Ethylene is a simple organic compound, a hydrocarbon composed solely of carbon and hydrogen atoms. This molecule is widely encountered in various industrial processes and naturally as a plant hormone. Despite its prevalence, ethylene exhibits a distinct characteristic: it does not readily mix with water, a property known as hydrophobicity.
Understanding Hydrophobicity
Hydrophobicity describes the physical property of a molecule that is seemingly repelled from a mass of water. This phenomenon is often explained by the principle that “like dissolves like,” meaning substances with similar intermolecular forces tend to dissolve or mix.
Water, a highly polar molecule, possesses an uneven distribution of electron density due to oxygen’s stronger pull on electrons compared to hydrogen. This creates partial negative charges on the oxygen atom and partial positive charges on the hydrogen atoms. The polarity of water allows it to form strong attractive forces, specifically hydrogen bonds, with other polar molecules and dissolve ionic compounds.
Molecules lacking this charge separation or the ability to form hydrogen bonds will not interact favorably with water. Their interactions with water molecules are energetically unfavorable compared to water molecules interacting with themselves.
The Molecular Structure of Ethylene
Ethylene has the chemical formula C2H4, consisting of two carbon atoms and four hydrogen atoms. These two carbon atoms are connected by a double bond, a strong covalent bond involving the sharing of four electrons. Each carbon atom is also bonded to two hydrogen atoms through single covalent bonds. This arrangement results in a planar molecule, where all six atoms lie in the same flat plane.
The carbon-hydrogen bonds within ethylene have a slight difference in electronegativity, with carbon being slightly more electronegative than hydrogen. This creates a small, localized polarity within each C-H bond. However, the carbon-carbon double bond involves atoms of the same element, leading to an even sharing of electrons and no polarity in that bond. The symmetrical arrangement of these atoms and bonds is a significant feature of ethylene’s overall character.
Ethylene’s Nonpolar Nature and Water Repulsion
Despite the minor polarity in individual carbon-hydrogen bonds, the overall structure of the ethylene molecule is nonpolar. This is because the symmetrical arrangement of the two carbon atoms and four hydrogen atoms ensures that any slight bond polarities within the C-H bonds effectively cancel each other out. The electron density is distributed evenly across the entire molecule, resulting in no net dipole moment.
Water molecules, with their distinct partial positive and negative ends, are strongly attracted to each other through hydrogen bonds. When a nonpolar molecule like ethylene is introduced into water, it cannot form these strong attractive interactions with the polar water molecules. Instead, water molecules are forced to rearrange themselves around the nonpolar ethylene, disrupting their preferred hydrogen bonding network.
This disruption is energetically unfavorable for the water molecules. As a result, water molecules tend to exclude nonpolar substances like ethylene, preferring to maximize their own strong intermolecular attractions. This energetic preference causes ethylene to be repelled by water, leading to its hydrophobic behavior.