CsF

Caesium fluoride

Caesium fluoride

Chemical compound

Caesium fluoride or cesium fluoride is an inorganic compound with the formula CsF. A hygroscopic white salt, caesium fluoride is used in the synthesis of organic compounds as a source of the fluoride anion.[5] The compound is noteworthy from the pedagogical perspective as caesium also has the highest electropositivity of all commonly available elements and fluorine has the highest electronegativity.

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Synthesis and properties

Crystalline CsF chains grown inside double-wall carbon nanotubes.[6]

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.[7] CsF reaches a vapor pressure of 1 kilopascal at 825 °C, 10 kPa at 999 °C, and 100 kPa at 1249 °C.[8]

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 alkali metal 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.[9] CsF gives higher yields in Knoevenagel condensation reactions than KF or NaF.[10]

Formation of C-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.[11] 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.[12] 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:[10]

Precautions

Like other soluble fluorides, CsF is moderately toxic.[13] 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.[14]

References

  1. [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. [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. [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. [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. [5]
    Friestad, Gregory K.; Branchaud, Bruce P.; Navarrini, Walter; Sansotera, Maurizio (2007). "Cesium Fluoride". Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rc050.pub2. ISBN 978-0-471-93623-7.
  6. [6]
    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)
  7. [7]
    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.
  8. [8]
    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.
  9. [9]
    Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. pp. 82–83. ISBN 978-0-08-022057-4.
  10. [10]
    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.
  11. [11]
    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.
  12. [12]
    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; Collected Volumes, vol. 10, p. 107.
  13. [13]
    "MSDS Listing for cesium fluoride". hazard.com. April 27, 1993. Archived from the original on 2012-02-09. Retrieved September 7, 2007.{{cite web}}: CS1 maint: unfit URL (link)
  14. [14]
    "MSDS Listing for cesium chloride". hazard.com. JT Baker. January 16, 2006. Archived from the original on 2012-03-13. Retrieved September 7, 2007.{{cite web}}: CS1 maint: unfit URL (link)
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