Hydrogen halides polarity

The Ar-HF molecule serves as both a prototype for inert gas atoms clustering or solvating around a hydrogen-halide polar center and as a simple test-bed for refining and testing how we create anharmonic potential energy surfaces. Specifically, both the Ar-HF and the Ar-Ar potential energy surfaces are extremely well characterized and have been verified extensively... 

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... 

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... 

In many addition reactions the attacking reagent, unlike H2, is a polar- molecule. Hydrogen halides are anong the simplest exanples of polar- substances that add to alkenes. 

Dipole moments depend on bond polarities. For example, the trend in dipole moments for the hydrogen halides follows the trend in electronegativity differences the more polar the bond (indicated by Ax), the larger the molecular polarity (indicated by the dipole moment, fi ... 

Of course, HF is actually a polar covalent molecule, but from the extent of the polarity, it behaves as if it were composed of the two structures shown above. A similar analysis can be carried out for all of the hydrogen halides, and the results are shown in Table 3.2. 

The properties of HF reflect the strong hydrogen bonding that persists even in the vapor state. As a result of its high polarity and dielectric constant, liquid HF dissolves many ionic compounds. Some of the chemistry of HF as a nonaqueous solvent has been presented in Chapter 10. Properties of the hydrogen halides are summarized in Table 15.9. 

It is important to be able to look at a molecular structure and deduce the possible reactions it can undergo. Take an alkene, for example. It has a 7t bond that makes it electron-rich and able to attack electrophiles such as water, halogens and hydrogen halides in electrophilic addition reactions. Haloalkanes, on the other hand, contain polar carbon-halogen bonds because the halogen is more electronegative than carbon. This makes them susceptible to attack by nucleophiles, such as hydroxide, cyanide and alkoxide ions, in nucleophilic substitution reactions. 

Addition Reactions. In general, polar molecules such as hydrogen halides add across the B—N bonds, the more electronegative group bonding to boron (91). The adducts are cydotriborazanes such as the product formed by reaction of B-trichloroborazine and hydrogen chloride (eq. 35). X-ray crystal analysis shows the structure exists in a chair conformation (124). 

The action of hydrogen halides on misoeUaiieocs other ethylene oxides containing polar atoms in the vicinity of the epoxide ring ha been described. Among substances examined in this connexion... 

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. 

Dipo/e Moment The evidence comes from an examination of the dielectric constant of the hydrogen halides. In an electric field, say between the plates of a condenser, molecules that have a charge separation within them will tend to orient themselves with the electric field. Such molecules behave like electric dipoles (Fig. 4.3) and are called polar molecules. The extent of orientation is reflected by a change in the dielectric... 

Tetraalkylammonium fluoride (R4N 1 F ) is well known as being highly receptive to protic compounds such as hydrogen halides and water, affording non-stoichiometric hydrogen-bonded adducts, R N F (HY) , in non-polar solvents. This property reasonably accounts for the hygroscopic nature of ammonium fluorides. However, under strictly anhydrous conditions, intramolecular interactions are predominant and result in self-destruction of the tetraalkylammonium cation via Hoffman elimination to furnish tetraalkylammonium bifluoride, trialkylamine, and olefin (Scheme 9.12) [22]. Consequently, the resulting tetraalkylammonium bifluoride,... 

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