Naphthalene is an organic compound best known as a component of traditional mothballs, but it is also widely used in industrial manufacturing to create plastics, dyes, and resins. This bicyclic aromatic hydrocarbon has a distinctive, pungent odor and is classified as a polycyclic aromatic hydrocarbon (PAH). Naphthalene is essentially insoluble, or only very sparingly soluble, in water. This fundamental chemical property dictates how the substance behaves in the environment and how it must be handled in both industrial and household settings.
The Chemistry of Solubility
The ability of one substance to dissolve in another is governed by a fundamental scientific guideline often summarized as “like dissolves like.” This principle relates to the electrical properties of the molecules involved, specifically their polarity. Molecules are classified as either polar (having a positive charge on one side and a negative charge on the other) or nonpolar (where the electrical charge is distributed symmetrically). Water is considered a highly polar molecule, possessing a strong positive end and a strong negative end.
A polar solvent, like water, readily dissolves other polar substances, such as table salt or sugar, because the opposing charges attract each other to form a stable solution. Conversely, nonpolar substances, like cooking oil or gasoline, do not dissolve in water. Their symmetrical charge distribution prevents the necessary attractive forces from forming with the polar solvent, leading to phase separation.
Naphthalene’s Molecular Structure and Polarity
Naphthalene’s chemical formula is C10H8, and its molecular structure consists of two six-carbon rings fused together. This configuration is a polycyclic aromatic hydrocarbon. The molecule is composed entirely of carbon and hydrogen atoms, which share electrons almost equally due to their similar electronegativities. This arrangement of bonds results in a highly symmetrical and planar molecule with a zero net dipole moment.
Because of this symmetry and the equal sharing of electrons, naphthalene is overwhelmingly nonpolar in nature. This nonpolar characteristic directly conflicts with the highly polar nature of the water molecule, leading to the observed insolubility. The weak attractive forces of naphthalene are not strong enough to overcome the powerful hydrogen bonding present between water molecules. When placed in water, the water molecules preferentially bond with each other, pushing the naphthalene aside. Naphthalene dissolves only to a concentration of about 31.6 milligrams per liter at 25 degrees Celsius, confirming its very trace solubility.
Environmental Fate and Safe Handling
The limited solubility of naphthalene in water has significant real-world implications for its movement and handling. When released into an aquatic environment, the substance does not truly dissolve but instead tends to either float on the surface or rapidly volatilize into the air. Volatilization is a primary route by which it leaves water, though some of the compound may also settle and persist in bottom sediments. In soil, naphthalene has a high affinity for organic matter, meaning it tends to stick to the soil particles rather than wash away easily with groundwater.
This behavior affects cleanup efforts, as water is ineffective for remediation; instead, organic solvents or specialized methods are required to extract the substance from contaminated soil and sediment. Exposure to naphthalene in household products, such as mothballs, occurs primarily through inhalation of the toxic vapor. In humans, exposure can lead to the breakdown of red blood cells, a condition known as hemolytic anemia, and the substance is classified as a possible human carcinogen. Therefore, products containing naphthalene must be used exactly as directed, typically in sealed containers, to minimize both inhalation exposure and environmental release.