Читать книгу Foundations of Chemistry - Philippa B. Cranwell - Страница 120

If a molecule has polar bonds, it may be polar overall and have an overall dipole moment, which is important as this can affect how one molecule interacts with another. For example, the molecule H—Cl has a polar bond and an overall dipole moment. This is depicted by an arrow drawn beside the molecule with a line through the flat end. The direction of the arrowhead shows the direction of electron charge in the molecule, and the arrow points from the less electronegative end to the more electronegative end of the molecule, as shown in Figure 2.24.

Figure 2.24 Hydrogen chloride molecule showing the charge separation and direction of overall dipole moment.

Another example is the molecule chloromethane, CH₃Cl, which has a tetrahedral shape. The C—Cl bond is polar because the chlorine atom is more electronegative than the carbon atom (C has χ = 2.5 and Cl has χ = 3.0). The C—H bonds are usually considered to be non‐polar because the electronegativities of carbon and hydrogen are similar (H has χ = 2.2). The molecule therefore has a dipole moment in the direction of the chlorine atom, as shown in Figure 2.25.

Figure 2.25 Chloromethane, CH₃Cl, is a polar molecule.

However, not all molecules with polar bonds have a dipole moment. We have seen that in carbon dioxide, CO₂, both C=O double bonds are polar. However, the molecule is linear, and Figure 2.26 shows that the two polar bonds pull charge from the carbon atom in opposite directions to each other. As the size of the dipole is equal in both C=O bonds, there is no net overall dipole moment. You can think of this like two teams in a tug of war pulling against each other. If both teams have exactly the same strength, neither team will win.

Figure 2.26 Carbon dioxide has polar bonds but no overall dipole moment.

Consider the series of molecules based on methane, CH₄, where the hydrogen atoms are replaced in turn to give fluoromethane, CH₃F, difluoromethane, CH₂F₂, trifluoromethane, CHF₃, and tetrafluoromethane, CF₄, shown in Figure 2.27. The molecule fluoromethane, Figure 2.27a, has a permanent dipole moment. It has the same shape as chloromethane, shown in Figure 2.25. The dipole moment is along the C—F bond from the carbon atom to the fluorine atom. In difluoromethane, there are two C—F bonds, and each is polar. The molecule is polar overall as the two dipoles along the bonds add together and give an overall dipole moment oriented between the C—F bonds. Trifluoromethane, CHF₃, also has an overall dipole in the molecule that points in the opposite direction to the remaining C—H bond: Figure 2.27c. Finally, tetrafluoromethane, CF₄, does not have an overall dipole because there is no overall charge separation in the molecule. The polar C—F bonds pull charge in opposite directions and cancel each other out: Figure 2.27d.

Figure 2.27 (a) Overall molecular dipole in fluoromethane; (b) overall molecular dipole in difluoromethane; (c) overall molecular dipole in trifluoromethane; (d) tetrafluoromethane with no overall dipole but polar bonds.