Hydrogen fluoride, ionization

The ease of ionization of binary protonic acids depends on both (1) the ease of breaking H—X bonds and (2) the stability of the resulting ions in solution. Let us consider the relative strengths of the Group VIIA hydrohalic acids. Hydrogen fluoride ionizes only slightly in dilute aqueous solutions. 

Pure liquid hydrogen fluoride ionizes in a way similar to that of water. 

Writing the equation in the usual way directs too much attention to the atoms and not enough to the electrons We can remedy that by deleting any spec tator ions and by showing the unshared electron pairs and covalent bonds that are made and broken Both sodium hydroxide and sodium fluoride are com pletely ionized in water therefore Na" which ap pears on both sides of the equation is a spectator ion Hydrogen fluoride is a weak acid and exists as undissociated HF molecules in water... 

Anhydrous hydrogen fluoride is an excellent solvent for ionic fluorides (Table 3). The soluble fluorides act as simple bases, becoming fully ionized and increasing the concentration of HF 2- Foi example,... 

The reaction of ferrocene and formaldehyde in either concentrated sulfuric acid or liquid hydrogen fluoride, followed by reduction, produces a compound containing two ferrocenyl and two methylene groups (57, 98, 123). After several incorrect assignments had been proposed for the structure of this condensation product, Rinehart and coworkers showed by an unequivocal synthesis that the product was 1,2-diferrocenylethane (XIX) (104). The mechanism of the reaction presumably involves the initial formation of ferrocenylcarbinol (XX) followed by ionization in the strongly acidic medium to the ferrocenylmethyl-carbonium ion (XXI). Conversion to radical ion XXII followed by dimerization and subsequent reduction produces the product. 

Hydrofluoric acid like water is an associated liquid, and even the gas, as we shall soon see, is associated. It has the power of uniting with fluorides. It also seems to be an ionizing solvent for a soln. of potassium fluoride in liquid hydrogen fluoride is an excellent conductor it also possesses marked solvent powers. According to E. C. Franklin,7 the liquid readily dissolves potassium fluoride, ehloride, and sulphate sodium fluoride, bromide, nitrate, chlorate, and bromate acetamide and urea. The solvent action is not so marked with barium fluoride, cupric chloride, and silver cyanide while calcium and lead fluorides copper sulphate and nitrate ferric chloride, mercuric oxide, and magnesium metal, are virtually insoluble in this menstruum. Glass also is not affected by the liquid if moisture be absent. The liquid scarcely acts on most of the metals or non-metals at ordinary temp., though it does act on the alkali metals at ordinary temp., much the same as does water, with the simultaneous production of flame. 

G. Gore, in his early work, showed that the presence of hydrogen fluoride in water lowered its f.p. E. Paterno and A. Peratoner11 measured the depression of the f.p. of water by soln. containing from 0057 to 2"80 mol. per litre and W. Ostwald showed that the results corresponded with the presence of molecules of HF, partially ionized, in the soln. 

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. 

Hydrogen fluoride is a weak acid in dilute solution in water but H0 measurements (Bell et al., 1956) show that in more concentrated solutions the acidity rises rapidly so that 100% HF has an H0 value only slightly less than that of 100% H2S04. Confirmation of this comes from cryoscopic measurements which show that the weak bases water, acetonitrile and p-nitrotoluene are somewhat less ionized in 100% HF than in 100% H2S04 (Gillespie and Humphreys, 1970). 

The existence of hydrogen bonds is not a necessary condition for association and self-ionization of solvents. Association of liquid hydrogen fluoride, for example, can be explained equally well by hydrogen bridges between fluorine atoms and fluorine bridges between hydrogen atoms (72) ... 

Other Electrolytic Solvents. Some liquids other than water can serve as ionizing solvents, with the power of dissolving electrolytes to give electrically conducting solutions. These liquids include liquid ammonia, hydrogen peroxide, and hydrogen fluoride. All of these liquids, like water, have very large dielectric constants. Liquids with... 

The relative fluoride ion donor ability, XeFe > XeF, XeF4, is compatible with the findings of Hyman and Quarterman, who have reported that XeFe and XeF2 are very soluble in anhydrous hydrogen fluoride, the former being extensively ionized, whereas XeFe is sparingly soluble. 

When metallic gold is exposed to elemental fluorine (F2) at room temperature in the presence of the ionizing solvent liquid anhydrous hydrogen fluoride (aHF) containing an alkali fluoride, the metal is quickly dissolved to give the alkali salt of the anion... 

Recently it was discovered (/) that AgF+ salts are powerful oxidizers capable of oxidizing xenon at ordinary temperatures, per-fluorobutene to perfluorobutane, and per-fluorobenzene to salts of, both of the latter at 208 K, in anhydrous hydrogen fluoride solution (2). These oxidations by AgF+ imply electron transfer to the cationic Ag(II) as the first step in the reaction. This is consistent with the high second ionization potential of Ag (21.49 eV) which is the highest... 

An important industrial process for the conversion of organic precursors to fully fluorinated derivatives (perfluo-rocarbon compounds) is the Simons Process. The Simons Process [1-9] uses an electrochemical cell to oxidize per-hydro-organic materials, at a nickel anode, to their perfluoro-relatives. The electrolyte for that process is liquid anhydrous hydrogen fluoride in which sodium fluoride, or other ionizing solutes, are dissolved (the latter, to provide for electrical conductance). 

Studies of the nonaqueous chemistry of xenon fluorides have also been initiated. Whereas xenon tetrafluoride is only sparingly soluble in hydrogen fluoride, xenon difluoride and xenon hexafluoride are very soluble indeed. Electrical conductivity, absorption and Raman spectroscopy, and nuclear magnetic resonance measurements have been employed in studying these solutions. The observations have been interpreted in terms of dissolution without appreciable dissociation for xenon difluoride and xenon tetrafluoride, and dissolution with extensive ionization for xenon hexafluoride. The extensive solubility and ionization of xenon hexafluoride contrast strikingly with the behavior of other hexafluorides (e.g., uranium hexafluoride). 

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