Sulfur_tetrafluoride

Sulfur tetrafluoride

Sulfur tetrafluoride

Chemical compound

"SF4" redirects here. For the video game, see Street Fighter IV.

Sulfur tetrafluoride is the chemical compound with the formula SF4. 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,[3] some of which are important in the pharmaceutical and specialty chemical industries.

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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–Fax = 164.3 pm and S–Feq = 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.[4]

Intramolecular dynamic equilibration of SF4.

Synthesis and manufacture

At the laboratory scale, fluorination of elemental sulfur with cobaltic fluoride suffices:[5]

S + 4CoF3 → SF4 + 4CoF2

For larger-scale syntheses, SF4 is produced by the reaction of SCl2 and NaF in acetonitrile:[6]

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).[7][8]

A low-temperature (e.g. 20–86 °C) alternative to the chlorinative process above uses liquid bromine (Br2) as oxidant and solvent:[9]

S(s) + 2 Br2(l; excess) + 4KF(s) → SF4↑ + 4 KBr(brom)

For the synthesis of organofluorine compounds

Main article: Fluorination by sulfur tetrafluoride

In organic synthesis, SF4 is used to convert COH and C=O groups into CF and CF2 groups, respectively.[10] The efficiency of these conversions are highly variable.

In the laboratory, the use of SF4 has been superseded by the more conveniently handled diethylaminosulfur trifluoride, Et2NSF3, "DAST":[11] This reagent is prepared from SF4:[12]

SF4 + (CH3)3SiN(C2H5)2 → (C2H5)2NSF3 + (CH3)3SiF

Other reactions

Sulfur chloride pentafluoride (SF
5Cl), a useful source of the SF5 group, is prepared from SF4.[13]

SF4 + Cl2 + CsF → SF5Cl + CsCl

Hydrolysis of SF4 gives sulfur dioxide:[14]

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.[6]

When amines are treated with SF4 and a base, iminosulfur difluorides result.[15]

Toxicity

SF
4 reacts inside the lungs with moisture, generating sulfur dioxide and hydrogen fluoride:[16]

References

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    NIOSH Pocket Guide to Chemical Hazards. "#0580". National Institute for Occupational Safety and Health (NIOSH).
  2. [2]
    Tolles, W. M.; W. M. Gwinn, W. D. (1962). "Structure and Dipole Moment for SF4". J. Chem. Phys. 36 (5): 1119–1121. Bibcode:1962JChPh..36.1119T. doi:10.1063/1.1732702.
  3. [3]
    Wang, C.-L. J. (2004). "Sulfur Tetrafluoride". In Paquette, L. (ed.). Encyclopedia of Reagents for Organic Synthesis. New York: J. Wiley & Sons. doi:10.1002/047084289X. hdl:10261/236866. ISBN 9780471936237.
  4. [4]
    Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. San Diego: Academic Press. ISBN 0-12-352651-5.
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    Kwasnik, W. (1963). "Fluorine compounds: Sulfur tetrafluoride". In Brauer, Georg (ed.). Handbook of Preparative Inorganic Chemistry. Vol. 1. Translated by Riley, Reed F. (2nd ed.). NY, NY: Academic Press. p. 168. LCCN 63-14307 – via the Internet Archive.
  6. [6]
    Fawcett, F. S.; Tullock, C. W. (1963). "Sulfur(IV) Fluoride" (PDF). Inorganic Syntheses. Vol. 7. pp. 119–124. doi:10.1002/9780470132388.ch33. ISBN 978-0-470-13166-4.
  7. [7]
    Tullock, C. W.; Fawcett, F. S.; Smith, W. C.; Coffman, D. D. (1960). "The Chemistry of Sulfur Tetrafluoride. I. The Synthesis of Sulfur Tetrafluoride". J. Am. Chem. Soc. 82 (3): 539–542. doi:10.1021/ja01488a011.
  8. [8]
    US 2992073, Tullock, C.W., "Synthesis of Sulfur Tetrafluoride", issued 1961
  9. [9]
    Winter, R.W.; Cook P.W. (2010). "A simplified and efficient bromine-facilitated SF4-preparation method". J. Fluorine Chem. 131: 780-783. doi:10.1016/j.jfluchem.2010.03.016
  10. [10]
    Hasek, W. R. (1961). "1,1,1-Trifluoroheptane". Organic Syntheses. 41: 104. doi:10.15227/orgsyn.041.0104.
  11. [11]
    Fauq, A. H. (2004). "N,N-Diethylaminosulfur Trifluoride". In Paquette, L. (ed.). Encyclopedia of Reagents for Organic Synthesis. New York: J. Wiley & Sons. doi:10.1002/047084289X. hdl:10261/236866. ISBN 9780471936237..
  12. [12]
    W. J. Middleton; E. M. Bingham (1977). "Diethylaminosulfur Trifluoride". Organic Syntheses. 57: 440. doi:10.15227/orgsyn.057.0050.
  13. [13]
    Nyman, F.; Roberts, H. L.; Seaton, T. (1966). "Sulfur Chloride Pentafluoride" (PDF). Inorganic Syntheses. Vol. 8. McGraw-Hill. p. 160. doi:10.1002/9780470132395.ch42. ISBN 9780470132395.
  14. [14]
    Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  15. [15]
    Grunwell, John R.; Dye, Sandra L. (1975). "Novel generation of benzonitrile-N-sulfide". Tetrahedron Letters. 16 (21): 1739–1740. doi:10.1016/s0040-4039(00)72247-7. ISSN 0040-4039.
  16. [16]
    Johnston, H. (2003). A Bridge not Attacked: Chemical Warfare Civilian Research During World War II. World Scientific. pp. 33–36. ISBN 981-238-153-8.
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