Caesium fluoride

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Caesium fluoride
Caesium fluoride
Systematic name Caesium fluoride
Other names Cesium fluoride
Molecular formula CsF
Molar mass 151.904 g/mol
Appearance white crystalline solid
CAS number [13400-13-0]
Density and phase 4.115 g/cm³, solid
Solubility in water 367 g/100 ml (18 °C)
In methanol soluble
In dioxane, pyridine insoluble
Melting point 682 °C (955 K)
Boiling point 1251 °C (1524 K)
Basicity (pKb)  ?
Crystal structure cubic
Dipole moment 7.9 D
MSDS External MSDS
Main hazards toxic, forms HF with acid
NFPA 704 image:nfpa_h3.pngimage:nfpa_f0.pngimage:nfpa_r0.png estimated
R/S statement R: 23/24/25, 34
S: 26, 36/37, 39, 45
RTECS number FK9650000
Supplementary data page
Structure and
n, εr, etc.
Thermodynamic data Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
Related compounds
Other anions caesium chloride
caesium bromide
caesium iodide
Other cations sodium fluoride
potassium fluoride
rubidium fluoride
Related compounds tetra-n-butyl-
ammonium fluoride
Except where noted otherwise, data are given for
materials in their standard state (at 25°C, 100 kPa)
Infobox disclaimer and references

Caesium fluoride (cesium fluoride in North America), is an ionic compound usually found as a hygroscopic white solid. It is more soluble and more readily dissociated than sodium fluoride or potassium fluoride. CsF is commercially available – on a lab scale it costs around $50 per 100g ( Synquest), cheaper than RbF. It is available in anhydrous form, and if water has been absorbed it is easy to dry by heating at 100 ° C for two hours in vacuo. It is therefore a useful, less hygroscopic alternative to tetra-n-butylammonium fluoride (TBAF) and TAS-fluoride (TASF) when anhydrous "naked" fluoride ion is needed. Like all soluble fluorides, it is mildly basic. Contact with acid should be avoided, as this forms highly toxic/corrosive hydrofluoric acid.

Chemical Properties

Caesium fluoride reacts usually as a source of fluoride ion, F-. It therefore undergoes all of the usual reactions associated with soluble fluorides such as potassium fluoride, for example:

2 CsF ( aq) + CaCl2 ( aq) → 2 CsCl ( aq) + CaF2 ( s)

Being highly dissociated it is quite reactive as a fluoride source under anhydrous conditions too, and it will react with electron-deficient aryl chlorides to form aryl fluorides ( halex reaction). Due to the strength of the Si-F bond, fluoride ion is useful for desilylation reactions (removal of Si groups) in organic chemistry; caesium fluoride is an excellent source of anhydrous fluoride for such reactions (see uses below). As with other soluble fluorides, CsF is moderately basic, due to the fact that HF is a weak acid. The low nucleophilicity of fluoride means it can be a useful base in organic chemistry (see uses below).


Caesium fluoride may be prepared by the action of hydrofluoric acid on caesium hydroxide or caesium carbonate, followed by removal of water.


Caesium fluoride is a useful base in organic chemistry, due the fact that fluoride ion is largely unreactive as a nucleophile. It is reported that CsF gives higher yields in Knoevenagel condensation reactions than KF or NaF.

Removal of silicon groups (desilylation) is a major application for CsF in the laboratory, as its anhydrous nature allows clean formation of water-sensitive intermediates. Caesium fluoride in THF or DMF can attack a wide variety of organosilicon compounds to produce an organosilicon fluoride and a carbanion, which can then react with electrophiles, for example:

Desilylation is also useful for the removal of silyl protecting groups.

Caesium fluoride is also a popular source of fluoride in organofluorine chemistry. For example, CsF reacts with hexafluoroacetone to form a caesium perfluoroalkoxide salt which is stable up to 60 ° C, unlike the corresponding sodium or potassium salt.

Single crystals of the salt are transparent into the deep infrared. For this reason it is often used as the windows of cells used for infrared spectroscopy.


Like soluble fluorides, CsF is moderately toxic, see Elf Atochem MSDS sheet. Contact with acid should be avoided, as this forms highly toxic/corrosive hydrofluoric acid. Caesium ion (Cs+) per se (for example as CsCl) is not generally considered toxic.

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