Patent Description:
<NUM>,<NUM>-Dichloro-<NUM>-methoxybenzoic acid (DICAMBA) is an important and selective systemic herbicide. It is effective against annual and perennial broad-leaved weeds and brushed species in cereals, maize, sorghum, sugarcane, turf, pastures, range land and non-crop areas. DICAMBA is absorbed through roots as well as leaves and translocates throughout the plant. It mimics auxin, a plant growth regulator and at adequate concentrations, is known to increase plant growth rate that outgrows its nutrient supplies leading to death of the plant. DICAMBA in combination with phenoxy or other herbicides is used in pastures, range land, and non-crop areas to control weeds.

The processes for the preparation of DICAMBA are complex and DICAMBA produced therefrom has low purity, and low yield.

There is, therefore, felt a need for preparing <NUM>,<NUM>-dichloro-<NUM>-methoxybenzoic acid (DICAMBA) with relatively high purity and developing a simple process with high yield.

<CIT> describes processes for the diazotization of <NUM>,<NUM>-dichloroanilines. <CIT> describes <NUM>-methoxy-<NUM>, <NUM>-dichlorobenzoates.

Some of the objects of the present disclosure, which at least one embodiment herein satisfies, are as follows.

It is an object of the present disclosure to ameliorate one or more problems of the prior art or to at least provide a useful alternative.

An object of the present disclosure is to provide a simple and easy process for the preparation of <NUM>,<NUM>-dichloro-<NUM>-methoxybenzoic acid (DICAMBA).

Other objects and advantages of the present disclosure will be more apparent from the following description.

A process for preparing <NUM>,<NUM>-dichloro-<NUM>-methoxybenzoic acid (DICAMBA) in accordance with the present disclosure comprises:.

wherein the step of diazotizing involves slow addition of a solution of <NUM>,<NUM>-dichloroaniline in the first fluid medium to nitrosylsulfuric acid, wherein the slow addition is carried out over a time period in the range of <NUM>-<NUM> hours by controlling the temperature of the reaction mixture, thereby preventing decomposition of the <NUM>,<NUM>-dichlorophenyldiazonium salt.

<NUM>,<NUM>-Dichloro-<NUM>-methoxybenzoic acid (DICAMBA) is an important herbicide. The present disclosure envisages a simple process for preparing DICAMBA with high purity.

The present disclosure provides a process for the preparation of DICAMBA (I) using a <NUM>,<NUM>-dichloroaniline (II) as a starting material.

The process involves the following steps.

Initially, <NUM>,<NUM>-dichloroaniline (II) is diazotized with nitrosylsulfuric acid in at least one first fluid medium at a temperature in the range of <NUM> to <NUM> to obtain <NUM>,<NUM>-dichlorophenyldiazonium salt (III).

The <NUM>,<NUM>-dichlorophenyldiazonium salt (III) is hydroxylated by contacting with sulfuric acid at a temperature in the range of <NUM> to <NUM> to obtain <NUM>,<NUM>-dichlorophenol (IV) and a residue comprising sulfuric acid. Concentration of the sulfuric acid used for hydroxylating is in the range of <NUM> % to <NUM> % w/w. Sulfuric acid is recovered from the residue comprising sulfuric acid.

An alkali metal <NUM>,<NUM>-dichlorophenolate (V) is formed by reacting <NUM>,<NUM>-dichlorophenol (IV) obtained in step-<NUM>, with an alkali metal hydroxide in at least one second fluid medium. Moisture content of the alkali metal <NUM>,<NUM>-dichlorophenolate (V) so obtained is in the range of <NUM> to <NUM> % w/w.

The alkali metal <NUM>,<NUM>-dichlorophenolate (V) obtained in step-<NUM>, is carboxylated with carbon dioxide (CO<NUM>) at a temperature in the range of <NUM> to <NUM>, to obtain alkali metal salt of <NUM>,<NUM>-dichlorosalicylic acid (VI).

The alkali metal salt of <NUM>,<NUM>-dichlorosalicylic acid (VI) obtained in step-<NUM>, in at least one third fluid medium, is methylated with a methylating agent selected from the group consisting of methyl chloride (CH<NUM>Cl), and dimethyl sulfate ((CH<NUM>)<NUM>SO<NUM>) at a temperature in the range of <NUM> to <NUM> to obtain methyl <NUM>,<NUM>-dichloro-<NUM>-methoxybenzoate (DICAMBA ester, VII).

The DICAMBA ester (VII) obtained in step-<NUM>, is hydrolysed at a temperature in the range of <NUM> to <NUM> to obtain DICAMBA (I).

The nitrosylsulfuric acid used in the step-<NUM> of diazotizing can be prepared by reacting nitric acid with SO<NUM>.

In accordance with the embodiments of the present disclosure, the molar ratio of <NUM>,<NUM>-dichloroaniline (II) and nitrosylsulfuric acid used for diazotizing can be in the range of <NUM>:<NUM> to <NUM>:<NUM>.

In accordance with the preferred embodiment of the present disclosure, the molar ratio of <NUM>,<NUM>-dichloroaniline (II) and nitrosylsulfuric acid used for diazotizing can be in the range of <NUM>:<NUM> to <NUM>:<NUM>.

In accordance with one embodiment of the present disclosure, the molar ratio of <NUM>,<NUM>-dichloroaniline (II) and nitrosylsulfuric acid used for diazotizing is <NUM>:<NUM>.

Nitrosylsulfuric acid is in the form of an aqueous solution.

In accordance with the embodiments of the present disclosure, the concentration of nitrosylsulfuric acid in an aqueous solution can be in the range of <NUM> % to <NUM> % w/w.

In accordance with the preferred embodiment of the present disclosure, the concentration of nitrosylsulfuric acid in the aqueous solution is in the range of <NUM> % to <NUM> % w/w.

In accordance with the present disclosure, in the step of diazotizing, a solution of <NUM>,<NUM>-dichloroaniline (II) in the first fluid medium is slowly added to nitrosylsulfuric acid over a period of time, in order to maintain the temperature of the reaction mass in the range of <NUM> to <NUM>. The slow addition helps in controlling the temperature of the reaction mixture thereby increasing the yield of <NUM>,<NUM>-dichlorophenyldiazonium salt by preventing its decomposition. In aqueous solution, <NUM>,<NUM>-dichlorophenyldiazonium salts are unstable at temperatures above <NUM>, as the diazonium group tends to decompose and liberate N<NUM> (nitrogen gas), thereby resulting in a reduced yield.

In accordance with the present disclosure, the addition of <NUM>,<NUM>-dichloroaniline (II) to nitrosylsulfuric acid is carried out over a period of time in the range of <NUM> hour to <NUM> hours.

In accordance with one embodiment of the present disclosure, the slow addition of <NUM>,<NUM>-dichloroaniline (II) to nitrosylsulfuric acid is carried out for <NUM> hours.

In accordance with the present disclosure, the first fluid medium is at least one selected from the group consisting of dichloromethane, <NUM>,<NUM>-dichloroethane (EDC), chloroform, and carbon tetrachloride. Other fluid media can be used for diazotization.

In accordance with the embodiments of the present disclosure, the amount of the first fluid medium used for diazotization can be in the range of <NUM>/mole to <NUM>/mole of <NUM>,<NUM>-dichloroaniline (II).

In accordance with one embodiment of the present disclosure, the first fluid medium used for diazotizing <NUM>,<NUM>-dichloroaniline (II) is <NUM>,<NUM>-dichloroethane (EDC) and the amount of <NUM>,<NUM>-dichloroethane (EDC) is <NUM>/mole of <NUM>,<NUM>-dichloroaniline (II).

The diazotization is carried out under an inert atmosphere with stirring in a suitable reactor.

In accordance with one embodiment of the present disclosure, the diazotization is carried out at a temperature of <NUM>.

On completion of the diazotization, the unreacted nitrosylsulfuric acid present in the diazotizing reaction mixture is quenched by adding sulfamic acid or urea to form a diazonium salt thereof, which decomposes. After quenching of excess nitrosylsulfuric acid, the resultant diazotized reaction mixture is separated into an aqueous bottom phase and an organic top phase. The aqueous bottom phase containing <NUM>,<NUM>-dichlorophenyldiazonium salt is taken directly to the step of hydroxylation.

In the hydroxylation step, the aqueous phase containing <NUM>,<NUM>-dichlorophenyldiazonium salt obtained from the diazotizing step is slowly added over a period of time to sulfuric acid having a concentration in the range of <NUM> % to <NUM> % w/w. The rate of addition is controlled, in order to maintain the temperature of the hydroxylating reaction mixture in the range of <NUM> to <NUM>. The reason for maintaining the temperature in this range is that, below <NUM>, <NUM>,<NUM>-dichloroaniline couples with <NUM>,<NUM>-dichlorophenyldiazonium salt leading to the formation of by-products, while above <NUM>, sulfuric acid degrades leading to SO<NUM> liberation, thereby causing safety hazards.

Steam is purged through the hydroxylating reaction mass and <NUM>,<NUM>-dichlorophenol is isolated from the reaction mass by steam distillation.

In accordance with the embodiments of the present disclosure, the addition of <NUM>,<NUM>-dichlorophenyldiazonium salt (III) to sulfuric acid is carried out over a period of time in the range of <NUM> hour to <NUM> hours.

In accordance with the preferred embodiment of the present disclosure, the addition of <NUM>,<NUM>-dichlorophenyldiazonium salt (III) to sulfuric acid is carried out over a period of time in the range of <NUM> hour to <NUM> hours.

In accordance with one embodiment of the present disclosure, the addition of <NUM>,<NUM>-dichlorophenyldiazonium salt (III) to sulfuric acid is carried out for <NUM> hours.

In accordance with the embodiments of the present disclosure, in the step of hydroxylating, the yield of <NUM>,<NUM>-dichlorophenol (IV) is <NUM> to <NUM> mole %.

In accordance with the embodiments of the present disclosure, recovery of sulfuric acid from the residue comprising sulfuric acid, obtained in the hydroxylating step, involves the following steps.

The residue comprising sulfuric acid is contacted with <NUM>,<NUM>-dichloroethane (<NUM> to <NUM>/mole) at a temperature in the range of <NUM> to <NUM> to obtain a biphasic mixture comprising an organic phase and an aqueous phase containing sulfuric acid.

The aqueous phase is separated from the biphasic mixture. The separated aqueous phase containing sulfuric acid, having concentration in the range of <NUM> to <NUM> % w/w, is concentrated by distilling out water to obtain sulfuric acid having a concentration in the range of <NUM> to <NUM> % w/w. The organic phase (EDC), obtained as a result of separation of the aqueous phase containing sulfuric acid from the biphasic mixture, is distilled to recover EDC.

The resulting sulfuric acid is further contacted with <NUM> % to <NUM> % w/w concentrated nitric acid, at a temperature in the range of <NUM> to <NUM>, to obtain recovered sulfuric acid.

In accordance with the embodiments of the present disclosure, the alkali metal hydroxide used for forming alkali metal <NUM>,<NUM>-dichlorophenolate (V) is at least one selected from the group consisting of sodium hydroxide, potassium hydroxide, and lithium hydroxide.

The formation of alkali metal <NUM>,<NUM>-dichlorophenolate (V) is carried out using an alkali metal hydroxide (M-OH) in a second fluid medium.

In accordance with the embodiments of the present disclosure, the molar ratio of <NUM>,<NUM>-dichlorophenol (IV) and the amount of the alkali metal hydroxide used for forming the alkali metal <NUM>,<NUM>-dichlorophenolate is in the range of <NUM>:<NUM> to <NUM>:<NUM>.

In accordance with one embodiment of the present disclosure, the alkali metal hydroxide used for forming the alkali metal <NUM>,<NUM>-dichlorophenolate (V) is potassium hydroxide, and the molar ratio of <NUM>,<NUM>-dichlorophenol (IV) and potassium hydroxide is <NUM>:<NUM>. The potassium hydroxide is in the form of flakes or lye having strength in the range from <NUM> % to <NUM> % w/w.

In accordance with the embodiments of the present disclosure, the second fluid medium may be selected from the group consisting of benzene, toluene, and xylene. Other fluid media can be used for forming alkali metal <NUM>,<NUM>-dichlorophenolate (V).

The step of forming alkali metal <NUM>,<NUM>-dichlorophenolate (V) is carried out under an inert atmosphere in a suitable reactor under stirring.

In accordance with the embodiments of the present disclosure, the step of forming the alkali metal <NUM>,<NUM>-dichlorophenolate (V) is carried out at a temperature in the range of <NUM> to <NUM>.

In accordance with the preferred embodiment of the present disclosure, the step of forming the alkali metal <NUM>,<NUM>-dichlorophenolate (V) is carried out at a temperature in the range of <NUM> to <NUM>.

In accordance with one embodiment of the present disclosure, the step of forming the alkali metal <NUM>,<NUM>-dichlorophenolate (V) is carried out at <NUM>.

The moisture content of the solution containing the alkali metal <NUM>,<NUM>-dichlorophenolate (V) is reduced by azeotropic removal of water. In accordance with one embodiment of the present disclosure, the moisture content of the alkali metal <NUM>,<NUM>-dichlorophenolate (V) is below < <NUM>%.

In accordance with the embodiments of the present disclosure, the period of time for forming the alkali metal <NUM>,<NUM>-dichlorophenolate (V) is in the range of <NUM> hour to <NUM> hours. In accordance with one embodiment of the present disclosure, the period of time for forming the alkali metal <NUM>,<NUM>-dichlorophenolate (V) is <NUM> hours.

In accordance with one embodiment of the present disclosure, the yield of potassium <NUM>,<NUM>-dichlorophenolate (V) is in the range of <NUM> to <NUM> mole %.

The potassium <NUM>,<NUM>-dichlorophenolate (V) is dissolved in xylene to form a clear solution and is carboxylated with carbon dioxide (CO<NUM>).

In accordance with the embodiments of the present disclosure, the step of carboxylating is carried out with CO<NUM> at a pressure in the range of <NUM>×<NUM><NUM> pascal to <NUM>×<NUM><NUM> pascal (<NUM>/cm<NUM> to <NUM>/cm<NUM>).

In accordance with the preferred embodiment of the present disclosure, the step of carboxylating is carried out with CO<NUM> at a pressure in the range of <NUM>×<NUM><NUM> pascal to <NUM>. 923x10<NUM> pascal (<NUM>/cm<NUM> to <NUM>/cm<NUM>).

In accordance with one embodiment of the present disclosure, the step of carboxylating is carried out with CO<NUM> at a pressure of <NUM>. 942x10<NUM> pascal (<NUM>/cm<NUM>).

In accordance with the embodiments of the present disclosure, the carboxylation is carried out at a temperature in the range of <NUM> to <NUM>.

In accordance with one embodiment of the present disclosure, the carboxylation is carried out at a temperature of <NUM>.

In accordance with the embodiments of the present disclosure, the carboxylation is carried out over a period of time in the range of <NUM> hours to <NUM> hours.

In accordance with the preferred embodiment of the present disclosure, the carboxylation is carried out over a period of time in the range of <NUM> hours to <NUM> hours.

In accordance with one embodiment of the present disclosure, the carboxylation is carried out for <NUM> hours.

The carboxylated mass is filtered at a temperature in the range of <NUM> to <NUM> to isolate the dipotassium salt of <NUM>,<NUM>-dichlorosalicylic acid (VI).

The yield of the dipotassium salt of <NUM>,<NUM>-dichlorosalicylic acid (VI) is in the range of <NUM> to <NUM> mole%.

The dipotassium salt of <NUM>,<NUM>-dichlorosalicylic acid (VI) is methylated using a methylating agent such as methyl chloride (CH<NUM>Cl) and dimethyl sulfate ((CH<NUM>)<NUM>SO<NUM>) in at least one third fluid medium.

The third fluid medium can be selected from the group consisting of methanol, ethanol, isopropanol, and butanol. Other fluid media can be used for methylation.

In accordance with one embodiment of the present disclosure, the third fluid medium is methanol.

In accordance with the embodiments of the present disclosure, the dipotassium salt of <NUM>,<NUM>-dichlorosalicylic acid (VI) can be methylated under CH<NUM>Cl pressure in the range of <NUM>. 961x10<NUM> pascal to <NUM>. 471x10<NUM> pascal (<NUM>/cm<NUM> to <NUM>/cm<NUM>).

In accordance with the preferred embodiments of the present disclosure, the dipotassium salt of <NUM>,<NUM>-dichlorosalicylic acid (VI) is methylated under CH<NUM>Cl pressure in the range of <NUM>. 884x10<NUM> pascal to <NUM>. 177x10<NUM> pascal (<NUM>/cm<NUM> to <NUM>/cm<NUM>).

In accordance with one embodiment of the present disclosure, the dipotassium salt of <NUM>,<NUM>-dichlorosalicylic acid (VI) is methylated under CH<NUM>Cl pressure of <NUM>. 884x10<NUM> pascal (<NUM>/cm<NUM>).

In accordance with one embodiment of the present disclosure, the dipotassium salt of <NUM>,<NUM>-dichlorosalicylic acid (VI) is methylated at a temperature of <NUM> to <NUM>.

In accordance with one embodiment of the present disclosure, the dipotassium salt of <NUM>,<NUM>-dichlorosalicylic acid (VI) is methylated for a period of <NUM> hours.

The methylated mass is filtered at a temperature in the range of <NUM> to <NUM> to remove potassium chloride (KCl). The filtrate of the methylated mass comprises methyl <NUM>,<NUM>-dichloro-<NUM>-methoxy benzoate (DICAMBA ester, VII), <NUM>,<NUM>-dichloro-<NUM>-methoxybenzoic acid (DICAMBA, I), dipotassium salt of <NUM>,<NUM>-dichlorosalicylic acid (VI), and <NUM>,<NUM>-dichloroanisole. The third fluid medium is distilled off from the filtrate to obtain a resultant mass.

Xylene and water are added to the resultant mass to obtain a biphasic mixture. The pH of the biphasic mixture is adjusted in the range of <NUM> to <NUM> by carefully adding an inorganic base such as NaOH, KOH or Na<NUM>CO<NUM>. Organic phase and aqueous phase of the biphasic mixture are separated. The organic phase is distilled to provide a mixture of DICAMBA ester (VII), and dichloroanisole. The distilled fraction has a composition of <NUM> % DICAMBA ester (VII), and <NUM>% dichloroanisole GLC area %.

Further, xylene is distilled out as a first fraction during this distillation. The distilled xylene is recovered and recycled in the step of forming the alkali metal <NUM>,<NUM>-dichlorophenolate.

The aqueous phase separated from the biphasic mixture contains alkali metal salts of DICAMBA (I), and <NUM>,<NUM>-dichlorosalicylic acid (VI), which can be recycled in methylation step.

The yield of DICAMBA ester (VII) is in the range of <NUM> to <NUM> mole %.

DICAMBA ester (VII) is hydrolyzed with an alkali selected from the group consisting of sodium hydroxide, potassium hydroxide, and lithium hydroxide.

In accordance with the embodiments of the present disclosure, the molar ratio of DICAMBA ester (VII) and the alkali used for hydrolysis can be in the range of <NUM>:<NUM> to <NUM>:<NUM>.

In accordance with one embodiment of the present disclosure, the alkali is NaOH and the molar ratio of DICAMBA ester (VII) and NaOH used for hydrolysis is <NUM>:<NUM>.

Mixture of DICAMBA ester (VII) and dichloroanisole obtained from the methylating step is subjected to selective hydrolysis with an alkali at a temperature in the range of <NUM> to <NUM>.

DICAMBA ester contains two hydrolysable groups i. , an ester group and an ether group. Further, <NUM>,<NUM>-dichloroanisole contains a hydrolysable ether group. By using one mole of alkali per mole of DICAMBA ester (VII), the ester group is selectively hydrolysed over the ether group with the alkali at a temperature in the range of <NUM> to <NUM> thereby, providing DICAMBA with higher purity.

In accordance with the embodiments of the present disclosure, in the step of hydrolyzing, the yield of DICAMBA (I) is in the range of <NUM> to <NUM>% with purity in the range of <NUM> to <NUM> mole %.

In accordance with the embodiments of the present disclosure, the process of the present disclosure further includes an additional step of dissolving the DICAMBA obtained in the step of hydrolysing, in a fourth fluid medium, to obtain a solution and recrystallizing DICAMBA from the solution. The recrystallized DICAMBA has a purity of <NUM>% or greater.

In accordance with embodiments of the present disclosure, the fourth fluid medium is at least one selected from the group consisting of water and xylene.

In accordance with one embodiment of the present disclosure, the DICAMBA obtained from the hydrolysing step is recrystallized from a mixture of water (<NUM>/mole) and xylene (<NUM>/mole). The DICAMBA crystals are washed with xylene (<NUM>/mole).

In accordance with embodiments of the present disclosure, the purity of the recrystallized DICAMBA is in the range of <NUM> to <NUM> %.

The process of the present disclosure uses commonly available and inexpensive reagents and fluid media and recycles the fluid media as well as some of the by-products. Hence, the process of the present disclosure is simple and economical.

The laboratory scale example provided herein can be scaled up to industrial or commercial scale.

Nitrosylsulfuric acid (<NUM> equivalents) was received in a stainless steel (SS) reactor and the reactor was cooled to <NUM>. A solution of <NUM>,<NUM> dichloroaniline (<NUM>, <NUM> mole) in <NUM>,<NUM>-dichloroethane (<NUM>) was fed to the reactor over <NUM> hours, while maintaining the reactor temperature below <NUM> by controlling the flow rate of addition of <NUM>,<NUM>-dichloroaniline (DCA) solution while agitating the solution at a speed of ~<NUM> rpm with a <NUM>% twisted turbine stirrer. On completion of addition of <NUM>,<NUM>-dichloroaniline (DCA) solution, the reaction mixture was further agitated for an hour. Completion of reaction was indicated by complete consumption of <NUM>,<NUM>-dichloroaniline. The reaction mixture was quenched by addition of sulfamic acid (HSO<NUM>NH<NUM>), in order to form a diazonium salt of sulfamic acid with excess of nitrosylsulfuric acid. The diazonium salt of sulfamic acid decomposed. The reaction mass was allowed to stand, wherein it separated into two phases; an aqueous phase containing <NUM>,<NUM>-dichlorophenyldiazonium salt and an organic <NUM>,<NUM>-dichloroethane phase. The aqueous phase containing <NUM>,<NUM>-dichlorophenyldiazonium salt was taken as such to the next step of hydroxylation. Yield of <NUM>,<NUM>-dichlorophenyldiazonium salt was found to be <NUM> mole %.

The aqueous phase containing the <NUM>,<NUM>-dichlorophenyldiazonium salt obtained in step-<NUM>, was fed over <NUM> hours into a glass lined reactor containing <NUM>% (wt/wt) sulfuric acid solution at <NUM> to obtain a reaction mass. Steam was continuously purged into reaction mass while maintaining the temperature at <NUM> to obtain <NUM>,<NUM>-dichlorophenol by steam distillation. During this operation, <NUM>,<NUM>-dichlorophenol was azeotropically collected from the reaction mass. The distillate was collected in a heated vessel (<NUM>), where the condensed distillate forms two phases with a bottom phase of molten <NUM>,<NUM>-dichlorophenol and an aqueous phase. The bottom phase of <NUM>,<NUM>-dichlorophenol was separated and the aqueous phase was extracted with xylene, and mixed with <NUM>,<NUM>-dichlorophenol. The xylene and the <NUM>,<NUM>-dichlorophenol organic phase were washed with water to remove mineral acidity to obtain <NUM>,<NUM>-dichlorophenol with a yield of <NUM> mole %.

The step of hydroxylating <NUM>,<NUM>-dichlorophenyldiazonium salt also yielded a residue comprising sulfuric acid. The residue comprising sulfuric acid was extracted with <NUM>,<NUM>-dichloroethane (<NUM> to <NUM>/mole), at a temperature of <NUM> to obtain a biphasic mixture comprising an organic phase (EDC) and an aqueous phase containing sulfuric acid. The aqueous phase containing sulfuric acid was separated from the biphasic mixture. The separated aqueous phase containing sulfuric acid, having concentration of <NUM>% w/w, was concentrated by distillation of water to obtain sulfuric acid with concentration of <NUM>% w/w. The resulting sulfuric acid was further treated with <NUM>% w/w concentrated nitric acid at a temperature of <NUM> to obtain recovered sulfuric acid.

The organic phase (EDC), obtained as a result of separation of the aqueous phase containing sulfuric acid from the biphasic mixture, is distilled to recover EDC.

<NUM>,<NUM>-dichlorophenol (<NUM>, <NUM> mole) obtained in step-<NUM>, was dissolved in xylene (<NUM>) in an SS reactor agitated with <NUM>% twisted turbine stirrer at <NUM> rpm. Aqueous potassium hydroxide solution (<NUM> equivalents) was added to the above <NUM>,<NUM>-dichlorophenol solution over <NUM> hour with continuous stirring to obtain a reaction mixture. On completion of the reaction, the reaction mixture was subjected to azeotropic distillation for removal of water by heating at <NUM> to obtain potassium salt of <NUM>,<NUM>-dichlorophenolate in xylene. The moisture content of the resultant dehydrated solution of potassium <NUM>,<NUM>-dichlorophenolate in xylene was < <NUM> %. Potassium <NUM>,<NUM>-dichlorophenolate content of xylene was <NUM> N. The solution of potassium <NUM>,<NUM>-dichlorophenolate in xylene was used as such for the next step of carboxylation.

The clear solution of potassium <NUM>,<NUM>-dichlorophenolate in xylene obtained in step-<NUM>, was transferred to a high pressure stainless steel (SS) reactor and the content was heated to <NUM>. The CO<NUM> pressure in the reactor was maintained at <NUM>. 942x10<NUM> pascal (<NUM>/cm<NUM>) for <NUM> hours until no more CO<NUM> was consumed by the reaction mixture. The reaction mixture was cooled to <NUM> by jacketed cooling, excess of CO<NUM> was vented out, recovered back and recycled. Under these conditions the reaction mixture formed a slurry which was filtered at <NUM> and washed (×<NUM>) with hot xylene at <NUM> to obtain dipotassium salt of <NUM>,<NUM>-dichlorosalicylic acid (VI) with <NUM> mole % yield.

In a high pressure stainless steel (SS) reactor, dipotassium <NUM>,<NUM>-dichlorosalicylic acid obtained in step-<NUM>, was re-slurred in methanol (<NUM>/mole) at <NUM> and methyl chloride was introduced into the reactor to attain a pressure of <NUM>. 884x10<NUM> pascal (<NUM>/cm<NUM>). The reaction was monitored for unreacted dipotassium <NUM>,<NUM>-dichlorosalicylic acid. After completion of the reaction, the reaction mass was filtered to collect KCl cake. The filtrate contained methyl, <NUM>,<NUM>-dicloro-<NUM>-methoxybenzoate (DICAMBA ester), <NUM>,<NUM>-dichlorosalicylic acid (DCSA), <NUM>,<NUM>-dichloro-<NUM>-methoxy benzoic acid (DICAMBA) and <NUM>,<NUM>-dichloroanisole (DCA). Methanol was recovered for reuse. Water and xylene were added to the residual mass and aqueous KOH solution was added to the reaction mass until the pH of the reaction mass was <NUM>. At the pH of <NUM>, dipotassium <NUM>,<NUM>-dichlorosalicylic acid and <NUM>,<NUM>-dichloro-<NUM>-methoxy benzoic acid (DICAMBA) dissolved in the aqueous phase as their salts while DICAMBA ester and dicholoroanisole remained in the organic xylene layer. The organic layer was subjected to fractional distillation at <NUM> under reduced pressure. Xylene was collected as first fraction followed by dicholoroanisole, and DICAMBA ester in that order. The distilled fraction of DICAMBA ester was composed of <NUM> % DICAMBA ester, and <NUM> % dichloroanisole. The overall yield of DICAMBA ester was <NUM> mole %.

The distilled fraction obtained in step-<NUM>, comprising DICAMBA ester and <NUM>,<NUM>-dichloroanisole, was mixed with aqueous NaOH (<NUM> N, <NUM> equi/mole) and the resultant mixture was heated at <NUM>. After completion of hydrolysis, the reaction mixture was cooled to <NUM>. The reaction mixture at <NUM> was mixed with xylene and stirred at <NUM>. Stirring was stopped and the reaction mixture was allowed to settle to obtain a biphasic mixture containing an organic phase and an aqueous phase. The aqueous phase was separated and acidified at <NUM> to pH = <NUM> with HCl. Upon adjusting pH, the resultant mixture formed a biphasic mixture comprising an aqueous phase and an organic phase. The organic phase at the bottom, containing molten DICAMBA, was separated at <NUM>, washed with water to free it from mineral acidity and dried under reduced pressure at <NUM> to remove trapped moisture. The DICAMBA was obtained with yield of <NUM> mole % and a purity of <NUM>%.

Water (<NUM>/mole) and xylene (<NUM>/mole) were added to the DICAMBA and the resultant mixture was heated under reflux. The reaction mixture was cooled to <NUM>. The precipitate formed was filtered and washed with <NUM>/mole xylene to obtain DICAMBA crystals with purity of <NUM>%.

Methanol formed during hydrolysis was recycled for reuse.

Claim 1:
A process for preparing <NUM>,<NUM>-dichloro-<NUM>-methoxybenzoic acid (DICAMBA), said process comprising:
a. diazotizing <NUM>,<NUM>-dichloroaniline with nitrosylsulfuric acid in at least one first fluid medium at a temperature in the range of <NUM> to <NUM> to obtain <NUM>,<NUM>-dichlorophenyldiazonium salt;
wherein the first fluid medium is selected from the group consisting of dichloromethane, <NUM>,<NUM>-dichloroethane (EDC), chloroform, and carbon tetrachloride;
b. hydroxylating the <NUM>,<NUM>-dichlorophenyldiazonium salt by contacting the <NUM>,<NUM>-dichlorophenyldiazonium salt with sulfuric acid at a temperature in the range of <NUM> to <NUM> to obtain <NUM>,<NUM>-dichlorophenol and a residue comprising sulfuric acid, wherein the concentration of the sulfuric acid used for hydroxylating is in the range of <NUM> % to <NUM> % w/w, and wherein the residue comprising sulfuric acid is subjected to recovery of sulfuric acid;
c. forming alkali metal <NUM>,<NUM>-dichlorophenolate by reacting <NUM>,<NUM>-dichlorophenol with an alkali metal hydroxide in at least one second fluid medium, wherein the moisture content of the alkali metal <NUM>,<NUM>-dichlorophenolate is in the range of <NUM> to <NUM> % w/w;
d. carboxylating the alkali metal <NUM>,<NUM>-dichlorophenolate at a temperature in the range of <NUM> to <NUM> to obtain alkali metal salt of <NUM>,<NUM>-dichlorosalicylic acid;
e. methylating the alkali metal salt of <NUM>,<NUM>-dichlorosalicylic acid, in at least one third fluid medium, with a methylating agent selected from the group consisting of methyl chloride (CH<NUM>Cl) and dimethyl sulfate ((CH<NUM>)<NUM>SO<NUM>) at a temperature in the range of <NUM> to <NUM> to obtain methyl <NUM>,<NUM>-dichloro-<NUM>-methoxybenzoate (DICAMBA ester); and
f. hydrolysing the DICAMBA ester at a temperature in the range of <NUM> to <NUM> to obtain DICAMBA;
wherein the step of diazotizing involves slow addition of a solution of <NUM>,<NUM>-dichloroaniline in the first fluid medium to nitrosylsulfuric acid, wherein the slow addition is carried out over a time period in the range of <NUM>-<NUM> hours by controlling the temperature of the reaction mixture, thereby preventing decomposition of the <NUM>,<NUM>-dichlorophenyldiazonium salt.