Other Hydrogen Halides

The hydrogen-halide lasers to be considered here besides HF are hydrogen chloride and hydrogen bromide and the corresponding deutero compounds. HC1 lasers are normally generated according to the following scheme  

Further details of this reaction are discussed in Sextion 9. An important alternative reaction used for pumping is that of a Cl atom with HI. 

Hydrogen bromide formation may be described in an equally global fashion by 

H2 + Cl2 / hv First report of a chem. laser, empirical investigation of emission details Kasper, Pimentel 193  

Ha(HD) + Clzlhv Spectroscopy, investigation of pumping processes, discussion of detailes rate constants Coraeil, Pimentel 

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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. 

Hydrogen fluoride also effects replacement reactions in organic compounds. For example, carbon tetrachloride yields a mixture of chlorofluoromethanes CCI3F, CCI2F2 and so on. Like all the other hydrogen halides, hydrogen fluoride adds on to olefins, for example ... 

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 ,... 

Although it is difficult to predict exactly which solute molecules will form clathrate solutions in any given host lattice, the general principle is quite clear. All molecules which fit into the cavities will be able to stabilize the host lattice, unless they show a specific chemical interaction with the solvent molecules. HC1 (or the other hydrogen halides), for instance, does not form a clathrate with water, but rather the stoichiometric compounds HC1 H20,... 

Similar mechanisms of course can be written for the reaction with the other hydrogen halide acids, and indeed for the relatively slow decomposition of N-chloroacetanilide in solutions of other strong acids15, viz. 

Similar reactions are likely to occur with the other hydrogen halides. 

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 iodide adds to double bond of alkenes undergoing Markovnikov addition. The reaction is faster than that with other hydrogen halides ... 

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 ... 

All thiopyrans not stabilized by electron-withdrawing substituents tend to disproportionate, especially in acid. This process is accelerated by hydrogen chloride,93,373 376 other hydrogen halides,93,360 a HCl-FeCl3 reagent,22,377... 

It is significant that no other hydrogen halides add to alkenes, contrary to the Markovnikov rule, even in the presence of free radicals. This appears to be due to the difficulty with which other halogen atoms, particularly fluorine and chlorine, are produced by reaction of the hydrogen halides with radicals ... 

The ionic addition of hydrogen bromide was discussed in Section 10-4 and will not be considered further at this point. Two questions with regard to the so-called abnormal addition will be given special attention. Why does the radical mechanism give a product of different structure than the ionic addition Why does the radical addition occur readily with hydrogen bromide but rarely with the other hydrogen halides (See Exercise 10-25.)... 

In this case, as with all other hydrogen halide lasers, only P branch transitions are observed, indicating that only partial inversion is attained. The vibrational transitions observed are 1 - 0 and 2 -+ 1. There is a definite threshold flash energy, below which no laser action is observed because the chain decomposition is not fast enough. The development in time of the emission spectrum was observed and discussed in terms of rotational relaxation. 

There are many examples of reactions where the proton adds to the less substituted carbon atom of the double bond in order to produce the more substituted carbocation. The addition of HBr (and other hydrogen halides) is said to be regioselective because in each case, one of the two possible orientations of addition results preferentially over the other. 

Liquid HF is miscible with H20 in all proportions. The other hydrogen halides are very soluble and form constant boiling mixtures with water. Therefore, constant boiling HC1 has been used as a primary standard for analytical purposes. At 760 torr, the compositions and boiling points of the constant boiling mixtures are as follows ... 

These types of processes are not, of course, of any consequence for the other hydrogen halides but rather are the result of the stability of the HF2 ion. The most important chemical property of the hydrogen halides is, nevertheless, their acidity. 

The other hydrogen halides add oxidatively to rhodium(I) complexes of ditertiary phosphines or arsines giving rise to numerous monohydrido complexes, whose physical properties are also listed in Table 79. However, it is possible to prepare certain monohydrido complexes from rhodium(III) halides. One interesting reaction, carried out under an atmosphere of CO, gives rise to dicar-bonyldichlororhodate(I) salts (equation 241).226... 

The course of reaction of alkenes with hydrogen fluoride occasionally differs from that with other hydrogen halides because of the strong bond between hydrogen and fluoride. The reaction products are monofluorides with Markovnikov orientation or polymers. ... 

So far we have two conflicting factors The large HF bond energy tends to make HF a weaker acid than the other hydrogen halides, but the enthalpy... 

The halide ion is believed to be associated with other hydrogen halide molecules, probably as a cluster averaging a composition near X(HX)3 [B3]-... 

The e.d. data for HF vapour are apparently best accounted for by a mixture of monomers and puckered (HF)g rings, with mean angle F—F—F, 104°, and F—H-F, 2-53 A. The behaviour of the other hydrogen halides in the crystalline state is complex, and careful n.d. studies have given the following results. ... 

To understand why HF is so much weaker an acid than the other hydrogen halides, let us consider the following factors. 

Carbene formation was mentioned in an earlier section. This elimination of HCl from 4-chlorophenol or elimination of other hydrogen halides from halophenols could have been inferred from earlier photochemical studies on this and other derivatives. Boule and his coworkers irradiated 4-chlorophenol under deoxygenated conditions and obtained the corresponding quinhydrone and the 2,4 -dihydroxy-5-chlorobiphenyl °. Other research demonstrated that its irradiation in neutral aqueous solutions gave the corresponding quinone " and also that de-aeration did not seem to affect the reaction. ... 

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