Sulfur tetrafluoride

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 S–F_ax = 164.3 pm and S–F_eq = 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, Cl₂, and NaF:

SCl₂ + Cl₂ + 4 NaF → SF₄ + 4 NaCl

Treatment of SCl₂ with NaF also affords SF₄, not SF₂. SF₂ is unstable, it condenses with itself to form SF₄ and SSF₂.

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The use of SF₄ is being superseded in recent years by the more conveniently handled diethylaminosulfur trifluoride, Et₂NSF₃, "DAST", where Et = CH₃CH₂.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 (SF
5Cl), 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

SF
4 reacts inside the lungs with moisture:

SF₄ + 2 H₂O → SO₂ + 4 HF