RU2149157C1 - Method of preparing purified 2,4-dichlorophenoxylacetic acid - Google Patents

Abstract

FIELD: chemical industry. SUBSTANCE: described is method of preparing purified 2,4-dichlorophenoxyacetic acid by condensation of industrial 2,4-dichlorophenol with monochloroacetic acid in alkaline medium followed by acidification of reaction mixture in the presence of solvent during heating operation and recovery of the desired product by crystallization during cooling operation. Acidification is carried out in the presence of organic solvent such as perchloroethylene or toluene from which the desired product is isolated by crystallization when cooled to 5-30 C. EFFECT: simplified technology of the process. 4 ex, 1 tbl

Description

 The invention relates to a method for producing pure 2,4-dichlorophenoxyacetic acid (2,4-D acid) used for special purposes, in particular as an active substance in a number of effective herbicides.

 The 2,4-D acid currently obtained by the condensation of monochloroacetic acid with technical dichlorophenol contains impurities in the form of 2-chloro, 4-chloro, 2,6-dichloro, 2,4,6-trichlorophenoxyacetic acids.

 A known method of producing 2,4-D acid (prototype) (copyright certificate N 118508, C 07 C 59/70, 1948) by condensation of monochloracetic acid with 2,4-dichlorophenol in an alkaline medium, followed by acidification of the reaction mass when heated in an aqueous medium and the selection of the target product by crystallization upon cooling to room temperature, after which it is recrystallized from an aqueous solution of acetic acid. The disadvantages of this method are the high aggressiveness of the environment, requiring special equipment, the need for purification of acetic acid itself from organic impurities (chlorophenols, chlorophenoxyacetic acids), which is a very difficult technological task due to the high toxicity of acetic acid.

The authors propose a method for producing 2,4-D acid by condensing monochloracetic acid with 2,4-dichlorophenol in an alkaline medium, followed by acidification of the reaction mass in the presence of a solvent to pH 1-2, crystallization of the isomer of 2,4-dichlorophenoxyacetic acid from an organic solvent at a temperature 5-30 ^o C, filtering and drying the resulting product.

 A distinctive feature of the proposed method is the isolation of the 2,4-D acid isomer by crystallization from a solvent (from a solution of a technical 2,4-D acid in an organic solvent).

 The invention consists in the following.

 It is known that isomers of chlorophenols have a different degree of dissociation in solutions (the degree of dissociation increases in the series from monochlorophenols to trichlorophenols). This allows them to be separated, for example, by dissociative extraction.

 We hypothesized that 2-chloro, 4-chloro, 2,4-dichloro, 2,6-dichloro, 2,4,6-trichlorophenoxyacetic acids have the same properties.

However, the order of their isolation during crystallization should be reversed in comparison with the dissociative extraction of chlorophenols, that is, when the organic solution of the above acids is cooled, 2-chloro, 4-chlorophenoxyacetic acids must first drop out of the solution, then 2,4-dichlorophenoxyacetic acids, then 2.6 β-dichlorophenoxyacetic acid and finally 2,4,6-dichlorophenoxyacetic acid. Optimal temperatures were found for the sequential isolation of acid isomers. Changing the temperature of the solution from 5 to 30 ^o C, we received purified 2,4-dichlorophenoxyacetic acid with a content of 2,4-D acid isomer from 96 to 99.71%.

 The following examples illustrate the possibilities of the proposed method for producing purified 2,4-D acid.

Example 1. A 41% aqueous solution of the sodium salt of technical 2,4-dichlorophenol in an amount of 91.35 g (containing 85 wt.% Of isomer 2.4) is loaded into the reactor. 56.44 g of a 41% aqueous solution of sodium salt of monochloracetic acid are charged there. The mixture is heated to 70-80 ^o C. Due to the heat of reaction, the temperature rises to 100-110 ^o C. At this temperature, the mixture is kept for about 3 hours The process control is carried out by changing the pH of the solution. The pH of the medium is maintained with a dosage of 30% aqueous alkali solution in the range of 10-11.

After exposure to the condensed mass at 80-84 ^o C add solvent, perchlorethylene in an amount of 194.4 g and hydrochloric acid of 27.5% concentration in an amount of 34.51 g. The resulting technical 2,4-D acid is dissolved in perchlorethylene , the organic layer at 80-84 ^o C is separated from the aqueous, cooled to 30 ^o C, filtered. The resulting wet cake was dried. The result is a dry 2,4-D acid with a content of 2,4-D isomer of 99.7%. Yield 80%.

Example 2. In the conditions of example 1, the resulting solution of technical 2,4-D acid in perchlorethylene is cooled to 5 ^o C, the crystals of 2,4-D acid precipitated from the solution are filtered off, dried, and dry 2,4-D acid is obtained with the content of isomer 2 4 - 96.4%. Yield 87%.

Example 3. The selection of 2,4-D acid obtained in example 1 from perchlorethylene is carried out at 15 ^o C, 2,4-D acid is filtered off, dried. Get dry 2,4-D acid with a content of 98.17%. Yield 84%.

Example 4. The selection of 2,4-D acid obtained in example 1 is carried out from toluene at 30 ^o C, 2,4-D acid is filtered off, dried. Get dry 2,4-D acid with a content of 99.8%. Yield 85%.

 Other examples are similarly carried out.

 The results of all experiments are shown in the table.

Claims (1)

The method of obtaining purified 2,4-dichlorophenoxyacetic acid by condensation of technical 2,4-dichlorophenol with monochloroacetic acid in an alkaline medium, followed by acidification of the reaction mass in the presence of a solvent by heating and isolation of the target product by crystallization by cooling, characterized in that the acidification is carried out in the presence of an organic solvent , such as perchlorethylene or toluene, from which the target product is isolated by crystallization upon cooling to 5 - 30 ^o C.

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