the simplest of the fused or condensed ring hydrocarbon compounds composed of two benzene rings sharing two adjacent carbon atoms; chemical formula, C₁₀H₈. It is an important hydrocarbon raw material that gives rise to a host of substitution products used in the manufacture of dyestuffs and synthetic resins. Naphthalene is the most abundant single constituent of coal tar, a volatile product from the destructive distillation of coal, and is also formed in modern processes for the high-temperature cracking (breaking up of large molecules) of petroleum. It is commercially produced by crystallization from the intermediate fraction of condensed coal tar and from the heavier fraction of cracked petroleum. The substance crystallizes in lustrous white plates, melting at 80.1° C (176.2° F) and boiling at 218° C (424° F). It is almost insoluble in water. Naphthalene is highly volatile and has a characteristic odour; it has been used as moth repellent.

In its chemical behaviour, naphthalene shows the aromatic character associated with benzene and its simple derivatives. Its reactions are mainly reactions of substitution of hydrogen atoms by halogen atoms, nitro groups, sulfonic acid groups, and alkyl groups. Large quantities of naphthalene are converted to naphthylamines and naphthols for use as dyestuff intermediates. For many years napthalene was the principal raw material for making phthalic anhydride.