CH646936A5 - Method for producing a high content dicamba mit. - Google Patents

Description

646936

2nd

PATENT CLAIMS

1. A process for the preparation of 2-methoxy-3,6-di-chlorobenzoic acid with a purity of more than 95% by weight from a mixture comprising 10-90 parts by weight of 2-methoxy-3,6-dichlorobenzoic acid and 90-9 Parts by weight of 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 acidic catalyst at a temperature which is sufficiently elevated to remove the water remove the esterification as soon as it is formed, and then the 2-methoxy-3,6-di-chlorobenzoic acid is obtained.

2. The method according to claim 1, characterized in that the alkanol has 2-10 carbon atoms.

3. The method according to claim 2, characterized in that the alkanol is n-butanol.

4. The method according to claim 1, characterized in that the reaction medium is an aromatic hydrocarbon from the alkylbenzene series.

5. The method according to claim 4, characterized in that the reaction medium is selected from the group consisting of toluene, xylene and ethylbenzene.

6. The method according to claim 1 ,. characterized in that the acid catalyst is selected from the group consisting of benzenesulfonic acid, p-toluenesulfonic acid, sulfuric acid, phosphoric acid, phosphonic acid and titanium - tetrapropoxide.

7. The method according to claim 1, characterized in that the reaction temperature is the reflux temperature of the reaction mixture.

8. The method according to claim 4, characterized in that the water of the esterification is removed by means of azeotropic distillation.

9. The method according to claim 1, characterized in that the purified 2-methoxy-3,6-dichlorobenzoic acid is obtained by cooling the reaction mixture to a temperature which is sufficient for the acid to precipitate out of the reaction medium.

10. The method according to claim 1, characterized in that said mixture contains 80-90 wt .-% 2-methoxy-3,6-dichlorobenzoic acid and 10-20% 2-methoxy-3,5-dichlorobenzoic acid, the Alkanol has 1-10 carbon atoms, the reaction medium is an aromatic hydrocarbon from the alkylbenzene series and the catalyst is p-toluenesulfonic acid.

Dicamba or 2-methoxy-3,6-dichlorobenzoic acid is a valuable herbicide, which is produced in large commercial quantities. Technical Dicamba, i.e. the product used to make commercial formulations is not of high purity and typically contains about 84% of the compound 2-methoxy-3,6-dichlorobenzoic acid. The predominant impurity is the isomer 2-methoxy-3,5-dichlorobenzoic acid. The reason for this is that Dicamba is made from 1,2,4-trichlorobenzene in a three-step process, which always results in the formation of a substantial quantity of the 3,5-isomer. In the first step of the process, the trichlorobenzene is converted into 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 give 2-hydroxy-3,6-dichlorobenzoic acid and the 3,5-dichloroisomer. The hydroxyl group is then methylated to give the technical dicamba. Each of the intermediate mixtures of isomers is difficult to separate due to the close boiling points. Furthermore, the end product, the isomeric mixture, is difficult to separate because it consists of solids with similar solution properties. For example, the 2,4- and 2,5-dichlorophenols boil at temperatures of 209-210 ° C and 211 ° C.

A method of overcoming this problem is described by Carlson in U.S. Patent 4,094,913, wherein pure 2,5-dichlorophenol is made by the reaction of l-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 produce high purity dicamba.

To use this method, however, it is necessary to use the long and commercially difficult method. Because technical dicamba will always be a commercial item and is readily available, it would be desirable to have a relatively simple process for producing relatively pure 2-methoxy-3,6-dichlorobenzoic acid from the technically pure product.

It has now surprisingly been found that dicamba with a high content, with a purity of more than 95% by weight and often with a purity of 99% can be produced in a single reaction step from a mixture of 2-methoxy-3,6- dichlorobenzoic acid and the 3,5-dichloroisomer using selective esterification under certain conditions.

More specifically, one embodiment of the present invention includes a process for making 2-methoxy-3,6-dichlorobenzoic acid in a purity greater than 95% by weight from a mixture comprising 2-methoxy-3,6-dichlorobenzoic acid and Contains 2-methoxy-3,5-dichlorobenzoic acid, which is characterized in that said mixture is reacted with the alkanol in a liquid inert organic reaction medium in the presence of an acid catalyst at a sufficiently elevated temperature to the water remove the esterification, and then the 2-methoxy-3,6-dichlorobenzoic acid is obtained.

In the above-mentioned process, the 3,5-isomer was surprisingly selectively esterified with the alkanol, with the exclusion of the 3,6-isomer. The alkane ester formed is typically a liquid product and can be easily separated from the remaining acid. Furthermore, the esters are more soluble in organic solvents and therefore the selective precipitation of the desired purified product from the reaction medium is also made possible.

Technical dicamba, as produced on a commercial scale, typically contains 80-90% by weight 2-methoxy-3,6-dichlorobenzoic acid and 10-20% by weight corresponding 3,5-dichloroisomer. It is this mixture which is preferably used as the starting mixture in the process of this invention. However, the invention is not limited to this, but any mixture with the ratio mentioned in claim 1 can be used to separate the said compounds. Accordingly, fractions which extend from 10:90 to 90: 9 of the isomers in question can be separated by means of the process according to the invention.

The alkanols useful in the process of the present invention are the lower alkanols, such as the alkanols, which have from 1-10 carbon atoms. Either straight chain or branched alkanols can also be used. Examples of such alkanols

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646936

are ethanol, n-propanol, isopropanol, n-butanol, sec-buta-nol, pentanol, 2-methylpentanoI, hexanol, heptanol, octa-nol, nonanol, decanol and the like. One molar or more than molar equivalent of the alkanol with respect to the 2-methoxy-3,5-dichlorobenzoic acid present in the starting mixture can be used.

The process of the present invention is carried out in a liquid inert organic reaction medium which is capable of dissolving the starting isomer mixture at the reaction temperatures. While the choice of reaction medium is not critical, 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 preferably dissolves the ester, simplifies the recovery of the desired product. Solvents that are particularly useful for this purpose are aromatic hydrocarbons such as benzene and alkylbenzenes, including toluene, ethyl benzene, xylene and the like, or mixtures thereof.

The process of the present invention is carried out in the presence of a catalytic amount of an acid catalyst. Typically, an amount ranging from about 0.001 to about 5% by weight of the starting isomeric mixture of substituted benzoic acids can be suitably used. Examples of acidic catalysts that can be used in the present process are sulfuric acid, para-toluenesulfonic acid, phosphoric acid, phosphonic acid, titanium alcoholates such as titanium tetraproproxide, benzenesulfonic acid, and anhydrous hydrochloric acid.

The process of the present invention is carried out at elevated temperatures which extend to the reflux temperature of the reaction mixture. Temperatures are required which are sufficiently high to remove the water from the esterification. This can be conveniently accomplished using azeotropic distillation when an aromatic hydrocarbon solvent is used. Accordingly, reflux temperatures can preferably be used and the water of the esterification can be removed by conventional means. The completeness reaction can also be followed by collecting and measuring the amount of water formed during the esterification.

After the reaction has ended, e.g. when all 2-methoxy-3,5-dichlorobenzoic acid has been selectively esterified, the desired product, 2-methoxy-3,6-dichlorobenzoic acid, can be easily obtained by conventional means. A convenient method of recovery involves cooling the reaction mixture sufficiently to precipitate the desired product, and the precipitate is recovered by filtration.

The process of the present invention is illustrated in more detail by means of the following examples.

example 1

Technical dicamba (578 g), which contained 85% by weight of 2-methoxy-3,6-dichlorobenzoic acid and 9% by weight of 2-methoxy-3,5-dichlorobenzoic acid, xylene (750 ml), n-Bu -tanol (62 ml) and p-toluenesulfonic acid (1.5 g) were placed in a glass reaction vessel, which was equipped with a mechanical stirrer, a thermometer and a reflux condenser, connected to a Dean-Stark separator. The reaction mixture was heated to reflux over a period of about 9 hours.

After this time, 6 ml of water had been collected. The reaction mixture was then concentrated by distilling off 250 ml of xylene and then cooled to a temperature of 0 ° C., whereby a precipitate was formed. The precipitate was collected by filtration and washed three times with cold xylene (100 ml; 8 ° C). The washed precipitate was then slurried in xylene and filtered. The filtered solid was washed again with cold xylene (100 ml) and then it was dried under reduced pressure (about 25 in a mercury column) at a temperature of 50 ° C. to give the desired product, 2-methoxy-3,6- dichlorobenzoic acid, which had a purity of 99.03% by weight and contained 0.86% by weight of 3,5-isomers, as determined by gas chromatography.

Example 2

Technical dicamba (264 g), which contained 85% by weight of 2-methoxy-3,6-dichlorobenzoic acid and 9% by weight of 2-methoxy-3,5-dichlorobenzoic acid, a mixture of xylene (340 ml ), n-butanol (30 ml) and p-toluenesulfonic acid (lg) were placed in a glass reaction vessel which was equipped with a magnetic stirrer, a thermometer and a reflux condenser which was connected to a Dean-Stark separator. The reaction mixture was heated to reflux over a period of about 24 hours, with 3.1 ml of water being obtained in the separator. The reaction mixture was then concentrated by distilling off about 170 ml of xylene and then cooled to a temperature of about 4 ° C, resulting in a precipitate. The precipitate was collected by filtration and was washed with the distilled xylenes obtained from the distillation, and cooled to a temperature of about 10 ° C, followed by air drying to obtain a first batch of the desired 2-methoxy-3 , 6-dichlorobenzoic acid (150.14 g), which had a purity of 97.58% by weight and contained 2.17% of 3,5-isomers. The mother liquor from the filtration was combined with the solvents from the washing processes and the mixture was concentrated to a volume of 150 ml by distillation. The resulting mixture was cooled to a temperature of about 4 ° C to give a second precipitate. This precipitate was separated by filtration, washed three times with cold toluene (50 ml portions; 8 ° C.) and then air-dried to give a further 30.6 g of the desired product, which had a content of 98.17% by weight. and contained 1.71% by weight of 3,5 isomers.

Example 3

Technical dicamba (1000 g), which contained about 85% by weight of 2-methoxy-3,6-dichlorobenzoic acid and about 9% by weight of 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 which was equipped with a stirrer, a thermometer and a reflux condenser which was connected to a water separator. The reaction mixture was heated to reflux over a period of about 18 hours. After this time the mixture was cooled to a temperature of 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 give the desired product, which had a purity of more than 95% by weight.

Example 4

A mixture containing equal parts by weight of 2-methoxy-3,6-dichlorobenzoic acid and 2-methoxy-3,5-dichlorobenzoic acid (600 g), toluene (800 ml), isopropanol

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646936

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(75 ml) and titanium tetrapropoxide (0.6 g) were placed in a glass reaction vessel equipped with an agitator, a thermometer and a reflux condenser connected to a water separator. The reaction mixture was heated to reflux over a period of about 12 hours. After this time the reaction mixture was concentrated using

Distill 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 5, washed with three portions of cold toluene (5 ° C; 100 ml) and dried to give 2-methoxy-3,6-dichloro-benzoic acid.

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