Most naphthalene is derived from coal tar. From the 1960s until the 1990s, significant amounts of naphthalene were also produced from heavy petroleum fractions during petroleum refining, but today petroleum-derived naphthalene represents only a minor component of naphthalene production. Naphthalene is the most abundant single component of coal tar.
While the composition of coal tar varies with the coal from which it is produced, typical coal tar is about 10% naphthalene by weight. In industrial practice, distillation of coal tar yields oil containing about 50% naphthalene, along with a variety of other aromatic compounds. This oil, after being washed with aqueous sodium hydroxide to remove acidic components, chiefly various phenols, and with sulfuric acid to remove basic components, is fractionally distilled to isolate naphthalene.
STRUCTURE AND REACTIVITY
A naphthalene molecule is composed of two fused benzene rings. (In organic chemistry, rings are fused if they share two or more atoms.) Accordingly, naphthalene is classified as a benzene polyaromatic hydrocarbon (PAH). Naphthalene has three resonance structures, which are shown in the drawing below. Naphthalene has two sets of equivalent hydrogen. The alpha positions are positions 1, 4, 5, and 8 on the drawing below. The beta positions are positions 2, 3, 6, and 7.
Unlike benzene, the carbon-carbon bonds in naphthalene are not of the same length. The bonds C1–C2, C3–C4, C5–C6 and C7–C8 are about 1.36 Å (136 pm) in length, whereas all the other carbon-carbon bonds are about 1.42 Å (142 pm) in length. This has been verified by x-ray diffraction and can be expected from the resonance structures, where the bonds C1–C2, C3–C4, C5–C6 and C7–C8 are double in two of the three structures, whereas all the others are double in only one.
Like benzene,naphthalene can undergo electrophilic aromatic substitution. For many electrophilic aromatic substitution reactions, naphthalene is more reactive than benzene, and reacts under milder conditions than does benzene.
Naphthalene’s most familiar use is as a household fumigant, such as in mothballs. In a sealed container containing naphthalene pellets, naphthalene vapors build up to levels toxic to both the adult and larval forms of many moths that are destructive to textiles. Other fumigant uses of naphthalene include use in soil as a fumigant pesticide, and in attic spaces to repel animals.
In the past, naphthalene was administered orally to kill parasitic worms in livestock. Larger volumes of naphthalene are used as a chemical intermediate to produce other chemicals. The single largest use of naphthalene is the industrial production of phthalic anhydride, although more phthalic anhydride is made from o-xylene than from naphthalene. Other naphthalene-derived chemicals include alkyl naphthalene sulfonate surfactants, and the insecticide carbaryl.
Naphthalenes substituted with combinations of strongly electron-donating functional groups, such as alcohols and amines, and strongly electron-withdrawing groups, especially sulfonic acids, are intermediates in the preparation of many synthetic dyes. The hydrogenated naphthalenes tetrahydronaphthalene (Tetralin) and decahydronaphthalene (Decalin) are used as low-volatility solvents. Naphthalene vapour can also slow the onset of rust, such as the use of moth balls in a tool box.