Hexane is a common organic compound used widely as a solvent in industrial processes, such as extracting vegetable oils, and in laboratory settings. Its effectiveness and physical characteristics, like its low boiling point, are determined by the forces between its individual molecules. Understanding these molecular attractions explains how this clear, colorless liquid behaves.
Defining the Requirements for Hydrogen Bonding
Hydrogen bonding is a particularly strong type of intermolecular attraction, not a true chemical bond, but a powerful dipole-dipole interaction. For a hydrogen bond to form, a molecule must contain a hydrogen atom covalently bonded to one of three highly electronegative atoms: nitrogen (\(\text{N}\)), oxygen (\(\text{O}\)), or fluorine (\(\text{F}\)).
The high electronegativity of these elements pulls shared electrons toward themselves, creating a significant charge separation. This leaves the hydrogen atom with a large partial positive charge (\(\delta+\)) and the electronegative atom with a large partial negative charge (\(\delta-\)). The resulting positive pole on the hydrogen atom is attracted to the lone pair of electrons on a nearby \(\text{N}\), \(\text{O}\), or \(\text{F}\) atom of a neighboring molecule. This combination distinguishes hydrogen bonding as the strongest intermolecular force.
Analyzing Hexane’s Molecular Structure
The chemical formula for hexane is \(\text{C}_6\text{H}_{14}\), identifying it as an alkane hydrocarbon. This structure means the molecule is composed exclusively of six carbon atoms single-bonded in a chain, with hydrogen atoms attached to every available position. To determine if hexane can participate in hydrogen bonding, one must examine the types of bonds present.
Hexane contains only carbon-carbon (\(\text{C}-\text{C}\)) and carbon-hydrogen (\(\text{C}-\text{H}\)) bonds. Although the \(\text{C}-\text{H}\) bond is slightly polar, carbon is not electronegative enough to create the large partial positive charge required for hydrogen bonding.
Crucially, the hexane molecule lacks the prerequisite highly electronegative atoms of nitrogen, oxygen, or fluorine. Since no hydrogen atom is covalently attached to these elements, hexane cannot act as a hydrogen bond donor. Therefore, hexane does not form hydrogen bonds with itself or with other molecules.
The Actual Intermolecular Forces in Hexane
In the absence of hydrogen bonding, hexane molecules are held together exclusively by London Dispersion Forces. These forces are present in all molecules, but they are the only significant intermolecular force acting between nonpolar molecules like alkanes. They arise from the constant, fleeting movement of electrons within the molecule’s electron cloud.
The random movement of electrons causes a temporary, uneven charge distribution, creating a momentary dipole. This temporary dipole then induces a corresponding dipole in a neighboring hexane molecule, leading to a weak, short-lived attraction. These forces are often described as induced dipole-induced dipole interactions.
While individually weak, London Dispersion Forces become more significant as the size and surface area of a molecule increase. Hexane is a relatively long molecule with six carbon atoms, providing a large surface area for numerous temporary attractions to occur simultaneously. The cumulative effect of these weak forces keeps hexane liquid at room temperature, with a boiling point between 68.5 and 69.1 degrees Celsius. This nonpolar nature also explains why hexane does not mix with polar liquids like water, following the rule of “like dissolves like.”