[OH:1][C:2]1[C:3]([F:13])=[C:4]([CH:8]=[C:9]([F:12])[C:10]=1[F:11])[C:5](O)=[O:6].Cl.[S:15]([O:20][CH3:21])([O:18][CH3:19])(=[O:17])=[O:16].[OH-].[Na+].[C:24]1(C)C(C)=CC=CC=1>>[S:15]([O:20][CH3:21])([O:18][CH3:19])(=[O:17])=[O:16].[CH3:24][O:1][C:2]1[C:3]([F:13])=[C:4]([CH:8]=[C:9]([F:12])[C:10]=1[F:11])[C:5]([O:20][CH3:21])=[O:6]The reaction proceeds according to the following reaction scheme: ##STR5##299 g of an aqueous solution which contains 15.6 g (81 mmol) of 3-hydroxy-2,4,5-trifluorobenzoic acid (B) are initially introduced into a glass flask with stirring and are treated with 10 g of xylene instead of the water-insoluble trialkylamine or amine mixture. A 30% strength by weight aqueous hydrochloric acid is added up to a pH of 7 and the pH is checked by means of a calibrated pH electrode which dips into the aqueous phase. A total of 156.8 g (1.41 mol) of dimethyl sulfate are then added dropwise at a temperature of 40° C. over a period of 3.5 hours and the pH is kept constant at 7 by addition of a total of 46.6 g of a 10% strength by weight aqueous sodium hydroxide solution. Despite a considerable excess of dimethyl sulfate, the dimethylated product, namely methyl 3-methoxy-2,4,5-trifluorobenzoate, is only formed in small amounts.3-Hydroxy-2,4,5-trifluorobenzoic acid (B) corresponding to a yield of 8.6%,methyl 3-hydroxy-2,4,5-trifluorobenzoate corresponding to a yield of 67% and methyl 3-methoxy-2,4,5-trifluorobenzoate (C) corresponding to a yield of only 8.7% are found, in each case based on 3-hydroxy-2,4,5-trifluorobenzoic acid employed.
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