Sulfur tetrafluoride

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Sulfur tetrafluoride
IUPAC name	Sulfur(IV) fluoride
Other names	Sulfur tetrafluoride
Identifiers
CAS number	[7783-60-0]
RTECS number	WT4800000
Properties
Molecular formula	SF4
Molar mass	108.07 g/mol
Appearance	colorless gas
Density	1.95 g cm−3, −78 °C
Melting point	−121.0 °C
Boiling point	−38 °C
Solubility in water	decomp
Structure
Coordination geometry	C2v
Dipole moment	0.632 D[1]
Hazards
Main hazards	highly toxic corrosive
NFPA 704	4 2 W
R-phrases	14-26-34-37
S-phrases	26-36/37/39-38-45
Related compounds
Related compounds	SeF4 SF6 SCl2 S2Cl2 Me2NSF3
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

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.

Contents 1 Structure of the SF4 molecule 2 Synthesis and manufacture 3 Use of SF4 for the synthesis of fluorocarbons 4 References

[edit] 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 see-saw shape, 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 has a see-saw molecular geometry. 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.

The¹⁹F NMR spectrum of SF₄ reveals only one signal, which indicates that the axial and equatorial F atom positions rapidly interconvert via "pseudorotation."^[3]

[edit] 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.

[edit] 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

[edit] References