Hydrogen, acceptors fluoride

Our next example will be the hydrogen fluoride dimer. It is a good example to show that the electrostatics restricted to the dipole-dipole interaction (or the equivalent charge interaction model) is not sufficient to determine the minimum orientation even for interactions of diatomic monomers. Such a model predicts the linear F-H- -F-H structure dictated by the optimal orientation of the dipoles, which disagrees with the observed structure in which the hydrogen acceptor is tilted away from the linear geometry by about 60°... 

As a powerful hydrogen-bond acceptor, fluoride is understandably a poor nucleophile in protic solvents. Iodide, as the worst hydrogen-bond acceptor, is thus a much more active nucleophile in protic solvents. In the absence of hydrogen-bonding interactions with the solvent, which is the case in dry polar aprotic solvents, fluoride is the strongest nucleophile. 

Fluorides. Tantalum pentafluoride [7783-71-3] TaF, (mp = 96.8° C, bp = 229.5° C) is used in petrochemistry as an isomerization and alkalation catalyst. In addition, the fluoride can be utilized as a fluorination catalyst for the production of fluorinated hydrocarbons. The pentafluoride is produced by the direct fluorination of tantalum metal or by reacting anhydrous hydrogen fluoride with the corresponding pentoxide or oxychloride in the presence of a suitable dehydrating agent (71). The ability of TaF to act as a fluoride ion acceptor in anhydrous HF has been used in the preparation of salts of the AsH, H S, and PH ions (72). The oxyfluorides TaOF [20263-47-2] and Ta02F [13597-27-8] do not find any industrial appHcation. 

Hydrogen chloride, 203,562-563 Hydrogen fluoride, 183,562-563 Hydrogen ion acceptors, donors of, 353 and balancing redox equations, 88-89 in buffer systems, 387-390 and hydroxide ion, 354—355 and indicator color, 391-393 and pH, 355... 

We can also examine isomeric pairs of H-bonded complexes in which the roles of electron-pair donor and acceptor are reversed, and the mutual dipole orientations are therefore fundamentally altered. As an example, Fig. 5.4 shows the isomeric pair of complexes between ammonia and hydrogen fluoride,... 

The ability of the fluoride ion to act as a hydrogen-bond acceptor, even towards the very weakest of hydrogen-bond donors, such as the C—H bond, has been investigated. McMahon and Roy (1985) computed the relative stabilities of C2H4F and CjHjF by ab initio methods and showed that... 

The action of strong aprotic Lewis acids (antimony(V) fluoride, arsenic(V) fluoride etc.) provokes the ionization of xenon difluoride, leading to the formation of fluoroxenonium salts XcF + MFn or Xe2F3 MFn less strong acceptors of the fluoride ion (hydrogen fluoride, boron trifluoride, etc.) polarize the xenon difluoride molecule. 

Although alcohols can be fluorinated directly with selenium tetrafluoride, yields can be disappointing because of rearrangement/polymerization due to the presence of hydrogen fluoride produced during the reaction. Higher yields have been obtained when the reaction is performed in the presence of 1 equivalent of pyridine, which forms a 1 1 donor-acceptor complex with selenium tetrafluoride. The reaction proceeds in two distinct steps first is the reaction of the alcohol with the complex to form an alkoxyselenium trifluoride, the second is the thermal decomposition to the alkyl fluoride. 

Chloroform plays a crucial role in the reaction. Among others its somewhat electrophilic hydrogen serves as an acceptor for the fluoride ion. 

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