Naphthalene

Naphthalene (not to be confused with naphtha), also known as naphthalin, naphthaline, tar camphor, white tar, or albocarbon, is a crystalline, aromatic, white, solid hydrocarbon, best known as the traditional, primary ingredient of mothballs. It is volatile, forming a flammable vapor, and readily sublimes at room temperature.

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History

In 1819–1820, at least two chemists reported a white solid with a pungent odor derived from the distillation of coal tar. In 1821, John Kidd described many of this substance's properties and the means of its production, and 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).^[1] Naphthaline's chemical formula was determined by Michael Faraday in 1826. The structure of two fused benzene rings was proposed by Emil Erlenmeyer in 1866,^[2] and confirmed by Carl Graebe three years later.

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 benzenoid polycyclic aromatic hydrocarbon (PAH). There are two sets of equivalent hydrogens: 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 highly-symmetrical aromatics, such as 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 the other carbon-carbon bonds are about 1.42 Å (142 pm) long. This has been verified by x-ray diffraction, and is consistent with the valence bond model of bonding in napthalene which involves three resonance structures (as shown below); while 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 is more reactive than benzene, reacting 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. Similarly, while 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.

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.

Naphthalene can be hydrogenated under high pressure with metal catalysts to give 1,2,3,4-tetrahydronaphthalene or tetralin (C₁₀H₁₄). Further hydrogenation yields decahydronaphthalene or decalin (C₁₀H₁₈). Oxidation with chromate or permanganate, or catalytic oxidation with O₂ 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. 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 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 (chiefly various phenols), and with sulfuric acid to remove basic components, is fractionally distilled to isolate naphthalene. The crude naphthalene resulting from this process is about 95% naphthalene by weight, often refered to as 78⁰C (melting point). The chief impurity is the sulfur-containing aromatic compound benzothiophene. 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, refered to as 80⁰C (melting point).

In North America, coal tar producers are Koppers Inc. and Recochem Inc., and petroleum-derived produer is Advanced Aromatics, L.P..

Incidence in nature

Trace amounts of naphthalene are produced by magnolias and specific types of deer. Naphthalene has also been found in the Formosan subterranean termite, possibly as a repellant against "ants, poisonous fungi and nematode worms." [1]

Uses

Naphthalene's most familiar use is as a household fumigant, such as in mothballs (although 1,4-dichlorobenzene (or p-dichlorobenzene) is now more widely used). 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, and in attic spaces to repel animals and insects.

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, naphthalene sulfonate polymer plasticizer 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^{[citation needed]}, such as the use of moth balls in a tool box.

Health effects

Exposure to large amounts of naphthalene may damage or destroy red blood cells. Humans, particularly 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 nausea, vomiting, diarrhea, blood in the urine, and jaundice (yellow coloration of the skin).

When the U.S. National Toxicology Program exposed male and female rats and mice to naphthalene vapors on weekdays for two years,^[3] 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 (IARC)^[4] classifies naphthalene as possibly carcinogenic to humans [Group 2B]. 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.

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.

See also

• Decahydronaphthalene (the fully saturated analog of naphthalene)
• Naphthol
• Classic naphthalene synthesis: the Wagner-Jauregg reaction

References

1. ^ John Kidd (1821). "Observations on Naphthaline, a peculiar substance resembling a concrete essential oil, which is apparently produced during the decomposition of coal tar, by exposure to a red heat". Philosophical Transactions 111: 209-221.
2. ^ Emil Erlenmeyer (1866). "Studien über die s. g. aromatischen Säuren". Annalen der Chemie und Pharmacie 137 (3): 327 - 359. doi:10.1002/jlac.18661370309.
3. ^ NTP Technical Reports 410 and 500. NTP Technical Reports 410 and 500, available from NTP: Long-Term Abstracts & Reports. Retrieved on March 6, 2005.
4. ^ IARC Monographs on the Evaluation of Carcinogenic Risks to Humans. Monographs on the Evaluation of Carcinogenic Risks to Humans, Some Traditional Herbal Medicines, Some Mycotoxins, Naphthalene and Styrene, Vol. 82 (2002) (p. 367). Retrieved on March 9, 2005.