Caesium fluoride or cesium fluoride is an

inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemi ...

with the formula CsF and it is a

hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance ...

white salt. Caesium fluoride can be used in

organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...

as a source of the fluoride anion.

Caesium Caesium ( IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that ...

also has the highest

electropositivity Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the d ...

of all non-radioactive elements and fluorine has the highest electronegativity of all known elements.

Synthesis and properties

Caesium fluoride can be prepared by the reaction of

caesium hydroxide Caesium hydroxide is a strong base (pKa= 15.76) containing the highly reactive alkali metal caesium, much like the other alkali metal hydroxides such as sodium hydroxide and potassium hydroxide. Caesium hydroxide is corrosive enough to quickly ...

(CsOH) with

hydrofluoric acid Hydrofluoric acid is a solution of hydrogen fluoride (HF) in water. Solutions of HF are colourless, acidic and highly corrosive. It is used to make most fluorine-containing compounds; examples include the commonly used pharmaceutical antidepr ...

(HF) and the resulting salt can then be purified by recrystallization. The reaction is shown below: :CsOH + HF → CsF + H₂O Using the same reaction, another way to create caesium fluoride is to treat

caesium carbonate Caesium carbonate or cesium carbonate is a white crystalline solid compound. Caesium carbonate has a high solubility in polar solvents such as water, alcohol and DMF. Its solubility is higher in organic solvents compared to other carbonates like ...

(Cs₂CO₃) with hydrofluoric acid and again, the resulting salt can then be purified by recrystallization. The reaction is shown below: :Cs₂CO₃ + 2 HF → 2 CsF + H₂O + CO₂ CsF is more soluble than

sodium fluoride Sodium fluoride (NaF) is an inorganic compound with the formula . It is used in trace amounts in the fluoridation of drinking water, in toothpaste, in metallurgy, and as a flux. It is a colorless or white solid that is readily soluble in water. ...

potassium fluoride Potassium fluoride is the chemical compound with the formula KF. After hydrogen fluoride, KF is the primary source of the fluoride ion for applications in manufacturing and in chemistry. It is an alkali halide and occurs naturally as the rare ...

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 A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often di ...

''. CsF reaches a

vapor pressure Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phase ...

of 1

kilopascal The pascal (symbol: Pa) is the unit of pressure in the International System of Units (SI), and is also used to quantify internal pressure, stress, Young's modulus, and ultimate tensile strength. The unit, named after Blaise Pascal, is defined ...

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 nanotube A scanning tunneling microscopy image of a single-walled carbon nanotube Rotating single-walled zigzag carbon nanotube A carbon nanotube (CNT) is a tube made of carbon with diameters typically measured in nanometers. ''Single-wall carbon nan ...

Structure

Caesium fluoride has the halite structure, which means that the Cs⁺ and F⁻ pack in a

cubic closest packed In geometry, close-packing of equal spheres is a dense arrangement of congruent spheres in an infinite, regular arrangement (or lattice). Carl Friedrich Gauss proved that the highest average density – that is, the greatest fraction of space occu ...

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 Tetra-''n''-butylammonium fluoride, commonly abbreviated to TBAF and ''n''-Bu4NF, is a quaternary ammonium salt with the chemical formula (CH3CH2CH2CH2)4N+F−. It is commercially available as the white solid trihydrate and as a solution in tetra ...

(TBAF) and TAS-fluoride (TASF).

As a base

As with other soluble fluorides, CsF is moderately basic, because HF is a

weak acid Acid strength is the tendency of an acid, symbolised by the chemical formula HA, to dissociate into a hydron (chemistry), proton, H+, and an anion, A-. The Dissociation (chemistry), dissociation of a strong acid in solution is effectively comple ...

. The low

nucleophilicity In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...

fluoride Fluoride (). According to this source, is a possible pronunciation in British English. is an inorganic, monatomic anion of fluorine, with the chemical formula (also written ), whose salts are typically white or colorless. Fluoride salts ...

means it can be a useful base in

organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clay ...

. CsF gives higher yields in

Knoevenagel condensation In organic chemistry, the Knoevenagel condensation () reaction is a type of chemical reaction named after German chemist Emil Knoevenagel. It is a modification of the aldol condensation. A Knoevenagel condensation is a nucleophilic addition of ...

reactions than KF or NaF.

Formation of Cs-F bonds

Caesium fluoride serves as a source of fluoride in

organofluorine chemistry Organofluorine chemistry describes the chemistry of the organofluorines, organic compounds that contain the carbon–fluorine bond. Organofluorine compounds find diverse applications ranging from oil and water repellents to pharmaceuticals, refri ...

. Similarly to

potassium Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosp ...

fluoride, CsF reacts with

hexafluoroacetone Hexafluoroacetone (HFA) is a chemical compound with the formula (CF3)2CO. It is structurally similar to acetone; however, its reactivity is markedly different. It a colourless, hygroscopic, nonflammable, highly reactive gas characterized by a mus ...

to form a stable perfluoroalkoxide salt. It will convert

electron-deficient Electron deficiency (and electron-deficient) is jargon that is used in two contexts: species that violate the octet rule because they have too few valence electrons and species that happen to follow the octet rule but have electron-acceptor properti ...

aryl chloride In organic chemistry, an aryl halide (also known as haloarene) is an aromatic compound in which one or more hydrogen atoms, directly bonded to an aromatic ring are replaced by a halide. The haloarene are different from haloalkanes because they exhi ...

s to

aryl In organic chemistry, an aryl is any functional group or substituent derived from an aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar" is used as ...

fluorides (

Halex process In chemistry, the Halex process is used to convert aromatic chlorides to the corresponding aromatic fluorides. The process entails ''Hal''ide ''ex''change, hence the name. The reaction conditions call for hot (150-250 °C) solution of the aryl ch ...

), although potassium fluoride is more commonly used.

Deprotection agent

Due to the strength of the Si– F bond, fluoride is useful for

desilylation Silylation is the introduction of one or more (usually) substituted silyl groups (R3Si) to a molecule. The process is the basis of organosilicon chemistry. Of organic compounds Alcohols, carboxylic acids, amines, thiols, and phosphates can be sil ...

reactions, i.e. cleavage of Si-O bonds in

. CsF is commonly used for such reactions. Solutions of caesium fluoride in THF or DMF attack a wide variety of

organosilicon Organosilicon compounds are organometallic compounds containing carbon–silicon bonds. Organosilicon chemistry is the corresponding science of their preparation and properties. Most organosilicon compounds are similar to the ordinary organic c ...

compounds to produce an organosilicon fluoride and a

carbanion In organic chemistry, a carbanion is an anion in which carbon is trivalent (forms three bonds) and bears a formal negative charge (in at least one significant resonance form). Formally, a carbanion is the conjugate base of a carbon acid: :R3CH ...

, which can then react with

electrophile In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that ca ...

s, for example: :

Precautions

Like other soluble fluorides, CsF is moderately toxic.MSDS Listing for cesium fluoride
.
www.hazard.com
.'' MSDS Date: April 27, 1993. Retrieved on September 7, 2007. Contact with acid should be avoided, as this forms highly toxic/corrosive

. The caesium

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

(Cs⁺) and

caesium chloride Caesium chloride or cesium chloride is the inorganic compound with the formula Cs Cl. This colorless salt is an important source of caesium ions in a variety of niche applications. Its crystal structure forms a major structural type where each c ...

are generally not considered toxic.MSDS Listing for cesium chloride
."
www.jtbaker.com
'' MSDS Date: January 16, 2006. Retrieved on September 7, 2007.

References

{{Fluorides Fluorides Caesium compounds Metal halides Alkali metal fluorides Rock salt crystal structure