Sulfur tetrafluoride

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Sulfur tetrafluoride

Structural formula of sulfur tetrafluoride, showing dimernsions
Space-filling model of sulfur tetrafluoride
Names
IUPAC name Sulfur(IV) fluoride
Other names Sulfur tetrafluoride
Identifiers
CAS Number	7783-60-0 Y
ChEBI	CHEBI:30495 Y
ChemSpider	22961 Y
Jmol 3D model	Interactive image
PubChem	24555
RTECS number	WT4800000
UN number	2418
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)
Structure
Molecular shape	Seesaw (C2v)
Dipole moment	0.632 D
Vapor pressure	{{{value}}}
Related compounds
Other anions	Sulfur dichloride Disulfur dibromide Sulfur trifluoride
Other cations	Oxygen difluoride Selenium tetrafluoride Tellurium tetrafluoride Polonium 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).
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Infobox 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, 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.

File:SF4dyns.png

Intramolecular dynamic equilibration of SF₄.

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. 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