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. |
Nitrosonaphthols or naphthoquinone-oximes, C10H6(OH)(NO) or C10H6(:NOH):O. Two are known, namely 4-nitroso-α-naphthol or α-naphthoquinane-oxime, formed by the action of nitrous acid on α-naphthol or of hydroxylamine hydrochloride on α-naphthoquinone (H. Goldschmidt and H. Schmidt, Ber., 1884, 17 p. 2064); and 2-nitroso-α-naphthol (β-naphthoquinone-oxime), formed by the action of hydroxylamine hydrochloride on β-naphthoquinone.