Intermolecular forces (IMFs) are the attractive or repulsive forces that exist between individual molecules, determining a substance’s physical state (gas, liquid, or solid). These forces are significantly weaker than the intramolecular forces, which are the chemical bonds holding atoms together within a molecule. We will examine chloromethane (\(\text{CH}_3\text{Cl}\)) to determine if its molecules exhibit dipole-dipole forces.
The Polarity Prerequisite
For a molecule to exhibit dipole-dipole forces, it must first possess a permanent, overall molecular polarity. This condition arises from the fundamental property of atoms known as electronegativity, which is a measure of an atom’s ability to attract a shared pair of electrons toward itself in a chemical bond. When two atoms with differing electronegativities form a covalent bond, the electron density is not shared equally, resulting in a polar bond. The atom with the higher electronegativity develops a partial negative charge (\(\delta-\)), while the less electronegative atom develops a partial positive charge (\(\delta+\)).
This uneven electron distribution creates a bond dipole moment, essentially a tiny electrical separation of charge. For example, in a molecule like hydrogen chloride (\(\text{HCl}\)), chlorine is significantly more electronegative than hydrogen, establishing a clear dipole. In contrast, a diatomic molecule like oxygen (\(\text{O}_2\)) is composed of two identical atoms, meaning the electron sharing is perfectly equal and the bond is nonpolar.
A molecule’s overall polarity is determined by both the polarity of its individual bonds and its three-dimensional molecular geometry. For a molecule to be considered polar, the sum of all its bond dipoles must result in a net, non-zero molecular dipole moment. If the individual bond dipoles are arranged symmetrically, they can effectively cancel each other out, leaving the molecule nonpolar despite having polar bonds.
Molecular Structure of Chloromethane (\(\text{CH}_3\text{Cl}\))
The structure of chloromethane must be analyzed to determine if it meets the polarity requirement. The central carbon atom in \(\text{CH}_3\text{Cl}\) is bonded to three hydrogen atoms and one chlorine atom, giving the molecule a tetrahedral geometry. This shape is inherently symmetrical if all the surrounding atoms are identical, such as in methane (\(\text{CH}_4\)), where the bond dipoles cancel perfectly.
However, the atoms surrounding the central carbon in \(\text{CH}_3\text{Cl}\) are not identical, creating a distinct difference between the chlorine atom and the three hydrogen atoms. Electronegativity values show that chlorine (3.16) is significantly higher than carbon (2.5) and hydrogen (2.2). This large difference makes the carbon-chlorine (\(\text{C-Cl}\)) bond highly polar, pulling electron density strongly toward the chlorine atom.
The carbon-hydrogen (\(\text{C-H}\)) bonds are only slightly polar, but the molecular asymmetry is the crucial factor. Since the highly electronegative chlorine atom is located on one side of the molecule, it pulls the electron density in that direction. This uneven pull prevents the individual bond dipoles from canceling each other out. Consequently, chloromethane has a permanent, overall net dipole moment, confirming it is a polar molecule.
Defining and Identifying Dipole-Dipole Forces
Dipole-dipole forces are the electrostatic forces of attraction that occur between two or more molecules that possess a permanent dipole moment. In this interaction, the partially positive end (\(\delta+\)) of one polar molecule is attracted to the partially negative end (\(\delta-\)) of a neighboring polar molecule. These attractions are significant only when molecules are close together, such as in the liquid or solid state.
Because chloromethane (\(\text{CH}_3\text{Cl}\)) is a polar molecule with a net dipole moment, it exhibits dipole-dipole forces. When liquid chloromethane molecules are near each other, the slightly negative chlorine end of one molecule aligns itself toward the slightly positive end of an adjacent molecule. These forces are a major factor in determining physical properties, such as a substance’s boiling point, since energy is required to overcome them.
All molecules, including chloromethane, also experience London Dispersion Forces (LDFs), which are temporary, weak attractions arising from instantaneous fluctuations in electron distribution. However, the dipole-dipole forces in \(\text{CH}_3\text{Cl}\) are permanent and generally stronger than the LDFs. The combination of both LDFs and the stronger dipole-dipole forces dictates the intermolecular behavior and physical characteristics of chloromethane.