Sulfur tetrafluoride

Sulfur tetrafluoride

Names
IUPAC name Sulfur(IV) fluoride
Other names Sulfur tetrafluoride
Identifiers
CAS Number	7783-60-0 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:30495 Y
ChemSpider	22961 Y
ECHA InfoCard	100.029.103
PubChem CID	24555
RTECS number	WT4800000
UNII	F4P8J39GOF Y
UN number	2418
CompTox Dashboard (EPA)	DTXSID40893074
InChI InChI=1S/F4S/c1-5(2,3)4 Y Key: QHMQWEPBXSHHLH-UHFFFAOYSA-N Y InChI=1/F4S/c1-5(2,3)4 Key: QHMQWEPBXSHHLH-UHFFFAOYAT
SMILES FS(F)(F)F
Properties
Chemical formula	SF4
Molar mass	108.07 g/mol
Appearance	colorless gas
Density	1.95 g/cm3, −78 °C
Melting point	−121.0 °C
Boiling point	−38 °C
Solubility in water	reacts
Vapor pressure	10.5 atm (22 °C)[1]
Structure
Molecular shape	Seesaw (C2v)
Dipole moment	0.632 D[2]
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	highly toxic corrosive
NFPA 704 (fire diamond)	3 0 2 W
NIOSH (US health exposure limits):
PEL (Permissible)	none[1]
REL (Recommended)	C 0.1 ppm (0.4 mg/m3)[1]
IDLH (Immediate danger)	N.D.[1]
Safety data sheet (SDS)	ICSC 1456
Related compounds
Other anions	Sulfur dichloride Disulfur dibromide Sulfur trifluoride
Other cations	Oxygen difluoride Selenium tetrafluoride Tellurium tetrafluoride
Related sulfur fluorides	Disulfur difluoride Sulfur difluoride Disulfur decafluoride Sulfur hexafluoride
Related compounds	Thionyl fluoride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

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.

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.^[4]

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

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