Dissociation Hydrogen Halides

The heats of formation of the gaseous atoms, 4, are not very different clearly, it is the change in the bond dissociation energy of HX, which falls steadily from HF to HI, which is mainly res ponsible for the changes in the heats of formation. 6. We shall see later that it is the very high H—F bond energy and thus the less easy dissoeiation of H—F into ions in water which makes HF in water a weak aeid in comparison to other hydrogen halides. 

Towards a simple Lewis base, for example the proton, phosphine is a poorer electron donor than ammonia, the larger phosphorus atom being less able to form a stable covalent bond with the acceptor atom or molecule. Phosphine is, therefore, a much weaker base than ammonia and there is no series of phosphonium salts corresponding to the ammonium salts but phosphonium halides. PH4X (X = Cl, Br, I) can be prepared by the direct combination of phosphine with the appropriate hydrogen halide. These compounds are much more easily dissociated than ammonium halides, the most stable being the iodide, but even this dissociates at 333 K PH4I = PH3 -t- HI... 

It is known that the order of acidity of hydrogen halides (HX, where X = F, Cl, Br, I) in the gas phase can be successfully predicted by quantum chemical considerations, namely, F < Cl < Br < I. However, in aqueous solution, whereas hydrogen chloride, bromide, and iodide completely dissociate in aqueous solutions, hydrogen fluoride shows a small dissociation constant. This phenomenon is explained by studying free energy changes associated with the chemical equilibrium HX + H2O + HjO in the solu-... 

The other hydrogen halides are less tractable as solvents, as might be expected from their physical properties (p. 813), especially their low bps, short liquid ranges, low dielectric constants and negligible self-dissociation into ions. Nevertheless, they have received some attention, both for comparison with HF and as preparative media with their own special advantages.In particular, because of their low bp and consequent ease of removal, the liquid HX solvent systems have provided convenient routes to BX4, BF3C1 ,... 

The most common hydrogen halides are HF (U.S. production = 3 X 108 kg/yr) and HC1 (3 X 109 kg/yr). They are most familiar as water solutions, referred to as hydrofluoric acid and hydrochloric acid, respectively. Recall (Chapter 13) that hydrofluoric acid is weak, incompletely dissociated in water, whereas HCl is a strong acid. 

FIGURE 2.18 The bond dissociation energies of the hydrogen halide molecules in kilojoules per mole of molecules. Note how the bonds weaken as the halogen atom becomes larger. 

In order to elucidate the causes of the increased stability of the hydrolyzed cluster ions compared with the unhydrolyzed ions, further studies were made of the behaviour of [Te2X8]3 (where X = Cl,Br, or I) in solutions of hydrogen halides [43,52,80,87]. The studies were performed mainly in relation to the most stable and most readily synthesized [Tc2C18]3- ion (Fig. la) kinetic methods with optical recording were employed. The identity of the reaction products was in most cases confirmed by their isolation in the solid phase. The studies showed that the stability of the [Tc2X8]3 ions (where X = Cl, Br, or I) in aqueous solutions is determined by the sum of competing processes acid hydrolysis complex formation with subsequent disproportionation and dissociation of the M-M bonds, and oxidative addition of atmospheric oxygen to the Tc-Tc multiple bond. 

Hydrogen halides will easily add to unsaturated compounds under radiolysis or photolysis. The free-radical chain reaction process is initiated by the dissociation of the halide or by the radiolytic production of radicals from the halide or the organic compound. Thus, for the radiolysis of a mixture of HBr and ethene the postulated initiation is... 

All of the hydrogen halides are very soluble in water, and acidic solutions result. Although HF is a weak acid, the others are strong and are almost completely dissociated in dilute solutions. HC1, HBr, and HI form constant-boiling mixtures with water that contain 20.2%, 47.6%, and 53% of the acid, respectively. 

These equilibrium constants vary with molarity of the HF solution. Measured values corrected for zero ionic strength at 25 °C are = 6.71 x 10 4 mol 1, K2=3.86 1 mol-1, and K3=2.71mor1 [BrlO, Iul, Wall], implying a dissociation of only a few percent. This unusual behavior is still controversial and has been attributed to the greater strength of the H-F bond compared to the other hydrogen halides [Pal], to the presence of the dimer (HF)2 [Wal], or to polymers that may... 

Hydrogen fluoride in aqueous solution is a weak acid, characterized by its pKa value of 3.2. By comparison, the other hydrogen halides are extremely strong acids in aqueous solution all three are fully dissociated in dilute solution, and their pA", values may be estimated by thermochemical cycle calculations. The thermochemical cycle shown in Figure 3.1 represents the various processes as the aqueous hydrogen halide, HX, is converted to a solution containing hydrated protons and hydrated halide ions. The enthalpy of acid dissociation of the HX(aq) compound is given by ... 

Table III.—Percentage Dissociation of the Hydrogen Halides-at Different...

Fig. 17.12 Bond dissociation energies and bond lengths of the hydrogen halides, methyl halides, and halogen molecules. Note that this figure, which is taken directly from Politzcr s work, portrays in a different way relationships that are closely rebled to Fig. 9.7. (From Politzer. P. J. Am. Chetn. See. 1969. 91.6235. Reproduced with permission.)...

Another variation of the palladium-catalyzed carbonylation reaction occurs when hydrogen is added rather than an alcohol or a primary or secondary amine. This variation leads to aldehyde formation the hydrogen reduces the acylpalladium intermediate to aldehyde and metal hydride (76). A basic tertiary amine is also added as in the ester-forming reaction to neutralize the hydrogen halide formed in the dissociation of the hydride ... 

A number of reactivity studies have been performed on 6 and 8 and indicate a strongly polar (if not ionic) Mn—E bond Mn "—E,+ (E = In, Tl). Thus heterolytic bond dissociation occurs in polar ligating solvents such as MeCN or DMF, and halogens, hydrogen halides, and alkyl halides readily add across the metal-metal bond in a manner consistent with the polarity described above (13,13a,18). In the thallium example, however, the reactions are generally more complicated and result in T1(I) salts [e.g., Eq. (3)], and metal exchange reactions are also more facile, e.g., the synthesis of 6 from 8 and indium metal. In general, therefore, the chemistry of 6 and 8 is consistent with predominantly ionic behavior. 

Most known chemical lasers oscillate on vibration-rotation transitions of a hydrogen halide. The first such laser was driven by the flash initiated explosion of H2 + C12 mixtures287. Here the flash dissociates the Cl2 to start the chain decomposition, and the population inversion is due to the subsequent reactions... 

In intramolecular arylations, a new bond is created between two aromatic moieties of the same molecule or between an aromatic nucleus and an atom of a side-chain. Many intramolecular arylation reactions of homocyclic and heterocyclic aromatic halides have been studied mainly in view of their synthetic applications, and it is not always clear which mechanistic pathway is followed. The reaction may start with homolytic or heterolytic dissociation of the carbon-halogen bond and proceed by attack of the aryl radical or aryl cation on another part of the molecule. Electrocyclization followed by elimination of hydrogen halide is another possibility. Especially when heteroatoms such as nitrogen, sulphur or phosphorus are involved, the initial step may be a nucleophilic attack on the carbon atom bearing the halide atom. 

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