In addition to sulphuric acid certain other sulphonating agents may be made use of, although they play no very important part in dye technology. The use of chlor-sulphonic acid as a sulphonating agent will not be discussed here, as it is only employed in very special cases. Its application to the preparation of the acid chlorides of the toluene sulphonic acids may be referred to; these may be prepared in the same apparatus as already described for other sulphonations. In addition to its use for the preparation of sulphonic chlorides chlor-sulphonic acid is still employed for the sulphonation of certain azo colours, as it forms a very mild non-oxidizing sulphonating medium, but it cannot be used for the sulphonation of bases; amino groups inhibit its action at lower temperatures so that it is more advantageous to use sulphuric acid or oleum. Hydroxy-lated compounds do not yield the sulphonic chlorides of the phenols or naphthols, but give the corresponding free sulphonic acids directly.

Bisulphite and neutral sulphite may also be used under certain conditions for introducing a sulphonic group. This method of introducing a sulphonic group into an aromatic nucleus was first employed by Nietzki (D. R. P. 89097). Thus nitraniline sulphonic acid 3:1:4 may be obtained by the action of sodium sulphite upon dinitro-benzene but in poor yield, and, so far as is known, this is not made use of practically.

Reaction:

More important is the introduction of the sulphonic group by replacing an easily removable chlorine atom with the aid of neutral sodium sulphite, as was proposed by Erdmann in D. R. P. 65240. By a slight modification of this interesting process it is possible to improve the yield considerably.

m-Phenylenediamine Sulphonic Acid 1:2:4.

Reaction:

202 Gms. dinitro-chlor-benzene (=1 molecule) are mixed with 500 gms. of methylated spirits which must not have been denatured with pyridine bases. (Caution is necessary owing to the unpleasant properties of dinitro-chlor-benzene.) To this is added 80 gms. So2 (=1 molecule) in the form of a concentrated solution of sodium sulphite. This concentrated solution is prepared from the commercial sodium bisulphite solution by the addition of the exactly equivalent quantity of strong soda lye. 40 % Caustic soda is added until phenolphthalein paper is just faintly reddened. The sulphite separates out to a certain extent even whilst warm, but this is of no consequence. The mixture of dinitro-chlor-benzene, sulphite, water, and spirit is now heated on the water-bath to boiling during 5 hours, with good stirring (Fig. 9, p. 47). The product is then cooled down as far as possible by standing in cold water, when the sodium salt of the dinitro-benzene sulphonic acid separates out in beautiful glistening yellow leaflets.

202 gms. Dinitro-chlor-benzene. 500 gms. 90 % Alcohol.

80 gms.

So2=ca. 320 gms. NaHso3. 25 % So2. ca. 100 gms. 40 % NaOH.

If the process be carried out exactly in accordance with the patent very unsatisfactory results will be obtained as, according to this, the alcohol is simply distilled off. It is far better to filter off the separated product on a nutsch, and to press out the mass in a screw-press. The leaflets of the sodium salt are then reduced exactly as given for the reduction of dinitro-benzene (q.v.). The solution of the m-phenylenediamine sulphonic acid is not, however, sufficiently pure for the purposes of the azo colour industry. It is therefore necessary to evaporate down the solution to about 400 c.cs. and to add 100 gms. common salt; on acidifying with hydrochloric acid the free sulphonic acid comes out in fine crystals. It is very important that just the right quantity of acid be taken, as the sulphonic acid will redissolve in any excess; Congo paper should not be turned pure blue, but just a definite faint violet. After two days the product is filtered off and washed in a very little water.

Fig. q. - Heating under reflux condenser with stirring.

The yield is about 125 gms. pure substance, or about 66 % of theory.

Notes on Works Practice. - Reactions of this type are best carried out in homogeneously lead-lined iron boilers (q.v.). It is very important that no trace of any other metal should come in contact with the liquid. A few milligrams of copper or iron suffice to prevent any trace of the desired compound being obtained. The dinitro-benzene sulphonic acid, as also the finished m-phenylenediamine sulphonic acid, is best not filtered off in a filter-press, but is centrifuged instead. The remainder of the alcohol is removed by hydraulic pressing, and after rectifying in a spirit column, may be used again; with careful working not more than 5 % of the alcohol should be lost during the whole operation.

100 gms. NaCl.

ca. 100 gms. Hc1, 30 %.

The mobility of the chlorine atom in negatively substituted aromatic hydrocarbons is frequently made use of for the synthesis of important dye intermediates. Further, actual dyes, particularly those of the anthraquinone series, may be produced by the aid of this reaction. Colouring matters cannot be discussed here in this connection, but a few interesting intermediate products obtained in this manner will now be described.

p-Nitraniline Sulphonic Acid from p-Nitro-chlor-benzene. Reaction:

The sulphonation of p-nitro-chlor-benzene is effected in a very similar manner to that of nitro-benzene. Thus, for example, 100 gms. p-nitro-chlor-benzene are mixed with 100 gms. of sulphuric acid monohydrate at 500, and to this are added, with stirring, 280 gms. of oleum containing 25 % So3. The product is then warmed to 100-110o until the nitro-chlor-benzene has disappeared. The mixture is poured on to 300 gms, of ice and 300 gms. of water and is salted out with 200 gms. of common salt; after 24 hours the product is filtered off and pressed. The yield is about 280 gms. moist press-cakes. The cakes are then broken up and heated with their own weight of strong ammonia (20 % Nh3) in autoclaves at 1500 during 8 hours, in order to obtain the p-nitraniline sulphonic acid. The pressure rises to about 6 atmospheres (steel tube mono-meter). On cooling, the ammonium salt of the desired sulphonic acid separates out in large, hard, amber-coloured cubes, which weigh about 100 gms. when isolated. On the large scale the mother-liquor from the ammonium salt is worked up by means of lime to recover the ammonia.

The p-nitro-chlor-benzene sulphonic acid finds, in addition, an extensive application in the preparation of diamino-diphenylamine sulphonic acid, and also of amino-diphenylamine sulphonic acid. The following schemes will indicate their modes of formation:

100 gms. p-Nitro-chlor-benzene. 100 gms. H2So4, 100 %. 280 gms. Oleum,

25 %.

1. Diamino-diphenylamine sulphonic acid.

and MgO (or CaCo3) in boiling aqueous solution.

(Wooden tubs.) p-Phenylene-diamine.

Easily soluble sodium salt.

I.

Diamino-diphenyl-amine sulphonic acid.

II.

Sparingly soluble sodium salt; easy separation.

The intermediate product (I.) gives valuable dark azo colours for cotton when combined with amino-naphthol sulphonic acids.

2. Amino-diphenylamine sulphonic acid (III.) or amino-phenyl-tolyl amine sulphonic acid.

Amino-diphenylamine sulphonic acid.

Substances of this type (III.) yield the Nerols of the Berlin Aniline Co., which are disazo dyes obtained by coupling with α-naphthylamine and subsequent further coupling with Schaffer salt or with other azo components.

We may note here that not only halogen atoms, but also nitro groups and sulphonic groups may be replaced by phenyl- or aryl-amines. In the anthraquinone molecule in particular the easy replaceability of the nitro groups makes it possible to obtain important derivatives in this manner, but we cannot enter into this question here. The use of various highly nitrated benzene derivatives also gives rise to interesting condensation products.