Organic Chemistry: Concepts and Applications - Headley Allan D. 2020
Bonding and Structure of Organic Compounds
1.7 Intermolecular Attractions
In this section, a visual description of the various interactions that exist among molecules will be examined. The types of attractions that exist among molecules (intermolecular attractions) are very weak compared to the bonding found within organic molecules, i.e. the ionic or covalent bond.
1.7.1 Dipole—Dipole Intermolecular Attractions
A very common intermolecular interaction encountered among molecules is dipole—dipole attraction. As we have seen in the earlier section of this chapter, some molecules are polar molecules, and different regions of the molecule have partial charges. Chloromethane (CH3Cl) is an example, it has four covalent bonds, three from the central carbon to hydrogens and one to chlorine. Since this molecule has a carbon—chlorine bond, this covalent bond is a polar covalent bond and hence based on the geometry, is a polar molecule, i.e. it has a dipole moment greater than zero. For chloromethane, as shown in Figure 1.32, the chlorine bears a partial negative charge (since it is electronegative), and the carbon bears a partial positive charge (since it is more electropositive than chlorine). If two of these molecules are in close contact with each other, there will be an attraction between them since opposite charges are attracted to each other. This type of attraction is called a dipole—dipole attraction as shown in Figure 1.32.
Figure 1.32 Illustration of the dipole—dipole attraction between two molecules of CH3—Cl molecules.
Problem 1.20
i. Using an illustration as shown in Figure 1.32, show the intermolecular attractions between each of the following molecules.
ii. For compounds (b) and (c) above, what factors should be considered to determine which will form a stronger intermolecular attraction?
1.7.2 Intermolecular Hydrogen Bond
If hydrogen is bonded to an electronegative atom in a molecule such as nitrogen or oxygen, a very polar covalent bond is created due to the difference in electronegativity between hydrogen and oxygen or nitrogen. As shown in Figure 1.33, there is a similar intermolecular attraction between two methanol molecules as that shown for chloromethane in Figure 1.32; in this case, however, the very electropositive hydrogen is involved.
Since the magnitude of charge separation between the electronegative oxygen and hydrogen is very large compared to other electropositive atoms, this type of intermolecular interaction is called a hydrogen bond. Thus, the hydrogen bond is a specific type of dipole—dipole interaction, which specifically involves hydrogen bonded to an electronegative atom.
Problem 1.21
i. Using an illustration as shown in Figure 1.33, show the hydrogen bond intermolecular attractions between each of the following molecules.
ii. Would you expect intermolecular hydrogen bonding to exist for the following molecule? Explain your answer.
Figure 1.33 Hydrogen bond between two methanol molecules.
1.7.3 Intermolecular London Force Attractions
Nonpolar molecules are also attracted to each other by very weak attractive forces called London forces; these attractive forces are also known as Van der Waals attraction. This type attraction is based on the induced dipole that is possible in nonpolar molecules based on an instantaneous shift of electrons. As a result of such instantaneous shifts, it is possible for nonpolar molecules to become instantaneously polarized with partial negative and positive charges distributed at different regions of the molecule. These partial charges can induce a similar charge distribution in another molecule, and as a result, an attraction exists between two fairly nonpolar molecules, as illustrated in Figure 1.34. Such molecules are often described as polarizable molecules and are typically large nonpolar molecules, compared to smaller molecules. Thus, this type of intermolecular attraction would be greater for pentane compared to ethane. In general, large polarizable molecules will result in stronger interactions, compared to smaller, compact less polarizable molecules.
Problem 1.22
Of the following pairs of molecules, determine which molecule would have the stronger Van der Waals intermolecular attraction. Explain your answer.
