IL124481A - Method of reducing the amount of by-products in carbendazim by adding diazotization agents - Google Patents

Abstract

A process for reducing the level of undesirable byproducts in impure carbendazim, which comprises treating the impure carbendazim in an acid medium with a diazotizing agent, and then isolating the carbendazim in a conventional manner.

Description

METHOD OF REDUCING THE AMOUNT OF BY-PRODUCTS IN CARBENDAZIM BY ADDING DIAZOTIZATION AGENTS For the Applicant, Eitan, Pearl, Latzer & Cohen-Zedek Advocates, Patent Attorneys & Notaries P-1642-IL PROCESS FOR REDUCING THE BYPRODUCT CONTENT OF CARBENDAZIM BY ADDITION OF DIAZOTIZING AGENTS.

The present invention relates to a process which converts carbendazim of insufficient quality into a product of high purity.

Carbendazim (methyl 1 H-benzimidazol-2-ylcarbamate) (I) is a valuable fungicide with a wide spectrum of activity [Charles R. Worthing, Pesticide Manual (1991 ) p. 123, The British Crop Protection Council].

It can be used as a systemic plant fungicide and as a technical fungicide. Furthermore, it serves as a starting material for the preparation of the important fungicide benomyl.

In all customary syntheses, the preparation of carbendazim is carried out advantageously by reacting 1 ,2-diaminobenzene with methyl cyanocarbamate according to DE-A-16 68 557 and DE-A-17 95 849. In this reaction, dimeric and polymeric condensates of 1 ,2-diaminobenzene are formed as byproducts, which cause undesirable discoloration of the product and in part can have toxic properties. These impurities can also have properties which in part may be linked to the 1 ,2-diaminobenzene employed.

These byproducts include, for example, 2,3-diaminophenazine (II) and 3-amino-2-hydroxyphenazine (III) whose residual levels in carbendazim prepared in this manner can be used as a measure of the purity of the product. To safeguard quality, it is therefore important to eliminate these staining and toxic concomitants of the carbendazim.

The reduction of these byproduct contents during the synthesis of carbendazim by addition of reducing agents (dithionites and thiosulfates) has already been described (DE-A-33 23 034). The formation of such byproducts can also be suppressed by consistently working under protective gas and using extremely pure 1 ,2-diaminobenzene in the synthesis. DE-A-44 02 043 describes the subsequent treatment of carbendazim with a large excess of aqueous formaldehyde solution.

It is a disadvantage of all existing processes, that in each case only starting materials of specified purity or batches of carbendazim with particular properties may be used. This can neither consistently and universally suppress the formation of the abovementioned byproducts, nor satisfactorly reduce their level in the reaction product. The processes are further characterized by high costs and/or difficult disposal of the resulting mother liquors.

It is therefore the object to significantly reduce the level of undesirable components in impure batches of carbendazim by a simple, generally applicable method in a cost-effective way and without damaging the environment, or to eliminate these components altogether.

The object is achieved by the present invention by admixing a suspension, solution or partial solution of the contaminated carbendazim in an acidic, aqueous medium with a small amount of a diazotizing reagent. After readjusting to a neutral pH, isolation can be performed in a customary manner. The preferred diazotizing agents are nitrous acid, its salts (nitrites) and their aqueous solutions, and nitrosylsulfuric acid and its solutions. The acids are preferably mineral acids such as hydrohalic acids (HCI, HBr), sulfuric acid, phosphoric acid and nitric acid. However, other diazotizing agents and acids may also be used.

The present invention therefore relates to a process for reducing the level of undesirable byproducts in impure carbendazim, which comprises treating the impure carbendazim in an acidic medium with a diazotizing agent, and subsequently isolating the carbendazim in a conventional manner.

The impure carbendazim charges mentioned are preferably charges of carbendazim obtained by the customary industrial production processes, in particular the product resulting from reaction of 1 ,2-diaminobenzene with methyl cyanocarbamate.

In the process according to the invention, the impure carbendazim is suspended or dissolved at 0 to 100°C, advantageously at 0 to 70°C, in dilute aqueous acid, preferably mineral acid (0 to 1.0 equivalent per mole of active compound), and admixed with up to 30 g, preferably 0.5 to 30 g, of the diazotizing agent per kg of active carbendazim. The diazotizing agents used are preferably alkali metal or alkaline earth metal nitrites or nitrosylsulfuric acid in the form of an aqueous or sulfuric acid solution or as a solid; the addition of mineral acids may be dispensed with entirely or partially when using nitrosylsulfuric acid. After a reaction time of 0.5 to 4 hours, the pH is adjusted to 5.5. to 7 using a base. The pure carbendazim is usually isolated by filtration/centrifugation in yields of normally more than 99%, based on the starting material. The aqueous mother liquor can be disposed of directly by way of a biological wastewater treatment plant, the process according to the invention thus offering significant advantages from an ecological point of view.

In general, the analysis of the carbendazim thus obtained shows levels of impurities of the formulae II and III of below 1 ppm in each case. It is therefore suitable for all areas of application.

The process is preferably carried out under an amosphere of inert gas. The solution to the problem provided by the method according to the invention is surprising because benzimidazole-2-carbamates can themselves react with diazotizing agents. However, this possible reaction is suppressed under the reaction conditions chosen; there is no loss of product.

Hereinafter, the process according to the invention is illustrated by examples.

Example 1 : 40 g of crude carbendazim powder (98.5% pure; II = 49 ppm, III = 2 ppm) are stirred under an N2 atmosphere with 100 ml of demineralized water to give a suspension. At 60°C, 0.5 ml of nitrosylsuifuric acid, in 40% strength solution in H2S04, is added dropwise with efficient stirring below the surface of the liquid within 30 minutes. During that period, the initially brown color lightens noticeably. Stirring is continued at the same temperature for a total of 4 hours, and the mixture is then neutralized to pH 6 to 7 using dilute sodium hydroxide solution. The product is filtered off under suction and washed with water (50°C). After drying, 39.5 g of pure carbendazim *99.4% of theory are obtained (HPLC: 99.2% pure; II < 1 ppm : III < 1 ppm).

Example 2: Under an N2 atmosphere, 40 g of crude carbendazim (98.5% pure; II = 49 ppm; III = 2 ppm) are heated to 50°C in 90 ml of water and 10 ml of 2 M hydrochloric acid with stirring. At this temperature, a solution of 0.4 g of sodium nitrite in 10 ml of water is added dropwise below the surface of the liquid slowly within 30 minutes and stirring is then continued at the same temperature for a further 3 hours. The pH is then adjusted to 6 by the dropwise addition of 20% strength sodium hydroxide solution. The product is filtered off and washed with water (50°C). After drying under reduced pressure at 65 to 70°C, 39.55 g of pure carbendazim are obtained _¾99.8% of theory (HPLC: 99.5% pure; II < 1 ppm; III < 1 ppm).

Example 3: Under an atmosphere of nitrogen, 40 g of crude carbendazim (99.7% pure; II = 14 ppm; III = 2 ppm) are dissolved in 120 ml of 2 M hydrochloric acid and 40 ml of water by heating to 55°C with stirring. At 50°C, 0.4 g of crystalline nitrosylsulfuric acid (E. Merck, Art. No. 820911 ) is then added a little at a time with thorough stirring. Stirring is continued at the same temperature for a further 30 minutes, and the mixture is then neutralized using 20% strength sodium hydroxide solution. The precipitated product is filtered off under suction using a Nutsche filter, and washed three times with a little water.

After drying (65°C/100 mbar), 39.7 g of product £99.3% of theory are obtained (HPLC: 99.8% pure; II < 1 ppm; III < 1 ppm).

Comparative Example 1 : Example 3 is repeated (same initial charge), but without the addition of diazotizing agent.

Product: 39.7 g *99.25% of theory (HPLC: 99.7% pure; II = 8 ppm; III = 1.5 ppm).

Comparative Example 2 (corresponds to DE-A-44 02 043): Example 1 is repeated (same initial charge), but with the addition of 5.0 g of 37% strength aqueous formalin solution and 0.05 g of a wetting agent (©Prawozell ND 11 ) instead of a diazotizing agent.

Product: 39.6 g 99.8% of theory (HPLC: 99.3% pure; II = 3 ppm; III = 1.5 ppm).

Claims (8)

124481/2 WO 97/19933 6 PCT/EP96/04799 What is claimed is:

1. A process for reducing the level of undesirable byproducts in impure carbendazim, which comprises treating the impure carbendazim in an acid medium with a diazotizing agent, and then isolating the carbendazim in a conventional manner.

2. A process as claimed in claim 1 , wherein the impure carbendazim mentioned therein is the product obtained from the reaction of 1 ,2- diaminobenzene with methyl cyanocarbamate.

3. A process as claimed in claim 1 or 2, wherein up to 30 g of diazotizing agent are used per kg of impure carbendazim.

4. A process as claimed in any of claims 1 to 3, wherein the treatment with the diazotizing agent is carried out at 0 to +100°C.

5. A process as claimed in any of claims 1 to 4, wherein the treatment with the diazotizing agent is carried out in solution or suspension in a mineral acid medium.

6. A process as claimed in any of claims 1 to 5, wherein the diazotizing agent is selected from group consisting of the salts of nitrous acid and their aqueous solutions and nitrosylsulfuric acid and its solutions.

7. A process as claimed in any of claims 1 to 6, wherein, in the treatment with the diazotizing agent, the mineral acid(s) is/are used in aqueous, dilute form in an amount of 0 to 1.0 equivalent of acid per mole of active compound.

8. A process as claimed in any of claims 1 to 7, wherein the treatment with the diazotizing agent is carried inert gas. ies