PREPARATION OF TETRACHLOROPYRIDINE

Abstract: Tetrachloropyridine, more specifically 2,3,5,6-­ tetrachloropyridine, is efficiently prepared by reacting pentachloropyridine in water with an oxidizable metal such as zinc, in the absence of an added mineral or organic hydrogen donating acid.

Description: Various processes are known for preparing tetrachloropyridine from pentachloropyridine. For example, U.S. Patent 4,259,495 discloses a process whereby symmetrical tetrachloropyridine is obtained from pentachloropyridine via zinc reduction in a dialkyl-alkanephosphonate solvent in the presence of an ammonium salt of an inorganic acid. The process is also described in J. Hetero. Chem. , l7 , 493 (l980). In U.S. Patent 3,993,654 is described a process for preparing tetra-chloropyridine by heating together pentachloropyridine, a mineral or organic hydrogen donating acid and an oxidizable metal such as zinc which combines readily with reactive chlorine. These known processes suffer the disadvantages in that they require the presence of flammable, toxic or otherwise corrosive substances, including hydrochloric acid, in order to effect the reaction. The use of such substances, in turn, effects a chemical reduction of zinc by the added mineral acid or base. Such chemical reduc-tions can cause reductions in yield due to undesirable by-product formation, such as the dichloropyridines and the trichloropyridines. It has been unexpectedly and surprisingly found that 2,3,5,6-tetrachloropyridine, can be advantageously prepared by reacting pentachloropyridine (PCP) in water with Zinc to convert the pentachloropyridine to tetrachloropyridine, the process being conducted in the absence of an added mineral or organic hydrogen donating acid. Subject matter of the present invention is therefore a process for preparing 2,3,5,6-tetrachloropyridine with 0.5 to 1.4 moles of zinc in 0.4 to 20 parts by weight of water, at a temperature of from 90° to 160°C, and in the absence of an added mineral or organic hydrogen donating acid. In a preferred embodiment the process is conducted with the zinc in a dust having a nominal particle size of from 7 to 15 µm. In a further preferred embodiment the process is carried out with a temperature from 100° to 145°C. In general, the invention is practiced by heating, with agitation, in a closed vessel, particulate pentachloropyridine, water and particulate zinc. The vessel is closed in order that the desired temperature can be reached by confining the water under at least autogenous pressure. The reaction may be generally illustrated as follows: The pentachloropyridine can be in particulate and dispersed form during the reaction and should be stirred or agitated in order to prevent the particles, when they become softened or molten, from agglomerating. The PCP may be first particulated and then dispersed in the aqueous medium or carrier prior to being heated to a softened or molten condition. The PCP can be particulated by grinding or by shearing. Alternatively, the PCP can be heated to its softened or molten condition before addition to the aqueous medium or carrier. The PCP may also be added in its particulate, softened or molten condition to the aqueous medium in the reaction vessel and stirred and heated at the same time, in which case a strong agitation, such as by a stirrer having a power input of at least about 0.005 horsepower/gallon, (1 joule sec⁻¹ liter⁻¹) preferably at least 0.015 horsepower/gallon, (3 joules sec⁻¹ liter⁻¹) has been found to be particularly advantageous. Any suitable high-intensity agitator, such as a high-shear recycling pump can be employed. Since the reaction works well at temperatures at which the PCP is molten, a temperature of 100-145°C is preferred. Temperatures higher than 145°C may be used, but such higher temperatures create higher autogenous pressure which require more expensive vessels to contain such pressures. Higher temperatures and pressures are more easily accommodated in a tube-type or coil reactor, useful in continuous flow reactions. Higher temperatures also have the advantage of allowing use of large particle size metal dust and still completing conversion of all the metal in a reasonable length of time. Greatest zinc efficiency, however, is achieved with decreasing temperatures, for example 100° to 120°C. The reaction pressure is at least autogenous since the reaction temperature is above the boiling point of water. Pressures greater than autogenous may be generated by adding additional pressure after the other ingredients in the reaction vessel have been brought up to reaction temperature. The reaction time may vary over a wide range depending on the temperature, the choice of metal and the ratio of ingredients to each other. Reaction times generally are a function of zinc particle size and temperature. The time required to completely react the zinc is a function of both reaction temperature and zinc particle size. As particle size becomes larger, more time is needed to consume the zinc charged to the reactor. For example, at 130°C it was found that roughly one hour of reaction time was required for each 5 microns of particle diameter. The optimum diameter from both kinetic and zinc efficiency considerations appears to be about 10 µm. The zinc dust can have a minimal particle size ranging between 1 to 50 µm, preferably from 7 to 15 µm. There is an exotherm which is easily controlled in small reactors and tube or coil reactors, but which, in large stirred vessels, should be regulated by heat transfer and by controlling the pressure, and, consequently the temperature. In one embodiment of the invention, PCP is dispersed in water in particulate, non-molten form by employing high-intensity agitation. The so-formed slurry is fed into a reaction vessel and particulate zinc is added. The vessel is sealed and the mixture is stirred and heated until the desired temperature is reached, which is preferably at least the melting point of the PCP. At the completion of the reaction mineral acid can be added to the reaction mixture to dissolve the water-insoluble metal salts. The chloropyridines are taken up in an inert, water immiscible solvent and the metal salt is separated from the water-immiscible solvent. In another suitable method, molten PCP is added to a heated slurry of particulate zinc in water. The slurry is agitated to disperse the PCP. The reaction mixture is brought to the desired reaction temperature and the reaction is continued at the appropriate rate until completed. In another embodiment the PCP, water, and zinc are charged to the reactor. The reactor is sealed and the mixture is heated, with strong agitation, until the PCP is sufficiently heated, preferably molten. The mole ratio of zinc to PCP is in the range of 0.5 to 1.4, preferably 1.0 to 1.2 (moles to moles). The process will operate with more or less of the metal, but if less is used there may not be enough to react with all the PCP; if more is used, there is an increased tendency to cause attack of the chlorine on the PCP in positions other than the fourth position, thereby producing di- and trichloropyridines. The weight ratio of water to PCP is 0.4 to 20, with the ratio 0.5 to 0.8 being preferred; most preferably a ratio of about 0.7 is preferred. Essentially, there should be enough water to help disperse the PCP and to take up the metal salt formed. Having a large excess of water merely adds to the amount of energy and equipment size required to operate the process and is detrimental to the yield of tetrachloropyridine. The following examples illustrate typical processes of the present invention. Example 1 In a 2-liter Parr reactor equipped with a turbine blade stirrer, thermowell, electrical heating jacket, knock-back condenser, and temperature and pressure controller were placed 377 grams (1.5 mole) penta-chloropyridine (PCP), 98 grams (1.5 mole) zinc metal as a dust having a nominal average particle diameter of 7 µm, and 420 grams (23 moles) of water. The reactor was sealed, agitation was set at 600 rpm, and the mixture was quickly heated to 130°C over a period of 25 minutes. The pressure controller was allowed to maintain the autogenous pressure of about 24 psig (266.8 kPa) which, along with heat input controlled by the temperature controller, kept the reactor system at 130°C. After 2 hours, 1000 grams of chilled water was pressured into the reactor with a nitrogen pad. The mixture was allowed to stir for 2 minutes, excess pressure was bled off, the reactor was opened and the contents were poured into 1200 grams of toluene. The organic compounds were extracted into the toluene phase with agitation, the entire mixture was filtered and the resulting filter cake washed with about 500 grams of water and 300 grams of toluene. The toluene and aqueous phases of the filtrate were separated, weighed and analyzed. The filter cake was dried, weighed and analyzed. Gas chromatographic analysis of the toluene phase indicated 74 percent PCP conversion with about 94 percent selectivity to 2,3,5,6-tetrachloropyridine (symmetrical tetrachloropyridine), 2 percent to 2,3,4,5- and 2,3,4,6-tetrachloropyridine, 3 percent to trichloro- pyridines and 1 percent to dichloropyridines. Analysis of the aqueous phase and the dried filter cake by standard analytical techniques indicated complete conversion of the zinc to a mixture of zinc chloride and zinc hydroxide. Example 2 A mixture of 190 grams PCP (0.75 mole), 50 grams (0.75 mole) zinc dust (7 µm average particle diameter), 210 grams (12 moles) of water, and 1 gram of toluene was heated to reflux with vigorous agitation in a 500 ml round-bottomed flask fitted with a reflux condenser. After 5 hours at l00 ± l°C, the product mixture was poured into 600 grams of toluene for extraction as described in Example l above. The entire mixture was filtered and the filter cake washed with 250 grams of water and l50 grams of toluene. The phases were separated and analyzed as described above. PCP conversion was 84 percent with about 96 percent selectivity to symmetrical tetrachloropyridine, 2 percent to other tetrachloropyridines, l.5 percent to trichloropyridines and 0.5 percent to dichloropyridines. Zinc conversion (consumption) was essentially complete. Examples 3-6 Table I shows run numbers l-4, corresponding to Examples 3-6, conducted in essentially the same manner as in Example l, except that the temperature was varied. Examples 7-10 Table II shows run numbers 5-8, corresponding to Examples 7-10, to show the effect, primarily, of employing a zinc dust with a nominal particle size of 7 µm. Examples 11-15 Table III shows run numbers 9-13, corresponding to Examples 11-15, to show the effect, primarily, of varying the reaction temperature. Examples 16-19 Table IV shows run numbers 14-17, corresponding to Examples 16-19, to show the effect, primarily, of varying the reaction temperature and source supply of the zinc dust with a nominal particle size of 15 µm from a different source supplier. Examples 20-23 Table V shows run numbers l8-2l, corresponding to Examples 20-23, to show the effect, primarily of reaction temperature. Examples 24-26 Table VI shows run numbers 22-24, corresponding to Examples 24-26, to show the effect, primarily of varying the molar ratio of reactants. Examples 27-31 Table VII shows run numbers 25-28, corresponding to Examples 27-31, to show the effect, primarily, of varying the water concentration.

Claims: A process for preparing 2,3,5,6-tetrachloropyridine which comprises reacting pentachloropyridine with 0.5 to 1.4 moles of zinc in 0.4 to 20 parts by weight of water, at a temperature of from 90 to 160° C, and in the absence of an added mineral or organic hydrogen donating acid. The process according to claim 1 wherein the zinc is a dust having a nominal particle size of from 7 to 15 µm. The process according to claim 1 or 2 wherein the temperature is from 100 to 145° C.

Inventors: KORANEK, DAVID JAMES
Applicants: THE DOW CHEMICAL COMPANY
Application year: 1987
Main classification: C07D213
IPC codes: C07B61, C07D213, B01J23