NAPHTHALENE (C10H8)
Naphthalene is 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 formed 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.
Like several other polycyclic aromatic hydrocarbons, it is carcinogenic.
Naphthalene is obtained from middle oil (170-230 C), from which
it separates, on cooling, as crystals, which are pressed, treated
with caustic soda solution, then with sulfuric acid, and finally
distilled or sublimed (vaporized and recondensed to solid form).
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.
Although coal tar remains an important source of aromatic hydrocarbons,
the petrochemicals industry is an increasingly important supplier;
in the United States it is the principal source. Distillation
of crude oil yields substances that can be reformed either by
thermal or catalytic processes and suitable purification into
the aromatic hydrocarbons. Largest users are the synthetic fibre,
synthetic rubber, detergent, and plastics industries, but sizable
quantities are consumed by the dye-making industry.
Source © Encyclopædia Britannica, Inc.
