixture. Alternatively, the 2-chloro-5-(trichloromethyl)­ pyridine starting material is supplied as one component of a mixture of chlorinated β-picolines and chlorinated pyridines. The mixture is the reaction product resulting from the vapor phase chlorination of β-picoline which is used to prepare 2-chloro-5-(trichloromethyl)pyridine. This mixture typically contains the following compounds on a weight percent (wt. %) basis: Instead of isolating the 2-chloro-5-(trichloromethyl)­ pyridine from this mixture, the above-described mixture is reacted with chlorine according to the conditions of the present invention as described herein to form 2,3-di­ chloro-5-(trichloromethyl)pyridine. The resulting 2,3-dichloro-5-(trichloromethyl)pyridine is separated from the reaction mixture as known in the art, e.g., by distillation, crystal refining, recrystallization from a solvent or a combination thereof. The following examples illustrate the practice of the present invention but should not be construed as limiting its scope. No attempt has been made to balance any chemical equations described herein. A number of experiments were carried out as set forth in the following tables. In each experiment the starting material, 2-chloro-5(trichloromethyl)pyridine was placed in the reactor and heated to the desired temperature after which chlorine gas was fed through a dip tube until the pressure reached 200 psig (1480 kPa). Chlorine and optionally, HCl were then metered into the reactor at the stated rates, with hydrogen chloride and excess chlorine being vented to a scrubber. The reaction mixtures were periodically sampled to monitor reaction progress. In similar operations, various pressure and temperature conditions as described herein are employed when reacting 2-chloro-5-(trichloromethyl)pyridine with an effective amount of Cl₂ in the absence of a catalyst resulting in the formation of 2,3-dichloro-­ 5-(trichloromethyl)pyridine. The desired product can be isolated by distillation from the reaction mixture. In other embodiments of the present invention, the reaction is conducted on a continuous recycle basis to prepare 2,3-dichloro-5-(trichloromethyl)pyridine. Such a process comprises the continuous, selective chlorination of 2-chloro-5-(trichloromethyl)­ pyridine at elevated temperature and pressure conditions as described herein. The 2,3-dichloro-5-(trichloromethyl)­ pyridine is recovered by distillation and any unreacted 2-chloro-5-(trichloromethyl)pyridine is recycled employing procedures well known in the art.

Claims: 1. A method of selectively chlorinating 2-­ chloro-5-(trichloromethyl)pyridine in the 3-position which comprises contacting 2-chloro-5-(trichloromethyl)­ pyridine in the liquid phase with an effective amount of Cl₂ at a pressure of from 446 to 6996 kPa and a tempera­ ture of from 60°C to 180°C in the absence of a catalyst whereby the 2,3-dichloro-5-­ (trichloromethyl)pyridine is produced in a ratio to 2,6-dichloro-3-(trichloromethyl)pyridine of at least 2.8 to 1. 2. The method of Claim 1 wherein an excess molar quantity of Cl₂ is employed, based on the molar quantity of 2-chloro-5-(trichloromethyl)pyridine. 3. The method of Claim 2 wherein the pressure is at least 790 kPa and the temperature is at least 100°C. 4. The method of Claim 3 wherein the pressure is 1480 kPa and the temperature is from 140°C to 160°C. 5. The method of Claim 4 wherein the temperature is about 150°C. 6. The method of Claim 5 further comprising the step of isolating the 2,3-dichloro-5-(trichloro- methyl)pyridine from the reaction mixture. 7. The method of Claim 6 which is conducted as a batch operation. 8. The method of Claim 6 which is conducted as a continuous, cyclical operation. 9. The method of Claim 2 wherein the 2-chloro-­ 5-(trichloromethyl)pyridine is supplied as a component in a mixture of chlorinated β-picolines and chlorinated pyridines. 10. The method of Claim 9 wherein the mixture of chlorinated β-picolines and chlorinated pyridines is the reaction product mixture resulting from the vapor phase chlorination of β-picoline. 11. The method of Claim 1 wherein HCl is continuously added to the reaction mixture.

Inventors: BIXBY, JAMES L., DIETSCHE, THOMAS J., HUMPHREYS, PAULA L.
Applicants: THE DOW CHEMICAL COMPANY
Application year: 1986
Main classification: C07D213
IPC codes: C07D213