Tetrafluoroalkyl ethers also form via reaction between difluoromethyl cation and acyl fluoride. Sulfur tetrafluoride can be used to fluorinate polymers efficiently. This often has a profound effect on polymer properties—fluorination of polyvinyl alcohol, for instance, improves its resistance to strong acids and bases. The synthesis of a prostaglandin bearing a trifluoromethyl group at C-16 is based on an intermediate obtained by fluorination of a carboxylic acid with sulfur tetrafluoride. Modern fluorination methods have rendered sulfur trifluoride essentially obsolete. Use of the more easily handled reagent DAST (diethylaminosulfur trifluoride) is now preferred to SF4 for nucleophilic fluorination reactions. DAST is more selective in fluorinations of alcohols and promotes fewer rearrangements and eliminations in acid-sensitive substrates. The inconvenience of handling gaseous SF4 is a disadvantage of fluorinations employing this reagent. Additionally, acid-labile functionality cannot be present in the substrate if HF is used. Electrophilic fluorinating reagents, such as Selectfluor and "N"-fluorosulfonimides, complement the reactivity of SF4 and avoid these issues. Sulfur tetrafluoride is a colorless gas at room temperature, although it boils at -38 °C. It is rapidly hydrolyzed in aqueous media and is corrosive and toxic; thus, reactions should be carried out in a well-ventilated fume hood with gloves and goggles. Sulfur tetrafluoride does not react with stainless steel, copper, or nickel. Ordinarily, fluorination reactions with SF4 are carried out in a stainless steel shaker apparatus. Gaseous reagents are introduced into the shaker by condensation and gaseous products are usually vented when the reaction is complete (unless the gaseous products are of interest, in which case they can be condensed into a separate vessel). Hydrogen fluoride can be separated from the desired products via pouring the reaction mixture into water followed by filtration. Solutions of calcium hydroxide can also be used in this context yielding harmless calcium fluoride.  5-Hydroxyhistamine dihydrochloride (1700 mg, 3.5 mmol) was dissolved in 25 mL of anhydrous liquid HF and the solution was evaporated in a stream of dry nitrogen to remove HCl.