Hydrogen fluoride covalent bonding

Representing a two electron covalent bond by a dash (—) the Lewis structures for hydrogen fluoride fluorine methane and carbon tetrafluoride become... 

Writing the equation in the usual way directs too much attention to the atoms and not enough to the electrons We can remedy that by deleting any spec tator ions and by showing the unshared electron pairs and covalent bonds that are made and broken Both sodium hydroxide and sodium fluoride are com pletely ionized in water therefore Na" which ap pears on both sides of the equation is a spectator ion Hydrogen fluoride is a weak acid and exists as undissociated HF molecules in water... 

One of the unshared pairs of the hydroxide oxygen is used to form a covalent bond to the positively polar ized proton of hydrogen fluoride The covalent bond betwen H and F in hydrogen fluoride breaks with the pair of electrons in this bond becoming an unshared pair of fluoride ion... 

Chemists refer to the bond in a molecule like sodium chloride as ionic , meaning that its electron pair resides entirely on chlorine. At the other extreme is the covalent bond in the hydrogen molecule, where the electron pair is shared equally between the two hydrogens. Intermediate cases, such as the bond in hydrogen fluoride which is clearly polarized toward fluorine, are generally referred to as polar covalent bonds (rather than partially ionic bonds). Are these situations really all different or do they instead represent different degrees of the same thing ... 

Two structures are possible for the interaction of aromatic hydrocarbons with acids.270 In the a-structures a covalent bond is established between the acidic reagent and a particular carbon atom of the benzene ring. The a-structures are essentially classical carbonium ions. In the -structures a non-classical bond is established, not to any particular atom, but to the -electron cloud in general. It is quite likely that both types of structure are represented by actual examples. Thus m-xylene interacts more strongly with hydrogen chloride than does o-xylene, but the difference between the two hydrocarbons is much more pronounced when their interactions with a boron trifluoride-hydrogen fluoride mixture are compared. This is readily understandable... 

The covalent bond that holds molecules of hydrogen, fluorine, and hydrogen fluoride together is a single bond. It involves a single bonding pair of electrons. Some molecules are bonded together with two shared pairs of electrons. These are called double bonds. Carbon dioxide is an example of a covalent molecule that consists of double bonds. 

Although the gas-phase hydrogen-bonded dimer (MeF)2H" is held by a strong hydrogen bond (McMahon and Kebarle, 1986) this is a rare exception to the previous statement regarding covalently bonded fluoride. More typical are the perfluorocarbons, which are among the weakest hydrogenbonding substances known, as their physical properties and uses clearly demonstrate. 

Many substances contain bonds that are intermediate in character— between pure covalent and pure ionic bonds. Such polar bonds occur when one of the elements attracts the shared electrons more strongly than the other element. In hydrogen fluoride, for instance, the shared electrons are so much more attracted by fluorine than hydrogen that the sharing is unequal. (See Figure 5-11.)... 

Eormula HE MW 20.006. A very stable polar covalent diatomic molecule H—E bond energy 136.1 kcaPmol at lower temperatures molecules are associated by hydrogen bonding H—bond length 0.92A partial ionic character 40% dipole moment 6.10 D hydrofluoric acid is an aqueous solution of hydrogen fluoride gas. 

Hydrogen fluoride (HF), a molecule that contains only two atoms linked by a covalent bond, exhibits heteronuclear spin/spin coupling. For example, in the absence of coupling between the nuclei, a single signal would be observed in the ]H NMR spectrum of HF because there is only one hydrogen atom. However, when the experiment is conducted, two signals of equal intensity are observed. 

The discussion of hydrogen fluoride in this paragraph is a little different from that in the first two editions of this book. In the first two editions the calculated energy curve for the extreme ionic structure was shown as falling below that for the normal covalent structure, and the conclusion was reached that the bond between the hydrogen atom and the fluorine atom in the molecule... 

In all of the molecules discussed above the bond is intermediate between theeovalent extreme M X and the ionic extreme M+X , varying from an essentially covalent bond with only a small amount of ionic character (hydrogen iodide), through a bond with about equal amounts of covalent and ionic character (hydrogen fluoride), to an essentially ionic bond with only a small amount of covalent character (cesium chloride). 

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