A kind of o-hydroxybenzamide synthesis method

Abstract

The present invention belongs to the field of fine chemical synthesis. A method for synthesizing o-hydroxybenzamide, the process of which includes: step S1, under the action of catalyst A, using o-hydroxybenzoic acid as the initial raw material, and synthesizing o-hydroxybenzoyl chloride in a reactor with a chlorinating agent; the catalyst A is dimethylformamide or dimethylacetamide; step S2, under the action of catalyst B, methanol or ethanol is substituted for chlorine in o-hydroxybenzoyl chloride to synthesize o-hydroxybenzyl ester; the catalyst B is 4-methylpyridine, 2,6-lutidine or pyridine; step S3, under the action of catalyst C, o-hydroxybenzyl ester and ammonia undergo amination reaction to generate o-hydroxybenzamide; the catalyst C is sodium pyrosulfite or ammonium chloride. The new process for synthesizing o-hydroxybenzamide of the present invention has the characteristics of simple operation, safety, environmental protection, low energy, high yield and high purity.

Description

Technical Field

[0001] The invention belongs to the field of fine chemical synthesis, and in particular relates to a new synthesis process of o-hydroxybenzamide. Background Art

[0002] o-Hydroxybenzamide (commonly known as salicylate) is an important intermediate widely used in chemicals, medicines, pesticides, fragrances, and liquid crystals. It is also a drug used to treat fever, headaches, joint pain, neuralgia, etc. In recent years, the demand for o-Hydroxybenzamide, as an important fine chemical intermediate, has been increasing.

[0003] The current industrial production process of o-hydroxybenzamide generally uses o-hydroxybenzoic acid as the starting material, which is esterified with methanol under the catalysis of sulfuric acid to produce o-hydroxybenzyl ester (for example: CN110563581A), and then o-hydroxybenzyl ester is aminated with ammonia water or liquid ammonia to obtain o-hydroxybenzamide. This production process has obvious shortcomings, mainly: (1) a large amount of concentrated sulfuric acid is used (the molecular ratio with o-hydroxybenzoic acid is 0.3), the equipment requirements are strict, and a large amount of wastewater is generated and the corrosion is strong; (2) since water is not easy to remove during esterification, the reversible reaction is not complete, the conversion rate of o-hydroxybenzyl ester is low (currently the industry is not higher than 95%), and the product purity is low. , poor appearance and color; the amination conversion rate is generally not higher than 95%, and the product purity is lower than 95%; the esterification and amination conversion rates are low, resulting in high costs, poor quality, and severe environmental pollution; (3) the esterification catalyst has poor selectivity for concentrated sulfuric acid and many side reactions; the esterification reaction cycle is long (the current industrial esterification time is 60-72h); there is no suitable catalyst for amination, the amination cycle is long (the current industrial esterification time is more than 96h), and the equipment utilization rate is low; (4) the esterification reaction has a serious excess of methanol, and methanol needs to be refluxed during the reaction; the subsequent recovery of methanol requires a large amount of power, resulting in high energy consumption; (5) the esterification reaction temperature is higher than the boiling point of methanol, which is prone to explosion accidents and has a low safety factor.

[0004] The invention patent with publication number CN113754553A discloses a method for continuously synthesizing salicylamide, which uses salicylic acid as raw material. Phosgene is used as an acylating agent to prepare salicyl chloride, which is esterified with methanol to prepare methyl salicylate, and then methyl salicylate is aminated with ammonia to prepare salicylamide. The obvious deficiencies of this synthesis method mainly include: (1) there is no suitable catalyst in each synthesis step, the reaction cycle is long, the selectivity is poor, and there are many by-products; (2) the acyl chloride process uses phosgene as a chlorinating agent, and the amination process uses ammonia as the main reaction raw material, which is easy to cause a major source of danger and a major safety accident. The state has expressly prohibited it; (3) the waste gas, waste liquid, and waste solid generated in the synthesis process have not been reasonably applied and reduced, and a green and clean production process has not been achieved.

[0005] With the tightening of national safety and environmental protection policies, the shortcomings of traditional processes can no longer meet the requirements of existing policies. It is urgent to develop a green synthesis route for o-hydroxybenzamide with a high safety factor, low environmental pollution and low energy consumption cost. Summary of the invention

[0006] In view of the shortcomings of the prior art, the invention provides a new process for synthesizing o-hydroxybenzamide, which has the characteristics of simple operation, safety, environmental protection, low energy consumption, high yield and high purity.

[0007] The technical solution adopted by the present invention is:

[0008] A new process for synthesizing o-hydroxybenzamide mainly includes the following three steps:

[0009] Step S1: o-hydroxybenzoic acid reacts with a chlorinating agent in a reaction vessel under the action of a catalyst A to synthesize o-hydroxybenzoyl chloride; in the chlorination process, the catalyst A used is dimethylformamide or dimethylacetamide; the chlorinating agent is any one of thionyl chloride, phosphorus trichloride, acetyl chloride, oxalyl chloride and chlorine;

[0010] Step S2: in the presence of catalyst B, o-hydroxybenzoyl chloride is reacted with methanol or ethanol to replace chlorine in o-hydroxybenzoyl chloride to synthesize o-hydroxybenzoyl ester; in the esterification reaction, the alcohol used is methanol, ethanol or other lower alcohols; in the esterification reaction, catalyst B is 4-methylpyridine, 2,6-lutidine or pyridine;

[0011] Step S3: Under the action of catalyst C, o-hydroxybenzyl ester reacts with ammonia water to rapidly generate o-hydroxybenzamide; catalyst C is one or both of sodium pyrosulfite and ammonium chloride;.

[0012] In step S1, the o-hydroxybenzoic acid: chlorinating agent: catalyst A = 1: 1.05-2.0: 0.05-0.1; the reaction temperature is 40-60 0 C; the reaction time is 1.5-3.0h.

[0013] In step S2, the o-hydroxybenzoyl chloride: alcohol: catalyst B = 1: 0.95-2.0: 0.01-0.05, and the reaction temperature is 20-35 0 C; the reaction time is 1.5-4.0h.

[0014] In the amination reaction of step S3, the o-hydroxybenzoate: catalyst C: ammonia water = 1: 0.01-0.1: 3-6; the reaction temperature is 45-55 0 C; the reaction time is 5-10h.

[0015] The chlorinating agent undergoes a chlorination reaction in the reactor, and the waste gases HCl and SO2 produced by the acyl chlorination synthesis are used as absorbents for excess ammonia water in the amination reaction to produce ammonium chloride or ammonium sulfate, without generating secondary pollution. The low-content solid produced after adjusting the pH value of the centrifugal mother liquor of o-hydroxybenzamide is added to the secondary reaction raw material of the acyl chlorination reactor without generating waste solids. The esterification reaction does not require the addition of excess alcohol, and the waste solids are recovered and reused in the centrifugal mother liquor, which greatly reduces the generation of wastewater, waste gas, and hazardous waste solids.

[0016] The reaction control end point is that the acyl chloride conversion rate is ≥98.5%, the esterification conversion rate is ≥99.0%, and the amination conversion rate is ≥99.5%. The yield control index is that the o-hydroxybenzamide molar yield is ≥96%, and the o-hydroxybenzamide content is ≥99.5%.

[0017] Beneficial effects of the invention:

[0018] 1. The o-hydroxybenzamide synthesis method of the present invention provides a new process for the synthesis of o-hydroxybenzamide, which avoids the environmental problems caused by the traditional synthesis method using concentrated sulfuric acid as a catalyst, has mild reaction conditions and low energy consumption; has the characteristics of simple operation, safety, environmental protection, and low energy, meets the requirements of environmental protection policies, and is suitable for industrial production.

[0019] 2. The o-hydroxybenzamide synthesis method of the present invention adopts pollutant (waste gas) treatment technology in the acyl chlorination epidemic prevention process, realizes the recycling of pollutants, and can reduce the discharge of three wastes; the esterification reaction does not need to add excessive alcohol, and the waste solids are recovered and reused by the centrifugal mother liquor, which greatly reduces the generation of wastewater, waste gas, and hazardous waste solids. The waste gas generation link is the acyl chlorination synthesis step to produce one or two of HCl and SO2, which are used as absorbents for excess ammonia water in the amination reaction to produce ammonium chloride or ammonium sulfate, and no secondary pollution is generated; the low-content solids produced after the o-hydroxybenzamide centrifugal mother liquor is adjusted to pH value are added to the acyl chlorination reactor as secondary reaction raw materials, and no waste solids are generated.

[0020] 3. The o-hydroxybenzamide synthesis method of the present invention has an esterification effect of more than 98.5%, a product purity of more than 99.7%, a yield of more than 96%, a product appearance and a crystal form that are significantly better than those of the traditional process, and a significantly improved catalyst reaction selectivity. The reaction time is shortened to 1 / 6 of the original, and the equipment utilization rate is increased by 600%. Implementation

[0021] In order to make the technical concept and advantages of the invention more clearly understood, the technical solution of the present invention is further described in detail below in conjunction with specific embodiments. It should be understood that the following embodiments are only used to explain and illustrate the preferred implementation methods of the present invention and should not constitute a limitation on the scope of the patent protection required by the present invention. Example

[0022] The present invention discloses a method for synthesizing o-hydroxybenzamide, and the process thereof comprises:

[0023] Step S1, under the action of catalyst A, o-hydroxybenzoic acid is used as the initial raw material and a chlorinating agent is used to synthesize o-hydroxybenzoyl chloride in a reaction kettle; during the chlorination reaction, dimethylformamide (DMF) or dimethylacetamide is used as the catalyst;

[0024] Step S2, in the presence of catalyst B, methanol or ethanol is used to replace chlorine in o-hydroxybenzoyl chloride to synthesize o-hydroxybenzoyl ester; in the esterification reaction, the catalyst B used is 4-methylpyridine, 2,6-lutidine or pyridine;

[0025] Step S3, under the action of catalyst C, o-hydroxybenzyl ester and ammonia water undergo an amination reaction to generate o-hydroxybenzamide; the catalyst C used is sodium pyrosulfite or ammonium chloride.

[0026] In the chlorination reaction of step S1, the chlorinating agent used is any one of thionyl chloride, phosphorus trichloride, acetyl chloride, oxalyl chloride and chlorine; the o-hydroxybenzoic acid: chlorinating agent: catalyst A = 1: 1.05-2.0: 0.05-0.2; the reaction temperature is 40-60 0 C; the reaction time is 1.5-3.0h.

[0027] In the esterification reaction of step S2, the o-hydroxybenzoyl chloride: alcohol: catalyst B = 1: 0.95-2.0: 0.01-0.05, and the reaction temperature is 20-35 0 C; the reaction time is 1.5-4.0h.

[0028] In the amination reaction of step S3, the o-hydroxybenzoate: catalyst C: ammonia water = 1: 0.01-0.1: 3-6; the reaction temperature is 45-55 0 C; the reaction time is 5-10h. Example

[0029] The o-hydroxybenzamide synthesis method of this embodiment is different from that of embodiment 1 in that: during the production process, HCl and SO2 produced in the chlorination synthesis process of step S1 are used as absorbents for excess ammonia water in the amination reaction to produce ammonium chloride or ammonium sulfate; the low-content solid produced after the pH value of the o-hydroxybenzamide centrifugal mother liquor is adjusted is added to the chlorination reaction kettle as a secondary reaction raw material. Waste gas treatment realizes the recycling of pollutants and can reduce the discharge of three wastes; the esterification reaction does not need to add excess alcohol, and the waste solid is recycled and reused by the centrifugal mother liquor, which greatly reduces the generation of wastewater, waste gas, and hazardous waste solids.

[0030] The o-hydroxybenzamide synthesis method of the present invention has the following control conditions for the reaction end points of the production process: acyl chloride conversion rate ≥ 98.5%, esterification conversion rate ≥ 99.0%, amination conversion rate ≥ 99.5%. The production control indexes are: o-hydroxybenzamide molar yield ≥ 96%, and o-hydroxybenzamide content ≥ 99.5%. Example

[0031] The o-hydroxybenzamide synthesis method of this embodiment is different from that of Embodiments 1 and 2 in that:

[0032] In the acyl chloride reaction of step S1, the catalyst A used is dimethylformamide (DMF); the chlorinating agent is thionyl chloride; the o-hydroxybenzoic acid: chlorinating agent: catalyst A = 1:1.05:0.05; the reaction temperature is controlled at 50 0 C; the reaction time is 1.5h.

[0033] In the esterification reaction of step S2, the catalyst B used is 4-methylpyridine; the alcohol is methanol; the ratio of o-hydroxybenzoyl chloride:alcohol:catalyst B in the esterification reaction is 1:0.95:0.01, and the reaction temperature is controlled at 30 0 C; reaction time 1.5h.

[0034] In the amination reaction of step S3, the catalyst C used is sodium pyrosulfite; the o-hydroxybenzoate: catalyst C: ammonia water = 1:0.01:3; the reaction temperature is controlled at 50 0 C; the reaction time is 5h. Example

[0035] The o-hydroxybenzamide synthesis method of this embodiment is different from that of Embodiments 1 and 2 in that:

[0036] In the acyl chloride reaction of step S1, the catalyst A used is dimethylacetamide; the chlorinating agent is phosphorus trichloride; the o-hydroxybenzoic acid: chlorinating agent: catalyst A = 1:2.0:0.2; the reaction temperature is 40 0 C; reaction time 3.0h.

[0037] In the esterification reaction of step S2, the catalyst B used is 2,6-lutidine or pyridine; the alcohol is ethanol; the o-hydroxybenzoyl chloride: alcohol: catalyst B in the esterification reaction is 1:2.0:0.05, and the reaction temperature is 35 0 C; the reaction time is 2.0h.

[0038] In the amination reaction of step S3, the catalyst C used is ammonium chloride; the o-hydroxybenzoate: catalyst C: ammonia water = 1:0.1:3; the reaction temperature is 45-55 0C; the reaction time is 5-10h. Example

[0039] The o-hydroxybenzamide synthesis method of this embodiment is different from that of Embodiments 1 and 2 in that:

[0040] In the acyl chloride reaction of step S1, the catalyst A used is dimethylformamide (DMF); the chlorinating agent is acetyl chloride; the o-hydroxybenzoic acid: chlorinating agent: catalyst A = 1:1.05:0.2; the reaction temperature is controlled at 40-60 0 C; the reaction time is 1.5-3.0h.

[0041] In the esterification reaction of step S2, the catalyst B used is pyridine; the alcohol is methanol; the o-hydroxybenzoyl chloride: alcohol: catalyst B in the esterification reaction is 1:2.0:0.01, and the reaction temperature is 20-35 0 C; reaction time is 1.5-4.0h.

[0042] In the amination reaction of step S3, the catalyst C used is sodium pyrosulfite; the o-hydroxybenzoate: catalyst C: ammonia water = 1:0.0.1:3; the reaction temperature is 45-55 0 C; the reaction time is 5-10h. Example

[0043] The o-hydroxybenzamide synthesis method of this embodiment is different from that of Embodiments 1 and 2 in that:

[0044] In the acyl chloride reaction of step S1, the catalyst A used is dimethylacetamide; the chlorinating agent is chlorine; the o-hydroxybenzoic acid: chlorinating agent: catalyst A = 1:1.5:0.0.1; the reaction temperature is 40-60 0 C; the reaction time is 1.5-3.0h.

[0045] In the esterification reaction of step S2, the catalyst B used is 4-methylpyridine; the alcohol is ethanol; the ratio of o-hydroxybenzoyl chloride:alcohol:catalyst B in the esterification reaction is 1:1.5:0.03, and the reaction temperature is 20-35 0 C; the reaction time is 1.5-4.0h.

[0046] In the amination reaction of step S3, the catalyst C used is ammonium chloride; the o-hydroxybenzoate: catalyst C: ammonia water = 1:0.05:3-6; the reaction temperature is 45-55 0 C; the reaction time is 5-10h.

[0047] The following is an example of the process of the o-hydroxybenzamide synthetic method of the present invention and the technical effect produced by the experimental example and the comparative example. The content of the product in each embodiment is detected by high performance liquid chromatography (HPLC), and the yield of the product is calculated by converting the molar amount of the measured o-hydroxybenzamide weight after drying.

[0048] Comparative Example 1

[0049] Add 420g methanol (99.5% content, 13.0mol), 30g concentrated sulfuric acid (98.1% content, 0.3mol), and 138g o-hydroxybenzoic acid (99.0% content, 1.0mol) to a 1000ml three-necked flask. Open the oil bath to control the temperature at 75-78℃, reflux reaction for 20h, chromatographic tracking, the conversion rate is 75.75%. Control the oil bath temperature at 80-85℃, collect 50g condensed methanol, reflux reaction for 10h, chromatographic tracking, the conversion rate is 85.63%. Collect 50g condensed methanol again, reflux reaction for 10h, chromatographic tracking, the conversion rate is 89.82%. Collect 100g condensed methanol again, reflux reaction for 10h, chromatographic tracking, the conversion rate is 94.91%. The reaction solution was filtered and the oil phase was washed with water and alkali, the pH value was adjusted to 8-9, 525 g (3.0 mol) of 25% ammonia water was added, the temperature was controlled at 30-35°C, and the chromatographic tracking after amination for 60 hours showed that the amination conversion rate was 93.68%. The pH value of the distilled amination solution was 8, the temperature was lowered to 5°C, filtered, dried and weighed to 96.13 g, the chromatographic tracking showed that the o-hydroxybenzamide content was 94.83%, and the theoretical yield was 66.54%.

[0050] Comparative Example 2

[0051] 300 g of toluene, 138 g of o-hydroxybenzoic acid (content 99.0%, 1.0 mol) and 2.0 mol of phosgene were added to a 1000 ml three-necked flask and continuously introduced into the chlorination reactor at a certain speed to react and generate o-hydroxybenzoyl chloride. The reaction temperature was 55 0 C, chromatographic tracking of o-hydroxybenzoyl chloride content, the reaction time 12h to the maximum value of 95.38%; o-hydroxybenzoyl chloride solution was added with 33g methanol (99.0%, 1.0mol), the reaction temperature was 25 0 C, the chromatographic tracking of the o-hydroxybenzyl ester content, the reaction time 9h the maximum value of 93.21%; layered washing with water, alkali washing, the oil phase and 3.0 mol of ammonia methanol solution into the amination reaction kettle to synthesize o-hydroxybenzamide, chromatographic tracking of the o-hydroxybenzamide content, the reaction time 48h the maximum value of 93.51%; negative pressure distillation adjusted pH to 4 when cooling crystallization, to obtain o-hydroxybenzamide dry product 118.68g, o-hydroxybenzamide content 95.87%, theoretical yield 82.45%.

[0052] Experimental Example 1

[0053] In a 1000ml three-necked flask, add 300g toluene, 7g (0.09mol) of dimethylformamide (DMF), and 138g of o-hydroxybenzoic acid (content 99.0%, 1.0mol), and control the temperature at 50-55 0 C, add 145g (1.2mol) of thionyl chloride dropwise, the addition time is 1.5h, the chromatographic tracking of o-hydroxybenzoyl chloride is 98.56%, and 280g of toluene is recovered under negative pressure. Add 2g of 4-methylpyridine, add 38g (1.2mol) of methanol dropwise, the addition time is 1.5h, and the chromatographic tracking of o-hydroxybenzyl ester is 97.77%. Wash with water and alkali in layers, add 525g (3.0mol) of ammonia water to the oil phase, add 10g of ammonium chloride, the temperature is 40-45℃, and after amination for 10h, the chromatographic tracking of o-hydroxybenzamide is 98.78%. Excess ammonia water is distilled, and the distillate is absorbed by the recovered hydrochloric acid. After the pH value of the distilled amination liquid is 8, it is cooled, filtered, and dried. The mother liquor is adjusted to pH 4 again, and the wet product is filtered and added to the amination kettle to obtain 125.5g of dry product, with an o-hydroxybenzamide content of 99.2% and a theoretical yield of 90.87%.

[0054] Experimental Example 2

[0055] In a 1000ml three-necked flask, add 300g toluene, 7g DMF (0.09mol), and 138g o-hydroxybenzoic acid (content 99.0%, 1.0mol), and control the temperature at 50-55 0 C, add 192g (1.25mol) of phosphorus oxychloride dropwise, the addition time is 2.0h, the chromatographic tracking of o-hydroxybenzoyl chloride is 98.31%, and 280g of toluene is recovered under negative pressure. Add 3g of 2,6-dimethylpyridine, add 40g (1.3mol) of methanol dropwise, the addition time is 2.0h, and the chromatographic tracking of o-hydroxybenzyl ester is 98.25%. Wash with water and alkali in layers, add 630g (4.5mol) of ammonia water to the oil phase, add 5g of sodium pyrosulfite, the temperature is 45-50℃, and after 8h of amination, the chromatographic tracking of o-hydroxybenzamide is 99.16%. Excess ammonia water is distilled, and the distillate is absorbed by the recovered hydrochloric acid. After the pH value of the distilled amination liquid is 8, it is cooled, filtered, and dried. The mother liquor is adjusted to pH 4 again, and the wet product is filtered and added to the amination kettle to obtain 131.65g of dry product, with an o-hydroxybenzamide content of 99.5% and a theoretical yield of 95.61%.

[0056] Experimental Example 3

[0057] In a 1000ml three-necked flask, add 300g toluene, 7g dimethylacetamide (0.09mol), and 138g o-hydroxybenzoic acid (content 99.0%, 1.0mol), and control the temperature at 50-550 C, add 110g (1.4mol) of acetyl chloride dropwise, the addition time is 2.5h, the chromatographic tracking of o-hydroxybenzoyl chloride is 99.02%, and 280g of toluene is recovered under negative pressure. Add 3-pyridine, add 45g (1.4mol) of methanol dropwise, the addition time is 2.5h, and the chromatographic tracking of o-hydroxybenzyl ester is 98.83%. Wash with water and alkali in layers, add 700g (5.0mol) of ammonia water to the oil phase, add 7g of sodium pyrosulfite, the temperature is 50-55℃, and after 6h of amination, the chromatographic tracking of o-hydroxybenzamide is 99.7%, the excess ammonia water is distilled, and the distillate is absorbed by the recovered hydrochloric acid. After the pH value of the distilled amination liquid is 8, it is cooled, filtered, and dried. The mother liquor is adjusted to pH 4 again, the wet product is filtered and added to the amination kettle, and 132.78g of dry product is obtained, the content of o-hydroxybenzamide is 99.75%, and the theoretical yield is 96.63%.

[0058] In the preparation process of o-hydroxybenzoic acid, the catalyst is dimethylformamide or dimethylacetamide, and the effect is equivalent. In the amination reaction of step S3, the catalyst C used is sodium pyrosulfite or ammonium chloride, and a mixed solution of sodium pyrosulfite and ammonium chloride can also be used. The use of ammonium chloride can increase the ammonium ion concentration in the solution to prevent the ionization of ammonia water.

[0059] Through the above-mentioned embodiments and comparative examples, it is shown that the present invention has better technical effects and yields than the prior art. In the production process of the present invention, increasing the low-value raw materials is beneficial to the conversion rate of the target product, increasing the temperature can shorten the reaction time; different varieties of chlorinating agents have equivalent effects, and different varieties of esterification catalytic effects are equivalent; sodium pyrosulfite is better than ammonium chloride as an amination catalyst; and extending the reaction time is beneficial to the conversion rate of the target product and product quality.

[0060] The various raw material ratios in the embodiments of the present invention are not intended to limit the present invention in other forms. Any technician familiar with the profession may use the technical content disclosed above to change or modify it into an equivalent embodiment with equivalent changes and apply it to other fields. Any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A method for synthesizing o-hydroxybenzamide, characterized in that: The process includes: Step S1, under the action of catalyst A, o-hydroxybenzoic acid is used as the initial raw material and a chlorinating agent is used to synthesize o-hydroxybenzoyl chloride in a reaction kettle; the catalyst A is dimethylformamide or dimethylacetamide; In the acyl chloride reaction of step S1, the o-hydroxybenzoic acid: chlorinating agent: catalyst A = 1: 1.05-2.0: 0.05-0.2; the reaction temperature is 40-60 0 C; the reaction time is 1.5-3.0h; Step S2, in the presence of a catalyst B, methanol or ethanol is used to replace chlorine in o-hydroxybenzoyl chloride to synthesize o-hydroxybenzoyl ester; the catalyst B is 4-methylpyridine, 2,6-lutidine or pyridine; In the esterification reaction of step S2, the o-hydroxybenzoyl chloride: alcohol: catalyst B in the esterification reaction is 1: 0.95-2.0: 0.01-0.05, and the reaction temperature is 20-35 0 C; the reaction time is 1.5-4.0h; Step S3, under the action of catalyst C, o-hydroxybenzyl ester and ammonia water undergo an amination reaction to generate o-hydroxybenzamide; the catalyst C is ammonium chloride; In the amination reaction of step S3, the o-hydroxybenzoate: catalyst C: ammonia water = 1: 0.01-0.1: 3-6; the reaction temperature is 45-55 0 C; the reaction time is 5-10h.

2. The method for synthesizing o-hydroxybenzamide according to claim 1, wherein: In the acyl chloride reaction of step S1, the chlorinating agent is any one of thionyl chloride, phosphorus trichloride, acetyl chloride, oxalyl chloride and chlorine gas.

3. The method for synthesizing o-hydroxybenzamide according to claim 1 or 2, characterized in that: The chlorination synthesis step produces HCl and SO2 as absorbents for excess ammonia water in the amination reaction to produce ammonium chloride or ammonium sulfate; the low-content solid produced after adjusting the pH value of the centrifugal mother liquor of o-hydroxybenzamide is added to the chlorination reactor as a secondary reaction raw material.

4. The method for synthesizing o-hydroxybenzamide according to claim 3, wherein: The reaction endpoints are controlled as follows: acyl chloride conversion rate ≥ 98.5%, esterification conversion rate ≥ 99.0%, amination conversion rate ≥ 99.5%.

5. The method for synthesizing o-hydroxybenzamide according to claim 3, wherein: The production control indicators are: o-hydroxybenzamide molar yield ≥ 96%, o-hydroxybenzamide content ≥ 99.5%.