NAPHTHALENE, a hydrocarbon, C,,1-1, formed when many organic compounds, e.g., acetylene, ethylene, acetic acid, ether or camphor, are passed as vapours through a red-hot tube. This accounts for its presence in certain fractions of the coal-tar dis tillate (see COAL-TAR), where it was originally observed in 1819 by Garden and independently by Brande. Its composition was first determined by Faraday, in 1826. The amount of naphthalene produced during the manufacture of coal-gas varies with the type of coal used and increases with rising temperature of carbon ization; it rarely exceeds io% of the tar distillate, and the usual amount is about 5%. Its presence among the products obtained by the modern low-temperature carbonization processes has not yet been established. Even when formed in only small amounts, naphthalene is frequently carried by the coal-gas through the various purifying chambers and escapes into the service pipes, where it gradually accumulates to such an extent as to necessitate its removal by mechanical means. Naphthalene also occurs in some natural mineral oils, but derivatives in nature are rare. Juglone (5-hydroxy-I :4-naphthoquinone), occurring in freshly gathered walnut shells, and santonin and artemisin, the active principles of wormseed, are the chief.
When the "carbolic oil" fraction (boiling point C) of the coal-tar distillate is allowed to cool, the crude naphthalene crystallizes out and is readily purified. The oily impurities are first removed by filtration; the naphthalene crystals are then hot pressed, washed with hot caustic soda to remove acidic phenols (see CARBOLIC ACID) and then heated at Ioo° C for an hour with 5-10% of strong sulphuric acid to remove basic substances and any residual phenols. After further washings with dilute alkali and hot water, the naphthalene is finally distilled with steam or sublimed.
Properties and Uses.—Pure naphthalene crystallizes in white, shining plates, melting at 8o° C, and boiling at 218° C. It pos sesses a characteristic tarry odour, a pungent taste, and is appre ciably volatile even at ordinary temperatures. It is sparingly solu ble in hot water and in petroleum spirit, but dissolves readily in benzene, alcohol and ether. With picric acid it forms a stable crystalline picrate, this, in the absence of other picrate-forming compounds, provides a method of estimat ing naphthalene. When heated in acetic acid solution with formal
dehyde and a little sulphuric acid, naphthalene gives a resin, which, unlike phenolic resins, does not become infusible when baked. (See RESINS, Synthetic.) The products obtained on oxidizing naphthalene vary considerably with the type of oxidizing agent employed. Potassium permanganate, in alkaline solution, gives phthalonic acid, but in acid solution phthalic acid is the chief product. Potassium dichromate and sulphuric acid, too, give phthalic acid, but with chromic acid in acetic acid solution, I naphthoquinone is mainly formed. The action of an excess of I00% sulphuric acid and mercuric sulphate at 20o° C produces mainly phthalic acid, which was formerly manufactured by this method.
Naphthalene burns with a luminous, smoky flame, and is in consequence used for increasing the illuminating power of coal gas. It is also utilized as a vermin killer (in the form of the so called "white carbon" balls), though as its antiseptic properties are slight, it more frequently functions as a solid solvent for other substances, e.g., para-dichlorobenzene, of higher germicidal power. It is also a useful solvent for sulphur, phosphorus, metallic sul phides, and indigo. The value of naphthalene depends most, how ever, on its extensive employment in the manufacture of dyestuffs, of explosives, and of phthalic acid (q.v.), the last-named being manufactured by the oxidation of naphthalene in the vapour phase by air, using vanadium pentoxide or vanadyl chloride as catalyst. The derivatives most important to the dyestuffs manu facturer are the sulphonic acids of the naphthylamines, naphthols, aminonaphthols, and dihydroxynaphthalenes, but not all are of equal value. Indeed, only relatively few form azo-dyes which are completely satisfactory as regards solubility, fastness, tinctorial power and delicacy of shade. The market price of naphthalene varies of course with the degree of purity, but approximates to ./14 per ton. Commercial naphthalene has setting point 79° C, and boiling point 217° C; it should volatilize completely, leaving no residue, and be free from phenols and quinoline bases; it should remain white in nitric acid (sp.gr. 1.42) for half an hour.