FIELD OF THE INVENTION
The present invention provides a process for the preparation of chlorinated pyridines. They are mainly used as intermediates for preparation of fungicides and insecticides.

BACKGROUND OF THE INVENTION
Chlorinated pyridines are highly valuable as intermediates in preparation of numerous compounds having application in agricultural chemical field.
Several methods for preparation of chlorinated pyridine are known in literature.
U.S. 3,555,032 discloses a continuous process for preparing 2,3,4,5-tetrachloropyridine by chlorinating 2-chloropyridine hydrochloride in the liquid phase. However, it is a time consuming process not viable for commercial scale.
U.S. 3,186,994 discloses a process for preparation of 2,3,5,6-tetrachloropyridine by chlorinating polychloro-(trichloromethyl)-pyridine reactant in a liquid state in absence of a catalyst. However, the process necessitate either irradiation with ultraviolet light or high temperatures of above 400?.
U.S. 4,810,797 discloses a process for preparation of 2,3,5,6-tetrachloropyridine by chlorination of 2-chloropyridine, 2,6-dichloropyridine, 2,3,6-trichloro-pyridine as starting materials under pressure and temperature conditions.
U.S. 4,483,993 discloses a process for preparation of polychlorinated pyridine by non-catalytic chlorination of beta-picoline or beta-picoline hydrochloride in a liquid phase. The process is extremely slow and does not provide selectivity towards 2,3,5,6-tetrachloropyridine.
The cited references have several drawback such as complex systems, long reaction time and lack of selectivity for desired 2,3,5,6-tetrachloropyridine.

Therefore, there is a need to develop a process, that is, economical, industrially viable, overcome drawback of existing processes and also provide improved selectivity.
Surprisingly the inventors of the present invention found that the selectivity towards 2,3,5,6-tetrachloropyridine can be obtained by chlorination of ß-picoline in the vapour phase chlorination in presence of a catalyst.

OBJECT OF THE INVENTION
The main object of the present invention is to provide a simple, cost-effective and an industrially applicable process for preparation of 2,3,5,6-tetrachloropyridine in vapour phase.

SUMMARY OF THE INVENTION
The present invention provides a process for the preparation of 2,3,5,6-tetrachloropyridine comprising the steps of chlorinating 3-methylpyridine in presence of a catalyst.

DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a process for preparation of chlorinated pyridine, comprising the step of chlorinating 3-methylpyridine in presence of a catalyst.
As used herein, the chlorination of methylpyridine is carried out by using chlorine.
As used herein, the catalyst is selected from high silica zeolite.
In one embodiment, the present invention provides a process for preparation of 2,3,5,6-tetrachloropyridine, comprising the steps of chlorinating 3-methyl pyridine in presence of a catalyst.
In an embodiment, the present invention provides a process for preparation of 2,3,5,6-tetrachloropyridine, comprising the step of chlorinating 3-methylpyridine in presence of high silica zeolite catalyst.
In a preferred embodiment, the present invention provides a process for preparation of 2,3,5,6-tetrachloropyridine, comprising the step of chlorinating 3-methylpyridine in presence of HSZ-320HOD1C as high silica zeolite catalyst.
In another embodiment, the chlorination of 3-methylpyridine is carried out in the presence of a catalyst, wherein the catalyst is heated with chlorine at a temperature of 250ºC to 450ºC.
In another embodiment, the chlorination of 3-methylpyridine in the presence of catalyst is optionally carried out in presence of an inert gas.
The inert gas is fed to the reactor at a flow rate of 30 to 80 ml/min.
In a preferred embodiment, the chlorination of 3-methylpyridine is carried out at a temperature of 250ºC to 350ºC.
In an embodiment, the process of present invention is carried out in absence of solvent.
The “absence of solvent” refers to the process in which no solvent is used to aid the reaction. The absence of solvent reduces cost of process and generate less effluent after reaction.
In another embodiment of the present invention, the process is carried out in vapor phase.
In another embodiment of the present invention, the process is carried out in a corrosion resistant reactor.
In another embodiment of the present invention, the corrosion resistant reactor comprised of materials which are resistant to corrosion such as Hastelloy, Inconel, Monel and/or fluoropolymers linings.
In another embodiment, the zeolite catalyst packed in an Inconel tubular reactor having a diameter of 2.54 to 5 cm and length of 100 to 150 cm.
In another embodiment, the zeolite catalyst packed in an Inconel tubular reactor having a diameter of 2.54 cm and length of 100 cm.
In another embodiment, 3-methylpyridine is preheated to about 250°C to 300°C.
The preheated 3-methylpyridine was fed to the reactor at a flow rate of 0.1 ml/min to 0.5 ml/min.
In another embodiment, a chlorine gas is preheated to about 280°C to 300°C.
In another embodiment of the present invention, the reaction mixture was condensed by passing through water scrubbing column upon completion of the reaction. Dichloromethane was added to reaction mass and proceeded for isolation of the product. The product was analysed, whereby it was found that it results in selectivity 80 to 90% of desired 2,3,5,6-tetrachloropyridine and 10 to 20% of undesired pentachloropyridine.
In another embodiment of the present invention, the purity of isolated 2,3,5,6-tetrachloropyridine is 80 to 95% at crude stage.
In another embodiment of the present invention, the purity of isolated 2,3,5,6-tetrachloropyridine is 95 to 99.5% at pure stage.
The isolation technique used for present invention may be selected from a group consisting of chemical separation, distillation, reactive distillation, condensation, filtration, crystallization or like or combination thereof.
The completion of the reaction may be monitored by any one of chromatographic techniques such as thin layer chromatography (TLC), high pressure liquid chromatography (HPLC), ultra-pressure liquid chromatography (UPLC), Gas chromatography (GC), liquid chromatography (LC) and alike.
The reagents used in the present invention are obtained commercially.
Unless stated to the contrary, any of the words “comprising”, “comprises” and includes mean “including without limitation” and shall not be construed to limit any general statement that it follows to the specific or similar items or matters immediately following it.
Embodiments of the invention are not mutually exclusive, but may be implemented in various combinations. The described embodiments of the invention and the disclosed examples are given for the purpose of illustration rather than limitation of the invention as set forth in the appended claims.
The following example is given by way of illustration and therefore should not be construed to limit the scope of the present invention.

EXAMPLE
Preparation of 2,3,5,6-tetrachloropyridine
A 320HOD1C zeolite catalyst (100g) was packed in an Inconel tubular reactor. The reaction tube was heated to a temperature of about 300°C. The preheated mixture of 3-methylpyridine (gaseous state) and nitrogen gas at about 280°C was fed into the reaction tube at such a rate that the former goes at 0.1 ml/min and the latter was at 50 ml/min for 3.5 hours to form a gaseous mixture. Thereafter preheated chlorine gas at about 280°C was fed into the reaction tube of the gaseous mixture at a rate of 10 liter per hour for 3.5 hours. The gas discharged from the reactor was condensed by passing it through a water scrubbing column followed by alkaline scrubbing column. The resulting product was extracted by dichloromethane, washed with potassium carbonate solution and dried over sodium sulfate. The crude product was analyzed by gas chromatography resulting a mixture of 2,3,5,6-tetrachloropyridine (85%) and pentachloropyridine (15%). This mixture was purified by column chromatography to obtain pure 2,3,5,6-tetrachloropyridine.
Purity: 98.9%

CLAIMS:WE CLAIM:
1. A process for preparation of 2,3,5,6-tetrachloropyridine, comprising the step of chlorinating 3-methylpyridine in presence of high silica zeolite catalyst.
2. The process as claimed in claim 1, wherein the chlorination is carried out at a temperature range of 250ºC to 350ºC.
3. The process as claimed in claim 1, wherein the process is carried out in vapor phase.
4. The process as claimed in claim 1, wherein the catalyst is preheated at a temperature range of 250ºC to 350ºC.
5. The process as claimed in claim 1, wherein 3-methylpyridine is preheated to 250°C to 300°C.
6. The process as claimed in claim 1, wherein the chlorine gas is preheated to 280°C to 300°C.
7. The process as claimed in claim 1, wherein the purity of isolated 2,3,5,6-tetrachloropyridine is 80 to 90% at crude stage.
8. The process as claimed in claim 1, wherein the purity of isolated 2,3,5,6-tetrachloropyridine is 90 to 99.5% at pure stage.
9. The process as claimed in claim 1, wherein the chlorination is carried out in absence of a solvent.