By oxidation this passes to a nitro compound of the composition
HC ============= CH
/\
C₆H₃—SO₂OK KOO₂S—C₆H₃
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NO₂O₂N
It was no doubt the formation of substances of this nature that occasioned the color observed in some of the oxidations.
The only effective method of separating this colored substance was found to be to pass to the neutral salt of paranitroorthosulphobenzoic acid, by making the solution slightly alkaline. The salt of this colored substance is also formed and the two can be separated by a few recrystallizations in a fairly satisfactory manner.
The yield in both of the transformations involved in the preparation of paranitroorthosulphobenzoic acid does not fall far short of the theoretical.
III. Preparation of the
Symmetrical Chloride of
Paranitroorthosulphobenzoic Acid.
This chloride was first separated from its unsymmetrical isomer by Gray[12]. It was obtained by allowing a chloroform solution of the mixed chlorides to evaporate until the chloroform had almost entirely disappeared. In the thick liquid so obtained, crystals of the symmetrical chloride were formed. It was also obtained by applying the method devised by Bucher in connection with the corresponding chloride of orthosulphobenzoic acid—i.e. by the action of dilute ammonia on the mixed chlorides. Gray also found that the best conditions for securing a relatively large proportion of the symmetrical chloride were, the employment of as low a temperature as possible in the formation of the chlorides, and of as small an excess of phosphorus pentachloride as would suffice for the reaction.
After many experiments, under widely differing conditions, the following method of procedure, embodying the results of Gray’s work, was adopted.
Dehydrated acid potassium salt of paranitroorthosulphobenzoic acid, and phosphorus pentachloride, in the ratio of 40: 55 grams, are brought together in a mortar and intimately mixed. The mixture is put into an evaporating dish, and placed on a sulphuric acid bath, previously heated to 150°. As soon as the action has been well started, the dish is removed, and the reaction allowed to proceed without further heating. When it is complete, and the contents of the dish has cooled down to the temperature of the room, the oily product is poured slowly into a salts bottle containing ice water, the bottle being frequently shaken during the process. The shaking is continued with renewed portions of water, as long as the wash water is cloudy. The water is then poured off, the brownish gummy chloride dissolved in chloroform, and the solution placed in a good-sized separating funnel. Ice water is then added, and the contents of the funnel treated with successive portions of ammonia (desk ammonia diluted one half). Shaking is continued after each addition until the odor of ammonia has disappeared, and ice is added from time to time as may be required.
When it is found that the odor of ammonia persists after several minutes’ shaking, the chloroform layer, which is usually filled with a solid substance that has separated during the process, is drawn off, filtered, and dried with calcium chloride.