Reduction by means of Zinc Dust. - 91 gms. (= 1/2 mol.) azobenzene is heated up with 250 gms. alcohol and 200 gms. soda lye (30 % NaOH) to 450 in an iron or glass vessel provided with a powerful stirrer and a reflux condenser. Zinc dust is then added by degrees until the solution is only a faint yellow. According to the purity of the zinc dust, about 200-250 gms. will be required. The temperature during the addition must not be allowed to exceed 6o°, otherwise aniline is readily formed. As soon as the liquid has been bleached it is filtered quickly through a nutsch, the zinc dust made into a paste with 100 c.cs. 90 % alcohol, quickly boiled up and filtered into the first portion; the extraction is then repeated. The zinc dust is spontaneously inflammable, and must not, therefore, be thrown into the dustbin.

91 gms. Azobenzene. 250 gms. Alcohol. 200 gms. NaOH (30 %)•

220-250 gms. Zinc dust.

100 c.cs.

Alcohol

(twice).

The aqueous-alcoholic solution separates into two layers, the upper containing the hydrazobenzene, and the lower the sodium zincate. The liquids are run off through a separating-funnel and the upper is saturated with carbon dioxide before evaporating off the alcohol. As much of the alcohol as possible is then distilled off, and 200 c.cs. water are then added to the residue with shaking. The hydrazobenzene comes out first of all as an oil, which then solidifies to coarse crystalline fragments. After filtering off it is quite pure enough for further working up. The yield of dry substance is quantitative = 92 gms.

Modifications. - 91 gms. of pure azobenzene are dissolved in 250 c.cs. alcohol, and 250 c.cs of 20 % ammonia are added. A rapid stream of hydrogen sulphide is passed into this suspension, which heats up considerably, becoming darker at first, and then rapidly colourless. The whole reduction occupies about 1/2-1 On cooling, the hydrazobenzene separates out in beautiful, glistening, colourless or pale-yellow crystals After standing 12 hours the product is filtered off and washed with a little water. Yield about 92 gms.

This method of preparation has the advantage that no aniline is produced so long as the temperature does not exceed 6o°, and the hydrogen sulphide is not allowed to act for too long.

91 gms. Azobenzene. 250 c.cs. Alcohol.

250 c.cs. 20 % Nh3.

Conversion of the Hydrazobenzene to Benzidine.

Owing to the easy oxidizability of the hydrazobenzene it should be dealt with so far as possible in the moist condition. The conversion must be effected by means of hydrochloric acid free from sulphuric acid since benzidine sulphate is insoluble.1 The finely divided hydrazobenzene is added cautiously to the purified acid, In the present case about 1.2 mol. technical acid (30 % Hc1) is used, the liquid at the end of the reaction remaining strongly acid to Congo. The temperature of the transformation is kept as low as possible by the cautious addition of 100 gms. of ice. The hydrazobenzene may be added quickly. The mass is then stirred continuously for 5 hours, and is heated up during 1 hour to 80°, all the benzidine and diphenyline going into solution. At this ca. 120 gms, Hc1 (30 %).

100 gms. Ice.

1 Hydrochloric acid, free from sulphuric acid, may be obtained by mixing 15 % commercial acid with barium chloride solution, until no further precipitate is formed.

stage of the operations oily drops of azobenzene frequently form, but only if the temperature has been too high or if much oxidized hydrazobenzene has been used.

The product is now allowed to cool until the precipitate becomes easily filterable, which is usually at about 6o°. Where the reduction has been effected by means of ammonium sulphide a fairly heavy precipitate of sulphur is formed which is filtered off warm. The residue is washed out with 50 c.cs. of water at 6o°. The solution of the benzidine is always coloured blue- to red-violet. The resultant benzidine hydrochloride is now precipitated with the calculated quantity of sulphuric acid or bisulphate (the cheapest form of sulphuric acid). For this purpose about 55 gms. of 66° Be. sulphuric acid are required. The benzidine sulphate is precipitated instantaneously as a thick, crystalline deposit. It may therefore be filtered off after a few minutes and thoroughly washed with water containing 1/2% sulphuric acid. It is then stirred up afresh with 400 c.cs. of water and made alkaline with about 50 gms. soda. The decomposition of the sulphate must be effected as quickly as possible, as it has been found that after a few hours the salt does not react so rapidly with soda. The benzidine sulphate mother-liquor is deeply coloured, and gives a precipitate of about 8 gms. diphenyline on making alkaline with soda. The free benzidine base, which always becomes a little darker on standing, separates out as a grey-white flocculent mass; it is filtered off and well washed with a little cold water. The dried product has an apparent purity of 98 %, but on distillation about 5 % of the total weight always remains behind in the form of pitch. With very exact working, which is by no means easy to carry out, a yield is obtained from 1 gm.-molecule of nitrobenzene of about 80 gms. purest distilled benzidine base (B.p. 405°; 240°/15 mm.).

Notes on Works Technique and Practice. - The manufacture of benzidine has developed into one of the most important operations in colour technology, as the direct cotton colours obtained from it are literally indispensable. The price of the product before the war was very low, owing to the keen competition of the different factories, namely about 2 fr. 90 per kilo. Whilst the reduction of nitrobenzene was carried out as recently as 15 years ago exclusively with caustic soda-lye, methyl alcohol, and zinc dust, the situation has altered completely at the present time, as there are now only two processes which can compete. The one which we have discussed replaces these expensive reducing agents by the cheaper iron, which is recovered as iron oxide and can then go back to the blast-furnace. For the last stage of the reduction zinc dust is often made use of in place of iron, but in the works it is more advantageous to complete this stage also by means of iron powder. Once the azobenzene has been isolated, the chief difficulties disappear, as the main bulk of the iron sludge has been got rid of. The reduction of the azobenzene with hydrogen sulphide, however, is also worth consideration as, in certain factories, this is a cheap by-product; it is impossible to say beforehand which process is to be preferred. The quality of the iron must be the same as that used for the Bechamp reduction, and, further, the borings must be carefully freed from grease.