Processes for preparing 3,5,6-trichlorosalicylic acid and esters thereof

A process for preparing 3,5,6-trichlorosalicylic acid comprising:

• (1) contacting a solution of salicylic acid in concentrated sulfuric acid with chlorine at a temperature below 35.degree. C. until essentially all of the salicylic acid is converted into monochlorosalicylic acid;
• further converting the monochlorosalicylic acid to 3,5-dichlorosalicylic acid by contacting it with chlorine at a temperature in the range 50.degree.-75.degree. C.,
• (2) adding to the product solution of 3,5-dichlorosalicylic acid about 4 moles of sulfur trioxide for each mole of salicylic acid that was used in step (1), with the temperature of the solution at or below 55.degree. C. during the addition of sulfur trioxide.
• (3) adding a catalytic amount of iodine to the solution obtained by step (2),
• (4) contacting the solution obtained in step (3) with gaseous chlorine while maintaining the solution temperature about 40.degree.-60.degree. C., until absorption of chlorine by the solution has ceased.
• (5) cooling the reaction mixture from step (4) to ambient temperature and adding the cooled reaction mixture to water at 0.degree. C., thereby precipitating 3,5,6-trichlorosalicylic acid as a solid.