Clean production method of 5-methyl-3-(trifluoromethyl)-1H-pyrazole

Abstract

The invention belongs to the field of chemical synthesis, and particularly relates to a method for clean production of 5-methyl-3-(trifluoromethyl)-1H-pyrazole. Methyl trifluoroacetate is used as a raw material and reacts with acetone under the conditions of a solvent and a catalyst, a reaction solution is subjected to acidification and negative pressure distillation to obtain a trifluoroacetylacetone methanol solution, the trifluoroacetylacetone methanol solution directly reacts with hydrazine hydrate, and 5-methyl-3-trifluoromethyl-lH-pyrazole is obtained. The content of the 5-methyl-3-(trifluoromethyl)-1H-pyrazole obtained through the method reaches 99% or above, the two-step synthesis yield reaches 70%, and the method has the advantages of being high in product content, few in impurities, simple in technological process, convenient in solvent recovery and the like, and meets the requirement of clean production.

Description

Technical Field

[0001] This invention belongs to the field of chemical synthesis, specifically relating to a clean production method for 5-methyl-3-(trifluoromethyl)-1H-pyrazole. Background Technology

[0002] 5-Methyl-3-(trifluoromethyl)-1H-pyrazole is an important intermediate in the synthesis of the fungicide fluthiazolium pyroxyfenozide. Fluthiazolium pyroxyfenozide's active group contains isoxazole, pyrazole, and thiazole rings, making it the first piperidinylthiazolium isoxazoline fungicide. Fluthiazolium pyroxyfenozide has a unique site of action against oomycete pathogens, and its mechanism of action is the inhibition of oxysterol-binding protein (OSBP). It significantly enhances the control effect against diseases of specialty crops such as potatoes, grapes, and vegetables, exhibiting good rapid-acting, long-lasting, and rain-wash-resistant properties. It demonstrates excellent control efficacy at extremely low dosages (only 1 / 100 to 1 / 5 of commonly used fungicides) and shows outstanding control efficacy against oomycete diseases such as late blight, downy mildew, root rot, stem rot, and blight.

[0003] WO2007 / 071900 describes a stepwise synthesis method for trifluoroacetylacetone and 5-methyl-3-trifluoromethyl-1H-pyrazole. This method uses ethyl trifluoroacetate, acetone, sodium methoxide, or sodium ethoxide in a one-pot reaction with ethanol under elevated temperature. After solvent removal under negative pressure, the mixture is acidified with sulfuric acid and distilled under reduced pressure to obtain trifluoroacetylacetone, with a yield of 40.1-45.4%. The obtained trifluoroacetylacetone is then added to methanol, followed by the slow addition of hydrazine hydrate, and reacted at room temperature for 16 h. The mixture is then concentrated to obtain 5-methyl-3-trifluoromethyl-1H-pyrazole. This one-pot heated reaction method for preparing trifluoroacetylacetone has a low yield. The excessive use of acetone leads to a complex composition of the removed solvent, requiring high precision in separation and purification. After solvent removal, the material solidifies and dries, and direct addition of sulfuric acid is exothermic, making temperature control difficult. A graduate thesis from Tianjin University of Science and Technology reported a two-step yield of only 38%. A major drawback is that both ethyl trifluoroacetate and acetone are highly unstable in the system and are prone to self-hydrolysis and condensation reactions. CN117736077A describes a synthesis process using toluene, tetrabutylammonium bromide, and ethyl trifluoroacetate as raw materials. Acetone and 40% sodium hydroxide are added dropwise while the temperature is lowered. After the reaction is complete, the mixture is separated, and the pH of the aqueous phase is adjusted to 4.0 with 10% hydrochloric acid. Trifluoroacetylacetone is then distilled off, and GC quantitative analysis shows a purity of 98.2% and a yield of 90.3%. This reaction is hydrophobic, yet this patent uses 40% sodium hydroxide. Furthermore, comparative examples 1 and 2 show that the yields of trifluoroacetylacetone are both poor.

[0004] Therefore, existing methods for synthesizing 5-methyl-3-trifluoromethyl-1H-pyrazole generally suffer from problems such as low yield, numerous side reactions, poor operational safety, complex post-processing, or poor reproducibility. Thus, developing a synthetic method with mild reaction conditions, simple operation, high yield, and suitability for large-scale production has significant industrial application value. Summary of the Invention

[0005] The purpose of this invention is to provide a clean production method for 5-methyl-3-(trifluoromethyl)-1H-pyrazole.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A clean production method for 5-methyl-3-(trifluoromethyl)-1H-pyrazole is disclosed, wherein methyl trifluoroacetate is used as a raw material and reacted with acetone under solvent and catalyst conditions. The reaction solution is acidified and distilled under negative pressure to obtain a trifluoroacetylacetone methanol solution, which is then directly reacted with hydrazine hydrate to obtain 5-methyl-3-trifluoromethyl-1H-pyrazole.

[0007] The synthesis of trifluoroacetylacetone uses the same solvent as 5-methyl-3-trifluoromethyl-1H-pyrazole, achieving solvent integration in the two-step system.

[0008] The reaction formula is:

[0009] Specifically, an organic base is used as a substrate. Methyl trifluoroacetate is added dropwise under the action of a catalyst, followed by the addition of acetone. The reaction solution is acidified with hydrogen chloride methanol solution. After acidification, the solution is distilled under negative pressure to obtain trifluoroacetylacetone methanol solution. The trifluoroacetylacetone methanol solution is then quantitatively reacted directly with hydrazine hydrate. The synthesized solution is then subjected to methanol recovery, water crystallization, filtration, and drying to obtain 5-methyl-3-trifluoromethyl-1H-pyrazole.

[0010] The organic base is sodium methoxide, methanol, or methanol solution; the catalyst is a quaternary ammonium salt catalyst, including but not limited to one or more of tetrabutylammonium bromide, tetramethylammonium chloride, and benzyltriethylammonium chloride.

[0011] The molar ratio of the organic base to methyl trifluoroacetate is 1~3:1; the molar ratio of acetone to methyl trifluoroacetate is 1~1.03:1; and the weight ratio of the catalyst to methyl trifluoroacetate is 0.01~0.1:1.

[0012] The dropping temperature for methyl trifluoroacetate and acetone is -5~10℃, preferably -5~5℃; the dropping time for methyl trifluoroacetate is 0.3~2h, and the dropping time for acetone is 0.5-4h.

[0013] The reaction solution is acidified with a hydrogen chloride methanol solution, wherein the concentration of the hydrogen chloride methanol solution is 25-38%; the acidification temperature is controlled at -5-10℃, preferably -5-5℃; the pH value at the acidification endpoint is 1-4, preferably 3-4.

[0014] The distillation at the end of acidification is carried out under negative pressure, with a final temperature of 45-50℃ / 5-20mmHg and a condensing medium temperature of ≤-10℃.

[0015] The molar ratio of trifluoroacetylacetone methanol solution to hydrazine hydrate is 1:1~1.03; the reaction temperature of trifluoroacetylacetone methanol solution and hydrazine hydrate is 15-30℃, preferably 15-25℃; the dropping time is 2-20h, preferably 5-8h; the heat preservation time after dropping is 16-48h.

[0016] The reaction solution is de-methanolized under negative pressure at a final temperature of 45-50℃ / 5-20 mmHg. The amount of water added for crystallization is 2-3 times the weight of trifluoroacetylacetone, and the crystallization temperature is 5-15℃ for 1-3 hours. The recovered methanol is purified and reused in the preparation of hydrogen chloride methanol solution, with excess methanol as a byproduct. 5-Methyl-3-(trifluoromethyl)-1H-pyrazole is dried under negative pressure at a temperature of 50-60℃ / 30 mmHg for 5 hours.

[0017] The residue obtained from distillation after acidification is dissolved in salt, extracted, adsorbed in an aqueous layer, concentrated, and dried to recover sodium chloride as a byproduct.

[0018] The beneficial effects of this invention are: 1. The 5-methyl-3-(trifluoromethyl)-1H-pyrazole obtained by the method of the present invention has a content of over 99%, and the two-step synthesis yield reaches 70%. It has the characteristics of high product content, low impurities, simple process, and convenient solvent recovery, and meets the requirements of clean production.

[0019] 2. The present invention uses the same solvent for the synthesis of trifluoroacetylacetone and 5-methyl-3-trifluoromethyl-1H-pyrazole, realizing the integration of solvent in the two-step system; the distilled trifluoroacetylacetone methanol solution does not need to be separated, and 5-methyl-3-trifluoromethyl-1H-pyrazole can be directly synthesized after quantification; the recovered methanol content is high, impurities are few, and it is easy to recover; the distillation residue is easy to recover by-product sodium chloride after water addition, extraction, and adsorption, which meets the requirements of clean production. Detailed Implementation

[0020] The following describes in detail the specific embodiments of the technical solution of the present invention, but the present invention is not limited to the following description. The present invention will be further illustrated below with reference to specific examples: Example 1 Add 172.8 g of 30% sodium methoxide methanol solution and 1.5 g of tetrabutylammonium bromide sequentially to a 1000 ml four-necked flask. Stir and cool to 2 °C. Maintain the temperature at 2 °C and begin adding 122.8 g of methyl trifluoroacetate dropwise over 1.5 h. After the addition is complete, continue adding 55.8 g of acetone dropwise over 3.5 h. After the addition is complete, maintain the reaction temperature for 5 h. Maintain the temperature of the synthesis solution at 5 °C and add 35% hydrogen chloride methanol solution (self-made, hydrogen chloride is bubbled into methanol) dropwise to adjust the pH of the system to 3. Distill under negative pressure water bath, controlling the condensing medium temperature at -12 °C, with a final solution removal temperature of 48 °C / 10 mmHg. Distillation yielded 101.0 g of trifluoroacetylacetone methanol solution (69.4% yield). The trifluoroacetylacetone methanol solution obtained by vacuum distillation was added to a 1000 ml four-necked flask. The temperature was controlled at 20°C, and 41.6 g of 80% hydrazine hydrate was added dropwise over 6 hours. After the addition was completed, the mixture was stirred at room temperature for 20 hours. Methanol was removed from the mixture in a negative pressure water bath to 48°C / 15 mmHg. The remaining material was dissolved in 280 g of water by stirring, cooled to 10°C, and kept at that temperature for 2 hours. Crystallization was then carried out upon cooling, and the mixture was filtered to obtain 94.3 g of a yellow solid, 5-methyl-3-trifluoromethyl-1H-pyrazole, with a purity of 99.4% and a yield of 66.1%.

[0021] The distillation residue from the synthesis of trifluoroacetylacetone was added to 300g of water-soluble salt and 50g of chloroform, and then stirred and extracted twice. The separated aqueous layer was adsorbed by a resin adsorption system, and then concentrated and dried to recover sodium chloride as a byproduct. The sodium chloride content reached over 99%.

[0022] Example 2 Add 208g of 30% sodium methoxide methanol solution and 1.5g of tetrabutylammonium bromide sequentially to a 1000ml four-necked flask. Stir and cool to 0℃. Maintain the temperature at 0℃ and begin adding 122.8g of methyl trifluoroacetate dropwise over 1.0h. After the addition is complete, continue adding 55.8g of acetone dropwise over 2.5h. After the addition is complete, maintain the reaction temperature for 5h. Maintain the temperature of the synthesis solution at 0℃ and add 35% hydrogen chloride methanol solution (self-made, hydrogen chloride is bubbled into methanol) dropwise to adjust the pH of the system to 2.0. Distill under negative pressure water bath, controlling the condenser temperature at -10℃, with a final temperature of 48℃ / 10mmHg. Distillation yields 110.0g of trifluoroacetylacetone methanol solution (75.6% yield). The trifluoroacetylacetone methanol solution obtained by vacuum distillation was added to a 1000 ml four-necked flask. The temperature was controlled at 25°C, and 45.4 g of 80% hydrazine hydrate was added dropwise over 6 hours. After the addition was completed, the mixture was stirred at room temperature for 15 hours. Methanol was removed from the mixture in a negative pressure water bath to 48°C / 10 mmHg. The remaining material was dissolved in 300 g of water by stirring, cooled to 8°C, and kept at that temperature for 2 hours. Crystallization was then carried out upon cooling, and the mixture was filtered to obtain 102.5 g of a yellow solid, 5-methyl-3-trifluoromethyl-1H-pyrazole, with a purity of 99.4% and a yield of 71.9%.

[0023] The distillation residue from the synthesis of trifluoroacetylacetone was added to 300g of water-soluble salt and 50g of chloroform for a second extraction with stirring. The separated aqueous layer was then adsorbed by a resin adsorption system, concentrated, and dried to recover sodium chloride as a byproduct. The sodium chloride content reached over 99%.

[0024] Example 3 Add 208g of 30% sodium methoxide solution and 2.5g of tetramethylammonium chloride sequentially to a 1000ml four-necked flask. Stir and cool to -2℃. Maintain the temperature at -2℃ and begin adding 122.8g of methyl trifluoroacetate dropwise over 1.5 hours. After the addition is complete, continue adding 55.8g of acetone dropwise over 2.5 hours. After the addition is complete, maintain the reaction temperature for 5 hours. Maintain the temperature of the synthesis solution at -2℃ and add 35% hydrogen chloride methanol solution (self-made, hydrogen chloride is bubbled into methanol) dropwise to adjust the pH of the system to 2.0. Distill under negative pressure in a water bath, controlling the condenser temperature at -15℃, with a final temperature of 48℃ / 10mmHg. Distillation yielded 103.1g of trifluoroacetylacetone methanol solution (70.9% yield).

[0025] The trifluoroacetylacetone methanol solution obtained by vacuum distillation was added to a 1000 ml four-necked flask. The temperature was controlled at 20°C, and 42.5 g of 80% hydrazine hydrate was added dropwise over 8 hours. After the addition was completed, the reaction was stirred at room temperature for 20 hours. Methanol was removed from the solution in a negative pressure water bath to 48°C / 10 mmHg. The remaining material was dissolved in 280 g of water by stirring, cooled to 10°C, and kept at that temperature for 2 hours. Crystallization was then carried out upon cooling, and the mixture was filtered to obtain 96.2 g of a yellow solid, 5-methyl-3-trifluoromethyl-1H-pyrazole, with a purity of 99.1% and a yield of 67.2%.

[0026] The distillation residue from the synthesis of trifluoroacetylacetone was added to 300g of water-soluble salt and 50g of chloroform for a second extraction with stirring. The separated aqueous layer was then adsorbed by a resin adsorption system, concentrated, and dried to recover sodium chloride as a byproduct. The sodium chloride content reached over 99%.

[0027] Example 4 Add 312g of 30% sodium methoxide solution and 2.5g of benzyltriethylammonium chloride sequentially to a 1000ml four-necked flask. Stir and cool to 0℃, maintaining the temperature at 0℃. Begin adding 122.8g of methyl trifluoroacetate dropwise over 2 hours. After the addition is complete, continue adding 55.8g of acetone dropwise over 4 hours. After the addition is complete, maintain the reaction temperature for 5 hours. Maintain the temperature of the synthesis solution at 0℃ and add 35% hydrogen chloride methanol solution (self-made, hydrogen chloride is bubbled into methanol) dropwise to adjust the pH to 2.0. Distill under negative pressure in a water bath, controlling the condenser temperature at -15℃, with a final temperature of 48℃ / 10mmHg. Distillation yields 110.0g of trifluoroacetylacetone methanol solution (75.6% yield).

[0028] The trifluoroacetylacetone methanol solution obtained by vacuum distillation was added to a 1000 ml four-necked flask. The temperature was controlled at 28°C, and 45.4 g of 80% hydrazine hydrate was added dropwise over 6 hours. After the addition was completed, the mixture was stirred at room temperature for 25 hours. Methanol was removed from the mixture in a negative pressure water bath to 48°C / 10 mmHg. The remaining material was dissolved in 280 g of water by stirring, cooled to 10°C, and kept at that temperature for 2 hours. Crystallization was then carried out upon cooling, and the mixture was filtered to obtain 103.2 g of a yellow solid, 5-methyl-3-trifluoromethyl-1H-pyrazole, with a purity of 99.3% and a yield of 72.3%.

[0029] The distillation residue from the synthesis of trifluoroacetylacetone was added to 300g of water-soluble salt and 50g of chloroform for a second extraction with stirring. The separated aqueous layer was then adsorbed by a resin adsorption system, concentrated, and dried to recover sodium chloride as a byproduct. The sodium chloride content reached over 99%.

[0030] Example 5 Add 208g of 30% sodium methoxide solution and 1.8g of tetramethylammonium chloride sequentially to a 1000ml four-necked flask. Stir and cool to -3℃. Maintain the temperature at -3℃ and begin adding 122.8g of methyl trifluoroacetate dropwise over 1.5 hours. After the addition is complete, continue adding 55.8g of acetone dropwise over 2.5 hours. After the addition is complete, maintain the reaction temperature for 5 hours. Maintain the temperature of the synthesis solution at -5℃ and add 35% hydrogen chloride methanol solution (self-made, hydrogen chloride is bubbled into methanol) dropwise to adjust the pH of the system to 3.5. Distill under negative pressure in a water bath, maintaining the condenser temperature at -15℃, with a final temperature of 48℃ / 10mmHg. Distillation yielded 104.9g of trifluoroacetylacetone methanol solution (72.1% yield).

[0031] The trifluoroacetylacetone methanol solution obtained by vacuum distillation was added to a 1000 ml four-necked flask. The temperature was controlled at 20°C, and 42.7 g of 80% hydrazine hydrate was added dropwise over 5 hours. After the addition was completed, the mixture was stirred at room temperature for 30 hours. Methanol was removed from the mixture in a negative pressure water bath at 45°C / 5 mmHg. The remaining material was dissolved in 300 g of water by stirring, cooled to 0°C, and kept at that temperature for 2 hours. Crystallization was then carried out upon cooling, and the mixture was filtered to obtain 98.7 g of a yellow solid, 5-methyl-3-trifluoromethyl-1H-pyrazole, with a purity of 99.3% and a yield of 69.1%.

[0032] The distillation residue from the synthesis of trifluoroacetylacetone was added to 300g of water-soluble salt and 50g of chloroform for a second extraction with stirring. The separated aqueous layer was then adsorbed by a resin adsorption system, concentrated, and dried to recover sodium chloride as a byproduct. The sodium chloride content reached over 99%.

[0033] Comparative Example 1 To a 1000 mL four-necked flask, add 320 g of toluene, 1.6 g of tetrabutylammonium bromide, and 80 g of ethyl trifluoroacetate sequentially. Cool to 12 °C, and simultaneously add 82 g of acetone and 112 g of 40% sodium hydroxide solution dropwise. After the addition is complete, maintain the temperature at 12 °C and react for 9 hours. After the reaction is complete, separate the phases. Adjust the pH of the aqueous phase to 4.0 with 10% hydrochloric acid. Distill off the trifluoroacetylacetone under negative pressure and collect the fraction from 31–33 °C at -0.09 MPa, yielding 36.4 g of trifluoroacetylacetone with a purity of 85.4% and a yield of 36.0%.

[0034] Add the above-mentioned trifluoroacetylacetone, then add 80 ml of methanol, heat to 25°C, and add 12.8 g of 80% hydrazine hydrate dropwise over 5 hours. After the addition is complete, stir the reaction at room temperature for 30 hours. Remove methanol from the mixture using a negative pressure water bath to 50°C / 5 mmHg. Add 70 g of water to the remaining material and stir to dissolve. Cool to 0°C and maintain the temperature for 2 hours. Allow to cool and crystallize, then filter to obtain 31.7 g of a yellow solid of 5-methyl-3-trifluoromethyl-1H-pyrazole, with a purity of 96.9% and a yield of 33.3%.

[0035] Comparative Example 2 To a 1000 mL four-necked flask, 320 g of toluene, 1.6 g of tetrabutylammonium bromide, and 72.1 g of methyl trifluoroacetate were added sequentially. The temperature was lowered to 12 °C, and 82 g of acetone and 112 g of 40% sodium hydroxide solution were added dropwise simultaneously. After the addition was complete, the reaction was maintained at 12 °C for 9 h. After the reaction was completed, the phases separated. The pH of the aqueous phase was adjusted to 4.0 with 10% hydrochloric acid. The trifluoroacetylacetone was then distilled off under negative pressure. The fraction collected at 31–33 °C / -0.09 MPa yielded 36.9 g of trifluoroacetylacetone with a purity of 88.2% and a yield of 37.5%.

[0036] Add the above-mentioned trifluoroacetylacetone, then add 80 ml of methanol, heat to 20°C, and add 13.2 g of 80% hydrazine hydrate dropwise over 5 hours. After the addition is complete, stir the reaction at room temperature for 30 hours. Remove methanol from the mixture using a negative pressure water bath to 50°C / 5 mmHg. Add 70 g of water to the remaining material and stir to dissolve. Cool to 5°C and maintain the temperature for 2 hours. Allow to cool and crystallize, then filter to obtain 30.2 g of a yellow solid, 5-methyl-3-trifluoromethyl-1H-pyrazole, with a purity of 96.3% and a yield of 34.9%.

[0037] Comparative Example 3 Add 500 mL of ethanol to a 1000 mL four-necked flask, followed by 204 g of sodium ethoxide, 142 g of ethyl trifluoroacetate, and 174 g of acetone while stirring. Heat to 50 °C and react under slight negative pressure (-0.01 MPa) for 4 h. Cool to 20 °C, and distill under vacuum until 50 °C to remove the solvent. Add 294 g of sulfuric acid to the remaining material, and distill under negative pressure to remove trifluoroacetylacetone. Collect the fraction at 31–33 °C / -0.09 MPa to obtain 64.2 g of trifluoroacetylacetone, with a purity of 97.6% and a yield of 41.7%.

[0038] Comparative Example 4 Add 500 mL of methanol to a 1000 mL four-necked flask, followed by stirring and the addition of 162 g of sodium methoxide, 128 g of methyl trifluoroacetate, and 174 g of acetone. Heat to 50 °C and react under slight negative pressure (-0.01 MPa) for 4 h. Cool to 20 °C, and distill under vacuum until 50 °C to remove the solvent. Add 294 g of sulfuric acid to the remaining material and distill under negative pressure to remove trifluoroacetylacetone. Collect the fraction at 31–33 °C / -0.09 MPa to obtain 69.6 g of trifluoroacetylacetone, with a purity of 98.2% and a yield of 44.3%.

[0039] The above examples are merely illustrative of the technical concept and features of the present invention and should not be construed as limiting the scope of protection of the present invention. All equivalent transformations or modifications made in accordance with the essence of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for the clean production of 5-methyl-3-(trifluoromethyl)-1H-pyrazole, characterized in that, Methyl trifluoroacetate was used as a raw material and reacted with acetone under organic base and catalyst conditions. The reaction solution was acidified and distilled under negative pressure to obtain a methanol solution of trifluoroacetylacetone, which was then reacted directly with hydrazine hydrate to obtain 5-methyl-3-trifluoromethyl-1H-pyrazole.

2. The method for the clean production of 5-methyl-3-(trifluoromethyl)-1H-pyrazole according to claim 1, characterized in that, Using an organic base as a substrate, methyl trifluoroacetate was added dropwise under the action of a catalyst, followed by the addition of acetone. The reaction solution was acidified with a hydrogen chloride methanol solution. After acidification, the solution was distilled under negative pressure to obtain a trifluoroacetylacetone methanol solution. The trifluoroacetylacetone methanol solution was directly reacted with hydrazine hydrate. The synthesized solution was then subjected to methanol recovery, water crystallization, filtration, and drying to obtain 5-methyl-3-trifluoromethyl-1H-pyrazole.

3. The method for the clean production of 5-methyl-3-(trifluoromethyl)-1H-pyrazole according to claim 2, characterized in that, The organic base is a sodium methoxide methanol solution.

4. The method for the clean production of 5-methyl-3-(trifluoromethyl)-1H-pyrazole according to claim 2, characterized in that, The catalyst is one or more of tetrabutylammonium bromide, tetramethylammonium chloride, and benzyltriethylammonium chloride.

5. The method for the clean production of 5-methyl-3-(trifluoromethyl)-1H-pyrazole according to claim 2, characterized in that, The molar ratio of the organic base to methyl trifluoroacetate is 1~3:1; the molar ratio of acetone to methyl trifluoroacetate is 1~1.03:1; and the weight ratio of the catalyst to methyl trifluoroacetate is 0.01~0.1:

1.

6. The method for clean production of 5-methyl-3-(trifluoromethyl)-1H-pyrazole according to claim 2, characterized in that, The dropping temperature for methyl trifluoroacetate and acetone is -5~10℃; the dropping time for methyl trifluoroacetate is 0.3~2h, and the dropping time for acetone is 0.5~4h.

7. The method according to claim 1, characterized in that: The concentration of the hydrogen chloride methanol solution is 25-38%; the acidification temperature is controlled at -5-10℃; and the pH value at the acidification endpoint is 1-4.

8. According to the method of claim 1, the distillation at the end of acidification is carried out under negative pressure, with a final temperature of 45-50℃ / 5-20mmHg and a condensing medium temperature of ≤-10℃.

9. The method for the clean production of 5-methyl-3-(trifluoromethyl)-1H-pyrazole according to claim 2, characterized in that, The molar ratio of trifluoroacetylacetone methanol solution to hydrazine hydrate is 1:1 to 1.

03.

10. The method for the clean production of 5-methyl-3-(trifluoromethyl)-1H-pyrazole according to claim 2, characterized in that, The temperature for adding trifluoroacetylacetone methanol solution to hydrazine hydrate is 15-30℃; the addition time is 2-20h; and the reaction time to complete the addition is 16-48h.

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