Naphthalene is an organic compound with formula C
10H 8. 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. As an aromatic hydrocarbon, naphthalene's
structure consists of a fused pair of benzene rings. It is best known as the main ingredient
of traditional mothballs.
History In the early 1820s, two separate reports described
a white solid with a pungent odor derived from the distillation of coal tar. In 1821,
John Kidd cited these two disclosures and then described many of this substance's properties
and the means of its production. He proposed the name naphthaline, as it had been derived
from a kind of naphtha. Naphthalene's chemical formula was determined by Michael Faraday
in 1826. The structure of two fused benzene rings was proposed by Emil Erlenmeyer in 1866,
and confirmed by Carl Gräbe three years later. Structure and reactivity
A naphthalene molecule can be viewed as the fusion of a pair of benzene rings. As such,
naphthalene is classified as a benzenoid polycyclic aromatic hydrocarbon. There are two sets of
equivalent hydrogen atoms: the alpha positions are positions 1, 4, 5, and 8 on the drawing
below, and 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 in length, whereas the other
carbon–carbon bonds are about 1.42 Å long. This difference, which was established by
X-ray diffraction, is consistent with the valence bond model of bonding in naphthalene
that involves three resonance structures; whereas the bonds C1–C2, C3–C4, C5–C6
and C7–C8 are double in two of the three structures, the others are double in only
one.
Like benzene, naphthalene can undergo electrophilic aromatic substitution. For many electrophilic
aromatic substitution reactions, naphthalene reacts under milder conditions than does benzene.
For example, whereas both benzene and naphthalene react with chlorine in the presence of a ferric
chloride or aluminium chloride catalyst, naphthalene and chlorine can react to form 1-chloronaphthalene
even without a catalyst. Likewise, whereas both benzene and naphthalene can be alkylated
using Friedel–Crafts reactions, naphthalene can also be alkylated by reaction with alkenes
or alcohols, with sulfuric or phosphoric acid as the catalyst.
Substituted derivatives Two isomers are possible for mono-substituted
naphthalenes, corresponding to substitution at an alpha or beta position. Usually, electrophiles
attack at the alpha position. The selectivity for alpha over beta substitution can be rationalized
in terms of the resonance structures of the intermediate: for the alpha substitution intermediate,
seven resonance structures can be drawn, of which four preserve an aromatic ring. For
beta substitution, the intermediate has only six resonance structures, and only two of
these are aromatic. Sulfonation, however, gives a mixture of the "alpha" product 1-naphthalenesulfonic
acid and the "beta" product 2-naphthalenesulfonic acid, with the ratio dependent on reaction
conditions. The 1-isomer forms predominantly at 25 °C, and the 2-isomer at 160 °C.
Naphthalene can be hydrogenated under high pressure in the presence of metal catalysts
to give 1,2,3,4-tetrahydronaphthalene or tetralin (C
10H 12). Further hydrogenation yields decahydronaphthalene
or decalin (C 10H
18). Oxidation with chromate or permanganate, or catalytic oxidation with O
2 and a vanadium catalyst, gives phthalic acid.
Production 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. Although 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 an 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, and with sulfuric acid
to remove basic components, undergoes fractional distillation to isolate naphthalene. The crude
naphthalene resulting from this process is about 95% naphthalene by weight. The chief
impurities are the sulfur-containing aromatic compound benzothiophene, indane, indene, and
methylnaphthalene. Petroleum-derived naphthalene is usually purer than that derived from coal
tar. Where required, crude naphthalene can be further purified by recrystallization from
any of a variety of solvents, resulting in 99% naphthalene by weight, referred to as
80 °C. Approximately 1.3M tons are produced annually.
In North America, coal tar producers are Koppers Inc., Ruetgers Canada Inc. and Recochem Inc.,
and petroleum-derived producer is Advanced Aromatics, L.P. In Western Europe most known
producers are Koppers, Ruetgers and Deza. In Eastern Europe, - variety of integrated
metallurgy complexes in Russia. Dedicated naphthalene and phenol maker INKOR and Yenakievsky
Metallurgy plant in Ukraine, and ArcelorMittal Temirtau in Kazakhstan.
Other sources and occurrences Aside from coal tar, trace amounts of naphthalene
are produced by magnolias and specific types of deer, as well as the Formosan subterranean
termite, possibly produced by the termite as a repellant against "ants, poisonous fungi
and nematode worms." Some strains of the endophytic fungus Muscodor albus produce naphthalene
among a range of volatile organic compounds, while Muscodor vitigenus produces naphthalene
almost exclusively. Naphthalene has been found in meteorites.
Naphthalene in the interstellar medium Naphthalene has been tentatively detected
in the interstellar medium in the direction of the star Cernis 52 in the constellation
Perseus. More than 20% of the carbon in the universe may be associated with polyaromatic
hydrocarbons, including naphthalene. Protonated cations of naphthalene (C
10H+ 9) are the source of part of the spectrum
of the Unidentified Infrared Emissions. Protonated naphthalene differs from neutral naphthalene
in that it has an additional hydrogen atom. The UIRs from "naphthalene cation" (C
10H+ 8) have been observed by astronomers. This
research has been publicized as "mothballs in space."
Uses As a chemical intermediate
Naphthalene is used mainly as a precursor to other chemicals. The single largest use
of naphthalene is the industrial production of phthalic anhydride, although more phthalic
anhydride is made from o-xylene. Other naphthalene-derived chemicals include alkyl naphthalene sulfonate
surfactants, and the insecticide 1-naphthyl-N-methylcarbamate. 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 and decahydronaphthalene
are used as low-volatility solvents. Naphthalene is also used in the synthesis of 2-naphthol,
a precursor for various dyestuffs, pigments, rubber processing chemicals and other miscellaneous
chemicals and pharmaceuticals. Naphthalene sulfonic acids are used in the
manufacture of naphthalene sulfonate polymer plasticizers, which are used to produce concrete
and plasterboard. They are also used as dispersants in synthetic and natural rubbers, and as tanning
agents in leather industries, agricultural formulations, dyes and as a dispersant in
lead–acid battery plates. Naphthalene sulfonate polymers are produced
by treating naphthalene with sulfuric acid and then polymerizing with formaldehyde, followed
by neutralization with sodium hydroxide or calcium hydroxide. These products are commercially
sold in solution or dry powder form. Sulfonation Step:
H 2SO
4 + C 10H
8 → C 10H
7-SO 3H + H
2O Polymerization Step:
C 10H
7-SO 3H + CH
2=O → SO 3H-C
10H 7-(-CH
2-C 10H
7-SO 3H)
n + H 2SO
4 Neutralization Step:
C 10H
7-SO 3H-(C
10H 7-SO
3H) n + NaOH → C
10H 7-SO
3Na-(C 10H
7-SO 3Na)
n + H 2O + Na
2SO 4
As a solvent for chemical reactions Molten naphthalene provides an excellent solubilizing
medium for poorly soluble aromatic compounds. In many cases it is more efficient than other
high-boiling solvent, such as dichlorobenzene, benzonitrile, nitrobenzene and durene. A reaction
of C60 with an equimolar amount of anthracene in refluxing naphthalene gives the 1:1 Diels-Alder
adduct in 67% yield.
A procedure developed for aromatization of porphyrins includes heating with DDQ in naphthalene
melt. It was shown that naphthalene not only provides sufficient solubility of reactants
but has an influence on a selectivity of the process.
Wetting agent and surfactant Alkyl naphthalene sulfonates are used in many
industrial applications as nondetergent wetting agents that effectively disperse colloidal
systems in aqueous media. The major commercial applications are in the agricultural chemical
industry, which uses ANS for wettable powder and wettable granular formulations, and the
textile and fabric industry, which utilizes the wetting and defoaming properties of ANS
for bleaching and dyeing operations. As a fumigant
Naphthalene has been used as a household fumigant. It was once the primary ingredient in mothballs,
though its use has largely been replaced in favor of alternatives such as 1,4-dichlorobenzene.
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 attack textiles. Other fumigant
uses of naphthalene include use in soil as a fumigant pesticide, in attic spaces to repel
animals and insects, and in museum storage-drawers and cupboards to protect the contents from
attack by insect pests. Naphthalene is a repellent to opossums and
could be used to deter them from taking up residency in people's homes.
Niche applications It is used in pyrotechnic special effects
such as the generation of black smoke and simulated explosions. In the past, naphthalene
was administered orally to kill parasitic worms in livestock. Naphthalene and its alkyl
homologs are the major constituents of creosote. Naphthalene is used in engineering to study
heat transfer using mass sublimation. Health effects
Exposure to large amounts of naphthalene may damage or destroy red blood cells. Humans,
in particular children, have developed this condition, known as hemolytic anemia, after
ingesting mothballs or deodorant blocks containing naphthalene. Symptoms include fatigue, lack
of appetite, restlessness, and pale skin. Exposure to large amounts of naphthalene may
cause confusion, nausea, vomiting, diarrhea, blood in the urine, and jaundice. Over 400
million people have an inherited condition called glucose-6-phosphate dehydrogenase deficiency.
Exposure to naphthalene is more harmful for these people and may cause hemolytic anemia
at lower doses. When the US National Toxicology Program exposed
male and female rats and mice to naphthalene vapors on weekdays for two years, male and
female rats exhibited evidence of carcinogenic activity based on increased incidences of
adenoma and neuroblastoma of the nose, female mice exhibited some evidence of carcinogenic
activity based on increased incidences of alveolar and bronchiolar adenomas of the lung,
and male mice exhibited no evidence of carcinogenic activity.
The International Agency for Research on Cancer classifies naphthalene as possibly carcinogenic
to humans and animals. The IARC also points out that acute exposure causes cataracts in
humans, rats, rabbits, and mice; and that hemolytic anemia, described above, can occur
in children and infants after oral or inhalation exposure or after maternal exposure during
pregnancy. Under California's Proposition 65, naphthalene is listed as "known to the
State to cause cancer". A probable mechanism for the carcinogenic effects of mothballs
and some types of air fresheners containing naphthalene has been identified.
Regulation US government agencies have set occupational
exposure limits to naphthalene exposure. The Occupational Safety and Health Administration
has set a permissible exposure limit at 10 ppm over an eight hour time-weighted average.
The National Institute for Occupational Safety and Health has set a recommended exposure
limit at 10 ppm over an eight hour time-weighted average, as well as a short-term exposure
limit at 15 ppm. Mothballs and other products containing naphthalene
have been banned within the EU since 2008. In China, the use of naphthalene in mothballs
is forbidden. It is due partly to the health effects as well as the wide use of natural
camphor as replacement. However naphthalene is widely produced for moth balls and currently
exported from China. See also
Camphor Dialin, Tetralin, Octalin, Decalin
List of interstellar and circumstellar molecules Mothballs
1-Naphthol, 2-Naphthol Sodium naphthalenide
Wagner-Jauregg reaction References
External links Mothballs Case Profile—National Pesticide
Information Center Naphthalene—EPA Air Toxics Web Site
Naphthalene—mostly on toxicity of naphthalene Naphthalene—CDC – NIOSH Pocket Guide
to Chemical Hazards Pesticide Properties Database record for Naphthalene