III. Preparation of the Symmetrical Chloride of Paranitroorthosulphobenzoic Acid.
Reading edition
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.
By this process all of the unsymmetrical chloride is converted into the ammonium salt of paranitroorthocyanbenzenesulphonic acid, according to the equation:
CCl₂ /\ /OCN /// C₆H₃—SO₂ + 4NH₃ = C₆H₃—SO₃NH₄ + 2NH₄Cl. \\ NO₂NO₂
while the symmetrical chloride remains for the most part unchanged, though some of it is converted into the ammonium salt of paranitrobenzoic sulphinide:
CO /\ COCl/N.NH₄ /// C₆H₃—SO₂Cl + 4NH₃ = C₆H₃—SO₂ + 2NH₄Cl. \\ NO₂NO₂
It was found that working in this way the symmetrical chloride could be prepared in pure condition, free from its isomer. The chloroform completely evaporates in a short time leaving fine crystals of the symmetrical chloride. In case the evaporation is slow and incomplete, it may be concluded that not all of the unsymmetrical chloride has been removed. The yield was uniformly about 40 per cent of the theoretical.
From the water used to wash the chlorides a considerable amount of the original salt can be recovered, as the reaction under the conditions employed, is never complete.
An examination was made of the substance mentioned as separating in the chloroform solution of the chlorides, during the treatment with ammonia, and it was found to possess the following properties. It is insoluble in benzene, chloroform, acetone, ether and ligroin; soluble in glacial acetic acid, from which it separates on cooling in colorless, crystalline condition; insoluble in the cold in water, alcohol and ammonia, but by boiling with these reagents, or by long standing in the cold, it is dissolved with decomposition. It was dissolved in hot water and the solution, which was acid in reaction, was neutralized with potassium carbonate. On adding an excess of hydrochloric acid to the solution, and allowing it to cool, characteristic crystals of acid potassium salt of paranitroorthosulphobenzoic acid separated. These properties identify the substance as the anhydride of this acid.
The formation of the corresponding anhydride of orthosulphobenzoic acid by the action of phosphorus pentachloride upon its acid potassium salt was observed by Sohon[13], who made use of the reaction to prepare this anhydride in quantity.