Potassium fluoride halogen exchange reaction with

Polymer-supported tetraphenylphosphonium bromide is a recyclable catalyst for halogen-exchange reactions. The reaction of 1 equivalent of chloro-2,4-dinitrobenzene with 1 5 equivalents of spray-dned potassium fluoride and 0.1 equivalent of this catalyst in acetonitnle at 80 C for 12 h gives 2,4-dinitro-fluorobenzene m 98% yield An 11% yield is obtained without the catalyst [3 /]. 

Reactions on or close to solid surfaces maybe inhibited by deposition ofinsoluble or poorly soluble products on the reactant surface, a phenomenon referred to as overgrowth. Examples include the reaction of amines with chloranil [7], the diazotisation of poorly soluble aromatic amines in which the product diazonium salt is also insoluble and halogen exchange reactions of chloroaromatic compounds using potassium fluoride in dipolar aprotic solvents where the potassium chloride product may coat the potassium fluoride [8]. 

For less activated aromatic systems (those without a nitro substituent), the halogen-exchange reaction has been investigated with potassium fluoride in a variety of polar aprotic solvents in the presence or absence of a catalyst (see Table 13). Many different types of catalysts have been investigated these include crown ethers.169 quaternary ammonium salts,163 164 phos-phonium salts,158167 aminophosphonium salts.162 compounds containing a phosphorus and an amino function,161 and inorganic fluorides of boron, aluminum, tin, phosphorus, titanium and zirconium.162 Different forms of potassium fluoride have been used these include spray-dried potassium fluoride,167 freeze-dried potassium fluoride,168 potassium fluoride recrystallized from methanol,165 and potassium fluoride dispersed oil calcium fluoride.166... 

Benzoyl fluoride can also be prepared by the reaction of anhydrous hydrogen fluoride or potassium fluoride with benzoic anhydride and by the halogen exchange of benzoyl chloride with alkali fluorides, such as NaF/ KF/ KHF2, Na2SiFe, or various other metal fluorides. ... 

Lithium fluoride cannot displace less reactive halogens from aryl chlorides and bromides. However, a high-temperature reaction of lithium fluoride with bromoform gives a few percent exchange along with considerable side products.3 By passing a stream of methyl chloride in a lithium fluoride/sodium fluoride/potassium fluoride melt at 500 C a conversion rate of 42% to methyl fluoride in 85% yield is observed.4... 

The synthesis of benzyl fluorides from benzyl halides is problematic due to the tendency to undergo Friedel Crafts reactions.The fluorination of benzyl bromide with thallium(l) fluoride gives impure benzyl fluoride and attempts to fluorinate benzyl halides with anti-mony(III) fluoride, silver fluoride, and potassium fluoride were all unsuccessful. Benzyl fluorides with substituents which deactivate the aromatic system towards Friedel-Crafts reactions are successfully obtained through halogen exchange with mercury(II) fluoride (Table... 

The pyrolytic decomposition of the sodium salts of various fluorinated carboxylic acids to give isomeric unsaturated compounds has also been reported. The products were identified as alkenes with the C = C bond inside the carbon chain, mainly alk-2-enes. This isomerization may be catalyzed by the coal-like products formed during the pyrolytic decarboxylation of the salts, but the metal fluoride formed in the reaction may also be responsible for the isomerization. When potassium perfluoro(5-chloropentanoate) is heated in a rocking autoclave at 300 C for 2 hours, perfluorobut-2-ene (2b) is isolated in 82% yield.This is only possible by migration of the double bond away from the terminal position after carbon dioxide elimination and halogen exchange to form potassium chloride. ... 

Perfluoroheteroaromatic compounds [25, 26], in which all the hydrogen atom substituents of the heterocyclic ring have been replaced by fluorine atoms, were first synthesized in the 1960s by reaction of potassium fluoride with appropriate perchlorinated heteroaromatic precursors [25, 26] and a range of perfluorinated heteroaromatic systems may be accessed by halogen exchange (Halex) techniques as shown in Figure 11.3. 

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