It is the simplest polycyclic aromatic hydrocarbon, and is a white crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass.

[15] As an aromatic hydrocarbon, naphthalene's structure consists of a fused pair of benzene rings.

In the early 1820s, two separate reports described a white solid with a pungent odor derived from the distillation of coal tar.

He proposed the name naphthaline, as it had been derived from a kind of naphtha (a broad term encompassing any volatile, flammable liquid hydrocarbon mixture, including coal tar).

The structure of two fused benzene rings was proposed by Emil Erlenmeyer in 1866,[17] and confirmed by Carl Gräbe three years later.

This difference, established by X-ray diffraction,[21] is consistent with the valence bond model in naphthalene and in particular, with the theorem of cross-conjugation.

This theorem would describe naphthalene as an aromatic benzene unit bonded to a diene but not extensively conjugated to it (at least in the ground state), which is consistent with two of its three resonance structures.

However, the solid shows semiconducting character below 100 K.[23][24] In electrophilic aromatic substitution reactions, naphthalene reacts more readily than benzene.

Oxidation with O2 in the presence of vanadium pentoxide as catalyst gives phthalic anhydride: This reaction is the basis of the main use of naphthalene.

From the 1960s until the 1990s, significant amounts of naphthalene were produced from heavy petroleum fractions during refining, but present-day production is mainly from coal tar.

In Eastern Europe, naphthalene is produced by a variety of integrated metallurgy complexes (Severstal, Evraz, Mechel, MMK) in Russia, dedicated naphthalene and phenol makers INKOR, Yenakievsky Metallurgy plant in Ukraine and ArcelorMittal Temirtau in Kazakhstan.

Molten naphthalene provides an excellent solubilizing medium for poorly soluble aromatic compounds.

Alkyl naphthalene sulfonates (ANS) are used in many industrial applications as nondetergent surfactants (wetting agents) that effectively disperse colloidal systems in aqueous media.

[25] Substituted naphthalenes serve as pharmaceuticals such as propranolol (a beta blocker) and nabumetone (a nonsteroidal anti-inflammatory drug).

[37] Humans, in particular children, have developed the condition known as hemolytic anemia, after ingesting mothballs or deodorant blocks containing naphthalene.

Exposure to large amounts of naphthalene may cause confusion, nausea, vomiting, diarrhea, blood in the urine, and jaundice (yellow coloration of the skin due to dysfunction of the liver).

[38] The US National Toxicology Program (NTP) held an experiment where male and female rats and mice were exposed to naphthalene vapors on weekdays for two years.

[39] Both male and female rats exhibited evidence of carcinogenesis with increased incidences of adenoma and neuroblastoma of the nose.

The International Agency for Research on Cancer (IARC)[40] classifies naphthalene as possibly carcinogenic to humans and animals (Group 2B).

A probable mechanism for the carcinogenic effects of mothballs and some types of air fresheners containing naphthalene has been identified.

The Occupational Safety and Health Administration has set a permissible exposure limit at 10 ppm (50 mg/m3) over an eight-hour time-weighted average.