Лако-красочные материалы — производство

The difficultly soluble sodium salt from the first step is dissolved in 300 cc. hot water, and the free sodium hydroxide present is neutralized with dilute hydrochloric acid. The resulting solution is added during the course of 30 minutes to 200 grams of iron turnings which have been etched with 20 cc. 40 per cent acetic acid. The reduction then proceeds in the normal way (see, e. g., page 75).

When reduction is complete, the reaction mixture is neutralized with soda, filtered to remove the iron sludge, and made strongly acid to Congo red with hydrochloric acid, precipitating diaminostilbenedi- sulfonic acid as a yellowish, finely crystalline material. After 10 hours, the product is filtered off and washed thoroughly.

The yield is about 70 grams of 100 per cent material from 100 grams of p-nitrotoluene. This compound cannot be diazotized satisfactorily by the indirect method, in contrast to the analogous benzidene-2,2′- disulfonic acid.

Technical Observations. The preparation given here for diaminostilbenedisul — fonic acid was first described by Green. With the decrease in the cost of chlorine, it has completely replaced the Leonhardt method which involves the reduction of Mikado yellow, formed by the action of concentrated sodium hydroxide solution on p-nitrotoluenesulfonic acid. At the best, this older method gave only about 48 per cent of the theoretical yield, and the reduction required large amounts of zinc dust or ammonium sulfide. Also the product from the Green synthesis is much purer if the reaction mixture is not too concentrated, the product contains almost no diaminodibenzyldisulfonic acid, which appreciably impairs the preparation of chrysophenine. The presence of the dibenzyl compound in the product can be detected easily by either of two reactions. The first test depends on the fact that the dye from H acid and the dibenzyl acid is much more red than that from H acid and the stilbene derivative. The second test is based on the fact that the ‘’chryso­phenine” from the dibenzyl derivative is not a pure blue, but almost red violet, in the presence of mineral acid. The presence of a relatively small proportion of the dibenzyl compound can be recognized immediately by a comparison test with pure diaminostilbenedisulfonic acid.

Oxidation of the dinitro acid is carried out in concrete tanks. It should be pointed out that a small amount of iron or copper decomposes the hypochlorite solu­tion immediately, and for this reason, wooden tanks cannot be used.

For this reaction, one can use the o-chlorobenzoic acid obtained in the preparation of o-chlorobenzaldehyde. Before using the technical acid, it should, in all cases, be dissolved in dilute sodium hydroxide and reprecipitated after filtering the alkaline solution.

A solution is made by stirring 32 grams of pure o-chlorobenzoic acid with 160 grams of 100 per cent sulfuric acid in a porcelain or iron con­tainer, and then cooled to below 0°C. in an ice-salt bath. A mixture of 16 grams of 80 per cent nitric acid and 40 grams of 100 per cent sulfuric acid is added dropwise over a period of about 1 hour, keeping the tem­perature below 0° to prevent the formation of undesired by-products. Near the end of the reaction, part of the nitro compound separates. The mixture is held at room temperature for 10 to 12 hours, and then heated slowly to 60°, after which it is poured onto 400 grams of ice. The nitro — chlorobenzoic acid is filtered off and preferably recrystallized twice from 1 liter of boiling water to remove unchanged chlorobenzoic acid. This purification is omitted in industry, but is to be recommended in laboratory preparations. A pure product is obtained in this way in a yield of about 37.5 grams, or 92 per cent of the theoretical amount. The material melts at 164-165°.

For the reduction, 20.2 grams (0.1 mole) of chloronitrobenzoic acid is dissolved in 70 cc. water containing 5.5 grams of soda ash. The solu­tion is then acidified by the addition of 10 cc. 40 per cent acetic acid and is added dropwise to a boiling suspension of 100 grams of zinc dust in 250 cc. water to which 4 cc. 40 per cent acetic acid has been added. The mixture is boiled and stirred continuously, and since bad foaming is often encountered, a large reaction vessel should be used (З-liter enameled crock).

The reduction takes about 2 hours, after which 5 grams of soda ash is added and the solution is filtered. The filtrate, after evaporation to 200 cc., can be used directly for the preparation of azo dyes without isolating the chloroaminobenzoic acid. The yield, determined by titration, is about 92 per cent of the theoretical amount.

If it is desired to isolate the chloroaminobenzoic acid as such, the solution is acidified with hydrochloric acid, avoiding an excess.