Sulfur tetrafluoride

Sulfur tetrafluoride is a chemical compound with the formula S F₄. It is a colorless corrosive gas that releases dangerous hydrogen fluoride gas upon exposure to water or moisture. Sulfur tetrafluoride is a useful reagent for the preparation of organofluorine compounds, some of which are important in the pharmaceutical and specialty chemical industries.

Structure

Sulfur in SF₄ is in the +4 oxidation state, with one lone pair of electrons. The atoms in SF₄ are arranged in a see-saw shape, with the sulfur atom at the center. One of the three equatorial positions is occupied by a nonbonding lone pair of electrons. Consequently, the molecule has two distinct types of F ligands, two axial and two equatorial. The relevant bond distances are = 164.3 pm and = 154.2 pm. It is typical for the axial ligands in hypervalent molecules to be bonded less strongly.

The ¹⁹F NMR spectrum of SF₄ reveals only one signal, which indicates that the axial and equatorial F atom positions rapidly interconvert via pseudorotation.

Synthesis and manufacture

At the laboratory scale, sulfur tetrafluoride is prepared from elemental sulfur and cobaltic fluoride
:S + 4CoF₃ → SF₄ + 4CoF₂

SF₄ is industrially produced by the reaction of SCl₂ and NaF with acetonitrile as a catalyst
:3 SCl₂ + 4 NaF → SF₄ + S₂Cl₂ + 4 NaCl
At higher temperatures (e.g. 225–450 °C), the solvent is superfluous. Moreover, sulfur dichloride may be replaced by elemental sulfur (S) and chlorine (Cl₂).

A low-temperature (e.g. 20–86 °C) alternative to the chlorinative process above uses liquid bromine (Br₂) as oxidant and solvent:
:S(s) + 2 Br₂(l; excess) + 4KF(s) → SF₄↑ + 4 KBr(brom)

Use in synthesis of organofluorine compounds

In organic synthesis, SF₄ is used to convert COH and C=O groups into CF and CF₂ groups, respectively. The efficiency of these conversions are highly variable.

In the laboratory, the use of SF₄ has been superseded by the safer and more easily handled diethylaminosulfur trifluoride, (C₂H₅)₂NSF₃, "DAST": This reagent is prepared from SF₄:
:

Other reactions

Sulfur chloride pentafluoride (), a useful source of the SF₅ group, is prepared from SF₄.
:
Hydrolysis of SF₄ gives sulfur dioxide:
:SF₄ + 2 H₂O → SO₂ + 4 HF
This reaction proceeds via the intermediacy of thionyl fluoride, which usually does not interfere with the use of SF₄ as a reagent.

When amines are treated with SF₄ and a base, aminosulfur difluorides result.

Toxicity

reacts inside the lungs with moisture, forming sulfur dioxide and hydrogen fluoride which forms highly toxic and corrosive hydrofluoric acid

References

{{Sulfur compounds

Sulfur fluorides
Fluorinating agents
Hypervalent molecules

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