Polar Covalent Bonds Dipole Moments

Just as individual bonds are often polar, molecules as a whole are often polar also. Molecular polarity results from the vector summation of all individual bond polarities and lone-pair contributions in the molecule. As a practical matter, strongly polar substances are often soluble in polar solvents like water, whereas nonpolar substances are insoluble in water. 

Net molecular polarity is measured by a quantity called the dipole moment and can be thought of in the fol low ing way assume that there is a center of mass of all positive charges (nuclei) in a molecule and a center of mass of all negative charges (electrons). If these two centers don t coincide, then the molecule has a net polarity. 

The dipole moment, /x (Greek mu), is defined as the magnitude of the charge Q at either end of the molecular dipole times the distance r between the charges, = Q x r. Dipole moments are expressed in debyes (D), where 1 1) = 3.336 X coulomb meter (C m) in SI units. For example, the unit 

Compound Dipole moment (D) Compound Dipole moment (D) 

In contrast with water, methanol, ammonia, and other substances in Table 2.1, carbon dioxide, methane, ethane, and benzene have zero dipole moments. Because of the symmetrical structures of these molecules, the individual bond polarities and lone-pair contributions exactly cancel. 

Make a three-dimensional drawing of methylainine, CH3NH2, a substance responsible for the odor of rotting fish, and show the direction of its dipole moment (/Lt = 1.31). 

Look for any lone-pair electifMis, and identify any atom with an electronegativity substantially different from that of carbon (Usually, this means O, X, F. Cl, or Br.) Electron density will be displaced in the general direction of the electronegative atoms and the lone pairs. 

Look at the following electrostatic potential map of chloromethane, and tell the direction of polarization of the C-CI bond  

Dipole moments for some common substances are given in Table 2.1. Of the compounds shown in the table, sodium chloride has the largest dipole moment (9.00 D) because it is ionic. Even small molecules like water (p. = 1.85 D), methanol (CH3OH /x = 1.70 D), and ammonia (/x = 1.47 D), have substantial dipole moments, however, both because they contain strongly 

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When two atoms of differing electronegativites are bonded, one end of the bond will be 5+ and the other will be 5—. This analysis leads to the notion of a bond dipole as the local moment that is associated with a polar covalent bond. A moment reflects the electrostatic force that would be exerted by a charge on a neighboring charge. The dipole moment pro-... 

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... 

Dipole moment (n ), 38 halomethanes, 335 polar covalent bonds and, 38-39 table of, 39... 

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. 

In Section 2.12, we saw that a polar covalent bond in which electrons are not evenly distributed has a nonzero dipole moment. A polar molecule is a molecule with a nonzero dipole moment. All diatomic molecules are polar if their bonds are polar. An HC1 molecule, with its polar covalent bond (8+H—Clfi ), is a polar molecule. Its dipole moment of 1.1 D is typical of polar diatomic molecules (Table 3.1). All diatomic molecules that are composed of atoms of different elements are at least slightly polar. A nonpolar molecule is a molecule that has no electric dipole moment. All homonuclear diatomic molecules, diatomic molecules containing atoms of only one element, such as 02, N2, and Cl2, are nonpolar, because their bonds are nonpolar. 

We met the concepts of a polar covalent bond and electric dipole moment in Section 2.14. 

Before looking at the forces between molecules, it s first necessary to develop the ideas of bond dipoles and dipole moments. We saw in Section 7.4 that polar covalent bonds form between atoms of different electronegativity. Chlorine is more electronegative than carbon, for example, and the chlorine atom in chloromethane (CH3C1) thus attracts the electrons in the C C1 bond toward itself. The C-Cl bond is therefore polarized so that the chlorine atom is slightly electron-rich (8—) and the carbon atom is slightly electron-poor (<5+). 

In contrast with water and ammonia, carbon dioxide and tetrachloromethane (CCI4) have zero dipole moments. Molecules of both substances contain individual polar covalent bonds, but because of the symmetry of their structures, the individual bond polarities exactly cancel. 

The presence of polar covalent bonds in a molecule can cause the molecule as a whole to have a net polarity, a property measured by the dipole moment. 

According to Mechanism 1, the ion-dipole interaction proposed by Shah and Schulman (II), Ca++ binds to the oxygen of a polarized P-O bond in the structure of the phosphorylcholine zwitterion. In spite of the speculative claims of Shah and Schulman (II) and the theoretical affirmations of Gillespie (20), this mechanism must be rejected on the basis of the theoretical arguments presented here and elsewhere (2,5,6). In brief, if such a bond existed (and the evidence (IR or NMR) is not available), it should not bear any direct relation to AV and surface dipole moments, which are generated only by ionized species and not by silent ion pairs nor by partial charges of polarized covalent bonds (2,5). 

How well can we tell the difference between an ionic bond and a polar covalent bond The only honest answer to this question is that there are probably no totally ionic bonds between discrete pairs of atoms. The evidence for this statement comes from calculations of the percent ionic character for the bonds of various binary compounds in the gas phase. These calculations are based on comparisons of the measured dipole moments for molecules of the type X—Y with the calculated dipole moments for the completely ionic case, X+Y. We performed a calculation of this type for HF in Section 13.3. The percent ionic character of a bond can be defined as... 

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