Halides, hydrogen

Hydrogen Halides.—The X-ray photoemission spectrum of the F Is region of gaseous HF has been measured, using A1 Ka 2 radiation.The spectrum was interpreted in terms of a many-electron theory with configurational interaction. The threshold photoelectron spectra of HF and DF have been reported and a number of vibronic levels of the HF and DF ions have been detected.  

A theoretical calculation of the force field of (HF)2 has been performed in the light of the structure previously reported for this dimer. Molecular orbital calculations of the structure of solid HF are not in particularly good agreement with that observed experimentally at 4 

The heat of neutralization of aqueous hydrofluoric acid has been remeasured and, hence, the value of AHf of the fluoride ion was calculated to be -80.03 0.08 kcalmor Giguere has argued that the anomalously low acidity of aqueous HF cannot be adequately accounted for by the high H—F bond strength and 

The liquid-vapour equilibrium in the HF-H2SO4 system has been investigated by a dynamic method over the temperature range 30—150 The solubility of 

The electrochemical behaviour of metals in anhydrous HF has been reviewed by Vijh, with particular attention to anodization, open-circuit corrosion, film formation, anodic dissolution, and evolution of F2. The dependence of the F2 overpotential at Ni in anhydrous HF on the current density has been investigated. At low current densities the overvoltage was mainly due to the potential difference across the anodic barrier film, whereas at high current density the electronic conduction of the film increased appreciably, resulting in a decrease in the potential drop. Other workers have shown that the process of H2 discharge in HF is affected by the addition of NaF, presumably by reducing the overvoltage on nickel. 

On the whole hydrogen halides are poor initiators of cationic polymerisation, except in the case of very nucleophilic olefins. The well-known order of acidity HI HBr HCl HF applies equally well in water and in many non-aqueous solvents, but of course 

7 and 10.8 for HI, HBr and HCl respectively One would expect these values to be even higher in such nonpolar solvents as aromatic and aliphatic hydrocarbons or carbon tetrachloride. In more polar media the reverse is tme tiius, in acetonitrile pK(HBr) = 

5 and pK(HCl) = 8.9 and in nitromethane pK(HCl) = 6.22 Other data are available for a variety of polar solvents Since all the evidence indicates that these acids do not show any tendency to molecular association in these media, it cam be concluded that in a cationic polymerisation system involving one of these initiators the predominant species will be the monomeric molecular acid. The weak dissociation must be viewed in two alternative schemes, one not involving the solvent (i.e., when the latter cannot accept a proton), 

The homo- and heteroconjugation of halide anions with hydrogen halides is well known and virtually complete in chlorinated hydrocarbons and nitromethane as shown by NMR spectroscopy. Thus, the presence of halide ions in a system containing one of these acids in such solvents, and most probably in less polar ones, produces the sequestration of an equivalent amount of acid. Higher homoconjugated aggregates have recently been reported in the case of HCl in propylene carbonate  

Aqueous Solutions of Other Inorganic Halides 5,1,4.1. Hydrogen Halides 

Schay [82] has reported on the reactions of both sodium and potassium with the hydrogen halides using the nozzle flame method. The rate coefficients increased for both metals in the order from chloride to bromide to iodide. A similar result was obtained [83] using the diffusion flame technique. The most accurate measurements [84] obtained by the diffusion flame apparatus give rate coefficients for reaction with sodium at 511°K of 4.1 X 10 (HCl) and 3.4 x 10 (DCl) cm mole sec. The mechanism of the reaction has been discussed [53] in terms of the energy surfaces involved and of the zero point energy contribution to the activation energy. 

The strengths of different acids in aqueous solutions tend to be discussed in elementary textbooks on a qualitative basis. In the case of the hydrogen halides, an exact treatment in terms of independently measurable thermod5mamic quantities is almost possible. Consider the dissociation of HX (X is F, Cl, Br or I) in aqueous solution (equilibrium 

Step (1) causes some experimental difficulty. It is the reverse of the dissolution of gaseous HX in water to form solvated undissociated HX. Since HCl, HBr and HI are 

CHEMICAL AND THEORETICAL BACKGROUND Box 7.2 Systematic oxoacid nomenclature 

In 2005, the lUPAC published a set of new guidelines for the systematic naming of inorganic acids and their derivatives. Many inorganic oxoacids possess non-systematic (trivial) names that are in everyday use, and the lUPAC recognizes that it is unrealistic to abandon names such as sulfuric acid, nitric acid, phosphoric acid, boric acid and perchloric acid. However, these names provide no information about composition and structure. 

The method of giving a systematic name to an inorganic oxoacid uses an additive name. This shows the connectivity of the central atom, as well as the groups attached to that central atom. The structure of a molecule of sulfuric acid is shown below  

The formula is usually written as H2SO4, but S02(0H)2 gives more information. This way of writing the formula tells you 

The factors that influence the degree of dissociation are summarized in Fig. 7.3. Equation 7.26 relates for the dissociation of HX in aqueous solution to AG°, and the latter depends on changes in both enthalpy and entropy (eq. 7.27). 

Step (1) causes some experimental difficulty. It is the reverse of the dissolution of gaseous HX in water to form solvated undissociated HX. Since HCl, HBr and HI are essentially fuUy dissociated in aqueous solution, measurement of enthalpy or entropy changes for step (1) must be estimated from somewhat unsatisfactory comparisons with noble gases and methyl halides. For HF, which is a weak acid in dilute aqueous solution, it might appear that values of AH° and AX° for step (1) could be obtained directly. However, IR spectroscopic data indicate that the species present in solution is the strongly hydrogen-bonded ion-pair F HOH2.  

We shall focus mainly on the conclusions drawn from calculations using the cycle in Fig. 7.3. Firstly, consider 

Menard, J. P. Masson, J. Devynck, and B. Tremillon, J. Electroanalyt. Chem. Interfacial 

Ionic transference numbers in liquid mixtures of NH3 and HF containing 80.85 wt % HF have been measured at room temperature the results for NH4 

Agranat, M. Rabinovitz, and H. Selig, Inorg. Nuclear Chem. Letters, 1975, 11, 185. 

Nikolaev et a/. have reviewed the thermodynamic properties of the alkali-metal and alkaline-earth-metal hydrofluorides. Boinon et aC have reinvestigated the NaF-HF system for 30—45 wt. % NaF and discussed their results in the light of earlier work. Examination of the 10 C isotherm of the RbF-HF-H2O system confirms the existence of a number of hydrates and hydrofluoride phases, and provides evidence for the formation of a new solvate RbF,4HF.  

The liquid-vapour equilibrium in the HF-H2SO4-H2O system at 22 °C has been determined by a dynamic method for the range of pressure 7.0—348.2 Torr. The extraction of HF into di-isoamyl methylphosphonate from the HF-HCl-HjO system increases as the HF content rises on the other hand, the extraction of HCl decreases as the HF content rises. 

The saturated vapour pressure of HF has been redetermined for the temperature range 273—303 K 167 this leads to a calculated boiling point of 292.90 K, at which temperature the association factor is estimated to be 3.75. Raman scattering by monomeric HF in the gaseous state and at low concentration in liquid SF6 has been studied by Le Duff and Holzer 168a Birnbaum has confirmed that rotational fine structure is evident in the far-i.r. spectrum in solution in SF6.1681 The Raman work also yielded some results on the HF polymer bands at 2900—3800 cm 1 these were said to be consistent with the presence of hexameric and tetrameric species.1680 The mean amplitudes of vibration of the cyclic hexamer (HF)6 have been calculated169 and the results compared with those from electron diffraction. 

Rozovskii, Z. Poskute, A. Prokopcikas, and P. Norkus, Zhur. neorg. Khim., 1973, 18, 

Inorganic Chemistry of the Main-group Elements as some enthalpies of dilution have been carried out in a reaction calorimeter.170 These results have been combined with some earlier data to obtain the enthalpy of solution of HF as a function of composition, between HF,ooH20 and HF,H20. Vasil ev and Kozlovskii171 have also determined some heats of dilution of aqueous HF calorimetrically as well as heats of neutralization, H++F — HF, and reaction, H+ + 2F — HF2. There are also some new values of the equilibrium constants for these two reactions.172 Russian workers have also investigated the same equilibria, with aqueous dioxan as solvent.173 Vaillant et al. have redetermined the acidity function for the HF-H20 system for a range of composition.174 Russian workers have reported solubility data for MF3 (M = Eu, Tb, Dy, or Ho)175 and some hexafluorostannates176 in aqueous HF. 

A patent from Uranit GmbH implies that the perfluoroalkylamines 

175 Sh. A. Abdukarimova, N. S. Nikolaev, and Sh. Dzhuraev, Izvest. Akad. Nauk Tadzh. S.S.R., Otdel. Fiz. Mat. Geol.-Khim. Nauk, 1973, 57. 

The enthalpy of solution of a-quartz (or low quartz) in aqueous hydrofluoric acid has been determined under standard conditions in order to provide a reference standard for HF solution calorimetry. ICresge and Chiang have investigated solvent (H2O and DgO) isotope effects on the 

The surface tension of the NH3-HF system has been measured as a function of the temperature and of the composition in the range of composition 78— 

86 weight % Using an accurate transpiration method, no reaction 

Ionic conduction has been shown to occur in MHFg (M = K, Rb, or Cs) in both the a (tetragonal) and the / (cubic) phases. Plots of the zero-frequency conductivities versus T show discontinuities at the a-/S transition temperatures moreover, the gradients for the phases are all essentially the same (A 20 kcal), whereas there are considerable differences between the 

HBr is known to inhibit combustion of hydrogen and hydrocarbon fuels and it is generally accepted that the inhibition results from reactions of HBr with the chain centres which are important for flame propagation, e.g. H and OH- radicals. Takacs and Glass have now investigated the absolute 

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Gattermann s reaction A variation of the Sandmeyer reaction copper powder and hydrogen halide are allowed to react with the diazonium salt solution and halogen is introduced into the aromatic nucleus in place of an amino group. 

MarkownikofT s rule The rule states that in the addition of hydrogen halides to an ethyl-enic double bond, the halogen attaches itself to the carbon atom united to the smaller number of hydrogen atoms. The rule may generally be relied on to predict the major product of such an addition and may be easily understood by considering the relative stabilities of the alternative carbenium ions produced by protonation of the alkene in some cases some of the alternative compound is formed. The rule usually breaks down for hydrogen bromide addition reactions if traces of peroxides are present (anti-MarkownikofT addition). 

Stephenson J C, Finzi J and Moore C B 1972 Vibration-vibration energy transfer in C02-hydrogen halide mixtures J. Chem. Phys. 56 5214-21... 

The unequal distribution of charge produced when elements of different electronegativities combine causes a polarity of the covalent bond joining them and, unless this polarity is balanced by an equal and opposite polarity, the molecule will be a dipole and have a dipole moment (for example, a hydrogen halide). Carbon tetrachloride is one of a relatively few examples in which a strong polarity does not result in a molecular dipole. It has a tetrahedral configuration... 

For the formation of the hydrogen halides by the direct combination of the elements, the enthalpies of formation are ... 

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... 

Numerous ionic compounds with halogens are known but a noble gas configuration can also be achieved by the formation of a covalent bond, for example in halogen molecules, X2, and hydrogen halides, HX. When the fluorine atom acquires one additional electron the second quantum level is completed, and further gain of electrons is not energetically possible under normal circumstances, i.e... 

The dipole moments of the hydrogen halides decrease with increasing atomic number of the hydrogen, the largest difference occurring between HF and HCl, and association of molecules is not an important factor in the properties of FICl, HBr and HI. This change in dipole moment is reflected in the diminishing permittivity (dielectric constant) values from HF to HI. 

All the hydrogen halides are freely soluble in water and react according to the general equation... 

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 ... 

Anhydrous halides, however, are obtained when the metal is heated with the dry hydrogen halide or the halogen. In the case of elements with more than one oxidation state, the hydrogen halide produces a lower halide and the halogen a higher halide, for example... 

A halogen atom directly attached to a benzene ring is usually unreactive, unless it is activated by the nature and position of certain other substituent groups. It has been show n by Ullmann, however, that halogen atoms normally of low reactivity will condense with aromatic amines in the presence of an alkali carbonate (to absorb the hydrogen halide formed) and a trace of copper powder or oxide to act as a catalyst. This reaction, known as the Ullmant Condensation, is frequently used to prepare substituted diphenylamines it is exemplified... 

As well as the cr-complexes discussed above, aromatic molecules combine with such compounds as quinones, polynitro-aromatics and tetra-cyanoethylene to give more loosely bound structures called charge-transfer complexes. Closely related to these, but usually known as Tt-complexes, are the associations formed by aromatic compounds and halogens, hydrogen halides, silver ions and other electrophiles. 

The majority of preparative methods which have been used for obtaining cyclopropane derivatives involve carbene addition to an olefmic bond, if acetylenes are used in the reaction, cyclopropenes are obtained. Heteroatom-substituted or vinyl cydopropanes come from alkenyl bromides or enol acetates (A. de Meijere, 1979 E. J. Corey, 1975 B E. Wenkert, 1970 A). The carbenes needed for cyclopropane syntheses can be obtained in situ by a-elimination of hydrogen halides with strong bases (R. Kdstcr, 1971 E.J. Corey, 1975 B), by copper catalyzed decomposition of diazo compounds (E. Wenkert, 1970 A S.D. Burke, 1979 N.J. Turro, 1966), or by reductive elimination of iodine from gem-diiodides (J. Nishimura, 1969 D. Wen-disch, 1971 J.M. Denis, 1972 H.E. Simmons, 1973 C. Girard, 1974),... 

The carbonylation of some alkyl halides such as iodocyclohexane (911) can be carried out under neutral conditions in the presence of N,N,N.N-tetre,-methylurea (TMU), which is a neutral compound, but catches generated hydrogen halide. Molecular sieves (MS-4A) are used for the same pur-pose[768]. Very reactive ethyl 3-iodobutyrate (912) is carbonylated to give ethyl methylsuccinate (913) in the presence of TMU. The expected elimination of HI to form crotonate, followed by carbonylation, does not occur. 

Table 1 3 lists the dipole moments of various bond types For H—F H—Cl H—Br and H—I these bond dipoles are really molecular dipole moments A polar molecule has a dipole moment a nonpolar one does not Thus all of the hydrogen halides are polar molecules To be polar a molecule must have polar bonds but can t have a shape that causes all the individual bond dipoles to cancel We will have more to say about this m Section 1 11 after we have developed a feeling for the three dimensional shapes of molecules... 

Bond Strength The effect of bond strength is easy to see by comparing the acidities of the hydrogen halides... 

The major portion of the present chapter concerns the conversion of alcohols to alkyl halides by reaction with hydrogen halides... 

Preparation of Alkyl Halides from Alcohols and Hydrogen Halides... 

PREPARATION OF ALKYL HALIDES FROM ALCOHOLS AND HYDROGEN HALIDES... 

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