Sulfur tetrafluoride

Sulfur tetrafluoride is the chemical compound with the formula S F₄. It is a colorless corrosive gas that releases dangerous HF upon exposure to water or moisture. Despite these unwelcome characteristics, this compound 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 formal +4 oxidation state. Of sulfur's total of six valence electrons, two form a lone pair. The structure of SF₄ can therefore be anticipated using the principles of VSEPR theory: it is a see-saw shape, with S 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. In contrast to SF₄, the related molecule SF₆ has sulfur in the 6+ state, no valence electrons remain nonbonding on sulfur, hence the molecule adopts a highly symmetrical octahedral structure. Further contrasting with SF₄, SF₆ is extraordinarily inert chemically.

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

SF₄ is produced by the reaction of SCl₂ and NaF in acetonitrile:
:3 SCl₂ + 4 NaF → SF₄ + S₂Cl₂ + 4 NaCl

SF₄ is also produced in the absence of solvent at elevated temperatures.

Alternatively, SF₄ at high yield is produced using sulfur (S), NaF and chlorine (Cl₂) in the absence of reaction medium, also at less-desirable elevated reaction temperatures (e.g. 225–450 °C).

A low temperature (e.g. 20–86 °C) method of producing SF₄ at high yield, without the requirement for reaction medium, has been demonstrated utilizing bromine (Br₂) instead of chlorine (Cl₂), S and KF:
:S + (2 + ''x'') Br₂ + 4 KF → SF₄↑ + ''x'' Br₂ + 4 KBr

Use of SF₄ for the synthesis of fluorocarbons

In organic synthesis, SF₄ is used to convert COH and C=O groups into CF and CF₂ groups, respectively. Certain alcohols readily give the corresponding fluorocarbon. Ketones and aldehydes give geminal difluorides. The presence of protons alpha to the carbonyl leads to side reactions and diminished (30–40%) yield. Also diols can give cyclic sulfite esters, (RO)₂SO. Carboxylic acids convert to trifluoromethyl derivatives. For example, treatment of heptanoic acid with SF₄ at 100–130 °C produces 1,1,1-trifluoroheptane. Hexafluoro-2-butyne can be similarly produced from acetylenedicarboxylic acid. The coproducts from these fluorinations, including unreacted SF₄ together with SOF₂ and SO₂, are toxic but can be neutralized by their treatment with aqueous KOH.

The use of SF₄ is being superseded in recent years by the more conveniently handled diethylaminosulfur trifluoride, Et₂NSF₃, "DAST", where Et = CH₃CH₂. This reagent is prepared from SF₄:

: SF₄ + Me₃SiNEt₂ → Et₂NSF₃ + Me₃SiF

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.

Toxicity

reacts inside the lungs with moisture, generating sulfur dioxide and hydrogen fluoride:
:SF₄ + 2 H₂O → SO₂ + 4 HF

References

{{Sulfur compounds

Sulfur fluorides
Fluorinating agents
Hypervalent molecules

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