Partial charge covalent bonds

Carbon-oxygen and carbon-halogen bonds are polar covalent bonds and carbon bears a partial positive charge in alcohols ( " C—0 ) and in alkyl halides ( " C—X ) Alcohols and alkyl halides are polar molecules The dipole moments of methanol and chloromethane are very similar to each other and to water... 

First, let us consider the formation of ions from covalently bound species, i.e., the heterolytic cleavage of the covalent (or partially covalent) bond. Charge separation under the influence of the solvent generates an ion pair in a process called ionization this ion pair may then separate into free ions in a dissociation step (Eq. 8-18). 

The charges on the atoms in HCI are called partial charges. We show the partial charges on the atoms by writing 8+l I—Cl8. A bond in which ionic contributions to the resonance result in partial charges is called a polar covalent bond. All bonds between atoms of different elements are polar to some extent. The bonds in homonuclear (same element) diatomic molecules and ions are nonpolar. 

Electronegativity is a measure of the pulling power of an atom on the electrons in a bond. A polar covalent bond is a bond between two atoms with partial electric charges arising from their difference in electronegativity. The presence of partial charges gives rise to an electric dipole moment. 

Opinions differ on the nature of the metal-adsorbed anion bond for specific adsorption. In all probability, a covalent bond similar to that formed in salts of the given ion with the cation of the electrode metal is not formed. The behaviour of sulphide ions on an ideal polarized mercury electrode provides evidence for this conclusion. Sulphide ions are adsorbed far more strongly than halide ions. The electrocapillary quantities (interfacial tension, differential capacity) change discontinuously at the potential at which HgS is formed. Thus, the bond of specifically adsorbed sulphide to mercury is different in nature from that in the HgS salt. Some authors have suggested that specific adsorption is a result of partial charge transfer between the adsorbed ions and the electrode. 

Because the Cl atom is more electronegative than the H atom, the bond pair is pulled towards the Cl atom. This results in a covalent bond with some partial ionic character the small charges present are indicated by a 8. 

As shown in Fig. 5.36, / R and vy become equal near s 0.6, whereas the charge transfer is half-complete near s — 0.3, and the energetic TS is at 5 = 0. Different criteria therefore lead to different estimates of the progress of reaction, but by all criteria the shifts of partial charge and covalency are quite pronounced in the H-bonded complexes. The net charge transfer gH to the evolving hydride Lewis base is seen to parallel bow closely, as the resonance picture (5.68a) suggests. 

Thus, by virtue of the continuity of the bond-order-bond-length relationship across the entire proton-transfer region, the interpretation of the H-bonded complexes in terms of partial proton transfer (with associated charge and covalent-bond transfer) can hardly be avoided. (Additional discussion of the properties of transition-state species in relation to the associated reactant and product species will be presented in Section 5.4.)... 

At shorter distances, particularly those characteristic of H-bonded and other charge-transfer complexes, the concepts of partial covalency, resonance, and chemical forces must be extended to intramolecular species. In such cases the distinction between, e.g., the covalent bond and the H-bond may become completely arbitrary. The concept of supramolecular clusters as fundamental chemical units presents challenges both to theory and to standard methods of structural characterization. Fortunately, the quantal theory of donor-acceptor interactions follows parallel lines for intramolecular and intermolecular cases, allowing seamless description of molecular and supramolecular bonding in a unified conceptual framework. In this sense, supramolecular aggregation under ambient thermal conditions should be considered a true chemical phenomenon. 

There are several, separate types of interaction in III both covalent bonds and dipoles. Induced dipoles involve a partial charge, which we called <5+ or S, but, by contrast, covalent bonds involve whole numbers of electrons. A normal covalent bond, such as that between a hydrogen atom and one of the carbon atoms in the backbone of III, requires two electrons. A double bond consists simply of two covalent bonds, so four electrons are shared. Six electrons are incorporated in each of the rare instances of a covalent triple bond . A few quadruple bonds occur in organometallic chemistry, but we will ignore them here. 

For example, in the HC1 molecule, the partial charge on the hydrogen atom is positive and the partial charge on the chlorine atom is negative. Between neighboring HC1 molecules there is an attraction between the hydrogen and chlorine ends of the molecules. Dipole - dipole forces between HC1 molecules are much weaker than the covalent bond within the molecule. 

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