NL8102501A - PROCESS FOR PREPARING HIGH PURITY DICAMBA. - Google Patents

Description

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Process for preparing high purity dicamba.

Dicamba or 2-methoxy-3,6-dichlorobenzoic acid is a valuable herbicide, which is produced in large technical quantities. Technical dicamba, that is, the product used for the preparation of technical preparations, does not have high purity, but typically contains about 84% of the compound 2-methoxy-3,6-dichlorobenzoic acid. The main impurity is the isomeric 2-methoxy-3,5-dichlorobenzoic acid. The reason for this is that dicamba is prepared from 1,2,4-trichlorobenzene by a three-step process, which has always resulted in the formation of a significant amount of the 3.5 isomer. In the first step of the process, the trichlorobenzene is converted to the 2,5-dichlorophenol by treatment with methanol and sodium hydroxide. 2,4-dichlorophenol is always formed during this reaction. The dichlorophenols are then treated with carbon dioxide under pressure to form 2-hydroxy-3,6-dichlorobenzoic acid and the 3,5-dichloro isomer. The hydroxyl group is then methylated to form the technical dicamba. Each of the intermediate isomeric mixtures are difficult to separate due to their close boiling points. Furthermore, the isomer mixture obtained as end product is difficult to separate because h-t consists of solids with analogous solubility properties. For example, the 2.4 and 2.5 dichlorophenols boil at 209-210 ° C and 211 ° C, respectively.

A method of overcoming this difficulty is described in U.S. Patent 4,094,913, which describes the preparation of pure 2,5-dichlorophenol by reacting 1-bromo-2,5-dichlorobenzene with alkali metal hydroxide and methanol in the presence of a copper. catalyst. This pure phenol can then be used to prepare high purity dicamba.

However, the execution of this process requires 8102501 - 2 - / the application of a lengthy and difficult technical procedure. Since technical dicamba is already a commercial product and readily available, it would be desirable to have a relatively simple procedure for preparing relatively pure 2-methoxy-3,6-dichlorobenzoic acid from the technical grade product.

It has now surprisingly been found that high purity dicamba, i.e., a purity of more than 95% by weight and often a purity of 99%, can be prepared in a single reaction step from a mixture of 2-methoxy-3.6- dichlorobenzoic acid and its 3,5-dichloro isomer thereof by selective esterification under certain conditions.

More particularly, according to an embodiment of the invention, there is provided a process for preparing 2-methoxy-3,6-dichlorobenzoic acid having a purity greater than 95% by weight from a mixture containing 2-methoxy-3,6-dichlorobenzoic acid and 2 -methoxy-3,5-dichlorobenzoic acid, characterized in that said mixture is reacted with an alkanol in a liquid inert organic reaction medium in the presence of an acid catalyst at a high enough temperature to remove the esterification water and then the 2-methoxy -3,6-dichlorobenzoic acid is recovered.

In the above process, the 3.5 isomer is surprisingly selectively esterified by the alkanol to the exclusion of the 3.6 isomer. The alkane ester formed is typically a liquid product and can be easily separated from the residual acid. In addition, the esters are more soluble in organic solvents and therefore also allow the selective precipitation of the desired purified product from the reaction medium.

Technical dicamba as prepared on technical plan typically contains 80-90 wt% 2-methoxy-3,6-dichlorobenzoic acid and 10-20 wt% of the corresponding 3,5-dichloro isomer. It is this mixture which is particularly intended as a starting mixture in the process of the invention. However, the process of the invention is not limited to the above 81 02 5 0 1 --3 mixture, but can be used to separate said components in any relative amount. Accordingly, amounts ranging from 10:90 to 90:10 of the respective isomers can be separated according to the method of the invention.

The alkanols useful in the process of the invention are the lower alkanols, such as the alkanols of 1-10 carbon atoms. In addition, both straight and branched alkanols can be used. Examples of such alkanols are ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, pentanol, 2-methylpentanol, hexanol, heptanol, octanol, nonanol, decanol and the like. A molar equivalent or more than 1 molar equivalent of the alkanol to the 2-methoxy-3,5-dichlorobenzoic acid present in the starting mixture can be used.

The process of the invention is carried out in a liquid inert organic reaction medium, which can dissolve the starting isomer mixture at reaction temperatures.

Although the choice of the reaction medium is not essential, it is desirable to use an organic solvent in which the alkanol ester is more soluble than the 2-methoxy-3,6-dichlorobenzoic acid. The use of a solvent which preferentially dissolves the ester will facilitate the recovery of the desired product. Solvents that are particularly useful for this purpose are aromatic hydrocarbons, such as benzene and alkylbenzenes, including toluene, ethylbenzene, xylene and the like, or mixtures thereof.

The process of the invention is conducted in the presence of a catalytic amount of an acidic catalyst. Typically, an amount ranging from about 0.001 to about 5% by weight based on the starting isomeric mixture of substituted benzoic acids can be suitably used. Examples of acidic catalysts useful in the process of the invention are sulfuric acid, p-tolu-35 sulfonic acid, phosphoric acid, phosphonic acid, titanium alcohols such as titanium tetrapropoxide, benzenesulfonic acid, and anhydrous chlorine 8102501.

b * ~ 'V

- 4 - hydrogen acid.

The process of the invention is carried out at elevated temperatures up to the reflux temperature of the reaction mixture. Temperatures high enough to remove the esterification water are necessary. This can be conveniently accomplished by azeotropic distillation when using an aromatic hydrocarbon solvent. Accordingly, reflux temperatures are preferably used, and the esterification water can be removed by conventional means. Furthermore, the reaction can be monitored to determine its completion by collecting and measuring the amount of esterification water formed.

After the reaction is complete, that is, when all of the 2-methoxy-3,5-dichlorobenzoic acid is selectively esterified, the desired product, namely 2-methoxy-3,6-dichlorobenzoic acid, can be easily recovered by conventional means. A suitable method of recovery involves cooling the reaction mixture to a sufficient degree to precipitate the desired product and recovering the precipitate by filtration.

The invention is further illustrated but not limited by the following examples.

Example I

Technical dicamba (578 g), containing 85 wt% 2-methoxy-3,6-dichlorobenzoic acid and 9 wt% 2-methoxy-3,5-dichlorobenzoic acid, xylene (750 ml), n-butanol (62 ml) and p-toluenesulfonic acid (1.5 g) were placed in a glass reaction vessel equipped with a mechanical stirrer, thermometer and reflux condenser containing a Dean-Stark water collector.

The reaction mixture was refluxed for a period of about 9 hours. At the end of this period, 6 ml of water had been collected. The reaction mixture was then concentrated by distilling off 250 ml of xylene and then cooled to 0 ° C resulting in the formation of a precipitate. The precipitate was collected by filtration and washed 81 02 5 0 1 - 5 - three times with cold xylene (100 ml; 8 ° C). The washed precipitate was then resuspended in xylene and filtered. The filtered solid was again washed with cold xylene (100 ml) and then dried under reduced pressure (about 25 mm Hg) at 50 ° C to yield the desired product, 2-methoxy-3,6-dichlorobenzoic acid, of 99 purity. 0.03 wt% and containing 0.86 wt% of the 3.5 isomer, as determined by gas chromatography, was obtained.

Example II

Technical dicamba (264 g), containing 85 wt% 2-methoxy-3,6-dichlorobenzoic acid and 9 wt% 2-methoxy-3,5-dichlorobenzoic acid, a mixture of xylenes (340 ml), n-butanol (30 ml) and p-toluenesulfonic acid (1 g) was placed in a glass reaction vessel equipped with a magnetic stirrer, thermo-15 meter and reflux condenser containing a Dean-Stark water collector. The reaction mixture was refluxed for a period of about 24 hours, resulting in the collection of 3.1 ml of water. The reaction mixture was then concentrated by distilling off about 170 ml of xylene and then cooled to about 4 ° C, resulting in the formation of a precipitate. The precipitate was collected by filtration and washed with the stripped xylenes obtained from the distillation and cooled to about 10 ° C and air dried to obtain a first yield of the desired 2-methoxy-3,6-dichlorobenzoic acid (150 14 g) with a purity of 97.58% by weight and containing 2.17% of the 3.5 isomer. The mother liquor from the filtration was combined with the washing liquid and the mixture was concentrated by distillation to a volume of 150 ml. The resulting mixture was cooled to about 4 ° G to form a second precipitate. This precipitate was collected by filtration, washed three times with cold toluene (50 ml portions; 8 ° C) and air dried to give an additional 30.6 g of the desired product with a purity of 98.17 wt% and containing 1.71 wt% of the 3.5 isomer.

81 025 01 - 6 -

Example Hl

Technical dicamba (1000 g), containing about 85 wt% 2-methoxy-3,6-dichlorobenzoic acid and about 9% 2-methoxy-3,5-dichlorobenzoic acid, n-octanol (120 ml), ethylbenzene (1500 ml) and benzenesulfonic acid (10 g) were placed in a glass reaction vessel equipped with a stirrer, thermometer and reflux condenser containing a water collector. The reaction mixture was refluxed for a period of about 18 hours. At the end of this period, the mixture was cooled to about 3 ° C to precipitate 2-methoxy-3,6-dichlorobenzoic acid. The precipitate was collected by filtration, washed with cold toluene and dried to yield the desired product with a purity of more than 95% by weight.

Example IV

A mixture containing equal parts by weight of 2-methoxy-3,6-dichlorobenzoic acid and 2-methoxy-3,5-dichlorobenzoic acid (600g), toluene (800ml), isopropanol (75ml) and titanium tetrapropoxide (0.6g) were placed in a glass reaction vessel equipped with a stirrer, thermometer and reflux condenser containing a water collector. The reaction mixture was refluxed for a period of about 12 hours. After this period, the reaction mixture was concentrated by distilling off about 300 ml of toluene. The remaining mixture was then cooled to a temperature of about 5 ° C to precipitate the desired product. This product was then collected by filtration, washed with three portions of cold toluene (5 ° C; 100 ml) and dried, yielding 2-methoxy-3,6-dichlorobenzoic acid.

Claims (11)

1. Process for preparing 2-methoxy-3,6-dichlorobenzoic acid with a purity of more than 95% by weight from a mixture containing 10-90 parts by weight of 2-methoxy-3,6-dichlorobenzoic acid and 90-10 wt. parts of 2-methoxy-3,5-dichlorobenzoic acid, characterized in that the mixture is reacted with an alkanol in a liquid inert organic reaction medium in the presence of an acid catalyst at a temperature high enough to remove the esterification water formed and then the 2-methoxy-3,6-dichlorobenzoic acid wins. 2. Process according to claim 1, characterized in that the alkanol contains 2-10 carbon atoms. Process according to claim 2, characterized in that the alkanol is n-butanol. Process according to claim 1, characterized in that the reaction medium is an aromatic hydrocarbon of the alkylbenzene series. Process according to claim 4, characterized in that the reaction medium is selected from toluene, 20 xylene and ethylbenzene. Process according to claim I, characterized in that the acid catalyst is selected from benzenesulfonic acid, p-toluenesulfonic acid, sulfuric acid, phosphoric acid, phosphonic acid and titanium tetrapropoxide. Process according to claim 1, characterized in that the reaction temperature is the reflux temperature of the reaction mixture. 8. Process according to claim 4, characterized in that the esterification water is removed by azeotropic distillation. Process according to claim 1, characterized in that the purified 2-methoxy-3,6-dichlorobenzoic acid is recovered by cooling the reaction mixture sufficiently to precipitate the acid from the reaction medium. Process according to claim 1, characterized in that the mixture contains about 80-90% by weight 2-methoxy-3,6-dichlorobenzoic acid and about 10-20% 2-methoxy-3,5-dichlorobenzoic acid, the alkanol contains 1-10 carbon atoms, the reaction medium is an aromatic hydrocarbon of the alkylbenzene-5 series and the catalyst is p-toluenesulfonic acid. 11. Methods as described in the description and / or examples. 81 025 0 1