Sulfur tetrafluoride
IUPAC name	Sulfur(IV) fluoride
Other names	Sulfur tetrafluoride
Identifiers
CAS number	7783-60-0 Y
UN number	2418
RTECS number	WT4800000
Properties
Molecular 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	decomp
Structure
Molecular shape	Seesaw (C2v)
Dipole moment	0.632 D[1]
Hazards
MSDS	ICSC 1456
EU Index	Not listed
Main hazards	highly toxic corrosive
NFPA 704	0 3 2 W
Related compounds
Other anions	Sulfur dichloride Disulfur dibromide
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
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
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,^[2] some of which are important in the pharmaceutical and specialty chemical industries.

[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. 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.^[3]

Intramolecular dynamic equilibration of SF₄.

[edit] Synthesis and manufacture

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

SCl₂ + Cl₂ + 4 NaF → SF₄ + 4 NaCl

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