1911 Encyclopædia Britannica/Naphthols

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.