Phenolsulphonic Acids

Picric acid, the 2 4 6-trinitro derivative of phenol, cannot be prepared in good yield by the action of nitric acid upon phenol since much of the latter is destroyed by oxidation and resinous products are also formed. It is more convenient to heat the phenol with concentrated sulphuric acid whereby a mixture of o- and p-phenolsulphonic acids is obtained upon treatment of the mixture with concentrated nitric acid, nitration occurs at the two positicsis mela to the —SOjH group in each compound, and finally, since sulphonation is reversible, the acid groups are replaced by a third iiitro group yielding picric acid in both cases ... 

A similar behaviour is observed in the case of the phenolsulphonic acids and in particular in that of anthraquinone, which, in its substitution reactions, is extraordinarily like naphthalene. Anthraquinone is sulphonated with more difficulty than is naphthalene, and in consequence the conditions of increased temperature which must be applied bring about the formation of the /8-acid, the important starting point for the synthesis of alizarin. In industrial practice, however, ways and means have been found for producing also anthraquinone-a-sulphonic acid, which was formerly not readily obtainable. a-Substitution takes place when the sulphonation is catalysed by mercury1 (R. E. Schmidt). 

Fig. 1.3 Rate of exchange of phenolsulphonic acid resin (in ammonium form) with various cations...

This method consists in the sulphonation of phenol to phenolsulphonic acid which, on heating with nitric acid, is converted into dinitrophenol. In addition some 2,6-dinitrophenolsulphonic acid is also formed. As the latter remains in solution the yield of dinitrophenol is lower than the calculated one. 

As this scheme indicates, phenolsulphonic acid is first nitrated to 2-nitro-4-sul-phonic and 4-nitro-2-sulphonic acids. The former yields 2,4-dinitrophenol and 2,6-dinitro-4-sulphonic acid, and the latter 2,4-dinitro-6-sulphonic acid. All these dinitro compounds are finally converted into picric acid. 

King [3] has shown that phenolsulphonic acid can be nitrated by the action of concentrated nitric acid to yield nitrophenolsulphonic acid. On the contrary, reaction with dilute nitric acid leads to the exchange of the sulphonic group by the nitro group ... 

The first stage of the production process - the preparation of phenolsulphonic acid - may be effected in various ways. 

Both liquids flow down to the sulphonator (4), designed for 150-250 kg of phenol. For 100 parts of phenol 500 parts of sulphuric acid is used. The phenol in the sulphonator being hot, the sulphonation reaction starts spontaneously, resulting in a temperature rise to 100°C. After some 10-20 min, sulphonation may be regarded as completed and hot sulphophenol is transferred by means of a pressure-egg (5), to the nitrator. The pipeline for conveying the product should be heated, otherwise the phenolsulphonic acids are likely to crystallize. 

The second stage in the manufacture of picric acid is the nitration of the phenolsulphonic acid (sulphophenol) obtained. The simplest reactors used for this process are stoneware jars, tourills , which may be of various size (e.g. 85 cm diameter... 

The mixture of phenolsulphonic acids is diluted with 737.5 1. of water. Usually wash-waters are used for this purpose. The concentration of phenolsulphonic acid in the resulting solution is 18.4% calculated on phenol. 

This method is not accompanied by the evolution of large quantity of nitrogen oxides. The nitration is ended when the content of the nitrator changes colour from red (the colour of phenolsulphonic acids) to yellow (the colour of picric acid). 

Also Reed [7] described a method of nitrating phenolsulphonic acids in a large pot nitrator of 5500 1. capacity made of acid-proof bricks. Picric acid was settled in a wooden, lead-lined slurry tank and eventually separated in a centrifuge. 

A polyfunctional cation-exchange resin prepared by heating a mixture of furfural and /)-phenolsulphonic acid with a prepolymer from furylacrylic add at 60 °C for 5-6 h is stable up to 100 °C, swells 135 % in water (in H form), and possesses static exchange capadty of 4.9-5.0, 2.3, 2.7, and 3.5 meq/g for O.IN NaOH, O.IN CaQj, —COOH groups, and O.IN salsolidine, respectively. 

Thus the diagram on p. 501, VoL I remains valid particularly as regards the nitration of p-phenolsulphonic acid. 

The mechanism of the attack of nitric acid upon phenolsulphonic acids was rationalized by T. Urbadski and Le niak as passing through the addition stages forming transient yellow coloured quinoid structures (16b)... 

The mechanism of the substitution of sulphonic groups in phenol by nitro groups was extensively studied by Lesniak and T. Urbartski [130 and to this purpose the chromato-polarographic method introduced by Kemula and associates 11311 was used. The trend of the nitration of o and p-phenolsulphonic acids with nitric acid can be depicted by diagram (16a) ... 

Sulphonic acids are generally obtained by combination of diazo-sulphonic acids with phenols or diazo-bases with phenolsulphonic acids, in a few cases also by heating the azo-compound with fuming sulphuric acid. 

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