Process for the preparation of pure, saturated perfluoroalkane-1-carboxylic acids from 1-iodoperfluoroalkanes

In the process for the preparation of pure, saturated perfluoroalkane-1-carboxylic acids having 6 to 14 C atoms from the corresponding 1-iodoperfluoroalkanes, by reacting the 1-iodoperfluoroalkanes with fuming sulfuric acid containing 15 to 45 percent by weight, relative to the solution, of dissolved sulfur trioxide, at temperatures of 100.degree. to 180.degree. C., the molar ratio of sulfur trioxide to 1-iodoperfluoroalkane being 2:1 to 15:1, the resulting two liquid phases being separated from one another when the reaction is complete and the separated lighter liquid phase being distilled, the improvement which comprises mixing, at room temperature or elevated temperature, the lighter liquid phase with 0.3 to 6 percent of its weight of water or with a corresponding amount of an aqueous liquid, and then distilling this mixture at a pressure of 0.1 to 100 kPa, and also mixing the heavier liquid phase, which has been separated off after the reaction with fuming sulfuric acid, with 0.7 to 1.5 moles of water, or with a corresponding amount of an aqueous sulfuric acid, per mole of sulfur trioxide employed, then cooling to 0.degree. to 30.degree. C., filtering off the solid iodine which has been precipitated, washing it with aqueous sulfuric acid and with water.