The physical state and behavior of any substance, including the common solvent hexane, are fundamentally determined by the forces that exist between its individual molecules. These attractions, known as intermolecular forces (IMFs), are distinct from the stronger covalent bonds that hold the atoms within a single molecule together. Hexane (\(\text{C}_6\text{H}_{14}\)) exists as a liquid at room temperature because of these persistent molecular interactions. Understanding the nature of these forces explains why hexane behaves as a nonpolar liquid.
Understanding Hexane’s Structure
Hexane is an alkane composed solely of carbon and hydrogen atoms. The electrons in the \(\text{C}-\text{H}\) bonds are shared almost equally because the electronegativity difference is very small, making the individual bonds nonpolar covalent. Although the molecule is flexible, its overall structure is generally linear. The slight polarities of the \(\text{C}-\text{H}\) bonds cancel out, resulting in a molecule with no net dipole moment. Consequently, stronger intermolecular forces—Dipole-Dipole interactions and Hydrogen Bonding—are completely absent in pure liquid hexane.
The Primary Intermolecular Force
Since hexane lacks permanent charge separation, the interaction between molecules relies on the weakest class of intermolecular attraction, known as London Dispersion Forces (LDFs). These LDFs are the only significant means of attraction holding hexane molecules together in the liquid phase. While LDFs are present in all molecules, they become the dominant force influencing the physical properties of nonpolar substances. These attractions are weak and fleeting, acting only over very short distances between neighboring molecules. The cumulative effect of these temporary attractions across the long hexane chain creates an overall cohesive force.
Mechanism of Dispersion Forces
London Dispersion Forces arise from the constant, random motion of electrons within a molecule’s electron cloud. At any given instant, the electrons orbiting the \(\text{C}_6\text{H}_{14}\) molecule may momentarily be distributed unevenly, resulting in a temporary, instantaneous dipole. This transient charge imbalance in one hexane molecule influences a neighboring molecule. The instantaneous dipole repels the nearby electrons in the neighbor, inducing a corresponding temporary dipole. This synchronized creation of temporary dipoles results in a weak, momentary attraction between the two hexane molecules.
Polarizability
The strength of these dispersion forces is directly related to polarizability, which is the ease with which a molecule’s electron cloud can be distorted. Because hexane is a relatively large molecule with a long, flexible carbon chain, its electron cloud is highly polarizable. This high polarizability allows for stronger LDFs compared to smaller hydrocarbons, enabling hexane to exist as a liquid at room temperature.
Influence on Physical Properties
The presence of only weak London Dispersion Forces directly dictates the macroscopic physical properties of hexane. A small amount of energy is required to overcome these temporary attractions and separate the molecules. This low energy requirement is reflected in hexane’s low boiling point, which is approximately \(69^\circ\text{C}\). The weak IMFs also make hexane a highly volatile substance, meaning it evaporates easily into a gas at room temperature. This characteristic is why hexane is widely used as a nonpolar solvent to dissolve other nonpolar substances, such as oils and greases.