Halogen exchange hydrogen fluoride

Vinyl halides are so inert that none has been converted to a fluoride by halogen exchange. Vinyl fluorides have been synthesized from saturated polyhalides by dehalogenation with zinc and by dehydrohalo- genation with alcoholic alkali, and from acetylene by addition of one molecule of hydrogen fluoride.12 18... 

Halogen exchange with KF is not successful ia acetic acid (10). Hydrogen bonding of the acid hydrogen with the fluoride ion was postulated to cause acetate substitution for the haUde however, the products of dissolved KF ia acetic acid are potassium acetate and potassium bifluoride (11). Thus KF acts as a base rather than as a fluorinating agent ia acetic acid. 

Hydrogen fluoride, HP, when used alone is a comparatively ineffective exchange agent and replaces only active halogens (13), eg, acyl fluorides from acyl chlorides and ben2otrifluoride [98-08-8] from hen2otrich1 oride (eq. 12). 

Another use of hydrogen fluoride, although not in halogen exchange, is the reaction with ethylenes or acetylenes to form the addition products, 1,1-difluoroethane [75-37-6] and vinyl fluoride [75-02-5]-. 

Vlayl fluoride [75-02-5] (VF) (fluoroethene) is a colorless gas at ambient conditions. It was first prepared by reaction of l,l-difluoro-2-bromoethane [359-07-9] with ziac (1). Most approaches to vinyl fluoride synthesis have employed reactions of acetylene [74-86-2] with hydrogen fluoride (HF) either directly (2—5) or utilizing catalysts (3,6—10). Other routes have iavolved ethylene [74-85-1] and HF (11), pyrolysis of 1,1-difluoroethane [624-72-6] (12,13) and fluorochloroethanes (14—18), reaction of 1,1-difluoroethane with acetylene (19,20), and halogen exchange of vinyl chloride [75-01-4] with HF (21—23). Physical properties of vinyl fluoride are given ia Table 1. 

Qua.driva.Ient, Zirconium tetrafluoride is prepared by fluorination of zirconium metal, but this is hampered by the low volatility of the tetrafluoride which coats the surface of the metal. An effective method is the halogen exchange between flowing hydrogen fluoride gas and zirconium tetrachloride at 300°C. Large volumes are produced by the addition of concentrated hydrofluoric acid to a concentrated nitric acid solution of zirconium zirconium tetrafluoride monohydrate [14956-11-3] precipitates (69). The recovered crystals ate dried and treated with hydrogen fluoride gas at 450°C in a fluid-bed reactor. The thermal dissociation of fluorozirconates also yields zirconium tetrafluoride. 

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. ... 

Moissan and his colleague Meslans then tried other methods for C —F bond synthesis.121 Alkyl fluorides could not be made via reactions of alcohols with hydrogen fluoride or phosphorus fluorides. However, silvcr(l) fluoride was found to function as a halogen-exchange reagent and several alkyl fluorides were made and characterized, all being fairly resistant to alkaline hydrolysis. 

Until recently, halogen substituted alkanes were thought to be inert to xenon difluoride, but detailed examination has shown that common solvents such as dichloromethane (3) and chloroform (4) undergo fluorine-chlorine and fluorine-hydrogen exchange at room temperature due to hydrogen fluoride catalysis.15... 

In contrast to aryl trichloromethyl ethers (see Section 12.1.2.), the replacement of chlorine atoms by fluorine in aryl trichloromethyl sulfides using antimony(III) fluoride (or hydrogen fluoride) proceeds under milder conditions, without a catalyst. Various substituents in the aromatic ring have little effect on the halogen exchange. The yields of aryl trifluoromethyl sulfides are 60 to 90% (see Table 1). 

Halogen exchange reactions, too, are valuable sources of fluorinated pyrimidines, especially 2-, 4-, and 6-fluoro derivatives. Sources of fluoride ion have included hydrogen, sodium, potassium, cesium, antimony, silver fluorides, and sulfur tetrafluoride. All of the reactions required heating. Prepared from nucleophilic fluorination processes have been... 

Table 2. Synthesis of Fluorohaloalkanes through Halogen Exchange with Antimony Halides/Hydrogen Fluoride...

A mixliire of copper(I) oxide/hydrogen fluoride in diethyl ether or tetrahydrofuran is a reagent for the exchange of tertiary and secondary halogens for fluorine without competing elimination to give alkenes (Table 5). ... 

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