Caesium fluoride


Caesium fluoride or cesium fluoride is an inorganic compound with the formula CsF and it is a hygroscopic white solid. Caesium fluoride can be used in organic synthesis as a source of the fluoride anion. Caesium also has the highest electropositivity of all non-radioactive elements and fluorine has the highest electronegativity of all elements.

Synthesis and properties

Caesium fluoride can be prepared by the reaction of caesium hydroxide with hydrofluoric acid and the resulting salt can then be purified by recrystallization. The reaction is shown below:
Using the same reaction, another way to create caesium fluoride is to treat caesium carbonate with hydrofluoric acid and again, the resulting salt can then be purified by recrystallization. The reaction is shown below:
CsF is more soluble than sodium fluoride or potassium fluoride in organic solvents. It is available in its anhydrous form, and if water has been absorbed, it is easy to dry by heating at 100 °C for two hours in vacuo. CsF reaches a vapor pressure of 1 kilopascal at 825 °C, 10 kPa at 999 °C, and 100 kPa at 1249 °C.
CsF chains with a thickness as small as one or two atoms can be grown inside carbon nanotubes.

Structure

Caesium fluoride has the halite structure, which means that the Cs+ and F− pack in a cubic closest packed array as do Na+ and Cl− in sodium chloride.

Applications in organic synthesis

Being highly dissociated, CsF is a more reactive source of fluoride than related salts. CsF is an alternative to tetra-n-butylammonium fluoride and TAS-fluoride.

As a base

As with other soluble fluorides, CsF is moderately basic, because HF is a weak acid. The low nucleophilicity of fluoride means it can be a useful base in organic chemistry. CsF gives higher yields in Knoevenagel condensation reactions than KF or NaF.

Formation of C-F bonds

Caesium fluoride serves as a source of fluoride in organofluorine chemistry. Unlike the corresponding sodium or potassium fluorides, CsF reacts with hexafluoroacetone to form a stable perfluoroalkoxide salt. It will convert electron-deficient aryl chlorides to aryl fluorides, although potassium fluoride is more commonly used.

Deprotection agent

Due to the strength of the Si–F bond, fluoride is useful for desilylation reactions, i.e. cleavage of Si-O bonds in organic synthesis. CsF is commonly used for such reactions. Solutions of caesium fluoride in THF or DMF attack a wide variety of organosilicon compounds to produce an organosilicon fluoride and a carbanion, which can then react with electrophiles, for example:

Precautions

Like other soluble fluorides, CsF is moderately toxic. Contact with acid should be avoided, as this forms highly toxic/corrosive hydrofluoric acid. The caesium ion and caesium chloride are generally not considered toxic.