Naphthalene Derivatives

Carboxylic Acids.

Only certain hydroxy-derivatives of a and j3-naphthoic acids are technically valuable. Their chief inter est, with one exception, lies in one of the methods of preparation, for Kolbe's synthesis (the action of carbon dioxide under pressure on the dry sodium or potassium salt of the naphthol) furnishes from a-naphthol the 2-carboxy-acid, and from 0-naphthol the I-, 3-, or 6-carboxy-acid, according to the temperature. Of these, 2-hydroxy-3-naphthoic acid, melting point C, is most impor tant, since it gives rise to various anilides, toluidides, naphthyl amides, etc., when heated in a solution of a tertiary base, such as dimethylaniline, with the appropriate amine in presence of phosphorus trichloride. These substances, of which the simplest, name "Naphthol AS" (Brenthol AS), possess a strong affinity for both natural and artificial fibres. When the material so impreg nated is passed through a diazo-solution from p-nitroaniline or other aromatic base, coupling takes place inside rather than on the surface of the fibre, and the colours are much faster to light and washing than those from azo-dyes produced by merely "padding" the fibre with (say) 0-naphthol. Various dyestuffs of this series are available for cotton, wool and artificial silks.

Hydrogenated Derivatives.

All the hydrogenated products of naphthalene, from dihydronaphthalene, to naphthalene, C.H., are known, each member having two hydrogen atoms more than its immediate predecessor. Three general methods are available for their preparation, in all of which, however, the products vary considerably with variations in the conditions. (1) Reduction with hydriodic acid: this method gives mixtures diffi cult to purify. Two intermediate products in the preparation of decahydronaphthalene by this method have been identified, viz., I :4-dihydro- and I :4 :5 : 8-tetrahydro-naphthalene. (2) Reduction with sodium and an alcohol : by this method no more than four hydrogen atoms can be introduced into the naphthalene nucleus, no matter what alcohol is employed. Amyl alcohol and sodium yield i :3 :4 :-tetrahydronaphthalene, but the product when any other alcohol is used depends to a certain extent on the concentra tion of the sodium alcoholate, and also on the boiling point of the solvent. I :4-Dihydronaphthalene I. is the first product and un less this isomerizes to i : 2-dihydronaphthalene II. hydrogenation can proceed no further. (3) Reduction with hydrogen in pres ence of a catalyst : the particular hydro-derivative obtained by this method depends upon a variety of conditions. Most impor tant is the nature and oxygen content of the catalyst, but the degree of hydrogenation is dependent also on the temperature, pressure and duration of the reaction, and on whether the naph thalene reacts in the vapour or liquid phase or in solution.

method adopted for the manufacture of 1:2 :3 Tetralin is a colourless liquid, boiling point 206°-208° C, flash point 78° C. It is insoluble in water and only sparingly soluble in ethyl alcohol (except after long standing, when oxidation products have affected its physical properties), but it mixes freely with amyl alcohol, butyl alcohol or benzene. It is a useful solvent for certain types of synthetic resins, essential and vegetable oils, waxes, rubber, sulphur and many organic substances and dye stuffs; it also finds application as a diluent in boot polishes. Both

it and "tetralin extra" (a mixture of decalin and tetralin) are used in the manufacture of lacquers and varnishes ; decalin alone has not such a powerful solvent action. Tetralin was used in Germany during the World War as a motor fuel in place of petrol, but under normal conditions its high flash point is a serious ob jection for such a purpose. When it is treated with concentrated sulphuric acid and the sulphonated oil neutralized with caustic soda, tetralin-soda sulphite, a white soapy substance, soluble in water and possessing a powerful emulsifying action, is formed. "Tetralol" (ar.-tetra-hydro-/3-naphthol) is used as a disinfecting soap; it is non-poisonous to man and may therefore also be em ployed for the preservation of fruit-trees.

When the naphthylamines and naphthols are reduced to their tetrahydro-derivatives with sodium in boiling amyl alcohol, two types of compounds are formed : (a) those in which the four hydrogen atoms are attached to the same ring as the original substituent, and (b) those in which the original substituent is in one ring and the four added hydrogen atoms are in the other. The members of class (a) are termed "alicyclic" (or ac.-) de rivatives, because they have become aliphatic in character; e.g., f3-naphthylamine is hydrogenated to ac.-tetrahydro-13-naphthyl alkaline, has an ammoniacal odour, absorbs carbon dioxide from the air, and cannot be diazotized. f3-Naphthol similarly gives a secondary alcohol. The members of class (b) are termed "aro matic" (or ar.-) because they still bear a close analogy to the corresponding compounds in the benzene series ; e.g., a -naphthyl amine and a -naphthol yield on hydrogenation ar.-tetrahydro at their works near Rosslau in Germany, where 120 tons per day can be produced, is briefly as follows : Commercially pure naph thalene, which contains sulphur compounds (e.g., thionaph thenes) equivalent to 0.25% of sulphur, is freed from them by heating with fuller's earth, kieselguhr or other absorbent ma terial. If this were not done, the catalyst would be quickly poi soned and unable to function. The purified naphthalene is dis tilled into the hydrogenating autoclaves, which contain the catalyst (usually finely-divided nickel and copper oxide deposited on an inert medium), and hydrogen (freed from carbon monoxide and sulphur dioxide) is pumped in until at 200° C a pressure of about 22o1b. per sq.in. is attained. The agitated molten naphthalene rapidly takes up hydrogen until the tetrahydro-stage is reached, when the rate of absorption diminishes. The product is then dis tilled under reduced pressure, a fresh charge of molten naph thalene introduced, and the process repeated. As many as 40 hydrogenations can be effected before the catalyst has to be re is required, more hydrogen is introduced when the reaction at tains the tetrahydro-stage, and the process continued.

-OH), the properties of which closely resemble those of ortho xylidine and ortho-xylenol respectively.