DESC:PROCESS FOR THE PREPARATION OF HIGHLY PURE SULFENTRAZONE
Field of the Invention
The present invention relates to an improved and industrially advantageous process for the preparation of highly pure sulfentrazone having assay more than 97% and impurity A (chloromethyl) and impurity B (chlorofluoromethyl) each present in an amount of not more than 0.3%.
Background of the Invention
Sulfentrazone having CAS name N-(2,4-dichloro-5-(4-(difluoromethyl)-4,5-dihydro-3-methyl-5-oxo-1H-1,2,4-triazol-1-yl) phenyl) methanesulfonamide belongs to the group of protophyrinogen oxidase inhibitors herbicide. It is an important commercial herbicide in agriculture and is used for pre-plant incorporated or pre-emergence control of annual broad-leaved weeds, some grass weeds and Cyperus spp. in soybeans, sugarcane and tobacco.
A number of processes for the preparation of sulfentrazone are already known from the literature. US patent no. 4818275 discloses a preparation of sulfentrazone with overall yield of 66% via bis methyl sulfonyl intermediate. The crude bis methyl sulfonyl intermediate is treated with base to convert it into desired product sulfentrazone as shown in below reaction scheme.

The disadvantages of this method is low yield, it includes one additional step and necessitates the use of larger amounts of methanesulfonyl chloride and acid scavenger which further increases costs.
Further US patent no. 5990315 disclosing the preparation of sulfentrazone with an assay ~90% and yield of 79% via amino intermediate in one step as shown in below reaction scheme. The disadvantage of this method is, it gives the low assay content of sulfentrazone.

The PCT publication WO2019141230A1 discloses the preparation of sulfentrazone using imidazole as base and other heterocyclic acyclic/cyclic base wherein assay content of final product is ~92% and overall yield is 95.7%.
Further, US publication 20220106277A1 discloses a process wherein purity is increased by acid and base partitioning method between inorganic base and followed by crystallization by acidification using inorganic acid. Further the US application 20220106277A1 mention that the crude product, N-(2,4-dichloro-5-(4-(difluoromethyl)-3-methyl-5-oxo-4,5-dihydro-1H-1,2,4-triazole-1-yl)methanesulfonamide has purity 94.6% and total impurities 5.6%. By using the process steps of US application 20220106277A1 the final product N-(2,4-dichloro-5-(4-(difluoromethyl)-3-methyl-5-oxo-4,5-dihydro-1H-1,2,4-triazole-1-yl) methanesulfonamide is obtained in purity of 99.4% with total impurities 0.6%. The disadvantage of this process is it requires multiple operations with higher temperature and also generates more effluent.
Therefore, there exists an urgent need to develop a simple, cost-effective, high yielding process for the preparation of sulfentrazone with higher assay content and less impurity desirable on commercial scale.
Object of the Invention
It is the foremost object of the present invention to provide a cost-effective and industrial advantageous process for the preparation of highly pure sulfentrazone.
Yet other object of the present invention is to provide a sulfentrazone having assay more than 97% and impurity A (chloromethyl) and impurity B (chlorofluoromethyl) each present in an amount of not more than 0.3%.
Yet another object of the present invention is to provide a sulfentrazone having assay more than 97% and with higher yield.
The one another object of the present invention is to identify impurity B (chlorofluoromethyl) and controlling the impurity A (chloromethyl) and impurity B (chlorofluoromethyl) each present in an amount of not more than 0.3%.
Summary of the Invention
Accordingly, the present invention provides an improved and industrially advantageous process for the preparation of highly pure sulfentrazone having assay more than 97% and impurity A (chloromethyl) and impurity B (chlorofluoromethyl) each present in an amount of not more than 0.3%.
Accordingly, the present invention provides an improved and industrially advantageous process for the preparation of highly pure sulfentrazone having assay more than 97% and impurity A (chloromethyl) and impurity B (chlorofluoromethyl) each present in an amount of not more than 0.3% comprising the steps of:
a) dissolving the crude sulfentrazone in organic solvent;
b) optionally distilling the solvent;
c) adding a water to the step b) mixture;
d) adding base to the step c) mixture;
e) adding acid to the step d) mixture; and
f) isolating pure sulfentrazone.
Accordingly, the present invention provides a process to lower the impurity A (chloromethyl) and impurity B (chlorofluoromethyl) each present in an amount of not more than 0.3%.

Accordingly, the present invention provides a process to reduce the impurity A (chloromethyl) and impurity B (chlorofluoromethyl) comprising the steps of:
a) dissolving the crude sulfentrazone in organic solvent;
b) optionally distilling the solvent;
c) adding a water to the step b) mixture;
d) adding base to the step c) mixture;
e) adding acid to the step d) mixture; and
f) isolating pure sulfentrazone.
Accordingly, the present invention provides a process for the preparation of highly pure sulfentrazone having assay more than 97% comprising the steps of:
a) dissolving the crude sulfentrazone in organic solvent;
b) optionally distilling the solvent;
c) adding a water to the step b) mixture; and
d) isolating pure sulfentrazone.
Detailed description of the Invention
The definitions provided herein for the terminologies used in the present disclosure are for illustrative purpose only and in no manner limit the scope of the present invention disclosed in the present disclosure.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which the invention pertains. Although other methods and materials similar, or equivalent, to those described herein can be used in the practice of the present invention, the preferred materials and methods are described herein.
It is to be noted that, as used in the specification and the appended claims, the singular forms "a", "an" and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a composition containing “a compound” includes a mixture of two or more compounds. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise.
As used herein, the terms "comprises", "comprising", "includes", "including", or any other variation thereof, are intended to cover a non-exclusive inclusion, subject to any limitation explicitly indicated.
The term ‘assay’ as used in present invention represents a quantitative measurement of the major components in agrochemical substance. It refers to content or potency to provide an exact result which allows an accurate statement on the content or potency of the analyte in a sample.
The term “crude sulfentrazone” as used herein can generically refer to any form of sulfentrazone such as amorphous form, solvates, polymorph forms, pseudo polymorph forms, crystalline form, mixture of crystalline forms or mixture of amorphous and crystalline form, or the sulfentrazone obtained from the conventional process/prior art process or the sulfentrazone known in art by the name of technical sulfentrazone.
The present invention provides an improved and industrially advantageous process for the preparation of highly pure sulfentrazone having assay more than 97% and impurity A (chloromethyl) and impurity B (chlorofluoromethyl) each present in an amount of not more than 0.3%.
In an embodiment, the present invention provides an improved and industrially advantageous process for the preparation of highly pure sulfentrazone having assay more than 97% and impurity A (chloromethyl) and impurity B (chlorofluoromethyl) each present in an amount of not more than 0.3% comprising the steps of:
a) dissolving the crude sulfentrazone in organic solvent;
b) optionally distilling the solvent;
c) adding a water to the step b) mixture;
d) adding base to the step c) mixture;
e) adding acid to the step d) mixture; and
f) isolating pure sulfentrazone.
In other embodiment the present invention provides a process to lower the impurity A (chloromethyl) and impurity B (chlorofluoromethyl) each present in an amount of not more than 0.3%.

Accordingly, the present invention provides a process to reduce the impurity A (chloromethyl) and impurity B (chlorofluoromethyl) comprising the steps of:
a) dissolving the crude sulfentrazone in organic solvent;
b) optionally distilling the solvent;
c) adding a water to the step b) mixture;
d) adding base to the step c) mixture;
e) adding acid to the step d) mixture; and
f) isolating pure sulfentrazone.
Accordingly, the present invention provides a process for the preparation of highly pure sulfentrazone having assay more than 97% comprising the steps of:
a) dissolving the crude sulfentrazone in organic solvent;
b) optionally distilling the solvent;
c) adding a water to the step b) mixture; and
d) isolating pure sulfentrazone.
The impurity A (chloromethyl) is known to by IUPAC name N-[2,4-Dichloro-5-[3-(chloromethyl)-4-(difluoromethyl)-4,5-dihydro-5-oxo-1H-1,2,4-triazol-1-yl]phenyl]-methanesulfonamide, herein after will be describe as (chloromethyl) impurity. The impurity B (chlorofluoromethyl) is known to by IUPAC name N-(2,4-Dichloro-5-(4-(chlorofluoromethyl)-3-methyl-5-oxo-4,5-dihydro-1H-1,2,4-triazol-1-yl) phenyl) methanesulfonamide, herein after will be describe as (chlorofluoromethyl) impurity.
In an embodiment of present invention the suitable solvent used in step a) is selected from toluene, acetonitrile, methylene chloride, chloroform, xylene, methyl ethyl ketone, benzene, dioxane, hexane, carbon tetrachloride, tetrahydrofuran, dichloromethane, bromodichloromethane, dibromochloromethane, trichlorofluoromethane, 1,1-dichloroethane, 1,2- dichloroethane, 1,1,1-trichloroethane, 1,1, 2-trichloroethane, 1,1,2,2- tetrachloroethane, 1,1-dichloroethylene, cis-1,2-dichloroethylene, trans-1,2- dichloroethylene, trichloethylene; Hydrocarbon solvents such as cyclohexane, heptane; Alcohol solvents such as n-butanol, tert-butanol, tert-amyl alcohol, benzyl alcohol, 1,4-butanediol, 1,2,4-butanetriol, butanol, 1-butanol, 2-butanol, tert-butyl alcohol, denatured alcohol, Di(propylene glycol) methyl ether, diethylene glycol, ethanol, ethylene glycol, 2-ethylhexanol, furfuryl alcohol, glycerol, isobutanol, isopropyl alcohol, methanol, 2-(2-methoxyethoxy)ethanol, 2-methyl-1-butanol, 2-methyl-1-pentanol, 3-methyl-2-butanol, neopentyl alcohol, 2-pentanol, 1,3-propanediol, propan-1-ol, propylene glycol, propylene glycol methyl ether; Ketonic solvents such as acetone, acetophenone, butanone, cyclopentanone, ethyl isopropyl ketone, 2-hexanone, isophorone, beta-isophorone, mesityl oxide, methyl isobutyl ketone, methyl isopropyl ketone, 3-methyl-2-pentanone, 2-pentanone, 3-pentanone or mixtures thereof, preferably acetone.
In one aspect of the embodiment, the solvent acetone used in step a) is in the range of 0.2 volume to 20 volumes.
In other aspect of the embodiment, the solvent used in step b) is optionally distilled. In the process if solvent acetone is distilled then distillation can be done more than 85% of used solvent amount. Generally the quality of product of step a) will be the deciding factor of distillation. It is observed that as per example 1 of present invention wherein solvent is not distilled yield is low, wherein when the solvent acetone is distilled upto 90% the yield of product is increased.
In another aspect of the embodiment, the water used in step c) is in the range of 0.1 volume to 100 volumes.
In another aspect of the embodiment, the suitable base used in step d) is selected from organic/inorganic bases such as cerium tetrahydroxide, potassium hydroxide, magnesium hydroxide, palladium hydroxide, cadmium hydroxide, barium hydroxide hydrate, gold hydroxide, lanthanum hydroxide, ammonium hydroxide, sodium hydroxide, sodium carbonate decahydrate, lead hydroxide, cupric hydroxide, bismuth hydroxide, barium hydroxide octahydrate, cobalt hydroxide, zinc hydroxide, cobalt hydroxide, aluminium hydroxide, cesium hydroxide, copper hydroxide, titanium hydroxide, sodium bicarbonate, potassium bicarbonate, ammonium bicarbonate, sodium tert-pentoxide, sodium trimethylsilanolate, methyllithium, sodium ethoxide, N,N-dimethylpyridin-4-amine, dimethylaminopyridine, monoethylamine, diethylamine, isopropyllithium solution, lithium trimethylsilanolate, 2,6-Dimethylpyridine, barium tert-butoxide, 2,6,6-tetramethylpiperidine lithium salt (1:1), lithium 2,2,6,6-tetramethylpiperidine, lithium ethoxide, lithium methoxide, potassium ethoxide, potassium methoxide, lithium ethoxide solution, potassium methylate, sodium amide, 1,5-Diazabicyclo[4.3. 0]non-5-ene, 1,5-Diazabicyclo[4.3.0]non-5-ene, choline hydroxide solution, potassium trimethylsilanolate, piperazine, potassium ethoxide solution, potassium hexamethyldisilazide, tetrabutylammonium ethoxide, tetrabutylphosphonium hydroxide, tetrapropylammonium hydroxide, potassium hexamethyldisilazide, sodium methoxide, methyl amine, hexamine, tetrabutylammonium methoxide solution, methyltriethylammonium hydroxide, 2,2,6,6-tetramethylpiperidine, pyridine, tri-ethyl amine, di-ethyl amine, 4-(Dimethylamino)pyridine, piperidine, 1,8-Diazabicyclo[5.4.0]undec-7-ene, tetrabutylammonium hydroxide, morpholine, 1,4-Diazabicyclo[2.2.2]octane solution, tetraoctylammonium hydroxide, tert-Butyl-1,1,3,3-tetramethylguanidine, tetramethylammonium hydroxide, 1,1-dimethylguanidine, di iso propylamine, di-N-butylamine, tri ethylene pentamine (TEPA), tri methylamine, tri-n-butylamine, triethanolamine, triethylene tetramine, diisopropylethylamine, tri-N-nutylamine, tri ethylene tetra amine, diethyl tetra amine or mixtures thereof, preferably ammonia.
In one aspect of embodiment, the base used in step d) is in the range of 0.1 mole equivalent to 5.0 mole equivalent with respect to sulfentrazone.
In one another aspect of the embodiment the suitable acid used in step e) is selected from in-organic/organic acids such as hydrochloric acid, nitric acid, phosphoric acid, sulphuric acid, boric acid, hydrofluoric acid, hydrobromic acid, perchloric acid, chromic acid, acetic acid, trifluoroacetic acid, citric acid, formic acid, lactic acid or mixtures thereof, preferably hydrochloric acid.
In another aspect of the embodiment, the acid used in step e) is to adjust the pH between 6.5 to 1, amount of acid used is not constraint and generally 0.01 to 5 mole equivalent is required with respect to sulfentrazone.
In another aspect of the embodiment, in step f) of present invention, the pure sulfentrazone is isolated by filtration and dried at a temperature in the range of 25-100oC.
In one other aspect of the embodiment, the product formed in step e) can be isolated in step f) by a suitable technique known in the art such as filtration, centrifugation, decantation and the like. Typically, the product is isolated by filtration. The isolation can be optionally done under the nitrogen atmosphere by providing a blanket of nitrogen.
The crude form of sulfentrazone used as starting material in the present invention can be prepared by methods known in the art such as by processes as described in U.S. Patent Nos. 4,818,275; 5,990,315 or any of the method known in prior art preferably by following the process of U.S. Patent No. 5,990,315.
The foregoing description of the embodiments has been provided for purposes of illustration and not intended to limit the scope of the present disclosure. Individual components of a particular embodiment are generally not limited to that particular embodiment, but, are interchangeable. Such variations are not to be regarded as a departure from the present disclosure, and all such modifications are considered to be within the scope of the present disclosure.
The present disclosure is further described in light of the following experiments which are set forth for illustration purpose only and not to be construed for limiting the scope of the disclosure. The following experiments are scalable to industrial/commercial process.
Examples
Example 1:
Crude sulfentrazone (10 gm on dry basis) was taken in acetone (10 ml) and stir the reaction mass and add water (50 ml). Heat the reaction mass at 80-85 oC. Add aqueous ammonia solution (1.9 gm) to the reaction mass and maintained for 5 hours at 80-85 oC. Gradually cool to 20-30 oC and acidify by hydrochloric acid to maintain pH 5-6, stirred and filter the product, wash the product with water (20 gm) and dried at 80 oC for 6-8 hours to get pure sulfentrazone.
Pure Sulfentrazone: yield=90%; HPLC Purity=98.82%; Impurity A=0.06 and Impurity B=0.03%; assay >97% by HPLC.
Crude Sulfentrazone: HPLC Purity=97.73%; Impurity A=0.18 and Impurity B=0.22%; assay=93.91% by HPLC.
Example 2:
Crude Sulfentrazone (25 gm on dry basis) was taken in acetone (85 gm) and stirred at 50 oC. Solvent is distilled out under vacuum up to 90%. Heat the reaction mass at 60-65 oC and water (125 gm) was added and maintained for 1 hour and cool to room temperature, filtered and wash with water (25 gm) and dried at 80 oC for 6-8 hours to get pure sulfentrazone.
Pure Sulfentrazone: yield=92.8%; HPLC Purity=99.32%; Impurity A=0.27 and Impurity B=0.23%, assay=98.01% by HPLC.
Crude Sulfentrazone: HPLC Purity=97.29%; Impurity A=0.28 and Impurity B=0.25%, assay=94.37% by HPLC.
Example 3:
Crude Sulfentrazone (25 gm on dry basis) was taken in acetone (85 gm) and stirred at 50 oC. Solvent is distilled out under vacuum up to 90%. Heat the reaction mass at 60-65 oC and water (125 gm) was added. Add aqueous ammonia solution (1.19 gm) and heat to 70-75 oC and maintained for 10 hours. Cool to room temperature, acidifying with 1 N HCl till pH between 5-6, stirred, filtered and washed with water (25 gm) and dried at 85 oC for 6-8 hours to get pure sulfentrazone.
Pure Sulfentrazone: yield=91%; HPLC Purity=99.29%; Impurity A=0.21 and Impurity B=0.12%, assay=97.95% by HPLC.
Crude Sulfentrazone: HPLC Purity=97.29%; Impurity A=0.28 and Impurity B=0.25%, assay=94.37% by HPLC.
Example 4:
Crude Sulfentrazone (25 gm on dry basis) was taken in acetone (125 ml) and stirred at 60-70 0C. Distilled out acetone completely under atmospherically pressure. Heat the reaction mass at 60-70 oC for 30 min. Charged acetone (17 gm) in it and cool it to 25-35 oC. Water (125 gm) was added and maintain for 1 hour, filtered, washed with water (25 gm), dried at 80 oC for 6-8 hours to get pure sulfentrazone.
Pure Sulfentrazone yield: 90.8%; HPLC purity=98.79%, Impurity A=0.21% and Impurity B= 0.52%, Assay 97.67% by HPLC.
Crude Sulfentrazone: HPLC Purity: 98.24%, Impurity A=0.22% and Impurity B=0.52%, assay=95.43% by HPLC.
Example 5:
Crude Sulfentrazone (15 gm on dry basis) was taken in acetone (75 ml) and stirred at 40-50 oC. Solvent is distilled out completely under vacuum. Charged acetone (10.2 gm) in it and cool it to 25-35 oC. (Filtered Mother Liquor of similar process batch containing acetone-water Mother Liquor) (125 gm) was added and maintain for 1 hour, filtered, washed with water (15 gm), dried at 80 oC for 6-8 hours to get pure sulfentrazone.
Pure Sulfentrazone yield: 93.14%; HPLC purity=98.75%, Impurity A=0.21% and Impurity B= 0.52%, Assay 97.03% by HPLC
Crude Sulfentrazone: HPLC Purity: 98.24%, Impurity A=0.22% and Impurity B=0.52%, assay=95.43% by HPLC
Comparative Examples
Reproduction of Step-A-1 of example 1 disclosed in US20220106277A1
A stirred solution of 40 grams of 1-(5-amino-2,4-dichlorophenyl)-4-(difluoromethyl)-3-methyl-1H-1,2,4-triazol-5(4H)-one (0.129 moles) in 160 ml of toluene was heated to reflux at 110-115 °C for one hour to remove the moisture azeotropically. 54 ml of toluene was recovered during the dehydration. The reaction mass was cooled to 85-90 °C over a period of 30-45 minutes and 11.7 grams of pyridine (0.148 mole) was added over a period of 30 minutes. The addition of pyridine was followed by dropwise addition of 19.2 grams of methanesulfonyl chloride (0.168 mole) over a period of 3 hours by maintaining the temperature of reaction mass at 85-90 °C and continued to cook the reaction mass at 85-90 °C for a period of 2 hours. After completion of reaction, the reaction mass was cooled to 75-80 °C and solvent toluene was distilled under vacuum. After completion of toluene recovery 169 ml of solvent ethylene dichloride and 70 ml of water was added to the reaction mass and stirred for 15 minutes. Then layers were separated. The ethylene dichloride layer was distilled to remove 80% of ethylene dichloride to get the crude product, N-(2,4-dichloro-5-(4-(difluoromethyl)-3-methyl-5-oxo-4,5-dihydro-1H-1,2,4-triazol-1-yl) methanesulfonamide. Purity=86.42% by GC; Total Impurities 13.58% by GC.
Reproduction of Step-A-2 of example 1 disclosed in US20220106277A1
Charged 290 ml of water under stirring in the crude mass and was heated to 60-65 °C. 61 ml of 20% sodium carbonate solution (14.6 grams of sodium carbonate dissolved in 57.2 ml of water) was added under stirring over a period of 1 hour and further maintained the reaction mass at 60-65 °C for a period of 1.5 hours followed by the addition of 20% sodium carbonate solution, 24.3 ml of concentrated hydrochloric acid (30% strength) was added dropwise over a period of 3-4 hours at 60-65 °C. The mass was stirred for 30 minutes. The precipitated solid was collected by filtration and washed with water. The dried solid weighed 45 grams of N-(2,4-dichloro-5-(4-(difluoromethyl)-3-methyl-5-oxo-4,5-dihydro-1H-1,2,4-triazol-1-yl) methanesulfonamide with yield 90% and purity 89.301% by GC; Assay: 85.28%; Total Impurities 10.699% by GC.
Technical advancement
The process according to the present invention has the following advantages over the known
prior art,
• The process of the present invention is carried out at ambient temperature.
• The process of the present invention is carried out at atmospheric pressure.
• The process is simple, efficient and environment friendly.
• The process provides highly pure sulfentrazone having assay more than 97%.
• The process provides highly pure sulfentrazone wherein impurity A (chloromethyl) and impurity B (chlorofluoromethyl) each present in an amount of not more than 0.3%.
• The process provides highly pure sulfentrazone in higher yield.
• The process of the present invention requires the minimum operations which is economical and efficient on a commercial scale.
• The technical advancement of the present invention is summarised herein below table:
Example Nos. Process parameters Process Variation Yield*
(%) Assay (%)
by HPLC Purity
(%)
by HPLC Impurity A
(%) Impurity B
(%)
Solvent Acid Base
Example 1 Acetone HCl Ammonia Less acetone with ammonia 90.0 97.03 98.82 0.06 0.03
Example 2 Acetone NA NA Without acid and base 92.8 98.01 99.32 0.27 0.23
Example 3 Acetone HCl Ammonia Higher acetone with ammonia 91.0 97.95 99.29 0.21 0.12
Example 4 Acetone NA NA Higher acetone qty. 90.8 97.67 98.79 0.21 0.52
Example 5 Acetone NA NA Recycled Mother liquor 93.14 97.03 98.75 0.21 0.52

Thus, from the foregoing description, it will be apparent to one of the person skilled in the art that many changes and modifications can be made thereto without departing from the scope of the invention as set forth in the description. Accordingly, it is not intended that the scope of the foregoing description be limited to the description set forth above, but rather that such description be construed as encompassing such features that reside in the present invention, including all the features and embodiments that would be treated as equivalents thereof by those skilled in the relevant art.
,CLAIMS:We Claim:
1. An improved process for the preparation of highly pure sulfentrazone having assay more than 97% and impurity A (chloromethyl) and impurity B (chlorofluoromethyl) each present in an amount of not more than 0.3% comprising the steps of:
a) dissolving the crude sulfentrazone in solvent acetone;
b) optionally distilling the solvent;
c) adding a water to the step b) mixture;
d) adding ammonia to the step c) mixture;
e) adding hydrochloric acid to the step d) mixture; and
f) isolating pure sulfentrazone.
2. The process as claimed in claim 1 wherein in step a) the solvent acetone is used in the range of 0.2 volume to 20 volumes.
3. The process as claimed in claim 1 wherein in step b) the solvent acetone is distilled more than 85% of used solvent amount.
4. The process as claimed in claim 1 wherein in step c) the water is used in the range of 0.1 volume to 100 volumes.
5. The process as claimed in claim 1 wherein in step d) the ammonia is used in the range of 0.1 mole equivalent to 5.0 mole equivalent with respect to sulfentrazone.
6. The process as claimed in claim 1 wherein in step e) the hydrochloric is used to adjust the pH between 6.5 to 1.
7. An improved process for the preparation of highly pure sulfentrazone having assay more than 97% comprising the steps of:
a) dissolving the crude sulfentrazone in solvent acetone;
b) optionally distilling the solvent;
c) adding a water to the step b) mixture; and
d) isolating pure sulfentrazone.
8. The process as claimed in claim 7 wherein in step a) the solvent acetone is used in the range of 0.2 volume to 20 volumes.
9. The process as claimed in claim 7 wherein in step b) the solvent acetone is distilled more than 85% of used solvent amount.
10. The process as claimed in claim 7 wherein in step c) the water is used in the range of 0.1 volume to 100 volumes.