Naphthalene

Naphthalene is an organic compound with the chemical formula C10H8. It is the simplest form of a polycyclic aromatic hydrocarbon, which means it consists of multiple rings of atoms joined together.

Visually, it appears as a white crystalline solid. It is also famous for its very strong, characteristic odor. This scent is so powerful that humans can detect it at concentrations as low as 0.08 parts per million by mass. Because of its ability to turn into a gas, it has historically been used as a fumigant in mothballs to protect clothing from insects.

At the molecular level, naphthalene is composed of two fused benzene rings. In organic chemistry, rings are considered fused when they share two or more atoms.

This structure makes naphthalene a benzenoid polycyclic aromatic hydrocarbon. The molecule is planar, meaning it is flat like a sheet of paper. While benzene has bonds that are all the same length, naphthalene is different. The carbon-carbon bonds alternate in length. Some bonds, such as C1-C2, are about 1.37 Å long. Other bonds are slightly longer, measuring about 1.42 Å.

This specific geometry is consistent with the theorem of cross-conjugation.

The symmetry of the molecule creates different types of positions for other atoms to attach. There are two sets of equivalent hydrogen atoms: the alpha positions and the beta positions. The alpha positions are numbered 1, 4, 5, and 8. The beta positions are numbered 2, 3, 6, and 7.

Because of this arrangement, scientists can create different isomers. An isomer is a molecule with the same formula but a different structure. For example, if you attach one group to an alpha position, you get a different result than if you attach it to a beta position. Other molecules, like azulene, also feature fused rings but use a different 5–7 ring system instead.

The history of discovering naphthalene involves several important scientists. In the early 1820s, researchers reported finding a pungent white solid through the distillation of coal tar. In 1821, John Kidd described the substance's properties and how to produce it. He named it "naphthaline" because it was derived from naphtha, a term for volatile hydrocarbon mixtures. In 1826, the scientist Michael Faraday determined its chemical formula. Later, Emil Erlenmeyer proposed the two-ring structure in 1866. This structural model was confirmed by Carl Gräbe in 1869.

Today, naphthalene is produced on a massive industrial scale. The global market for naphthalene has reached approximately 2.25 million tons. Most of this comes from the distillation of coal tar. Typical coal tar contains about 10% naphthalene by weight. During industrial processing, workers distill the tar to create an oil containing about 50% naphthalene. After washing the oil with sodium hydroxide and sulfuric acid, fractional distillation is used. This results in crude naphthalene that is about 95% pure.

In North America, companies like Koppers Inc. and Monument Chemical Inc. are major producers.

Naphthalene serves as a vital chemical precursor for many industries. Its single largest use is the production of phthalic anhydride. This chemical is an intermediate used to make resins for paints and varnishes. It is also used to create plasticizers for polyvinyl chloride, known as PVC. Beyond plastics, naphthalene derivatives are used to make various synthetic dyes and pigments. Some specific derivatives, such as naphthalenesulfonic acids, are used to create rubber processing chemicals. They are even used in the production of lead-acid battery plates and agricultural pesticides.

In the medical and specialized fields, naphthalene plays a surprising role. Certain derivatives are used in the synthesis of pharmaceuticals. For example, some naphthalene-based structures help create beta blockers, which are medicines used to treat heart conditions.

Naphthalene also acts as a powerful solvent. In its molten, liquid state, it can dissolve aromatic compounds that are otherwise very difficult to melt or mix. This makes it a useful tool in complex laboratory reactions, such as the reaction between C60 and anthracene. Even in nature, trace amounts of naphthalene can be found in certain plants, like magnolias, and even in some species of deer.