Ethane (\(\text{C}_2\text{H}_6\)) is one of the simplest hydrocarbons, composed entirely of carbon and hydrogen atoms. It is a colorless, odorless gas used as a fundamental building block in the petrochemical industry. Understanding how ethane molecules interact is important for explaining its physical properties, such as its very low boiling point. This analysis will determine if \(\text{C}_2\text{H}_6\) possesses the necessary structural elements to engage in hydrogen bonding.
Defining the Requirements for Hydrogen Bonding
Hydrogen bonding is a strong type of interaction that occurs between molecules, distinct from standard covalent or ionic bonds. For this attraction to form, a hydrogen atom must be involved in a highly polarized covalent bond. This means the hydrogen atom needs to be covalently attached to a highly electronegative atom.
Only three elements possess the necessary high electronegativity and small atomic size to create this strong polarization: Nitrogen (\(\text{N}\)), Oxygen (\(\text{O}\)), or Fluorine (\(\text{F}\)). When hydrogen is bonded to one of these three atoms, the electronegative atom pulls the shared electrons strongly toward itself, giving the hydrogen atom a significant partial positive charge (\(\delta+\)).
This partially positive hydrogen atom is then strongly attracted to a lone pair of electrons on a neighboring \(\text{N}\), \(\text{O}\), or \(\text{F}\) atom in a different molecule. The resulting attraction is considerably stronger than other common intermolecular forces, influencing properties such as boiling points and solubility. Therefore, the presence of an \(\text{N}-\text{H}\), \(\text{O}-\text{H}\), or \(\text{F}-\text{H}\) bond is the mandatory prerequisite for a molecule to be a hydrogen bond donor.
Analyzing Ethane’s Molecular Structure
Ethane is classified as an alkane, a simple organic molecule where all carbon-carbon bonds are single bonds. Its structure consists of two carbon atoms connected to each other, with three hydrogen atoms bonded to each carbon, forming the \(\text{C}_2\text{H}_6\) formula. The molecule is structurally symmetrical, meaning the distribution of its atoms and electrons is largely balanced across its shape.
The molecule contains two types of bonds: a single carbon-carbon (\(\text{C}-\text{C}\)) bond and six carbon-hydrogen (\(\text{C}-\text{H}\)) bonds. The polarity of a bond is determined by the difference in electronegativity between the two atoms involved. Carbon has an electronegativity value of approximately 2.5, while hydrogen has a value of about 2.1.
This results in an electronegativity difference of only about 0.4 for the \(\text{C}-\text{H}\) bond. Because this difference is so small, the electrons are shared nearly equally between the carbon and hydrogen atoms. Consequently, the \(\text{C}-\text{H}\) bond is classified as non-polar or only very weakly polar.
Applying the Criteria to Ethane
When the strict requirements for hydrogen bonding are compared against the structure of ethane, the conclusion is straightforward. Ethane lacks any \(\text{N}\), \(\text{O}\), or \(\text{F}\) atoms within its molecular framework. Specifically, there are no hydrogen atoms covalently attached to these three highly electronegative elements.
The hydrogen atoms in ethane are only bonded to carbon atoms, forming \(\text{C}-\text{H}\) bonds. The weak polarity of the \(\text{C}-\text{H}\) bond is insufficient to create the large partial positive charge on the hydrogen atom that is necessary for hydrogen bonding. Without this highly polarized bond, the hydrogen atom cannot act as a strong donor to attract the electrons of a neighboring molecule.
Ethane molecules cannot form hydrogen bonds. The molecule does not possess the structural features required to generate the powerful, directed dipole-dipole interaction that defines a hydrogen bond. This absence means ethane is structurally incapable of participating in the strong attractions that characterize molecules like water or ammonia.
The Intermolecular Forces Ethane Does Exhibit
Since hydrogen bonding is ruled out, the attractive forces that hold ethane molecules together in condensed states, such as liquid ethane, must be weaker. Ethane is a non-polar molecule, meaning it does not have a permanent separation of positive and negative charge across the entire structure. The primary intermolecular force governing the interaction between ethane molecules is the London Dispersion Force (LDF).
London Dispersion Forces are the weakest of all intermolecular forces and are present in all molecules, regardless of their polarity. They arise from the constant, random movement of electrons within a molecule. At any given moment, the electrons may temporarily cluster on one side of the molecule, creating a fleeting, induced dipole moment.
This temporary dipole can then induce a corresponding dipole in a neighboring molecule, leading to a weak, momentary attraction. Because ethane is a relatively small molecule with a low number of electrons, these dispersion forces are quite weak. This explains why ethane has a very low boiling point of approximately \(-88.5\) degrees Celsius and exists as a gas at standard room temperature.