[F:1][C:2]1[C:7]([CH3:8])=[CH:6][C:5]([S:9](Cl)(=[O:11])=[O:10])=[CH:4][C:3]=1[CH3:13].S([O-])([O-])=O.[Na+].[Na+].C(=O)(O)[O-].[Na+]>O>[F:1][C:2]1[C:7]([CH3:8])=[CH:6][C:5]([S:9]([OH:11])=[O:10])=[CH:4][C:3]=1[CH3:13]4-Fluoro-3,5-dimethylbenzenesulphonyl chloride (5.56 g, 25 mmol) was added in portions to a solution of anhydrous sodium sulphite (6.0 g, 47.6 mmol) and sodium bicarbonate (4.2 g, 50 mmol) in water (25 ml) at 70° C. with vigorous stirring. When the addition was complete, the mixture was heated and stirred at 70° C. for a further 2 hours. The mixture was allowed to cool. The solid which crystallised from the solution was collected by filtration, dissolved in the minimum amount of water, and acidified to pH 1 with 2M hydrochloric acid. The resultant precipitate was extracted with ethyl acetate (3×75 ml) and the extracts were combined and dried (MgSO4). The solvent was evaporated. The solid residue was triturated with ether, collected by filtration, washed with ether and dried in vacuo to give 4-fluoro-3,5-dimethylbenzenesulphinic acid (4.7 g). This was converted directly to its sodium salt by addition to a solution of sodium methoxide (1 equivalent) in methanol and evaporation of the solution.
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