Sulfur tetrafluoride

Sulfur tetrafluoride is the chemical compound with the formula SF₄. This species exists as a gas at standard conditions. It is a corrosive species 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,^[2] some of which are important in the pharmaceutical and specialty chemical industries. Aside from SF₄, two other "binary" sulfur fluorides are well known, SF₆ and S₂F₁₀.

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Structure of the SF₄ molecule

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 trigonal bipyramid, with S at the center. One of the three equatorial positions is occupied by a nonbonding lone pair of electrons. Ignoring this lone pair, SF₄ resembles a "see-saw." Consequently, the molecule has two distinct types of F ligands, two axial and two equatorial. The relevant bond distances are S-F_ax = 1.643 Å and S-F_eq = 1.542 Å. 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."^[3]  

Synthesis and manufacture

SF₄ is produced by the reaction of SCl₂, Cl₂, and NaF:

SCl₂ + Cl₂ + 4NaF → SF₄ + 4NaCl

Interestingly, 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.^[4] 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. 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.^[5] This reagent is prepared from SF₄:^[6]

SF₄ + Me₃SiNEt₂ → Et₂NSF₃ + Me₃SiF

References

1. ^ Tolles; W. M. Gwinn, W. D. “Structure and Dipole Moment for SF4” The Journal of Chemical Physics 1962, Volume 36, pp. 1119-1121. doi:10.1063/1.1732702
2. ^ C.-L. J. Wang, "Sulfur Tetrafluoride" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.
3. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
4. ^ W. R. Hasek "1,1,1-Trifluoroheptane" Organic Syntheses, Coll. Vol. 5, p.1082; Vol. 41, p.104. http://www.orgsyn.org/orgsyn/pdfs/CV5P1082.pdf
5. ^ A. H. Fauq, "N,N-Diethylaminosulfur Trifluoride" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.
6. ^ W. J. Middleton, E. M. Bingham" Diethylaminosulfur Trifluoride” Organic Syntheses, Coll. Vol. 6, p.440; Vol. 57, p.50.http://www.orgsyn.org/orgsyn/pdfs/CV6P0440.pdf