Process for preparation of 3,6-dichloro-2-methoxybenzoic acid (DICAMBA)

A process for preparing 3,6-dichloro-2-methoxybenzoic acid (DICAMBA), said process comprising:

• (a) diazotizing 2,5-dichloroaniline with nitrosylsulfuric acid in at least one first fluid medium at a temperature in the range of 15° C. to 50° C. to obtain 2,5-dichlorophenyldiazonium salt;
• wherein the first fluid medium is selected from the group consisting of dichloromethane, 1,2-dichloroethane (EDC), chloroform, and carbon tetrachloride;
• (b) hydroxylating the 2,5-dichlorophenyldiazonium salt by contacting the 2,5-dichlorophenyldiazonium salt with sulfuric acid at a temperature in the range of 150° C. to 170° C. to obtain 2,5-dichlorophenol and a residue comprising sulfuric acid, wherein the concentration of the sulfuric acid used for hydroxylating is in the range of 60% to 75% w/w, and wherein the residue comprising sulfuric acid is subjected to recovery of sulfuric acid, and wherein yield of 2,5-dichlorophenol is in the range of 95 mole % to 98 mole %;
• (c) forming alkali metal 2,5-dichlorophenolate by reacting 2,5-dichlorophenol with an alkali metal hydroxide in at least one second fluid medium, wherein the moisture content of the alkali metal 2,5-dichlorophenolate is in the range of 0.005 to 0.05% w/w;
• (d) carboxylating the alkali metal 2,5-dichlorophenolate at a temperature in the range of 60° C. to 160° C. to obtain alkali metal salt of 3,6-dichlorosalicylic acid;
• (e) methylating the alkali metal salt of 3,6-dichlorosalicylic acid, in at least one third fluid medium, with a methylating agent selected from the group consisting of methyl chloride (CH3Cl) and dimethyl sulfate ((CH3)2SO4) at a temperature in the range of 60° C. to 160° C. to obtain methyl 3,6-dichloro-2-methoxybenzoate (DICAMBA ester);
• and
• (f) hydrolysing the DICAMBA ester at a temperature in the range of 50° C. to 130° C. to obtain DICAMBA;
• wherein the step of diazotizing involves slow addition of a solution of 2,5-dichloroaniline in the first fluid medium to nitrosylsulfuric acid, wherein the slow addition is carried out over a time period in the range of 4-50 hours by controlling a temperature of the reaction, thereby preventing decomposition of diazonium salt;
• and
• wherein said DICAMBA has a yield in the range of 97% to 98% and purity in the range of 97.5% to 99.6%.