35 gms. Nitrite.

2:5:7-Naphthyl-amine disul-phonicacid. ca. 180 gms. 220 gms. NaOH. 160 gms. H2O.

Nitrobenzene Sulphonic Acid and Metanilic Acid from

Nitrobenzene.

Reaction:

Sulphone.

1 Gm.-molecule of nitrobenzene is allowed to run cautiously into three times the quantity of oleum (25 % So3) at 700, contained in a cast-iron pot similar to that used for the sulphonation of the naphthalene sulphonic acid The mixture warms up rapidly to 100-no0, but must not be allowed to rise any higher, or else there is danger of sudden carbonization. When all has been added, the mixture is heated at 110-115° until a test portion poured into water no longer gives any odour of nitrobenzene. If complete sulphonation has not occurred within half an hour of the mixing, then insufficient So3 has been used. In this case 50 gms. more oleum are added drop by drop, and if necessary a further quantity after half an hour more, but if the oleum used really contained 25 % So3 no more man the original quantity will be required. The mixture is then allowed to cool, and is poured on to 500 gms. ice with good mechanical stirring. The nitrobenzene sulphonic acid goes completely into solution with the exception of a small proportion of sulphone.

The further working up of the acid may be effected in various ways, e.g. as given under H-acid on pp. 15 - 20. ' The method recommended is to salt out the acid, as the sodium salt is practically insoluble in saturated brine; 200 gms. common salt, in small quantities at a time, are slowly sprinkled in with continuous stirring. The sodium salt of nitrobenzene sulphonic acid separates out as a thick paste, and stirring must be continued for some time, until the mass again liquefies. After about 10 hours the solid is filtered off through paper on a large suction funnel, and is then well pressed (in cotton cloth) in a screw-press. The sodium salt can be used technically without further treatment, or may be obtained pure by recrystallizing from water.

The reduction is carried out as described for H-acid, with the one difference that the iron used need not be subjected to a preliminary " etching," as the free acid contained in the press-cake is sufficient to start the reaction. The reduction product is then neutralized and filtered, as described on p. 18. On evaporating down to 600 c.cs. and acidifying the solution with hydrochloric acid until acid to Congo, the metanilic acid comes out as a finely crystalline precipitate. Many works prefer to use the concentrated solution directly, as the metanilic acid is extremely soluble, and 10-15 % is always lost on separating out, although this loss is more than made up for by the higher yield of finished dye. The yield is determined by simply titrating the mineral acid solution with sodium nitrite, and is about 90 %= approximately 140 gms. of 100 % product.

Other similar Sulphonations. - The following substances are sulphonated in similar manner to the foregoing,p-nitro-chlorbenzene, p-nitro-toluene, o-nitro-chlorbenzene, chlorbenzenes, etc. Dinitro bodies, on the other hand, cannot be sulphonated in this way. Dinitrochlorbenzene, on treatment in this way with fuming sulphuric acid, decomposes with explosive violence, as do the dinitro-toluenes. If it be desired to prepare dinitrochlorbenzene disulphonic acid, for example, one starts with p-nitrochlorbenzene: this is sulphonated according to the above method, and the sulphonic acid then converted into the dinitrochlorbenzene sulphonic acid by means of 50 % mixed acid (50 % H2So4 + 50 % Hno3) at a low temperature; it has, however, no technical importance. Dinitro-naphthalenes are converted into naphthazarine on treatment with oleum.

123 gms. Nitrobenzene. 375 gms. Oleum

(25 %).

200 gms. Salt.

500 gms. Ice.

250 gms. Iron Powder. 400 c.cs. Water.

ca. 100 c.cs. Hc1 (30 %), and, if necessary, 100 gms. NaCl.

Notes on Works Technique and Practice. - Sulphonations similar to the foregoing are carried out on the works scale in steam-jacketed vessels through which either steam or cold water may be allowed to flow. The substances frequently heat up very considerably, so that it is necessary to use caution in working, as otherwise dangerous rises in temperature and even explosions may take place. The salting out is done in wooden vats, the pressing of the filter cakes being effected first in filter-presses and then (in hair cloths) in hydraulic presses at 250 atmospheres. The reduction, evaporation, and further working up are carried out as already described.