The method employed in the preparation of paranitroorthosulphobenzoic
acid was essentially that described by Hart,[7] Kastle,[8] Gray[9]
and Hollis.[10] The details of it are repeated here for the purpose of
calling attention to certain facts that came under the author’s notice.
[7] Am. Chem. Journ. I, 350.
[8] Ibid XI, 177.
[9] Inaug. Diss. J. H. Univ. 1845.
[10] Inaug. Diss. J. H. Univ. 1896.
100 grams of paranitrotoluene are added to 400 grams of fuming sulfuric
acid, and the mixture heated in a balloon flask at 100° on a water
bath. The heating is continued until a few drops of the mixture, added
to cold water, dissolves completely to a clear solution. The time
required for this operation varies much with the conditions. Continued
stirring very considerably hastens the reaction, as paranitrotoluene
forms a layer on the acid, which presents a small surface to its
action. With constant stirring the reaction is complete in a few hours,
whereas if no stirring is resorted to, as much as several days may be
required, especially when large quantities are employed at one time.
When the reaction is complete, the mixture is poured into a
large volume of water, and neutralized with calcium carbonate.
In the filtrate from calcium sulphate, the calcium salt of
paranitroorthotoluene sulphonic acid is found, and this is converted
into the potassium salt in the usual way.
The oxidation of the potassium salt is effected as follows. 50 grams of
the salt are dissolved in 2½ litres of water, and to this is added a
solution of 15 grams of potassium hydroxide. The mixture is heated to
100° on a water-bath, and when this temperature is reached, 110 grams
of potassium permanganate are added. Heating is continued until the
solution is decolorized, care being taken to prevent the evolution of
free oxygen.
The oxides of manganese are then filtered off, the filtrate neutralized
with hydrochloric acid, and evaporated to about one fifth of its
original volume. Strong hydrochloric acid is them added in excess, and
on cooling the acid potassium salt of paranitroorthosulphobenzoic acid
separates in very slender colorless needles completely filling the
liquid.
For the success of this operation it is important that the potassium
salt of paranitroorthotoluenesulphonic acid and the potassium hydroxide
should both be perfectly dissolved before they are heated together.
If the two substances lie together in solid form at the bottom of the
flask, a very slight elevation of temperature leads to the formation
of an extremely troublesome red substance, which is very difficult
to remove. It is almost impossible to remove it from the oxidation
product by recrystallization, since any considerable amount of it has
a marked influence on the solubility of the salt, rendering it much
more soluble. It persists throughout all subsequent transformations of
paranitroorthosulphobenzoic acid, and should therefore be carefully
avoided.
Otto Fischer[11] has shown that in concentrated solution, potassium
hydroxide acts on nitro derivatives of toluene, with the formation
of various colored substances derived from stilbene. In the case
of paranitroorthotoluenesulphonic acid, he describes the substance
formed as possessing a cherry red color. The reactions involved in its
formation are:
CH₃ HC ============ CH
/ / \
2C₆H₃——SO₂OK = C₆H₃——SO₂OK KO.O₂S——C₆H₃ + 2H₂O
\ \ /
NO₂ \ /
\ /-- O --\ /
N N
\-- O --/