IN a 5-l. round-bottom flask, fitted with a mechanical stirrer,
are placed 680 g. of sodium dichromate, 1500 cc. of water,
and 230 g. of _p_-nitrotoluene. Stirring is started, and 1700 g.
of concentrated sulfuric acid are allowed to flow in during about
thirty minutes. The heat of dilution of the sulfuric acid will cause
the nitrotoluene to melt, and rapid oxidation will soon take place.
The last half of the sulfuric acid must be added gradually,
in order to prevent too violent a reaction. Since a small amount
of nitrotoluene is volatilized, it is advisable to carry on this
work under a hood.
After the sulfuric acid has been added and the spontaneous heating
of the reaction mixture has subsided, the mixture is heated to gentle
boiling for about half an hour. After the reaction mixture has cooled,
2 l. of water are added, the cooled solution is filtered through
a cloth filter, and the product washed with about 1 l. of water.
In order to remove the chromium salts as completely as possible,
the crude nitrobenzoic acid is warmed on the water bath and agitated
with 1 l. of dilute (5 per cent) sulfuric acid solution.
After cooling, the product is again filtered. It is then dissolved
in 5 per cent sodium hydroxide solution, filtered from any chromium
hydroxide remaining, and also from unchanged nitrotoluene.
The filtrate, which should be light yellow or greenish in color,
is acidified with dilute sulfuric acid, with stirring.
It is usually preferable to run the alkaline solution into
the dilute sulfuric acid, rather than to use the reverse procedure,
for the precipitation of the nitro acid. The precipitated product
is filtered with suction, washed thoroughly, and dried. The product
should possess only a light-lemon color. The yield should be 230-240 g.
(80-85 per cent of the theoretical amount).
For a product of special purity, crystallization from benzene
is advisable. For most purposes, however, the nitrobenzoic acid
may be used without crystallization, since its melting point is found
to be within 2'0 of the correct value of 238'0.