It is a colourless solid, which melts at 80° C., and boils at
218° C. It crystallizes in the monoclinic system; it is to be
noted that α- and β-naphthol assume almost identical forms, so
that these three compounds have been called isomorphous. It is
insoluble in water, but is readily soluble in alcohol, and ether.
It has a characteristic smell, and is very volatile, distilling
readily in a current of steam. It acts as a weak antiseptic. It is
used for enriching coal gas, as a vermin killer, in the manufacture
of certain azo dyes, and in the preparation of phthalic acid (q.v.).
When passed through a red-hot tube packed with carbon it
yields ββ-dinaphthyl, (C10H7)2. It forms a crystalline compound
with picric acid. It readily forms addition products with
chlorine and with hydrogen; the dichloride, C10H8Cl2, is obtained
as a yellow liquid by acting with hydrochloric acid and potassium
chlorate; the solid tetrachloride, C10H8Cl4, results when chlorine
is passed into naphthalene dissolved in chloroform. Numerous
hydrides are known; heated with red phosphorus and hydriodic
acid the hydrocarbon yields mixtures of hydrides of composition
C10H10 to C10H20. Sodium in boiling ethyl alcohol gives the
α-dihydride, C10H10 (E. Bamberger, Ber., 1887, 20, p. 1705);
and with boiling amyl alcohol the β-tetrahydride, C10H12
(E. Bamberger, Ber., 1890, 23, p. 1561). The α-tetrahydronaphthalene
is formed when naphthalene is heated with phosphonium
iodide at 170°–190° (A. v. Baeyer). Structurally
naphthalene may be represented as a fusion of two benzene
nuclei, the hydrogen atoms being numbered as in the inset
formula 
; 1, 4, 5, 8 are α-positions, 2, 3, 6, 7 are β;
1-5 or 4-8 diderivatives are ana, whilst 1-8 or 4-5 are peri (see
Chemistry, Organic).
α-Nitronaphthalene, C10H7·NO2, is formed by the direct nitration of naphthalene. For its commercial preparation see O. Witt, Die chemische Industrie, 1887, 10, p. 215. It crystallizes in yellow needles, which melt at 61° C., and are readily soluble in alcohol. By the action of nitro-sulphuric acid it is converted into a mixture of 1·5 and 1·8 dinitronaplithalenes (P. Friedlander, Ber., 1829, 32, p. 3531). When heated with aniline and its salts it yields phenylrosindulin (German patent 67339 (1888)). β-Nitronaphthalene is prepared by acting with ethyl nitrite on an alcoholic solution of 2-nitro-α-naphthylamine in the presence of sulphuric acid (E. Lellmann and A. Remy, Ber., 1886, 19, p. 237), or with freshly prepared potassium cupronitrite on β-naphthalene diazonium sulphate (A. Hantzsch, Ber., 1900, 33, p. 2553). It crystallizes in small yellow needles which melt at 78° C. and are volatile in steam.
Sulphonic Acids.—Two monosulphonic acids (α and β) result by acting with sulphuric acid on the hydrocarbon, the α-acid predominating at low temperatures (80° C. and under) and the β-acid at higher temperatures (170°–200° C.). They are crystalline, hygroscopic compounds and are employed for the manufacture of the naphthols. Numerous di- and tri-sulphonic acids are known.
α-Naphthoquinone, C10H6O2, resembles benzoquinone, and is formed by the oxidation of many α-derivatives of naphthalene with chromic acid. It crystallizes in yellow needles which melt at 125° C. It sublimes readily, is volatile in steam and reduces to the corresponding dihydroxynapthalene. β-Naphthoquinone is formed by oxidizing 2-amino-α-naphthol (from β-napthol-orange by reduction) with ferric chloride. It crystallizes in red needles which melt at 115° C; it has no smell and is non-volatile (cf. phenanthrenequinone). Alizarin black, C10H4(OH)2O2·NaHSO3, the sodium bisulphite compound of 7·8 dioxy-α-naphthoquinone, is a dyestuff used for printing on cotton in the presence of a chromium mordant. The naphthoquinone is prepared by the action of zinc and concentrated sulphuric acid on α-dinitronaphthalene. A 2·6 naphthoquinone results on oxidizing 2·6 dihydroxynaphthalene with lead peroxide.
α-Naphthoic acid, C10H7·CO2H, is formed by hydrolysis of the nitrile, obtained by distilling potassium-α-naphthalene sulphonate with potassium cyanide (V. Merz, Zeit. f. Chemie, 1868, p. 34), or by heating the sulphonate with sodium formate (V. Meyer, Ann., 1870, 156, p. 274). It forms needles which melt at 160° C. β-Naphthoic acid, obtained by boiling β-methylnaphthalene with dilute nitric acid, or by hydrolysis of its nitrile (formed when formyl-β-naphthalide is heated with zinc dust), crystallizes from alcohol in needles which melt at 184° C.
NAPHTHOLS, or Hydroxynaphthalenes, C10H7OH, the
naphthalene homologues of the phenols. The hydroxyl group
is more reactive than in the phenols, the naphthols being converted
into naphthylamines by the action of ammonia, and
forming ethers and esters much more readily.
α-Naphthol may be prepared by fusing sodium-α-naphthalene sulphonate with 'caustic soda; by heating α-naphthylamine sulphate with water to 200° C. (English Patent 14301 (1892)); and by heating phenyl isocrotonic acid (R. Fittig and H. Erdmann, Ann. 1885, 227, p. 242): C6H5CH:CH·CH2·CO2H＝C10H7OH+H2O. It forms colourless needles which melt at 94° C.; and is readily soluble in alcohol, ether, chloroform, and caustic alkalis. A It is volatile in steam. With ferric chloride it gives a dark-blue precipitate of α-dinaphthol, HO·C10H6·C10H6·OH. Alkaline potassium permanganate oxidizes it to phenyl-glyoxyl-ortho-carboxylic acid, HO2C·C6H4·CO·CO2H. It is reduced by sodium in boiling amyl alcohol solution to “aromatic” tetrahydro-α-naphthol (reduction occurring in the ring which does not contain the hydroxyl group). When heated with hydrazine hydrate at 160° C. it gives α-naphthyl hydrazine, C10H7NH·NH2 (L. Hoffmann, Ber., 1898, 31, p. 2909). Nitric acid converts it into nitro-compounds, which are occasionally used for dyeing silk and wool.
Martius yellow, C10H5(NO2)2ONa·H2O, the sodium salt of 2·4 dinitro-α-naphthol (for notation see Naphthalene), is prepared by the action of nitric acid on α-naphthol-2·4-disulphonic acid. It forms orange-yellow plates and dyes wool a golden yellow (from an acid bath). Naphthol yellow S., C10H4(ONa)(NO2)2SO3Na, prepared by the action of nitric acid on α-naphthol-2·4·7-trisulphonic acid, is an orange-yellow powder which dyes wool and silk yellow (from an acid bath).
Numerous mono-, di- and trisulphonic acids of α-naphthol are employed in the preparation of azo dyes. The most important is Nevile and Winther’s acid, C10H6(OH)(SO3H)(1·4), formed when diazotized naphthionic acid (α-naphthylamine-4-sulphonic acid) is boiled with dilute sulphuric acid (Nevile and Winther, Ber.,1880, 13, p. 1949), or when sodium naphthionate is heated with concentrated caustic soda solution under pressure at 240°–260° C. (German patent 46307 (1888)). It melts at 170° C., and is readily soluble in water. With ferric chloride it gives a blue coloration.
β-Naphthol, C10H7OH, prepared by fusing sodium β-naphthalene sulphonate with caustic soda, crystallizes in plates which melt at 122° C. With ferric chloride it gives a green colouration, and after a time a white flocculent precipitate of a dinaphthol. With sodium in boiling amyl alcohol solution it gives a mixture of alicyclic and aromatic tetrahydro-β-naphthols (E. Bamberger, Ber., 1890, 23, p. 197). When heated with ammonium formate to 150° C. it forms β-naphthylamine. With nitrosodimethylaniline hydrochloride it forms Meldola’s Blue (dimethylamino-naphthophenoxazonium chloride), C18H15N2OCl (R. Meldola, Ber., 1879, 12, p. 2065).
The β-naphthol sulphonic acids find extensive application in the colour industry. The most important members are shown in the table:—
| Formula. | Method of Preparation. | Remarks. |
| 2-oxy-8-sulphonic (Baeyer’s acid) |
From β-naphthol and concentrated sulphuric acid at 50°–60° C. | Sodium salt soluble in strong alcohol. |
| 2-oxy-6-sulphonic (Schaffer’s acid) |
From β-naphthol and concentrated sulphuric at 100° C. | Sodium salt insoluble in alcohol. |
| 2-oxy-7-sulphonic (F-acid) |
By fusion of naphthalene 2·7-disulphonic acid with caustic soda at 200°–250° C. | Very soluble in water and alcohol. |
| 2-oxy-3·6-disulphonic (R-acid) |
Both R- and G-acid from β-naphthol and concentrated sulphuric acid at 100°–100° C. | The sodium salts separated by crystallization. R-salt insoluble in alcohol; G-salt soluble. |
| 2-oxy-6·8-disulphonic (G-acid). | ||
| 2-oxy-3·6·8-trisulphonic | From β-naphthol and fuming sulphuric acid at 140°–160° C. | Alkaline solutions show green fluorescence. |
