Thionyl Fluoride, SOF₂

Thionyl Fluoride, SOF₂, was first obtained pure by Moissan and Lebeau in 1900 by heating a mixture of arsenic pentafluoride and thionyl chloride in a glass tube at 100° C. for half an hour. The tube was cooled to -80° C. before re-opening, the liquid then being allowed to evaporate by cautiously allowing the temperature to rise, the thionyl fluoride, which boiled away at a little below 30° C., being collected over mercury. The gas was freed from traces of thionyl chloride and arsenic pentafluoride by passage through a spiral tube at -23° C., the fluoride passing over uncondensed:

2AsF₅ + 5SOCl₂ = 5SOF₂ + 2AsCl₅.

A modification of this method has been used by Steinkopf and Herold. An ice-cooled brass flask containing arsenic fluoride was fitted with a reflux condenser connected to a second condenser which led to a leaden vessel cooled to -50° or -60° C. The calculated quantity of thionyl chloride was gradually added to the arsenic fluoride and the flask slowly warmed to about 80° C., when the thionyl fluoride distilled into the cooled lead receiver, whilst the arsenic fluoride and chloride and thionyl chloride were held back by the reflux condenser.

Thionyl fluoride has also been obtained in excellent yield by the interaction of liquid hydrogen fluoride and nitrogen sulphide in the presence of a little copper oxide, the reaction being best effected in a copper bomb at 100° C. The gas, which issues on opening the bomb, can be collected in a receiver cooled by liquid air.

The fluoride is a colourless gas which fumes in moist air and has a pungent, unpleasant odour, recalling that of carbonyl chloride. It condenses at -30° C. to a liquid which on further cooling gives a solid of m.pt. -110° C. Its vapour density corresponds with the formula SOF₂. It is soluble in arsenic chloride, ether, benzene and turpentine.

When submitted, in a glass vessel, to the electric spark discharge, or when heated to 400° C., gradual decomposition sets in, with formation of sulphur dioxide and fluorine, the latter giving rise to silicon tetrafluoride, and the total change being representable by the equation:

SiO₂ + 2SOF₂ = SiF₄ + 2SO₂.

When not in contact with glass, e.g. in a tube of platinum, thionyl fluoride can be heated to a white heat without appreciable decomposition. Water causes a slow hydrolysis:

SOF₂ + 2H₂O = H₂SO₃ + 2HF.

At high temperatures hydrogen acts on the gas with formation of hydrogen fluoride, water, hydrogen sulphide and free sulphur. Chlorine in sunlight, or in contact with charcoal in a glass tube, gives rise to sulphuryl chloride and silicon fluoride. A mixture of thionyl fluoride with moist nitrogen trioxide in the presence of silica undergoes chemical change with formation of silicon fluoride and nitrosulphonic acid.

Sulphur and phosphorus do not affect the fluoride even at 500° C., but hot sodium causes gradual decomposition, with complete absorption.