Лако-красочные материалы — производство

The reduction most frequently encountered by the dye chemist is the transformation of a nitro compound into an amine, but numerous other reductions play an important role in dye chemistry. The following methods are employed.

(1) Reduction with iron and water, with the addition of small amounts of acid (hydrochloric, sulfuric, or acetic acid, and occasionally mixtures of these). This “neutral” reduction method of Bechamp-Brimmeyr can be carried out only with certain kinds of iron, notably gray-iron casting, which must always be tested pre­viously to establish its usability. In general, other kinds of iron are not usable (many examples have been given).

(2) Reduction with iron and enough acid so that all of the iron used in the reduction goes into solution as the ferrous salt. For this purpose, any kind of iron can be used but it is desirable to use a variety low m carbon, for example, iron nails, iron plate, steel, etc., so that no contaminating graphite particles are formed (see H acid, page 212).

(3) Reduction with hydrogen sulfide or its salts (many examples are given in this book).

(4) Reduction with zinc dust and acid or alkali (rarely with tin).

(5) Reduction with hydrosulfite (e. g., p-aminosalicylic acid from phenylazo — salicylic acid, page 157).

(6) Electrolytic reduction, as in the preparation of hydrazobenzene from nitrobenzene, or of p-aminophenol from nitrobenzene. The latter reduction involves a simultaneous rearrangement of the intermediate phenylhydroxylamine.

(7) Reduction with ferrous hydroxide, Fe(OH)2, now seldom used.

(8) Reduction with aluminum powder, for example, in the preparation of benzanthrone, or of quinizarin from purpurin (seldom used).

(9) Catalytic reduction with hydrogen.

(10) Reduction with sulfur dioxide, often giving simultaneous sulfonation.