THE crude trinitrobenzoic acid obtained by oxidation of 360 g.
of trinitrotoluene (prep. XXV, p. 95) is mixed with 2 l.
of water at 35'0 in a 5-l. flask provided with a stirrer.
Fifteen per cent sodium hydroxide solution is added,
with continuous stirring, until a FAINT red color is just produced.
(See Notes.) The color is then immediately discharged by means
of one or two drops of acetic acid, and the liquid is filtered
from unchanged trinibrotoluene. The filtrate is transferred
to a 5-l. flask, and 70 cc. of glacial acetic acid are added.
The mixture is then gently heated, with continuous stirring,
when trinitrobenzene separates in crystalline condition,
and floats on the surface of the liquid as a frothy layer.
After about one and a half hours the evolution of gas ceases;
at this point the crystals begin to stir into the solution.
The heating and stirring is continued for three-quarters of an hour,
when the mixture is allowed to cool, and the crystals filtered off.
A sample of the filtrate should be tested for undecomposed
trinitrobenzoic acid: if a precipitate is produced by
the addition of sulfuric acid the process must be continued.
After recrystallization from glacial acetic acid, the product
melts at 121-122'0. The yield is 145-155 g. (43 to 46 per cent
of the theoretical amount calculated from the trinitrotoluene). 2.
Notes
During the solution of the trinitrobenzoic acid, the temperature should
not be below 35'0, owing to the slight solubility of trinitrobenzoic
acid in cold water. The heat of neutralization raises the temperature
to 45-55'0, but the latter temperature should not be exceeded,
since any trinitrobenzene formed at this point would later be removed
with the unreacted trinitrotoluene.
Care must be taken that no more alkali is added than is just sufficient
to produce the faint red color. If an excess of alkali is added it
produces a permanent color, which is not removed by acid and colors
the final product.
When once the evolution of carbon dioxide sets in, the flame must
be cut down so as to avoid the formation of a thick layer of froth
which might foam over.