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2.2 Polar Covalent Bonds: Dipole Moments

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  • Page ID
    67053
  • Objectives

    After completing this section, you should be able to

    1. explain how dipole moments depend on both molecular shape and bond polarity.
    2. predict whether a molecule will possess a dipole moment, given only its molecular formula or Kekulé structure.
    3. use the presence or absence of a dipole moment as an aid to deducing the structure of a given compound.

    Key Terms

    Make certain that you can define, and use in context, the key term below.

    • dipole moment

    Study Notes

    You must be able to combine your knowledge of molecular shapes and bond polarities to determine whether or not a given compound will have a dipole moment. Conversely, the presence or absence of a dipole moment may also give an important clue to a compound’s structure. BCl3, for example, has no dipole moment, while NH3 does. This suggests that in BCl3 the chlorines around boron are in a trigonal planar arrangement, while the hydrogens around nitrogen in NH3 would have a less symmetrical arrangement (e.g., trigonal pyramidal, T-shaped). Remember that the $\ce{\sf{C-H}}$ bond can usually be assumed to be nonpolar.

    Molecular Dipole Moments

    You previously learned how to calculate the dipole moments of simple diatomic molecules. In more complex molecules with polar covalent bonds, the three-dimensional geometry and the compound’s symmetry determine whether there is a net dipole moment. Mathematically, dipole moments are vectors; they possess both a magnitude and a direction. The dipole moment of a molecule is therefore the vector sum of the dipole moments of the individual bonds in the molecule. If the individual bond dipole moments cancel one another, there is no net dipole moment. Such is the case for CO2, a linear molecule (part (a) in Figure 2.2.8). Each C–O bond in CO2 is polar, yet experiments show that the CO2 molecule has no dipole moment. Because the two C–O bond dipoles in CO2 are equal in magnitude and oriented at 180° to each other, they cancel. As a result, the CO2 molecule has no net dipole moment even though it has a substantial separation of charge. In contrast, the H2O molecule is not linear (part (b) in Figure 2.2.8); it is bent in three-dimensional space, so the dipole moments do not cancel each other. Thus a molecule such as H2O has a net dipole moment. We expect the concentration of negative charge to be on the oxygen, the more electronegative atom, and positive charge on the two hydrogens. This charge polarization allows H2O to hydrogen-bond to other polarized or charged species, including other water molecules.

     

    Figure 2.2.8 How Individual Bond Dipole Moments Are Added Together to Give an Overall Molecular Dipole Moment for Two Triatomic Molecules with Different Structures. (a) In CO2, the C–O bond dipoles are equal in magnitude but oriented in opposite directions (at 180°). Their vector sum is zero, so CO2 therefore has no net dipole. (b) In H2O, the O–H bond dipoles are also equal in magnitude, but they are oriented at 104.5° to each other. Hence the vector sum is not zero, and H2O has a net dipole moment.

    Other examples of molecules with polar bonds are shown in Figure 2.2.9. In molecular geometries that are highly symmetrical (most notably tetrahedral and square planar, trigonal bipyramidal, and octahedral), individual bond dipole moments completely cancel, and there is no net dipole moment. Although a molecule like CHCl3 is best described as tetrahedral, the atoms bonded to carbon are not identical. Consequently, the bond dipole moments cannot cancel one another, and the molecule has a dipole moment. Due to the arrangement of the bonds in molecules that have V-shaped, trigonal pyramidal, seesaw, T-shaped, and square pyramidal geometries, the bond dipole moments cannot cancel one another. Consequently, molecules with these geometries always have a nonzero dipole moment.

    Figure 2.2.9: Molecules with Polar Bonds. Individual bond dipole moments are indicated in red. Due to their different three-dimensional structures, some molecules with polar bonds have a net dipole moment (HCl, CH2O, NH3, and CHCl3), indicated in blue, whereas others do not because the bond dipole moments cancel (BCl3, CCl4, PF5, and SF6).

    Note

    Molecules with asymmetrical charge distributions have a net dipole momentEdit section

    Example 2.2.1

    Which molecule(s) has a net dipole moment?

    1. H2S
    2. NHF2
    3. BF3

    Given: three chemical compounds

    Asked for: net dipole moment

    Strategy:

    For each three-dimensional molecular geometry, predict whether the bond dipoles cancel. If they do not, then the molecule has a net dipole moment.

    Solution:

    1. The total number of electrons around the central atom, S, is eight, which gives four electron pairs. Two of these electron pairs are bonding pairs and two are lone pairs, so the molecular geometry of H2S is bent (Figure 2.2.6). The bond dipoles cannot cancel one another, so the molecule has a net dipole moment.

    2. Difluoroamine has a trigonal pyramidal molecular geometry. Because there is one hydrogen and two fluorines, and because of the lone pair of electrons on nitrogen, the molecule is not symmetrical, and the bond dipoles of NHF2 cannot cancel one another. This means that NHF2 has a net dipole moment. We expect polarization from the two fluorine atoms, the most electronegative atoms in the periodic table, to have a greater affect on the net dipole moment than polarization from the lone pair of electrons on nitrogen.

    3. The molecular geometry of BF3 is trigonal planar. Because all the B–F bonds are equal and the molecule is highly symmetrical, the dipoles cancel one another in three-dimensional space. Thus BF3 has a net dipole moment of zero:

    Exercise 2.2.1

    Which molecule(s) has a net dipole moment?

    1. CH3Cl
    2. SO3
    3. XeO3

    Answer: CH3Cl; XeO3

    In 1923, chemists Johannes Brønsted and Martin Lowry independently developed definitions of acids and bases based on compounds abilities to either donate or accept protons (H+ ions). Here, acids are defined as being able to donate protons in the form of hydrogen ions; whereas bases are defined as being able to accept protons. This took the Arrhenius definition one step further as water is no longer required to be present in the solution for acid and base reactions to occur.

    Exercises

    1. Determine whether each of the compounds listed below possesses a dipole moment. For the polar compounds, indicate the direction of the dipole moment.
      1. $\ce{\sf{O=C=O}}$
      2. ICl
      3. SO2
      4. $\ce{\sf{CH3-O-CH3}}$
      5. $\ce{\sf{CH3C(=O)CH3}}$

    Answers:

      1. carbon dioxide is nonpolar

      2. net dipole moment on iodine monochloride

      3. net dipole moment on sulfur dioxide

      4. net dipole moment on dimethyl ether

      5. net dipole moment on 2-propanone

    Questions

    Q2.2.1

    The following molecule has no dipole moment in the molecule itself, explain.

    Q2.2.2

    Which of the following molecules has a net dipole?

    Q2.2.3

    Within reactions with carbonyls, such as a reduction reaction, the carbonyl is attacked from the carbon side and not the oxygen side. Using knowledge of electronegativity explain why this happens.

    Solutions

    S2.2.1

    The hydroxyl groups are oriented opposite of one another and therefore the dipole moments would “cancel” one another out. Therefore having a zero net-dipole.

    S2.2.2

    1, 3, and 4 have a net dipoles.

    S2.2.3

    The oxygen is more electronegative than the carbon and therefore creates a dipole along the bond. This leads to having a partial positive charge on the carbon and the reduction can take place.

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