Dissociation energy of fluorides

Plots of mean dissociation energies of fluorides against molecular weights illustrate these mainly nonlinear trends, the patterns of which can be used to predict missing heats of formation (Fig. 7). 

Comparison of Dissociation Energies of Fluorides in Different Oxidation States... 

A further check on the hardness ordering can be made using a different set of reference reactions. The bond dissociation energies of fluorides and iodides were used in the earliest attempt to order the metal ions ... 

Of course we can assign relative values of q for a series of acids or bases, as we have already done, using the data in Table 1. There are two objections to this procedure. One is that we would get somewhat diflerent orders, depending on what reference reactions are selected. For example, one could use bond dissociation energies of fluorides and iodides ... 

The bond dissociation energy of the hydrogen-fluorine bond in HF is so great that the above equilibrium lies to the left and hydrogen fluoride is a weak acid in dilute aqueous solution. In more concentrated solution, however, a second equilibrium reaction becomes important with the fluoride ion forming the complex ion HFJ. The relevant equilibria are ... 

The last example represents a fairly rare elimination of hydrogen fluoride in preference to hydrogen chloride, a reaction that deserves a more detailed discussion A comparison of bond dissociation energies of carbon-halogen bonds shows that the carbon-fluorine bond is much stronger than the carbon-chlorine, carbon-bromine, and carbon-iodme bonds 108-116, 83 5, 70, and 56 kcal/mol, respec-... 

Halo-2-methylbutanes (X = F, Cl, Br) all produce the same carbocation upon dissociation, yet one of these halides is unreactive. Calculate the dissociation energies for 2-metbyl-2-butyl fluoride and 2-metbyl-2-butyl-cbloride. (Energies of fluoride and chloride are given at right.) Compare these to the dissociation energy of the corresponding bromo derivative. Which hahde is most likely to be unreactive ... 

The variety of fluoride compounds that exist and the wide spectrum of their preparation methods are related to the properties of fluorine, and above all to fluorine s high electronegativity. Low dissociation energy of the fluorine molecule, F2, relatively high energies of bond formation found in most fluoride compounds, as well as fluorine s strong oxidizing ability lead, in some cases, to spontaneous fluorination. 

For the most electronegative ligand, fluorine, we expect a relativistic destabilization in the Au—F bond, which was indeed determined to be —0.36eV at the coupled cluster level [182,183], Nevertheless, AuF has a sufficiently high dissociation energy of about 3.17 eV and has been identified recently in the gas phase [184]. In solution or in the solid state it would disproportionate to metallic Au and compounds of Au (AuF3 for the solid). However, a carbene-stabilized Au(I) fluoride was synthesized only very recently (see discussion in the next section) [185]. 

In fluorine thermochemistry, two key heat values frequently occur. They are the dissociation energy of difluorine, required for evaluation of fluorine bond energies and the heat of formation of hydrogen fluoride, a product in hydrolysis, hydrogenation, fluorine combustion, or neutralization reactions. These values have been difficult to measure and have changed considerably over the years. A recommended set of values has been reported in recent CODATA bulletins (60) which are collected in Table I together with older values and corrections to update them. 

V, A. Further experimental work on dissociation energies in fluoride series underlines the warning given on the variability of high-temperature data. A new AHf(BF,) is 79 kJ different from the old value (6), and a maximum difference of 79 kJ appears in the dissociation energies of the sulfur fluorides (9). Values for tantalum (17) and platinum fluorides (14) are also now available. 

Similarly, the original bond dissociation energy of hydrogen fluoride (147.6 kcal/mol (618 kJ) [/]) was later corrected to 136 kcal/mol (570 kJ) [31. 

An alternative view of the interaction of an alkali metal cation with a fluoride-containing anion is one of Lewis acid/base competition. The reactions discussed in the preceding section involved the reaction of an alkali fluoride salt with a Lewis acid with subsequent fluoride ion transfer to the Lewis acid. However, the alkali metal cation is a Lewis acid as well, and the degree of perturbation of the anion by the cation may be dependent on the differences in fluoride ion affinity of the Lewis acid and the alkali metal cation. The fluoride ion affinities for a variety of Lewis acids are well known from ICR (53,54,64) studies, while the fluoride ion affinities for alkali metal cations are the heterolytic bond dissociation energies of the gas phase alkali fluoride molecules... 

The order of ease of reductive dehalogenation of organic halides in the same type of structural environment is I > Br > Cl F. This order is parallel with the dissociation energy of carbon-halogen bonds (HsC—I 234 kJ mol- H3C—Br 293 kJ mol" H3C—Cl 351 kJ mol- H3C—F 452 kJ moL ) and is generally observed in the reduction of alkyl halides. Consequently, selective reduction of di- or polyhalides containing different halogen atoms is possible. Fluorides are often removed only with difficulty and examples of such reductions are comparatively limited. 

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