Naphthalene is an organic compound with the chemical formula C10H8. It is a white crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 ppm. As an aromatic hydrocarbon, naphthalene's structure consists of a fused pair of benzene rings, making it a simple and rather symmetrical polycyclic aromatic hydrocarbon (PAH). It is the main ingredient of traditional mothballs.
History Alexander Garden first reported he had crystalised from the distillate (of coal tar) "a silvery solid that resembled Camphor and Benzoic Acid" in 1819. Further sources credit Garden with the discovery. Two days after Garden's paper was submitted, William Thomas Brande submitted his paper, and John Kidd followed with "a longer and more thorough examination of the same solid, finding that it was composed of carbon and a little hydrogen". Garden and Brande's separate discoveries were both published in 1820. In 1821, John Kidd 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, a broad term for various volatile, flammable liquid hydrocarbon mixtures, including coal tar. 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 A naphthalene molecule can be viewed as the fusion of a pair of benzene rings. (In organic chemistry, rings are fused if they share two or more atoms.) The eight carbon atoms that are not shared by the two rings carry one hydrogen atom each. For purpose of the standard IUPAC nomenclature of derived compounds, those eight atoms are numbered 1 through 8 in sequence around the perimeter of the molecule, starting with a carbon atom adjacent to a shared one. The shared carbon atoms are labeled 4a (between 4 and 5) and 8a (between 8 and 1). The molecule is planar, like benzene. 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 1.37 Å (137 pm) in length, whereas the other carbon–carbon bonds are about 1.42 Å (142 pm) long. This difference, established by X-ray diffraction, is consistent with the valence bond model in naphthalene and in particular, with the theorem of cross-conjugation. This theorem would describe naphthalene as an aromatic benzene unit bonded to a diene but not extensively conjugated to it (at least in the ground state), which is consistent with two of its three resonance structures.
Because of this resonance, the molecule has bilateral symmetry across the plane of the shared carbon pair, as well as across the plane that bisects bonds C2-C3 and C6-C7, and across the plane of the carbon atoms. Thus there are two sets of equivalent hydrogen atoms: the alpha positions, numbered 1, 4, 5, and 8, and the beta positions, 2, 3, 6, and 7. Two isomers are then possible for mono-substituted naphthalenes, corresponding to substitution at an alpha or beta position.
Structural isomers of naphthalene that have two fused aromatic rings include azulene, which has a 5–7 fused ring system, and Bicyclo[6.2.0]decapentaene which has a fused 4–8 ring system.
Electrical conductivity Pure crystalline naphthalene is a moderate insulator at room temperature, with resistivity of about 1012 Ω·m. The resistivity drops more than a thousandfold on melting, to about 4×108 Ω·m. Both in the liquid and in the solid, the resistivity depends on temperature as:
ρ = ρ0 × expE/kT Where ρ0 (Ω·m) and (eV) are constant parameters, k is the Boltzmann constant (8.617×10−5 eV/K), and T is absolute temperature (K). The parameter E is 0.73 in the solid. However, the solid shows semiconducting character below 100 K (−280 °F; −173 °C).
Chemical properties
Reactions with electrophiles In electrophilic aromatic substitution reactions, naphthalene reacts more readily than benzene. For example, chlorination and bromination of naphthalene proceeds without a catalyst to give 1-chloronaphthalene and 1-bromonaphthalene, respectively. Likewise, whereas both benzene and naphthalene can be alkylated using Friedel–Crafts reaction conditions, naphthalene can also be easily alkylated by reaction with alkenes or alcohols, using sulfuric or phosphoric acid catalysts. Anhydrous aluminium chloride reacts with naphthalene to give a polymer, in which one ring of each naphthalene monomer loses aromaticity, linking to the other monomers at the 1 and 4 positions. In terms of regiochemistry, 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 gives the "alpha" product naphthalene-1-sulfonic acid as the kinetic product but naphthalene-2-sulfonic acid as the thermodynamic product. The 1-isomer forms predominantly at 25 °C (77 °F), and the 2-isomer at 160 °C (320 °F). Sulfonation to give the 1- and 2-sulfonic acid occurs readily:
H2SO4 + C10H8 → C10H7SO3H + H2O Further sulfonation give di-, tri-, and tetrasulfonic acids.
Reduction and oxidation With alkali metals, naphthalene forms the dark blue-green radical anion salts such as sodium naphthalenide, Na+[C10H8]−. The naphthalene anions are strong reducing agents. Naphthalene can be hydrogenated under high pressure in the presence of metal catalysts to give tetralin (C10H12). Further hydrogenation yields decalin (C10H18). Oxidation with O2 in the presence of vanadium pentoxide (V2O5) as catalyst gives phthalic anhydride:
C10H8 + 4.5 O2 → C6H4(CO)2O + 2 CO2 + 2 H2O This reaction is the basis of the main use of naphthalene. Oxidation can also be effected using conventional stoichiometric chromate or permanganate reagents.
Production
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