Caesium fluoride

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

Caesium fluoride
Caesium fluoride
Caesium fluoride
Names
IUPAC name
Caesium fluoride
Other names
Cesium fluoride
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.033.156
EC Number
  • 236-487-3
RTECS number
  • FK9650000
UNII
  • InChI=1S/Cs.FH/h;1H/q+1;/p-1 checkY
    Key: XJHCXCQVJFPJIK-UHFFFAOYSA-M checkY
  • InChI=1/Cs.FH/h;1H/q+1;/p-1
    Key: XJHCXCQVJFPJIK-REWHXWOFAY
  • [F-].[Cs+]
Properties
CsF
Molar mass 151.903 g/mol[1]
Appearance white crystalline solid
Density 4.64 g/cm3[1]
Melting point 703 °C (1,297 °F; 976 K)[1]
Boiling point 1,251 °C (2,284 °F; 1,524 K)
573.0 g/100 mL (25 °C)[1]
Solubility Insoluble in acetone, diethyl ether, pyridine and ethanol
191 g/100 mL in methanol.
-44.5·10−6 cm3/mol[2]
1.477
Structure
cubic, cF8
Fm3m, No. 225[3]
a = 0.6008 nm[3]
0.2169 nm3[3]
4
Octahedral
7.9 D
Thermochemistry
51.1 J/mol·K[4]
92.8 J/mol·K[4]
-553.5 kJ/mol[4]
-525.5 kJ/mol[4]
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
 toxic
GHS labelling:
GHS05: CorrosiveGHS06: ToxicGHS08: Health hazard
Danger
H301, H311, H315, H318, H331, H361f
P201, P202, P260, P261, P264, P270, P271, P280, P281, P301+P310, P301+P330+P331, P302+P352, P303+P361+P353, P304+P340, P305+P351+P338, P308+P313, P310, P311, P312, P321, P322, P330, P332+P313, P361, P362, P363, P403+P233, P405, P501
NFPA 704 (fire diamond)
NFPA 704 four-colored diamond
3
0
0
Flash point Non-flammable
Safety data sheet (SDS) External MSDS
Related compounds
Other anions
 Caesium chloride
Caesium bromide
Caesium iodide
Caesium astatide
Other cations
 Lithium fluoride
Sodium fluoride
Potassium fluoride
Rubidium fluoride
Francium fluoride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)
Infobox references

Synthesis and properties
Crystalline CsF chains grown inside double-wall carbon nanotubes.[5]

Caesium fluoride can be prepared by the reaction of caesium hydroxide (CsOH) with hydrofluoric acid (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 (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 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.[6] CsF reaches a vapor pressure of 1 kilopascal at 825 °C, 10 kPa at 999 °C, and 100 kPa at 1249 °C.[7]

CsF chains with a thickness as small as one or two atoms can be grown inside carbon nanotubes.[5]

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.[3] Unlike sodium chloride, caesium fluoride's anion is smaller than its cation, so it is the anion size that sterically inhibits larger coordination numbers than six under normally encountered conditions. A larger halide ion would allow for the eight-coordination seen in other caesium halide crystals.

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 (TBAF) and TAS-fluoride (TASF).

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.[8] CsF gives higher yields in Knoevenagel condensation reactions than KF or NaF.[9]

Formation of Cs-F bonds

Caesium fluoride serves as a source of fluoride in organofluorine chemistry. Similarly to potassium fluoride, CsF reacts with hexafluoroacetone to form a stable perfluoroalkoxide salt.[10] It will convert electron-deficient aryl chlorides to aryl fluorides (Halex process), 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.[11] 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:[9]

Precautions

Like other soluble fluorides, CsF is moderately toxic.[12] Contact with acid should be avoided, as this forms highly toxic/corrosive hydrofluoric acid. The caesium ion (Cs+) and caesium chloride are generally not considered toxic.[13]

References
  1. Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). Boca Raton, FL: CRC Press. p. 4.57. ISBN 1-4398-5511-0.
  2. Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). Boca Raton, FL: CRC Press. p. 4.132. ISBN 1-4398-5511-0.
  3. Davey, Wheeler P. (1923). "Precision Measurements of Crystals of the Alkali Halides". Physical Review. 21 (2): 143–161. Bibcode:1923PhRv...21..143D. doi:10.1103/PhysRev.21.143.
  4. Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). Boca Raton, FL: CRC Press. p. 5.10. ISBN 1-4398-5511-0.
  5. Senga, Ryosuke; Suenaga, Kazu (2015). "Single-atom electron energy loss spectroscopy of light elements". Nature Communications. 6: 7943. Bibcode:2015NatCo...6.7943S. doi:10.1038/ncomms8943. PMC 4532884. PMID 26228378. (Supplementary information)
  6. Friestad, G. K.; Branchaud, B. P. (1999). Reich, H. J.; Rigby, J. H. (eds.). Handbook of Reagents for Organic Synthesis: Acidic and Basic Reagents. New York: Wiley. pp. 99–103. ISBN 978-0-471-97925-8.
  7. Lide, D. R., ed. (2005). "Vapor Pressure" (PDF). CRC Handbook of Chemistry and Physics (86th ed.). Boca Raton (FL): CRC Press. p. 6.63. ISBN 0-8493-0486-5.
  8. Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. pp. 82–83. ISBN 978-0-08-022057-4.
  9. Fiorenza, M; Mordini, A; Papaleo, S; Pastorelli, S; Ricci, A (1985). "Fluoride ion induced reactions of organosilanes: the preparation of mono and dicarbonyl compounds from β-ketosilanes". Tetrahedron Letters. 26 (6): 787–788. doi:10.1016/S0040-4039(00)89137-6.
  10. Evans, F. W.; Litt, M. H.; Weidler-Kubanek, A. M.; Avonda, F. P. (1968). "Formation of adducts between fluorinated ketones and metal fluorides". Journal of Organic Chemistry. 33 (5): 1837–1839. doi:10.1021/jo01269a028.
  11. Smith, Adam P.; Lamba, Jaydeep J. S. & Fraser, Cassandra L. (2002). "Efficient Synthesis of Halomethyl-2,2'-bipyridines: 4,4'-Bis(chloromethyl)-2,2'-bipyridine". Organic Syntheses. 78: 82.; Collective Volume, vol. 10, p. 107
  12. MSDS Listing for cesium fluoride Archived 2012-02-09 at the Wayback Machine. www.hazard.com Archived 2013-06-05 at the Wayback Machine. MSDS Date: April 27, 1993. Retrieved on September 7, 2007.
  13. "MSDS Listing for cesium chloride Archived 2012-03-13 at the Wayback Machine." www.jtbaker.com. MSDS Date: January 16, 2006. Retrieved on September 7, 2007.


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