View source for Caesium fluoride

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{{Chembox
| Verifiedfields = changed
| Watchedfields  = changed
| verifiedrevid  = 444641443
| Name           = Caesium fluoride
| ImageFile      = Caesium fluoride.jpg
| ImageName      = Caesium fluoride
| ImageFile1     = Caesium-fluoride-3D-ionic.png
| ImageName1     = Caesium fluoride
| IUPACName      = Caesium fluoride
| OtherNames     = Cesium fluoride
| Section1       = {{Chembox Identifiers
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID = 24179
| InChI = 1/Cs.FH/h;1H/q+1;/p-1
| SMILES = [F-].[Cs+]
| InChIKey = XJHCXCQVJFPJIK-REWHXWOFAY
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/Cs.FH/h;1H/q+1;/p-1
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = XJHCXCQVJFPJIK-UHFFFAOYSA-M
| CASNo = 13400-13-0
| CASNo_Ref = {{cascite|correct|CAS}}
| RTECS = FK9650000
| PubChem = 25953
| EC_number = 236-487-3
| UNII = T76A371HJR
}}
| Section2       = {{Chembox Properties
| Formula = CsF
| MolarMass = 151.903 g/mol<ref name=crc/>
| Appearance = white crystalline solid
| Density = 4.64 g/cm<sup>3</sup><ref name=crc/>
| Solubility = 573.0 g/100 mL (25 °C)<ref name=crc>{{RubberBible92nd|page=4.57}}</ref>
| SolubleOther = Insoluble in [[acetone]], [[diethyl ether]], [[pyridine]] and [[ethanol]]<br>191 g/100 mL in [[methanol]].
| MeltingPtC = 703
| MeltingPt_ref = <ref name=crc/>
| BoilingPtC = 1251
| BoilingPt_notes = (2,284 °F; 1,524 K)
| pKb = −744 kJ/mol
| RefractIndex = 1.477
| MagSus = −44.5·10<sup>−6</sup> cm<sup>3</sup>/mol<ref>{{RubberBible92nd|page=4.132}}</ref>
}}
| Section3       = {{Chembox Structure
| Coordination = [[Octahedron|Octahedral]]
| CrystalStruct = [[Cubic crystal system|cubic]],  [[Pearson symbol|cF8]]
| SpaceGroup = Fm{{overline|3}}m, No. 225<ref name=str>{{cite journal|doi=10.1103/PhysRev.21.143|title=Precision Measurements of Crystals of the Alkali Halides|journal=Physical Review|volume=21|issue=2|pages=143–161|year=1923|last1=Davey|first1=Wheeler P.|bibcode=1923PhRv...21..143D}}</ref>
| LattConst_a =  0.6008 nm<ref name=str/>
| UnitCellFormulas = 4
| UnitCellVolume = 0.2169 nm<sup>3</sup><ref name=str/>
| Dipole = 7.9 [[Debye|D]]
}}
| Section4       = {{Chembox Thermochemistry
| DeltaHf = −553.5 kJ/mol<ref name=b92t>{{RubberBible92nd|page=5.10}}</ref>
| Entropy = 92.8 J/mol·K<ref name=b92t/>
| DeltaGf = −525.5 kJ/mol<ref name=b92t/>
| HeatCapacity = 51.1 J/mol·K<ref name=b92t/>
}}
| Section7       = {{Chembox Hazards
| MainHazards = toxic
| GHSPictograms = {{GHS05}}{{GHS06}}{{GHS08}}
| GHSSignalWord = Danger
| HPhrases = {{H-phrases|301|311|315|318|331|361f}}
| PPhrases = {{P-phrases|201|202|260|261|264|270|271|280|281|301+310|301+330+331|302+352|303+361+353|304+340|305+351+338|308+313|310|311|312|321|322|330|332+313|361|362|363|403+233|405|501}}
| NFPA-H = 3
| NFPA-F = 0
| NFPA-R = 0
| ExternalSDS = [https://www.fishersci.com/store/msds?partNumber=AC189510250&productDescription=CESIUM+FLUORIDE%2C+99%25+25GR&vendorId=VN00032119&countryCode=US&language=en External MSDS]
| FlashPt = Non-flammable
}}
| Section8       = {{Chembox Related
| OtherAnions = [[Caesium chloride]]<br/>[[Caesium bromide]]<br/>[[Caesium iodide]]<br/>Caesium astatide
| OtherCations = [[Lithium fluoride]]<br/>[[Sodium fluoride]]<br/>[[Potassium fluoride]]<br/>[[Rubidium fluoride]]<br/>Francium fluoride
}}
}}

'''Caesium fluoride''' ('''cesium fluoride''' in [[American English]]) is an [[inorganic compound]] with the formula CsF. A [[hygroscopic]] white salt, caesium fluoride is used in the [[organic synthesis|synthesis of organic compounds]] as a source of the fluoride anion.<ref>{{cite book |doi=10.1002/047084289X.rc050.pub2 |chapter=Cesium Fluoride |title=Encyclopedia of Reagents for Organic Synthesis |date=2007 |last1=Friestad |first1=Gregory K. |last2=Branchaud |first2=Bruce P. |last3=Navarrini |first3=Walter |last4=Sansotera |first4=Maurizio |isbn=978-0-471-93623-7 }}</ref>  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.

==Synthesis and properties==
[[File:CsF@DWNT.png|thumb|left|150px|Crystalline CsF chains grown inside double-wall [[carbon nanotube]]s.<ref name=chains>{{cite journal|doi=10.1038/ncomms8943|pmid=26228378|pmc=4532884|title=Single-atom electron energy loss spectroscopy of light elements|journal=Nature Communications|volume=6|pages=7943|year=2015|last1=Senga|first1=Ryosuke|last2=Suenaga|first2=Kazu|bibcode=2015NatCo...6.7943S}} (Supplementary information)</ref>]]
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 + H<sub>2</sub>O

Using the same reaction, another way to create caesium fluoride is to treat [[caesium carbonate]] (Cs<sub>2</sub>CO<sub>3</sub>) with hydrofluoric acid and again, the resulting salt can then be purified by recrystallization. The reaction is shown below:
:Cs<sub>2</sub>CO<sub>3</sub> + 2 HF → 2 CsF + H<sub>2</sub>O + CO<sub>2</sub>

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&nbsp;°C for two hours ''[[in vacuo]]''.<ref name="Friestad">{{cite book|author1=Friestad, G. K. |author2=Branchaud, B. P. |title=Handbook of Reagents for Organic Synthesis: Acidic and Basic Reagents|editor1=Reich, H. J. |editor2=Rigby, J. H. |publisher=Wiley|location=New York|year=1999|pages=99–103|isbn=978-0-471-97925-8}}</ref> CsF reaches a [[vapor pressure]] of 1 [[kilopascal]] at 825&nbsp;°C, 10 kPa at 999&nbsp;°C, and 100 kPa at 1249&nbsp;°C.<ref>{{cite book | editor = Lide, D. R. | title = CRC Handbook of Chemistry and Physics | edition = 86th | location = Boca Raton (FL) | publisher = CRC Press | year = 2005 | isbn = 0-8493-0486-5 |page=6.63|chapter=Vapor Pressure|chapter-url=http://www.physics.nyu.edu/kentlab/How_to/ChemicalInfo/VaporPressure/morepressure.pdf}}</ref>

==Structure==
Caesium fluoride has the [[halite]] structure, which means that the Cs<sup>+</sup> and F<sup>&minus;</sup> pack in a [[cubic closest packed]] array as do Na<sup>+</sup> and Cl<sup>&minus;</sup> in sodium chloride.<ref name=str/> 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 [[TASF reagent|TAS-fluoride]] (TASF).

===As a base===
As with other soluble fluorides, CsF is moderately basic, because [[hydrofluoric acid|HF]] is a [[weak acid]]. The low [[nucleophile|nucleophilicity]] of [[fluoride]] means it can be a useful base in [[organic chemistry]].<ref name="greenwood">{{Greenwood&Earnshaw1st|pages=82–83}}</ref> CsF gives higher yields in [[Knoevenagel condensation]] reactions than [[potassium fluoride|KF]] or [[sodium fluoride|NaF]].<ref name="Fiorenza">{{cite journal|year=1985|journal=Tetrahedron Letters|volume=26|pages=787–788|doi=10.1016/S0040-4039(00)89137-6|title=Fluoride ion induced reactions of organosilanes: the preparation of mono and dicarbonyl compounds from β-ketosilanes|author1=Fiorenza, M|author2=Mordini, A|author3=Papaleo, S|author4=Pastorelli, S|author5=Ricci, A|issue=6}}</ref>

===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 {{chem name|perfluoroalkoxide}} salt.<ref>{{cite journal|author1=Evans, F. W. |author2=Litt, M. H. |author3=Weidler-Kubanek, A. M. |author4=Avonda, F. P. |title=Formation of adducts between fluorinated ketones and metal fluorides|year=1968|journal=Journal of Organic Chemistry|volume=33|pages=1837–1839|doi=10.1021/jo01269a028|issue=5}}</ref> It will convert [[electron-deficient]] [[aryl chloride]]s to [[aromatic|aryl]] fluorides ([[Halex process]]), although potassium fluoride is more commonly used.

===Deprotection agent===
Due to the strength of the [[silicon|Si]]–[[fluorine|F]] bond, fluoride is useful for [[desilylation]] reactions, i.e., cleavage of Si-O bonds in [[organic synthesis]].<ref>{{OrgSynth |author=Smith, Adam P. |author2=Lamba, Jaydeep J. S. |author3=Fraser, Cassandra L. |name-list-style=amp |title=Efficient Synthesis of Halomethyl-2,2'-bipyridines: 4,4'-Bis(chloromethyl)-2,2'-bipyridine|volume=78|page=82 |year=2002|collvol= 10|collvolpages=107 |prep=v78p0082}}</ref> CsF is commonly used for such reactions. Solutions of caesium fluoride in [[THF]] or [[Dimethylformamide|DMF]] attack a wide variety of [[organosilicon]] compounds to produce an organosilicon fluoride and a [[carbanion]], which can then react with [[electrophile]]s, for example:<ref name="Fiorenza"/>

:[[Image:CsF desilylation.png|500px]]

==Precautions==
Like other soluble fluorides, CsF is moderately toxic.<ref name="msds-csf">{{cite web |url=http://www.hazard.com/msds/f2/bms/bmsqc.html |title=MSDS Listing for cesium fluoride |date=April 27, 1993 |website=hazard.com |access-date=September 7, 2007 |url-status=usurped |archive-url=https://web.archive.org/web/20120209103840/http://www.hazard.com/msds/f2/bms/bmsqc.html |archive-date=2012-02-09 }}</ref> Contact with [[acid]] should be avoided, as this forms highly toxic/corrosive [[hydrofluoric acid]]. The caesium [[ion]] (Cs<sup>+</sup>) and [[caesium chloride]] are generally not considered toxic.<ref name="msds-cscl">{{cite web |url=http://hazard.com/msds/mf/baker/baker/files/c1903.htm |title=MSDS Listing for cesium chloride |date=January 16, 2006 |website=hazard.com |publisher=JT Baker |access-date=September 7, 2007 |url-status=usurped |archive-url=https://web.archive.org/web/20120313200630/http://hazard.com/msds/mf/baker/baker/files/c1903.htm |archive-date=2012-03-13 }}</ref>

==References==
{{reflist}}


{{Authority control}}
{{Caesium compounds}}
{{Fluorides}}

[[Category:Fluorides]]
[[Category:Caesium compounds]]
[[Category:Metal halides]]
[[Category:Alkali metal fluorides]]
[[Category:Rock salt crystal structure]]