KR850001156B1 - Process for the preparation of 2-methoxy-3,6-dichlorobenzoic - Google Patents

Abstract

A process for preparing 2-methoxy-3,6-dichlorobenzoic acid of a purity in excess of 95 wt.% comprises reacting a mixt. of 10-90 wt.% 2-methoxy-3,6-dichlorobenzoic acid and 90-10 wt.% 2-methoxy-3,5- dichlorobenzoic acid with an alkanol. The reaction occurs in an inert organic reaction medium in the presence of an acid catalyst, at a temp. sufficiently elevated to remove the esterification water as it forms and, thereafter, recovering the 2-methoxy-3,6-dichlorobenzoic acid.

Description

Preparation of 2-methoxy-3, 6-dichlorobenzoic acid

DICAMBA or 2-methoxy-3, 6-dichloro benzoic acid are valuable herbicides produced in commercial mass. Industrial "Dicamba," a product used in the manufacture of commercial preparations, typically contains about 84% of compound 2-methoxy-3, 6-dichlorobenzoic acid, although not of high purity.

Primarily impurities are isomeric 2-methoxy-3, 5-dichloro benzoic acid.

This is because “dimamba” is prepared from 1, 2, 4-trichlorobenzene by a three step process and always generates a significant amount of 3, 5-isomer. In the first step of this process, "trichlorobenzene" is converted to 2,5-dichlorophenol by treatment with methanol and sodium hydroxide.

2, 4-dichlorophenol is always produced during this reaction. Dichlorophenol is then treated with carbon dioxide under pressure to obtain 2-hydroxy-3, 6-dichloro benzoic acid and the 3, 5-dichloro isomer. The hydroxyl group group is then methylated to obtain the industrial "dimamba".

Each of the isomeric mixtures of the intermediates is difficult to separate because there is no difference in their boiling points. Moreover, the isomeric mixture of the final product is difficult to separate because it is a solid with similar solubility properties. For example, 2, 4- and 2, 5-dichlorophenols boil at 209-210 ° C. and 210 ° C., respectively.

One way to solve this problem is described in US Pat. No. 4,094,913 to Kalson, wherein 1-bromo-2, 5-dichlorobenzene is reacted with "alkali" metal hydroxides and methanol in the presence of a cocatalyst to obtain high purity 2 And 5-dichlorophenol are manufactured.

This high purity phenol is then used for the production of high purity dicamba. However, in order to use this method, it is necessary to use a complicated procedure for a long time.

Since industrial dicamba is already commercially available and readily available, it is desirable to have a relatively simple process for producing relatively pure 2-methoxy-3, 6-dichlorobenzoic acid from industrial grade products. Surprisingly high purity dicamba of at least 95% by weight, sometimes 99% purity, can be prepared in a single reaction process by selective esterification under certain conditions from 2-methoxy-3, 6-dichlorobenzoic acid and its 3, 5-dichloro isomers. I found it possible.

One embodiment of the invention is a mixture of 2-methoxy-3, 6-dichlorobenzoic acid and 2-methoxy 3,5 dichloro benzoic acid having a purity of at least 95% by weight of "2-methoxy-3, 6-dichloro benzoic acid." In the process for the preparation, the mixture is reacted with alkanol at a temperature where ester water can be removed at reflux in the presence of an acid catalyst in a liquid inert organic reaction medium, and 2methoxy-3 and 6-dichlorobenzoic acid are reacted. It is a method characterized by recovering.

Surprisingly the 3, 5 isomers are esterified with alkanols except for the 3, 6 isomers in the above-described process. The resulting alkane esters are typically liquid products and are easily separated from the remaining acid. Moreover, this ester can be dissolved by the organic solvent, and thus can selectively precipitate the desired purified material from the reaction medium. Representative industrial dicambas produced on a commercial scale contain 80-90% by weight of 2-methoxy-3, 6-dichloro benzoic acid and 10-20% by weight of the 3,5-dichloro isomer.

The starting mixture intended in the process of the invention is this mixture. However, the present invention is not limited thereto and can be used to separate the above-described components in any relative ratio. Therefore, the ratio of each isomer may be disadvantaged by the present invention from 10:90 to 90:10.

Alkanols useful in the process of the invention are lower alkanols having 1-10 carbon atoms. Moreover, any of linear or branched alkanols can be used. Such representative alkanols are ethanol, normal propanol, isopropanol, normal butanol, second butanol, pentanol, 2-methylpentanol, hexanol, heptanol, octanol, nonaol, decanol and the like.

One mole or excess molar equivalent of "alkanol" is used relative to 2-methoxy-3, 5-dichloro benzoic acid present in the starting mixture.

The process of the invention is carried out in a liquid inert organic reaction medium capable of dissolving the starting mixture at the reaction temperature. The choice of reaction medium is not critical but it is preferred to use an organic solvent which dissolves the alkanol ester more than 2-methoxy 3, 6-dichloro benzoic acid. The use of a solvent which dissolves the ester preferentially facilitates the recovery of the desired product.

Particularly useful solvents for this purpose are aromatic hydrocarbons such as benzene, alkylbenzenes and toluene, ethylbenzene, xylene, or the like or mixtures thereof.

The process of the invention is carried out in the presence of a catalytic amount of acid catalyst. Typically, it is appropriate to use an amount ranging from about 0.001-5% of the isomeric starting mixture of substituted benzoic acids.

Representative acid catalysts useful in the process of the present invention are lactic acid "para-toluenesulfonic acid, phosphoric acid, phosphonic acid, titanium alcoholates such as titanium tetraproproxide, benzenesulfonic acid and hydrochloric anhydride.

The process of the invention is carried out at high temperatures up to the reflux temperature of the reaction mixture. It is necessary to raise the temperature sufficiently and remove the moisture of the esterification.

This can be achieved by azeotropic azeotropy using a solvent of aromatic hydrocarbon. Therefore, the use of reflux temperature is preferred, whereby the water of esterification can be removed by conventional means. Completion of the reaction can be monitored by collecting and measuring the moisture content of the resulting esterification.

After the reaction is complete, i.e. when 2-methoxy-3, 5-dichlorobenzoic acid is all selectively esterified, 2-methoxy 3, 6-dichlorobenzoic acid of the desired product is easily recovered by conventional means. . One convenient method of recovery is to sufficiently cool the reaction mixture to precipitate the desired product and recover the precipitate by filtration.

The method of the present invention is explained in more detail by the following examples.

Example 1

85% by weight of 2-methoxy-3, 6-dichloro benzoic acid, 9% by weight of 2-methoxy-3, 5-dichloro benzoic acid (578 g), xylene (750 ml) and normal butanol (62 ml) And paratoluenesulfonic acid (15 g) are added to a magnetic reaction vessel equipped with a mechanical stirrer, a thermometer, and a DEAN-STARK trap and a reflux cooler. The reaction mixture is heated to reflux for about 9 hours. After this time 6 ml of water was collected. The reaction mixture is then concentrated by distilling 250 ml of "xylene" and then cooling to 0 [deg.] C. to produce a precipitate. The precipitate is recovered by filtration and washed three times with cold xylene (100 ml; 8 ° C.).

The washed precipitate is slurried again in xylene and filtered. The filtered solid was washed again with cold xylene (100 ml) and dried under reduced pressure (about 25 inches of mercury) at 25 ° C. to obtain 99.03 according to the measurement of the desired product 2-methoxy-3, 6-dichlorobenzoic acid sess chromatography. It can be obtained by including the 3, 5- isomer of 0.86% by weight with a purity of% by weight.

Example 2

Industrial Dicamba (264 g), xylene (340 ml), and normal butanol (30 ml) containing 85 wt% of 2-methoxy-3, 6-dichloro benzoic acid and 9 wt% of 2-methoxy-3, 5-dichloro benzoic acid And paratoluenesulfonic acid (1 g) are added to a mechanical reaction vessel equipped with a mechanical stirrer, a thermometer and a DEAN STARK trap, and a reflux condenser.

The reaction mixture was heated to reflux for about 24 hours to obtain 3.1 ml of water in the trap.

The reaction mixture is then concentrated by distilling out about 170 ml of xylene and cooling to about 4 ° C. to produce precipitates. The precipitate was taken out by filtration and stripped with distillation and washed with xylene obtained, cooled to about 10 ° C. and air dried to give a purity of 97.58% by weight and to contain 2.17% of 3,5 isomers. A first crop of oxy-3, 6-dichloro benzoic acid (150.14 g) was obtained.

The mother liquor of the filtration is washed, mixed, distilled and concentrated to a volume of 150 ml. The mixture obtained here was cooled to about 4 ° C. to obtain a second precipitate.

This precipitate was washed three times with cold toluene (50 ml part: 8 ° C.) and air dried to obtain 30.6 g of the desired product with an analytical purity of 98.18 wt.

Example 3

About 85% by weight of 2-methoxy-3, 6-dichlorobenzoic acid, about 9% by weight of 2-methoxy-3, 5-dichlorobenzoic acid, industrial dicamba (1000 g), normaloctanol (120 ml) and ethylbenzene ( 1500 ml) and benzenesulfonic acid (10 g) are added to a magnetic reaction vessel equipped with a stirrer, a thermometer and a strap, and a reflux cooler. The reaction mixture is heated to reflux for about 18 hours. After this time elapses, the mixture is cooled to about 3 ° C. to precipitate 2-methoxy-3, 6-dichloro benzoic acid.

The precipitate was collected by filtration, washed with cold toluene and dried to obtain a desired product having a purity of at least 95% by weight.

Example 4

A mixture (600 g) consisting of 2-methoxy-3, 6-dichloro benzoic acid, the same weight part of 2-methoxy-3, 5-dichloro benzoic acid, toluene (800 ml), isopropanol (75 ml) and titanium tetrapropoxide (0.6 g) was added to the vitreous reaction with the stirrer, thermometer and water trap installed and the reflux cooler installed. The reaction mixture is heated to reflux for about 12 hours. After this time has elapsed, the reaction mixture is maintained at about 300 ml of toluene and concentrated.

The remaining mixture is then cooled to about 5 ° C. to precipitate the desired product.

This product is then recovered by filtration, washed with 3 parts of 3 parts of cold toluene (5 ° C .: 100 ml) and dried to obtain 2-methoxy-3, 6-dichlorobenzoic acid.

Claims (1)

2-methoxy- with a purity of at least 95% by weight in a mixture containing 50-85% by weight of 2-methoxy-3,6-dichlorobenzoic acid and 8-50% by weight of 2-methoxy-3,5-dichlorobenzoic acid A process for preparing 3, 6-dichlorobenzoic acid, wherein the mixture is a liquid inert medium selected from toluene, xylene or ethylbenzene and benzenesulfonic acid, P-toluenesulfonic acid, sulfuric acid, phosphoric acid, phosphonic acid or titanium tetrapropoxide. Reacting with alkanol at reflux in the presence of a selected acid catalyst to remove esterified water to produce 2-methoxy-3, 6-dichloro benzoic acid.