SK87994A3 - Method of preparationbenzothiazol-2-yl salts sulfonic acid - Google Patents

Abstract

The benzothiazol-2-yl sulfonic acid alkali salts are prepared by continuous oxidation of 2-mercaptobenzothiazole or an alkaline salt thereof with hydrogen peroxide in an aqueous alkali hydroxide solution. Reaction at a temperature of 40 to 75 ° C, a molar ratio of peroxide of hydrogen and 2-mercaptobenzothiazole 2.2-2.8 moles the ratio of alkali metal hydroxide to 2-mercaptobenzothiazole 1-3, product yield can be achieved up to 88% at 4.4 to 12, 1kg.h '- dm'3 reaction the zone while maintaining the pH of the solution above 7.5. Benzothiazol-2-yl alkali metal sulfonates are intermediates chemical and pharmaceutical industry.

Description

A process for preparing alkali salts of benzothiazol-2-yl sulfonic acid

Technical field

The invention relates to a process for the preparation of alkali salts of benzothiazol-2-yl sulfonic acid by the continuous oxidation of 2-mercaptobenzothiazole or its alkali salt with hydrogen peroxide in an aqueous solution of an alkali metal hydroxide.

BACKGROUND OF THE INVENTION

One method of preparing alkali salts of benzothiazol-2-yl sulfonic acid is by oxidation of 2-mercaptobenzothiazole or salts thereof. Potassium permanganate [Eíros L.S. et al .: Žur. obšč. chim. 21, 2046-8 (1951)] or hydrogen peroxide and the reaction is carried out in an alkaline medium [Kimijima T. et al., J. Soc. Chem. Ind. Japan 45, 957-60 (1942) and Gvozdjakova A. et al., Acta Fac. Rerum. Nat. Univ. Comenice Chim., 33, 137-46 (1985)]. The advantage of hydrogen peroxide as an oxidizing agent is the higher selectivity of the reaction. Depending on the reaction conditions, oxidation of 2-mercaptobenzothiazole with hydrogen peroxide affords benzothiazole [Kimijima T. et al., J. Soc. Chem. Ind. Japan 45, 957-60 (1942)], 2,2'-bis (benzothiazolyl) sulfide [Gurjanova T.N. et al .: Doc. Akad. USSR 94, 53-56 (1954)] and in the environment of methanol (DE 2 349 314) or an alkali metal hydroxide [HU 32 361 (1984)] 2,2'-bis (benzothiazolyl) disulide.

The processes described to date for the preparation of alkali salts of benzothiazol-2-yl sulfonic acid by oxidation of 2-mercaptobenzothiazole are discontinuous. Due to the exothermic nature of the oxidation reaction, the use of hydrogen peroxide as an oxidizing agent encounters several disadvantages in the discontinuous process. This is primarily a process safety problem, especially when working with larger amounts of reactants. Due to the constantly changing ratio of hydrogen peroxide to 2-mercaptobenzothiazole in the discontinuous process, the consumption of hydrogen peroxide is always substantially higher than that corresponding to stoichiometry, which also negatively affects the selectivity of the oxidation.

SUMMARY OF THE INVENTION

The above-mentioned disadvantages are eliminated by the process for the preparation of the alkali metal salts of the benzothiazol-2-yl sulfonic acid according to the invention, which consists in the fact that the reaction zone is stirred and under a load of 4.4 to 12.1 kg.h ¹ .dm ^{-3 of the} reaction zone. continuously and simultaneously introduces hydrogen peroxide in the form of 15 to 80% by weight. aqueous solution and a mixture of 2-mercaptobenzothiazole and an alkali metal hydroxide dissolved in water at a molar ratio of 2-mercaptobenzothiazole: hydrogen peroxide: alkali metal hydroxide of 1: 2.2 to 5.8: 1 to 3, maintaining the pH of the solution above 7.5.

The course and selectivity of hydrogen peroxide oxidation of 2-mercaptobenzothiazole is influenced by the pH of the reaction medium, the temperature, the molar ratio of hydrogen peroxide and 2-mercaptobenzothiazole, and the molar ratio of alkali metal hydroxide and 2-mercaptobenzothiazole. 2-Mercaptobenzothiazole or its alkali metal salts can be used as starting materials. The presence of impurities in the raw material reduces the purity of the final product. Hydrogen peroxide may preferably be used in the form of 30 to 35% by weight. aqueous solution.

At a molar ratio of hydrogen peroxide to 2-mercaptobenzothiazole of 3.3 to 5.0 and a molar ratio of alkali metal hydroxide to 2-methyl-benzothiazole of 1.4 to 2, a product yield of up to 88 mol% is achieved under a reaction zone load of 4.4 to 12 1 kg.h ^-1 .dm ^-3 . The reaction zone load is defined as the weight of the reaction mixture containing 15% by weight. an alkali metal salt of mercaptobenzothiazole, which resides in the volume of the reaction zone filled by the liquid phase per unit of time.

Preferably, the oxidation is carried out at a molar ratio of hydrogen peroxide to 2-mercaptobenzothiazole of about 3.5. A molar ratio of hydrogen peroxide to 2-mercaptobenzothiazole of greater than 4.7 is no longer advantageous since the portion of unreacted hydrogen peroxide remaining in the reaction mixture promotes the course of side reactions and at the same time worsens the process economy. Due to the accumulation of by-products in the reaction product, it is difficult to isolate the final product from the reaction mixture in the desired quality.

The pH of the reaction medium, which is closely related to the molar ratio of alkali metal hydroxide to 2-mercaptobenzothiazole, has a significant effect on the selectivity of the formation of the alkali metal salt of benzothiazol-2-yl sulfonic acid. At a molar ratio of alkali metal hydroxide to 2-mercaptobenzothiazole of less than 1.2, problems may arise in the continuous course of the reaction, resulting in a decrease in the pH of the environment below 7.5 during the reaction, causing undesirable by-products. Conversely, increasing the molar ratio of alkali metal hydroxide to 2-mercaptobenzothiazole above 2 leads to an increase in the solubility of the benzothiazol-2-yl sulfonic acid alkali salt in the reaction mixture. This results in difficulties in isolation and reduces product yield.

The alkali salts of benzothiazol-2-yl sulfonic acid are white to pale yellow solids. They are sparingly soluble in the reaction mixture at a temperature below the reaction temperature. The solubility of the alkali salts of benzothiazol-2-yl sulfonic acid in the reaction mixture at about 10 ° C is 5-8% by weight. Continuous regulation and control of the pH of the reaction mixture is of great importance for the selective course of oxidation. Lowering the pH of the reaction mixture below 7.5 leads to increased formation of by-products, thereby reducing the yield and purity of the product. The product becomes yellow to brown in color.

Intensive mixing of the reaction mixture and removal of the heat of reaction play an important role in the continuous oxidation process. The proper choice of reactor and heat dissipation prevents solids from depositing on the reactor walls and cooling surfaces since the alkali salts of benzothiazol-2-yl sulfonic acid tend to crystallize at temperatures below 50 ° C.

For safety reasons, the oxidation process should be conducted such that the unreacted hydrogen peroxide content of the reaction mixture is low.

DETAILED DESCRIPTION OF THE INVENTION

Example 1

Continuous oxidation of 2-mercaptobenzothiazole with hydrogen peroxide was carried out in a glass stirred reactor with a free volume of 55 cm ³ , measuring the temperature and pH of the reaction medium. Intensive stirring was provided with a paddle stirrer. The reactor was immersed in a water bath. At 50 ° C, a 10 g teflon hose was fed into the reactor via a peristaltic pump. h ^-1 30% by weight. aqueous hydrogen peroxide solution and 33.3 gh ^-1 aqueous-alkaline solution containing 15.3 wt%. potassium salt of 2-mercaptobenzothiazole. The molar ratio of potassium hydroxide and 2-mercaptobenzothiazole was 1.49, the molar ratio of hydrogen peroxide and 2-mercaptobenzothiazole and the load of 3.47 kg · h ^-1 4.4 reactor .dm ^-3. The reaction was allowed to proceed for 9 hours. The product was continuously removed from the reactor by overflow from the side of the reactor to a storage vessel where it was cooled to a temperature of about 10 ° C. After filtering off the precipitated crystalline product and drying and weighing it was analyzed by HPLC. Under these conditions, potassium benzothiazol-2-yl sulfonate was obtained in a yield of 88% with a conversion of 2-mercaptobenzothiazole of 98%. The pH of the reaction mixture was continuously monitored using a combined glass and calomel electrode and maintained at 7.5 to 8.0. Unreacted hydrogen peroxide in the reaction mixture, as determined by iodometric titration, was virtually undetected. The purity of the obtained benzothiazol-2-yl potassium sulfonate was 99.9%.

Example 2

The procedure and apparatus were the same as in Example 1. The reactor was fed continuously with a flow rate of 25.3 gh ^-1 30% by weight. aqueous hydrogen peroxide solution and 59.4 g / ^l aqueous-alkaline solution containing 15.3 wt%. potassium salt of 2-mercaptobenzothiazole. The reaction temperature was 55 ° C and the pH of the reaction mixture moved between 7.9 and 8.5. The molar ratio of potassium hydroxide and 2-mercaptobenzothiazole was 1.49, the molar ratio of hydrogen peroxide and 2-merkaptobenzot.iazolu 5 and the load of the reactor 12.1 kg · h ^-1 .dm ^-3. Under the above conditions, potassium benzothiazol-2-yl sulfonate was obtained in a yield of 78% with a conversion of 2-mercaptobenzothiazole of 98%. Its purity was 99.9%. The concentration of hydrogen peroxide in the reaction mixture was maintained at about 11% by weight.

Example 3

The procedure and apparatus were the same as in Example 1. 13.3 gh ^-1 30% by weight were fed into the reactor. an aqueous solution of hydrogen peroxide and 33.3 gh ^{-1 of an} aqueous-alkaline solution containing 15.3% by weight of the potassium salt of 2-mercaptobenzothiazole. At a reaction temperature of 55 ° C and a pH of the reaction mixture of 8.1 to 8.5. The molar ratio of potassium hydroxide and 2-merkpatobenzotiazolu was 2, the molar ratio of hydrogen peroxide and 2-mercaptobenzothiazole and the load of 4.69 kg · h ^-1 9.3 reactor .dm ^-3. The hydrogen peroxide content of the reaction mixture in the steady state reaction was virtually zero. Under these conditions, benzothiazol-2-yl potassium sulfonate was obtained in a yield of 72% with a conversion of 2-mercaptobenzothiazole of 98%. His purity was. 96%.

Example 4

The procedure and apparatus were the same as in Example 1. At a temperature of 55 ° C and a pH of the reaction mixture of 7.7 to 8.3, 30% by weight were charged to the reactor. hydrogen peroxide solution at a flow rate of 13.0 gh ^-1 and 15.3% by weight. aqueous-alkaline solution of sodium 2-mercaptobenzothiazole at a flow rate of 33.3 gh ^-1 . the molar ratio of sodium hydroxide and 2-mercaptobenzothiazole was 1.5, the molar ratio of hydrogen peroxide and 2-mercaptobenzothiazole and the load of 3.45 kg · h ^-1 8.2 reactor .dm ^-3. At steady state, the hydrogen peroxide content of the reaction mixture was not detected. Under these conditions benzothiazole6 was formed

-2-yl sodium sulfonate in a yield of 76% with a purity of 99.7%, with a conversion of 2-mercaptobenzothiazole of 96%.

Example 5

A glass reaction vessel equipped with a thermometer, stirrer and dropping funnel, which was tempered in a water bath, was charged with 650 cm ^{3 of} water and 50 g of solid potassium hydroxide. After dissolution 100 g of 2-mercaptobenzothiazole was added. After the mixture was heated to 50 ° C, 34% by weight of 200 cm ^{3 were} added dropwise with vigorous stirring. aqueous hydrogen peroxide solution. The molar ratio of potassium hydroxide to 2-mercaptobenzothiazole was 1.49 and the molar ratio of hydrogen peroxide to 2-mercaptobenzothiazole was 3.87. Under these conditions, after 3 hours of reaction, potassium benzothiazol-2-yl sulfonate was obtained in a yield of 58% with a conversion of 2-mercaptobenzothiazole of 65%.

Comparing the results of Example 5 with the results of Examples 1-4, it can be seen that at the same molar ratio of 2-methylbenzothiazole to hydrogen peroxide and the same other reaction conditions, the yield of benzothiazol-2-yl sulfonic acid alkali salt is lower than in the case of continuous oxidation.

Industrial usability

The alkali salts of benzothiazol-2-yl-sulfonic acid are intermediates used in the chemical and pharmaceutical industries. Their hydrolysis under acidic conditions affords 2-hydroxybenzothiazole which is used as a plant growth regulator, a rubber stabilizer, a rubber modifier, a vulcanization accelerator, and also as an intermediate in the production of herbicides, microbicides, medicaments, flame retardants and others.

Claims (1)

PATENT CLAIMS Process for preparing alkali salts of benzothiazol-2-yl sulfonic acid by oxidation of 2-mercaptobenzothiazole with hydrogen peroxide at a temperature of 40-75 ° C, characterized in that the reaction zone is stirred and under a load of 4.4 to 12.1 kg · h. ^-1 .dm ^-3 reaction zone is introduced at the same time the continuity and hydrogen peroxide as a 15 to 80 wt.% aqueous solution and a mixture of 2-mercaptobenzothiazole and alkali metal hydroxide dissolved in water at a molar ratio of 2-mercaptobenzothiazole: hydrogen peroxide: alkali metal hydroxide of 1: 2.2 to 5.8 1 to 3, maintaining the pH of the solution above 7.5 .