Hydrocarbons are organic compounds composed exclusively of carbon and hydrogen atoms. They vary widely in structure and properties. This article explores the chemical principles determining if hydrocarbons are polar or nonpolar.
What Makes a Molecule Polar?
Molecular polarity is determined by the distribution of electrical charge, which arises from electronegativity differences between atoms in a chemical bond. Electronegativity measures an atom’s ability to attract shared electrons. When atoms with differing electronegativities bond, electrons pull closer to the more electronegative atom, creating partial negative and positive charges, resulting in a bond dipole.
Overall molecular polarity depends on both polar bonds and molecular geometry. If bond dipoles are present, their effects can either add up or cancel based on molecular symmetry. A symmetrical arrangement of polar bonds may cause individual dipoles to cancel, leading to an overall nonpolar molecule. Conversely, an asymmetrical arrangement results in a net molecular dipole, making the molecule polar.
The Nature of Hydrocarbon Bonds
Hydrocarbons primarily feature carbon-carbon (C-C) and carbon-hydrogen (C-H) covalent bonds. Their polarity influences the overall character of the hydrocarbon molecule.
Carbon-carbon bonds are nonpolar because they involve two identical atoms with the same electronegativity, sharing bonding electrons equally. This results in no charge separation or bond dipole. The carbon-hydrogen bond is also considered nonpolar or very weakly polar. The small electronegativity difference between carbon (2.55) and hydrogen (2.20) is approximately 0.35, falling below the typical threshold of 0.4 to 0.5 for a polar bond.
Overall Polarity of Hydrocarbons
Hydrocarbons are nonpolar molecules due to their bond characteristics and symmetrical structures. Since both C-C and C-H bonds are nonpolar or minimally polar, there are no significant individual bond dipoles to contribute to overall molecular polarity.
The typical symmetrical arrangement of atoms in hydrocarbon molecules causes any minor C-H bond dipoles to cancel. For example, in methane (CH4), the carbon atom is at the center, bonded to four hydrogen atoms arranged in a tetrahedral geometry. This highly symmetrical structure ensures any slight charge imbalances across the C-H bonds negate each other.
Similarly, in larger hydrocarbons like ethane (C2H6), the consistent tetrahedral geometry around each carbon, combined with the nonpolar nature of the C-C and C-H bonds, results in overall nonpolar character. Even in longer chains or cyclic structures, the balanced distribution of hydrogen atoms around the carbon backbone leads to a negligible net dipole moment.
Why Hydrocarbon Nonpolarity Matters
The nonpolar nature of hydrocarbons has practical implications for their solubility. A fundamental principle in chemistry is “like dissolves like,” stating that substances with similar polarities tend to dissolve in each other.
Because hydrocarbons are nonpolar, they readily dissolve in other nonpolar substances. For example, oils, largely composed of hydrocarbons, mix well with other nonpolar organic solvents like gasoline or paint thinners. Conversely, nonpolar hydrocarbons do not mix with polar substances such as water. When oil and water are combined, they separate into distinct layers, with the less dense oil typically floating on top.
This characteristic explains why grease, a nonpolar substance, cannot be easily washed away with water alone. Instead, nonpolar solvents or detergents, which have both polar and nonpolar parts, are required to effectively remove hydrocarbon-based stains. This insolubility in water influences their behavior in various natural and industrial contexts.