text: caesium fluoride caesium fluoride or cesium fluoride is an inorganic compound usually encountered as a hygroscopic white solid . it is used in organic synthesis as a source of the fluoride anion . caesium 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 ( csoh ) with hydrofluoric acid ( hf ) . the resulting salt can then be purified by recrystallization . the reaction is shown below : csoh ( aq ) + hf ( aq ) → csf ( aq ) + h2o ( l ) another way to make caesium fluoride is to react caesium carbonate ( ) with hydrofluoric acid . the resulting salt can then be purified by recrystallization . the reaction is shown below : ( aq ) + 2 hf ( aq ) → 2 csf ( aq ) + h2o ( l ) + co2 ( g ) in addition , elemental fluorine and caesium can be used to form caesium fluoride as well , but doing so is very impractical because of the expense . while this is not a normal route of preparation , caesium metal reacts vigorously with all the halogens to form caesium halides . thus , it burns with fluorine gas , f2 , to form caesium fluoride , csf according to the following reaction : 2 cs ( s ) + f2 ( g ) → 2 csf ( s ) csf is more soluble than sodium fluoride or potassium fluoride . it is available in 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 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 it is a more reactive source of fluoride than related salts . csf is less hygroscopic alternative to tetra - n - fluoride ( ) and tas - fluoride ( ) when anhydrous " naked " fluoride ion is needed . 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 is also a popular source of fluoride in organofluorine chemistry . for example , csf reacts with to form a caesium salt , which is stable up to 60 ° c , unlike the corresponding sodium or potassium salt . it will convert electron - deficient aryl chlorides to aryl fluorides ( reaction ) . agent due to the strength of the si – f bond , fluoride ion is useful for reactions ( removal of si groups ) in organic chemistry ; caesium fluoride is an excellent source of anhydrous fluoride for such reactions . removal of silicon groups ( ) is a major application for csf in the laboratory , as its anhydrous nature allows clean formation of water - sensitive intermediates . solutions of caesium fluoride in thf or attack a wide variety of organosilicon compounds to produce an organosilicon fluoride and a carbanion , which can then react with electrophiles , for example : is also useful for the removal of silyl protecting groups . other uses single crystals of the salt are transparent into the deep infrared . for this reason it is sometimes used as the windows of cells used for infrared spectroscopy . 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 ( cs + ) and caesium chloride are generally not considered toxic .