Hexane is a common organic solvent and hydrocarbon with the chemical formula \(\text{C}_6\text{H}_{14}\). Classified as an alkane, its structure consists entirely of single bonds between its six carbon atoms and fourteen hydrogen atoms. Understanding how hexane behaves requires examining the non-covalent attractions between its individual molecules, known as intermolecular forces (IMFs). These forces dictate a substance’s physical properties, such as its boiling point and solubility.
The Three Types of Intermolecular Forces
Intermolecular forces are generally categorized into three main types, which vary significantly in strength. The strongest is hydrogen bonding, which occurs exclusively when a hydrogen atom is directly bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine. This arrangement creates a strong, permanent dipole that allows for a powerful attraction with an adjacent molecule.
The second category is the dipole-dipole force, which is an attraction that exists between molecules possessing a permanent net dipole moment. These forces are present in polar molecules where there is an unequal sharing of electrons, resulting in one end of the molecule having a partial positive charge and the other a partial negative charge. Dipole-dipole interactions are generally weaker than hydrogen bonds but stronger than the final type of force.
The third and weakest category is the London Dispersion Force (LDF), also known as induced dipole forces. Unlike the other two types, LDFs are present in all molecules, regardless of their polarity, because they rely only on the movement of electrons. This makes LDFs the universal attractive force found in all matter.
Molecular Geometry and Polarity of Hexane
The hexane molecule, \(\text{C}_6\text{H}_{14}\), is structurally a long, continuous chain of carbon atoms saturated with hydrogen atoms. While the molecule’s overall shape is somewhat flexible, its backbone consists of carbon atoms that are each surrounded by four bonds in a roughly tetrahedral geometry.
Although there is a very slight difference in electronegativity between carbon and hydrogen atoms, the \(\text{C}-\text{H}\) bonds are considered only very weakly polar. The symmetrical, non-directional arrangement of these numerous \(\text{C}-\text{H}\) bonds along the carbon chain causes any slight bond polarity to cancel out. This cancellation means that the hexane molecule possesses no net charge separation or permanent dipole moment.
Consequently, hexane is classified as a nonpolar molecule. Because it lacks permanent charge separation, it cannot form the strong attractions seen in polar compounds. This structural fact immediately excludes the possibility of dipole-dipole forces or hydrogen bonding between hexane molecules.
Identifying the Dominant Force: London Dispersion
Since hexane is a nonpolar molecule, the only intermolecular attraction that occurs between its molecules is the London Dispersion Force. These forces are temporary, arising from the random movement of electrons within the molecule’s electron cloud.
This momentary charge imbalance in one hexane molecule influences the electron cloud of a neighboring molecule, inducing a corresponding temporary dipole. This induction creates a short-lived electrostatic attraction between the two adjacent molecules, which is the mechanism of the London Dispersion Force. The constant, rapid formation and disappearance of these dipoles result in a sustained overall attractive force within the liquid.
The strength of the London Dispersion Force is highly dependent on both molecular size and surface area. Larger molecules with more electrons are more easily polarized, meaning their electron clouds are more easily distorted to create temporary dipoles. Hexane is a relatively long molecule, which significantly increases the total surface area available for molecular contact compared to smaller alkanes.
This extended shape allows a greater number of LDF interactions to occur simultaneously between adjacent hexane molecules, making the cumulative force substantial. Although LDF is the weakest type of intermolecular force individually, the cumulative effect in this six-carbon chain is strong enough to hold the molecules together as a liquid at standard room temperature.
How Intermolecular Forces Govern Hexane’s Properties
The presence of only London Dispersion Forces dictates many of hexane’s observable physical characteristics. Since LDFs are comparatively weak, the energy required to overcome them is low, resulting in a low boiling point of approximately \(69^\circ\text{C}\). This low boiling point means that hexane is highly volatile, readily evaporating into a gaseous state.
The nonpolar nature of hexane also determines its solubility, following the chemical principle that “like dissolves like.” Hexane readily dissolves other nonpolar substances, such as oils, waxes, and fats, because similar weak LDFs can form between the solvent and solute molecules. This makes it an effective solvent for nonpolar extractions.
Conversely, hexane is virtually insoluble in highly polar solvents such as water. Water molecules are held together by strong hydrogen bonds. The energetic cost of breaking these strong attractions to accommodate the weakly interacting, nonpolar hexane molecules is too high, as the weak LDFs hexane can form with water are insufficient to overcome the dedicated forces already present in the water.