Naphthalene

Naphthalene (also known as naphthalin, naphthaline, tar camphor, white tar, albocarbon, or naphthene) is a crystalline, aromatic, white, solid hydrocarbon, best known as the primary ingredient of mothballs. Naphthalene is volatile, forming a flammable vapor. Its moleculesconsist of two fused benzenerings. It is manufactured from coal tar, and converted to phthalic anhydridefor the manufacture of plastics, dyesand solvents. It is also used as an antisepticand insecticide, especially in mothballs. p-Dichlorobenzeneis now often used instead of naphthalene as a mothball substituent. Naphthalene easily sublimesat room temperature without vacuum.

Name

The name naphthalene is derived from the Latin word of Iranian origin naphtha. Naphthais a term for any volatile and usually flammable liquid hydrocarbon mixture, normally in the context of a solvent. It was earlier spelled naphthaline. The name phthalic acid is a shortened naphthalic acid, which is named from 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 benzenoid polycyclic aromatic hydrocarbon(PAH). Naphthalene has three resonance structures, which are shown in the drawing below. Naphthalene has two sets of equivalent hydrogens. 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 diffractionand 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.

Image:Naphthalene resonance.PNG

Like benzene, naphthalene can undergo electrophilic substitution. The major product generally has the electrophile in the "alpha" position. 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 or with a special catalyst to give decalin(C₁₀H₁₈, also known as bicyclo[4.4.0]decane). Oxidationof naphthalene with chromateor permanganate, or catalytic oxidation with O₂ and a vanadiumcatalyst, gives phthalic acid.

Health effects

In humans, exposure to large amounts of naphthalene may damage or destroy red blood cells. This could cause the body to have too few red blood cells until it replaces the destroyed cells. Humans, particularly children, have developed this condition after ingesting mothballs or deodorant blocks containing naphthalene. Some of the symptoms of this condition are fatigue, lack of appetite, restlessness, and pale skin. Exposure to large amounts of naphthalene may also cause nausea, vomiting, diarrhea, bloodin the urine, and a yellow color 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 (1), male and female rats exhibited: clear 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) (2) classifies naphthalene as possibly carcinogenic to humans [Group 2B]. It also points out that acute exposure causes cataracts in humans, rats, rabbits, and mice and, that haemolytic anaemia, described above, can occur in children and infants after oral or inhalation exposure or after maternal exposure during pregnancy.

See also

• Decahydronaphthalene(the fully saturated analog of naphthalene)
• Naphthol

References

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External links

• Naphthalene (PIM 363)- mostly on toxicity of naphthaleneda:Naftalin

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Categories: Hydrocarbons| Simple aromatic rings| Insecticides| Antiseptics