A process for the preparation of 2-chloro-3-methylbenzoic acid

Abstract

This invention discloses a method for preparing 2-chloro-3-methylbenzoic acid. The method comprises the following steps: 2,6-dimethylchlorobenzene undergoes an oxidation reaction under the action of an oxidant, a catalyst, and an initiator at a temperature of 60–200°C and a pressure of 0.1 MPa–1.0 MPa; wherein the oxidant is a gas with an oxygen content of 10–100%; and the initiator is 2-chloro-3-methylbenzaldehyde. This invention uses 2-chloro-3-methylbenzaldehyde as the initiator, eliminating the need for KBr or NaBr-containing initiators and reducing production costs. Furthermore, the reaction conditions of this invention can be applied to tubular reactions, thereby reducing safety risks.

Description

Technical Field

[0001] This invention relates to a method for preparing 2-chloro-3-methylbenzoic acid. Background Technology

[0002] Chlorobenzoic acids are an important class of chemical intermediates with wide applications in the synthesis of pesticides, pharmaceuticals, and fine chemicals. Among them, 2-chloro-3-methylbenzoic acid is an important intermediate in the synthesis of the next-generation insecticide chlorantraniliprole, and can also be used to synthesize various folic acid analogs and anticancer drugs.

[0003] Recent patents CN1088572A, CN108530297A, and CN107698436A have all reported methods for preparing 2-chloro-3-methylbenzoic acid. However, these patents all require the addition of an inorganic salt initiator to the reaction system. For example, CN108530297A reports the preparation of 2-chloro-3-methylbenzoic acid by oxidizing 2,6-dimethylchlorobenzene with oxygen under the conditions of cobalt acetylacetonate as a catalyst and KBr as an initiator, as shown in the reaction formula below. However, the addition of inorganic salts increases the post-processing procedures of the final product, increasing the difficulty of industrialization.

[0004]

[0005] There is an urgent need in this field for a method for preparing 2-chloro-3-methylbenzoic acid that is more suitable for industrial production. Summary of the Invention

[0006] The technical problem this invention aims to solve is to overcome the limitation that existing methods for preparing 2-chloro-3-methylbenzoic acid require the addition of inorganic salts as initiators, resulting in high requirements for industrial production. Therefore, this invention provides a method for preparing 2-chloro-3-methylbenzoic acid. The method of this invention uses 2-chloro-3-methylbenzaldehyde as the initiator, replacing KBr to reduce reaction costs (2-chloro-3-methylbenzaldehyde can ultimately be converted into the product). Furthermore, the method of this invention can be applied to tubular reactions, has low safety risks, and is easy to industrialize.

[0007] This invention provides a method 1 for preparing 2-chloro-3-methylbenzoic acid, which includes the following steps: 2,6-dimethylchlorobenzene undergoes an oxidation reaction in the presence of an oxidant, a catalyst, and an initiator under conditions of a temperature of 60-200°C and a pressure of 0.1 MPa-1.0 MPa.

[0008]

[0009] The oxidant is a gas with an oxygen content of 10-100%.

[0010] The initiator is 2-chloro-3-methylbenzaldehyde.

[0011] Using 2-chloro-3-methylbenzaldehyde as an initiator can reduce the risk of inorganic residues in the product.

[0012] The reactor in method 1 for preparing 2-chloro-3-methylbenzoic acid can be a conventional reactor in the art, preferably a continuous flow reactor, an oxidation tower, or an autoclave. The continuous flow reactor can be a conventional tubular continuous flow reactor, such as a Hastelloy loop reactor. The autoclave is preferably a stainless steel autoclave. When the reactor is a continuous flow reactor, the reaction solution using 2-chloro-3-methylbenzaldehyde as an initiator has higher fluidity than that using an inorganic initiator, reducing the risk of pipeline blockage.

[0013] In the preparation method 1 of 2-chloro-3-methylbenzoic acid, when the reactor of the preparation method 1 of 2-chloro-3-methylbenzoic acid is a continuous flow reactor, the frequency of the circulation pump of the continuous flow reactor can be 30-60Hz, preferably 40-45Hz, for example 40Hz.

[0014] In the preparation method 1 of 2-chloro-3-methylbenzoic acid, when the reactor of the preparation method 1 of 2-chloro-3-methylbenzoic acid is a continuous flow reactor or an oxidation tower, the oxygen content in the oxidant is preferably 15% to 40%; more preferably 18% to 25%, for example 20%.

[0015] In the preparation method 1 of 2-chloro-3-methylbenzoic acid, when the reactor of the preparation method 1 of 2-chloro-3-methylbenzoic acid is an autoclave, the oxygen content in the oxidant is preferably 60-100%, for example 100%.

[0016] In method 1 for preparing 2-chloro-3-methylbenzoic acid, the gas includes inert gas and oxygen. The inert gas is preferably selected from one or more of nitrogen, argon, and argon, for example, nitrogen.

[0017] In method 1 for preparing 2-chloro-3-methylbenzoic acid, the gas is preferably air or a mixture of an inert gas and oxygen. The air is preferably compressed air.

[0018] In the preparation method 1 of 2-chloro-3-methylbenzoic acid, when the reactor of the preparation method 1 of 2-chloro-3-methylbenzoic acid is a continuous flow reactor or an oxidation tower, the reaction temperature is preferably 110-200℃, more preferably 160-180℃, for example 160-170℃.

[0019] In the preparation method 1 of 2-chloro-3-methylbenzoic acid, when the reactor of the preparation method 1 of 2-chloro-3-methylbenzoic acid is an autoclave, the reaction temperature is preferably 60-110°C, more preferably 60-80°C, for example 70-75°C.

[0020] In the preparation method 1 of 2-chloro-3-methylbenzoic acid, the catalyst can be a catalyst commonly used in this type of oxidation reaction in the art, preferably a cobalt salt, more preferably one or more of cobalt acetylacetonate, cobalt acetate, cobalt isooctanoate and cobalt dichloride, such as cobalt acetylacetonate or cobalt isooctanoate.

[0021] In method 1 for preparing 2-chloro-3-methylbenzoic acid, the mass ratio of 2,6-dimethylchlorobenzene to the catalyst can be a conventional mass ratio for such oxidation reactions in the art, more preferably 1:(0.001-0.007), for example 1:(0.005-0.007), and even more preferably 1:0.006.

[0022] In method 1 for preparing 2-chloro-3-methylbenzoic acid, the mass ratio of 2,6-dimethylchlorobenzene to 2-chloro-3-methylbenzaldehyde can be a conventional mass ratio for this type of oxidation reaction in the art, preferably 1:(0.001-0.02), for example 1:0.01. 2-chloro-3-methylbenzaldehyde can ultimately be converted into the product 2-chloro-3-methylbenzoic acid. Using 2-chloro-3-methylbenzaldehyde as an initiator can omit the step of removing other inorganic initiators, effectively reducing material costs, and also omitting the step of removing inorganic salts, saving process costs.

[0023] In the preparation method 1 of 2-chloro-3-methylbenzoic acid, the pressure is preferably 0.3 MPa to 0.9 MPa; for example, 0.4 MPa, 0.5 MPa, 0.6 MPa, 0.7 MPa or 0.8 MPa.

[0024] In method 1 for preparing 2-chloro-3-methylbenzoic acid, the molar ratio of the oxidant to the 2,6-dimethylchlorobenzene can be (0.23 to 15.00):1, for example, 5.5:1.

[0025] In the preparation method, the gas introduction rate can be 120–900 mL / min, preferably 600–900 mL / min; for example, 700–800 mL / min. Too fast a gas introduction rate results in excessive material being blown out and severe flooding; too slow a rate results in prolonged reaction time.

[0026] In method 1 for preparing 2-chloro-3-methylbenzoic acid, the reaction time of the oxidation reaction is 1 to 10 hours, preferably 4 to 5 hours.

[0027] The preparation method 1 of 2-chloro-3-methylbenzoic acid also includes filtration and direct reuse of the filtrate in the next single reaction after the single reaction is completed.

[0028] Preferably, when adding catalyst, the mass ratio of 2,6-dimethylchlorobenzene to the catalyst can be a conventional mass ratio for such oxidation reactions in the art, preferably 1:(0.001 to 0.01), more preferably 1:(0.001 to 0.007), for example 1:(0.005 to 0.007), and even more preferably 1:0.006.

[0029] In the method 1 for preparing 2-chloro-3-methylbenzoic acid, when the reactor in the method 1 for preparing 2-chloro-3-methylbenzoic acid is a continuous flow reactor, the yield of the single-pass reaction of the oxidation reaction is controlled at about 20-30%, for example, 25%.

[0030] In the preparation method 1 of 2-chloro-3-methylbenzoic acid, when the reactor of the preparation method 1 of 2-chloro-3-methylbenzoic acid is a continuous flow reactor, the conversion rate of the single-pass reaction of the oxidation reaction is controlled to be less than or equal to 60%, for example 25%.

[0031] In the preparation method 1 of 2-chloro-3-methylbenzoic acid, the filtered reaction solution is returned to the continuous flow reactor as a raw material.

[0032] In method 1 for preparing 2-chloro-3-methylbenzoic acid, the filtered filter cake is dried to obtain 2-chloro-3-methylbenzoic acid with a purity greater than 90%.

[0033] In the preparation method 1 of the 2-chloro-3-methylbenzoic acid, the drying temperature can be a conventional temperature in the art, preferably 120-160°C.

[0034] In the preparation method 1 of the 2-chloro-3-methylbenzoic acid, the vacuum degree of drying can be a conventional vacuum degree in the art, preferably 100-200 mbar.

[0035] In method 1 for preparing 2-chloro-3-methylbenzoic acid, the liquid obtained from drying is returned to the reactor as a raw material.

[0036] The preparation method 1 of the 2-chloro-3-methylbenzoic acid includes: adding 2,6-dimethylchlorobenzene, the cobalt salt, and the initiator into a Hastelloy loop reactor; raising the reaction temperature to 110-170°C; continuously introducing compressed air; controlling the reaction pressure at 0.4-0.5 MPa; controlling the gas velocity at 700-800 mL / min; and the reaction time at 4-5 h. After a single reaction, cooling to room temperature and filtering are performed. The mother liquor is returned to the loop reactor as raw material. Cobalt salt is added, and the filter cake is melt-dried at 120-160°C and a vacuum of 100-200 mbar. The resulting distillate is returned to the reactor as raw material.

[0037] This invention provides the application of 2-chloro-3-methylbenzaldehyde as an initiator in the preparation of 2-chloro-3-methylbenzoic acid.

[0038] In the aforementioned application, the reaction conditions, reaction steps, and reaction raw materials are as described above.

[0039] The present invention also provides a method 2 for preparing 2-chloro-3-methylbenzoic acid, which includes the following steps: 2,6-dimethylchlorobenzene undergoes an oxidation reaction in the presence of an oxidant, cobalt isooctanoate and an initiator under conditions of a temperature of 60 to 200°C and a pressure of 0.1 MPa to 1.0 MPa.

[0040]

[0041] The oxidant is a gas with an oxygen content of 10-100%.

[0042] The reaction steps of method 2 for preparing 2-chloro-3-methylbenzoic acid are as described in method 1 for preparing 2-chloro-3-methylbenzoic acid.

[0043] In method 2 for preparing 2-chloro-3-methylbenzoic acid, the temperature and pressure are as described in method 1 for preparing 2-chloro-3-methylbenzoic acid.

[0044] In method 2 for preparing 2-chloro-3-methylbenzoic acid, the type and amount of the oxidant and the amount of the initiator are as described in method 1 for preparing 2-chloro-3-methylbenzoic acid.

[0045] In the preparation method 2 of 2-chloro-3-methylbenzoic acid, the initiator can be a commonly used initiator for such oxidation reactions in the art, preferably one or more of 2-chloro-3-methylbenzaldehyde, KBr and NaBr, such as KBr, NaBr or 2-chloro-3-methylbenzaldehyde.

[0046] Without violating common sense in the field, the above-mentioned preferred conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0047] The reagents and raw materials used in this invention are all commercially available.

[0048] The positive and progressive effects of this invention are as follows: The preparation method disclosed herein, which uses 2-chloro-3-methylbenzaldehyde as the initiator for 2-chloro-3-methylbenzoic acid, does not use inorganic salts as initiators, reducing post-processing steps and lowering production costs. Furthermore, the preferred reaction conditions of this invention can be applied to tubular reactions, thereby reducing safety risks. Detailed Implementation

[0049] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0050] Example 1

[0051] In a 2L Hastelloy loop reactor, 1000g of 2,6-dimethylchlorobenzene, 6g of cobalt isooctanoate catalyst, and 10g of 2-chloro-3-methylbenzaldehyde were added. The circulation pump was turned on (frequency 40Hz), and the reaction temperature was raised to 160-170℃. Then, compressed air was continuously introduced into the system, with the pressure controlled at 0.8-0.9MPa and the gas velocity controlled at 700-800mL / min. The reaction time was 4-5h. The product was sampled and analyzed, with 24wt% of the product, 56.8wt% of the raw material, and 15% of the intermediate 2-chloro-3-methylbenzaldehyde. The mixture was cooled to room temperature, filtered, and the mother liquor was returned to the loop reactor as raw material (only 0.6% catalyst needed to be added). The filter cake was melt-dried at 120-160℃ / 100-200mbar, and the resulting distillate was returned to the loop reactor as raw material. The bottom liquid was the target product with a purity of 98% and a yield of 87.3% (based on the consumed raw material).

[0052] Example 2

[0053] In a 2L Hastelloy loop reactor, 1000g of 2,6-dimethylchlorobenzene, 6g of cobalt isooctanoate catalyst, and 10g of 2-chloro-3-methylbenzaldehyde were added. The circulation pump was turned on (frequency 40Hz), and the reaction temperature was raised to 100-110℃. Then, compressed air was continuously introduced into the system, with the pressure controlled at 0.8-0.9MPa and the gas velocity controlled at 500-600mL / min. The reaction time was 6-7h. A sample was taken to test the product (15wt%), the raw material (59.5wt%), and the intermediate 2-chloro-3-methylbenzaldehyde (12%). The mixture was cooled to room temperature, filtered, and the mother liquor was returned to the loop reactor as raw material (only 0.6% catalyst needed to be added). The filter cake was melt-dried at 120-160℃ / 100-200mbar, and the resulting distillate was returned to the loop reactor as raw material. The bottom liquid was the target product with a purity of 98% and a yield of 54.4% (based on the consumed raw material).

[0054] Example 3

[0055] In a 2L Hastelloy loop reactor, 1000g of 2,6-dimethylchlorobenzene, 6g of cobalt isooctanoate catalyst, and 10g of 2-chloro-3-methylbenzaldehyde were added. The circulation pump was turned on (frequency 40Hz), and the reaction temperature was raised to 60-70℃. Then, compressed air was continuously introduced into the system, with the pressure controlled at 0.8-0.9MPa and the gas velocity controlled at 500-600mL / min. The reaction time was 6-7h. A sample was taken to test the product (5wt%), the raw material (80.0wt%), and the intermediate 2-chloro-3-methylbenzaldehyde (12%). The mixture was cooled to room temperature, filtered, and the mother liquor was returned to the loop reactor as raw material (only 0.6% catalyst needed to be added). The filter cake was melt-dried at 120-160℃ / 100-200mbar, and the resulting distillate was returned to the loop reactor as raw material. The bottom liquid was the target product with a purity of 98% and a yield of 48.4% (based on the consumed raw material).

[0056] Example 4

[0057] In a 2L Hastelloy loop reactor, 1000g of 2,6-dimethylchlorobenzene, 6g of cobalt isooctanoate catalyst, and 10g of 2-chloro-3-methylbenzaldehyde were added. The circulation pump was turned on (frequency 40Hz), and the reaction temperature was raised to 160-170℃. Then, compressed air was continuously introduced into the system, with the pressure controlled at 0.1-0.2MPa and the gas velocity controlled at 700-800mL / min. The reaction time was 4-5h. A sample was taken for analysis, and the product was 12wt%, the raw material accounted for 56.8wt%, and the intermediate 2-chloro-3-methylbenzaldehyde accounted for 30%. The mixture was cooled to room temperature, filtered, and the mother liquor was returned to the loop reactor as raw material (only 0.6% catalyst needed to be added). The filter cake was melt-dried at 120-160℃ / 100-200mbar, and the resulting distillate was returned to the loop reactor as raw material. The bottom liquid was the target product with a purity of 98% and a yield of 84.1% (calculated based on consumed raw materials).

[0058] Example 5

[0059] In a 2L Hastelloy loop reactor, 1000g of 2,6-dimethylchlorobenzene, 6g of cobalt acetylacetone catalyst, and 10g of 2-chloro-3-methylbenzaldehyde were added. The circulation pump was turned on (frequency 40Hz), and the reaction temperature was raised to 160-170℃. Then, compressed air was continuously introduced into the system, with the pressure controlled at 0.8-0.9MPa and the gas velocity controlled at 700-800mL / min. The reaction time was 4-5h. The product was sampled and analyzed, with 23wt% of the product, 57wt% of the raw material, and 16% of the intermediate 2-chloro-3-methylbenzaldehyde. The mixture was cooled to room temperature, filtered, and the mother liquor was returned to the loop reactor as raw material (only 0.6% catalyst needed to be added). The filter cake was melt-dried at 120-160℃ / 100-200mbar, and the resulting distillate was returned to the loop reactor as raw material. The bottom liquid was the target product with a purity of 98% and a yield of 87% (calculated based on the consumed raw material).

[0060] Example 6

[0061] In a 500 mL stainless steel autoclave, 140 g (1.0 mol) of 2,6-dimethylchlorobenzene, 0.84 g of cobalt isooctanoate catalyst, and 0.14 g of 2-chloro-3-methylbenzaldehyde were added. After purging twice with nitrogen, oxygen was added until the pressure reached 0.3–0.5 MPa. The system was heated to 70–75 °C, and stirring was started. When the pressure fell below 0.3 MPa, oxygen was continuously added. The reaction time was 5–6 hours. A sample was taken for analysis, and the product was 20 wt%, the raw material was 55 wt%, and the intermediate 2-chloro-3-methylbenzaldehyde accounted for 20%. The mixture was cooled to room temperature, filtered, and the mother liquor was returned to the loop reactor as raw material (only 0.6% catalyst was added). The filter cake was melt-dried at 120–160 °C / 100–200 mbar, and the resulting distillate was returned to the loop reactor as raw material. The bottom liquid was the target product with a purity of 98% and a yield of 95% (based on the consumed raw material).

Claims

1. A method for preparing 2-chloro-3-methylbenzoic acid, characterized in that, It includes the following steps: 2,6-dimethylchlorobenzene undergoes an oxidation reaction in the presence of an oxidant, a catalyst, and an initiator under conditions of 60–200°C and 0.1 MPa–1.0 MPa. The oxidant is a gas with an oxygen content of 10-100%. The initiator is 2-chloro-3-methylbenzaldehyde; The catalyst is a cobalt salt.

2. The method for preparing 2-chloro-3-methylbenzoic acid as described in claim 1, characterized in that, When the reactor in method 1 for preparing 2-chloro-3-methylbenzoic acid is a continuous flow reactor or an oxidation tower, the oxygen content in the oxidant is 15-40%. And / or, when the reactor in method 1 for preparing 2-chloro-3-methylbenzoic acid is an autoclave, the oxygen content in the oxidant is 60-100%.

3. The method for preparing 2-chloro-3-methylbenzoic acid as described in claim 2, characterized in that, When the reactor in method 1 for preparing 2-chloro-3-methylbenzoic acid is a continuous flow reactor or an oxidation tower, the oxygen content in the oxidant is 18%-25%. And / or, when the reactor in method 1 for preparing 2-chloro-3-methylbenzoic acid is an autoclave, the oxygen content in the oxidant is 100%.

4. The method for preparing 2-chloro-3-methylbenzoic acid as described in claim 3, characterized in that, When the reactor in method 1 for preparing 2-chloro-3-methylbenzoic acid is a continuous flow reactor or an oxidation tower, the oxygen content in the oxidant is 20%.

5. The method for preparing 2-chloro-3-methylbenzoic acid as described in claim 1, characterized in that, When the reactor in method 1 for preparing 2-chloro-3-methylbenzoic acid is a continuous flow reactor or an oxidation tower, the temperature is 110–200°C. And / or, when the reactor in method 1 for preparing 2-chloro-3-methylbenzoic acid is an autoclave, the temperature is 60–110°C.

6. The method for preparing 2-chloro-3-methylbenzoic acid as described in claim 5, characterized in that, When the reactor in method 1 for preparing 2-chloro-3-methylbenzoic acid is a continuous flow reactor or an oxidation tower, the temperature is 160-180°C. And / or, when the reactor in method 1 for preparing 2-chloro-3-methylbenzoic acid is an autoclave, the temperature is 60-80°C.

7. The method for preparing 2-chloro-3-methylbenzoic acid as described in claim 6, characterized in that, When the reactor in method 1 for preparing 2-chloro-3-methylbenzoic acid is a continuous flow reactor or an oxidation tower, the temperature is 160-170°C. And / or, when the reactor in method 1 for preparing 2-chloro-3-methylbenzoic acid is an autoclave, the temperature is 70-75°C.

8. The method for preparing 2-chloro-3-methylbenzoic acid as described in claim 1, characterized in that, The cobalt salt is a divalent cobalt salt; And / or, the mass ratio of the 2,6-dimethylchlorobenzene to the catalyst is 1:(0.001 to 0.02).

9. The method for preparing 2-chloro-3-methylbenzoic acid as described in claim 8, characterized in that, The cobalt salt is one or more of cobalt acetylacetonate, cobalt acetate, cobalt isooctanoate, and cobalt dichloride; And / or, the mass ratio of the 2,6-dimethylchlorobenzene to the catalyst is 1:(0.001 to 0.007).

10. The method for preparing 2-chloro-3-methylbenzoic acid as described in claim 9, characterized in that, The cobalt salt is cobalt acetylacetonate or cobalt isooctanoate; And / or, the mass ratio of the 2,6-dimethylchlorobenzene to the catalyst is 1:(0.005 to 0.007).

11. The method for preparing 2-chloro-3-methylbenzoic acid as described in claim 10, characterized in that, The mass ratio of 2,6-dimethylchlorobenzene to the catalyst is 1:0.

006.

12. The method for preparing 2-chloro-3-methylbenzoic acid according to claim 1, characterized in that, The mass ratio of 2,6-dimethylchlorobenzene to the initiator is 1:(0.001-0.02).

13. The method for preparing 2-chloro-3-methylbenzoic acid according to claim 1, characterized in that, The mass ratio of 2,6-dimethylchlorobenzene to the initiator is 1:0.

01.

14. The method for preparing 2-chloro-3-methylbenzoic acid according to claim 1, characterized in that, The gas includes inert gas and oxygen; And / or, the pressure of the oxidation reaction is 0.3 MPa to 0.9 MPa.

15. The method for preparing 2-chloro-3-methylbenzoic acid as described in claim 14, characterized in that, The gas is air or a mixture of an inert gas and oxygen; And / or, the pressure of the oxidation reaction is 0.4 MPa, 0.5 MPa, 0.6 MPa, 0.7 MPa or 0.8 MPa.

16. The method for preparing 2-chloro-3-methylbenzoic acid as described in claim 15, characterized in that, The inert gas is selected from one or more of nitrogen, argon, and argon.

17. The method for preparing 2-chloro-3-methylbenzoic acid as described in claim 16, characterized in that, The inert gas is nitrogen.

18. The method for preparing 2-chloro-3-methylbenzoic acid as described in claim 14, characterized in that, The gas is compressed air.

19. The method for preparing 2-chloro-3-methylbenzoic acid according to any one of claims 1-18, characterized in that, The preparation method 1 of 2-chloro-3-methylbenzoic acid satisfies one or more of the following conditions: (1) The molar ratio of the oxidant to the 2,6-dimethylchlorobenzene is (0.23–15.00):1; (2) In the preparation method, the gas introduction rate is 600-900 mL / min; (3) The reaction time of the oxidation reaction is 1 to 10 hours. The reactor in method 1 for preparing 2-chloro-3-methylbenzoic acid as described in (4) is a continuous flow reactor, an oxidation tower, or a stainless steel autoclave.

20. The method for preparing 2-chloro-3-methylbenzoic acid as described in claim 19, characterized in that, The preparation method 1 of 2-chloro-3-methylbenzoic acid satisfies one or more of the following conditions: (1) The molar ratio of the oxidant to the 2,6-dimethylchlorobenzene is (4.67~6.67):1; (2) In the preparation method, the gas introduction rate is 700-800 mL / min; (3) The reaction time of the oxidation reaction is 4 to 5 hours; The continuous flow reactor described in (4) is a Hastelloy loop reactor.

21. The method for preparing 2-chloro-3-methylbenzoic acid as described in claim 20, characterized in that, The preparation method 1 of 2-chloro-3-methylbenzoic acid satisfies one or two of the following conditions: (1) The molar ratio of the oxidant to the 2,6-dimethylchlorobenzene is 5.5:1; and (2) When the reactor of the preparation method 1 of 2-chloro-3-methylbenzoic acid is a continuous flow reactor, the frequency of the circulation pump of the continuous flow reactor is 30-60Hz.

22. The method for preparing 2-chloro-3-methylbenzoic acid according to claim 21, characterized in that, When the reactor in method 1 for preparing 2-chloro-3-methylbenzoic acid is a continuous flow reactor, the frequency of the circulating pump of the continuous flow reactor is 40-45 Hz.

23. The method for preparing 2-chloro-3-methylbenzoic acid as described in claim 22, characterized in that, When the reactor in method 1 for preparing 2-chloro-3-methylbenzoic acid is a continuous flow reactor, the frequency of the circulation pump of the continuous flow reactor is 40 Hz.

24. The method for preparing 2-chloro-3-methylbenzoic acid as described in claim 23, characterized in that, The preparation method 1 of 2-chloro-3-methylbenzoic acid includes filtration and direct reuse of the filtrate in the next single reaction after the completion of a single reaction.

25. The method for preparing 2-chloro-3-methylbenzoic acid as described in claim 24, characterized in that, The preparation method 1 of 2-chloro-3-methylbenzoic acid satisfies one or more of the following conditions: (1) When adding catalyst, the mass ratio of 2,6-dimethylchlorobenzene to the catalyst is 1:(0.001~0.01); (2) The filtered reaction liquid is returned to the continuous flow reactor as a raw material; (3) In the preparation method 1 of 2-chloro-3-methylbenzoic acid, when the reactor of the preparation method 1 of 2-chloro-3-methylbenzoic acid is a continuous flow reactor, the yield of the single-pass reaction of the oxidation reaction is controlled at about 20-30%. (4) When the reactor of the preparation method 1 of 2-chloro-3-methylbenzoic acid is a continuous flow reactor, the conversion rate of the single-pass reaction of the oxidation reaction is controlled to be less than or equal to 60%. (5) The filtered cake was dried to obtain 2-chloro-3-methylbenzoic acid with a purity greater than 90%.

26. The method for preparing 2-chloro-3-methylbenzoic acid as described in claim 25, characterized in that, The preparation method 1 of 2-chloro-3-methylbenzoic acid satisfies one or more of the following conditions: (1) When adding catalyst, the mass ratio of 2,6-dimethylchlorobenzene to the catalyst is 1:(0.001~0.007); (2) In method 1 for preparing 2-chloro-3-methylbenzoic acid, when the reactor in method 1 for preparing 2-chloro-3-methylbenzoic acid is a continuous flow reactor, the yield of the single-pass oxidation reaction is controlled at 25%; and (3) When the reactor of the preparation method 1 of 2-chloro-3-methylbenzoic acid is a continuous flow reactor, the conversion rate of the single-pass reaction of the oxidation reaction is controlled at 25%. (4) The drying temperature is 120-160℃; (5) The vacuum degree of the drying process is 100-200 mbar; The liquid obtained from the drying process described in (6) is returned to the reactor as a raw material.

27. The method for preparing 2-chloro-3-methylbenzoic acid as described in claim 26, characterized in that, When adding catalyst, the mass ratio of 2,6-dimethylchlorobenzene to the catalyst is 1:(0.005-0.007).

28. The method for preparing 2-chloro-3-methylbenzoic acid according to claim 27, characterized in that, When adding catalyst, the mass ratio of 2,6-dimethylchlorobenzene to the catalyst is 1:0.

006.

29. The application of 2-chloro-3-methylbenzaldehyde as an initiator in the preparation of 2-chloro-3-methylbenzoic acid; In the application described herein, the reaction conditions, reaction steps, and reaction raw materials are as described in any one of claims 1-28.

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