Ethene, commonly known as ethylene, is an organic compound with the chemical formula C2H4. The molecule consists of two carbon atoms double-bonded to each other, with two hydrogen atoms attached to each carbon. Ethene is a nonpolar molecule. This classification is determined by the molecule’s internal bond characteristics and its overall symmetrical structure.
Fundamentals of Molecular Polarity
Molecular polarity arises from the uneven sharing of electrons between atoms, a property primarily governed by electronegativity. Electronegativity is the measure of an atom’s ability to attract shared electrons in a chemical bond. When two atoms with different electronegativities bond, the electrons spend more time near the more attractive atom, creating a bond dipole, which is a separation of charge.
The difference in electronegativity between carbon and hydrogen is relatively small. Carbon has a value of approximately 2.5 and hydrogen has a value of about 2.1 on the Pauling scale. This minor difference suggests that the carbon-hydrogen bond itself is considered nonpolar or only weakly polar. A polar molecule, however, requires more than just polar bonds; it must possess a net dipole moment.
A net dipole moment is the vector sum of all the individual bond dipoles within the molecule. Think of bond dipoles as arrows pointing toward the more electronegative atom, with the length of the arrow representing the magnitude of the charge separation. For a molecule to be polar, these individual dipoles must not cancel each other out, resulting in a distinct positive and negative end. Conversely, if the molecular geometry causes the individual dipoles to perfectly oppose and cancel one another, the molecule is considered nonpolar.
The Unique Structure and Bonding of Ethene (C2H4)
The structure of the ethene molecule is defined by a double bond between the two central carbon atoms. Each carbon atom is also single-bonded to two hydrogen atoms, connecting each carbon to three other atoms. This arrangement forces a specific shape around each carbon atom.
The carbon atoms in ethene exhibit \(sp^2\) hybridization, which is the result of one \(s\) orbital and two \(p\) orbitals mixing. This hybridization results in a geometry known as trigonal planar around each carbon atom. Consequently, the entire C2H4 molecule is flat, with all six atoms lying in the same plane.
This planar structure dictates that the bond angles between the atoms are all approximately 120 degrees. The double bond between the carbons consists of one sigma bond and one pi bond, preventing the free rotation of the carbon halves. This rigidity locks the atoms into their symmetrical positions.
Why Ethene is Nonpolar: The Role of Symmetry
Ethene is nonpolar because of the perfect symmetry of its molecular geometry. Although the carbon-hydrogen bonds possess a small degree of polarity, the way these bonds are oriented causes their slight electrical effects to neutralize. The overall molecular shape is highly symmetrical.
The individual bond dipoles created by the C-H bonds point away from the center of the molecule toward the hydrogen atoms. Because the molecule is trigonal planar and symmetrical, the dipoles from the two C-H bonds on one carbon atom are perfectly balanced by the two C-H bonds on the other carbon atom. This situation is analogous to a tug-of-war where opposing teams of equal strength pull in exactly opposite directions, resulting in no net movement.
The symmetrical distribution of the electron density across the entire molecule means that the molecule has no distinct positive or negative pole. The vector sum of all the individual C-H bond dipoles is zero, resulting in a net dipole moment of zero for the C2H4 molecule. The nonpolar nature of ethene is a classic example of how molecular geometry can override the presence of slightly polar bonds.
Common Uses and Significance of Ethene
Ethene is a colorless, flammable gas and the most-produced organic chemical globally. Its nonpolar nature allows it to serve as a fundamental building block in the petrochemical industry. The primary industrial use of ethene is as a monomer in the production of polyethylene.
Polyethylene is the world’s most widely used plastic, accounting for approximately 60% of the global demand for ethene. This plastic is used to manufacture many items, such as:
- Food packaging.
- Shopping bags.
- Industrial containers.
Ethene is also converted into compounds like ethylene oxide (used for antifreeze) and ethylene dichloride (a precursor for PVC plastic).
Beyond industrial applications, ethene plays a significant biological role as a natural plant hormone. It regulates various processes in plants, notably controlling the ripening of fruits. This dual function in both industry and nature underscores its importance.