The synthesis of tetrafluoro-sulphobenzoic acids and their derivatives

The tetrafluorosulphobenzoic acids 1,2-HO₂C.C₆F₄.SO₃H (I) and 1,4-HO₂C.C₆F₄.SO₃H (II) have been synthesised [1] by oxidation of respectively 4,5,6,7-tetrafluorobenzo [b] thiophen- 2-carboxylic acid (III) [2] and 4-mercaptotetrafluorobenzoic acid (IV) [3].By-products were obtained when (III) was oxidised with either trifluoroperacetic acid or potassium permanganate.A number of reactions of these intermediates have been investigated and the products characterized, eg reaction of (I) with SoCl₂ gave tetrafluoro-1,2-sulphobenzoic acid cyclic anhyride (V). Although no significant change was observed when (II) was similarly treated reaction of (II) with DMF/SOCl₂ gave 1,4-Cl.C₆F₄.COCl. The diacid fluorides 1,2-FOC.C₆F₄.SO₂F (VI) and 1,4-FOC.C₆F₄.SO₂F (VII) were obtained when (I) and (II) respectively were reacted with sulphur tetrafluoride. The use of excess SF₄ gave 1,2- or 1,4- [F₃C.C₆F₄.SO₂F].