NITRO DERIVATIVES OF HALOGENOBENZENES 457If the test is satisfactory, the contents of the nitrator are left for 3 hr to settle,
then the spent acid is drawn off through a sight glass into an extraction vessel of
10 m^3 capacity. The spent acid after cooling and settling, is transferred to a spent
acid vessel of 18 m^3 capacity and from here, after standing, it is finally sent to the
acid concentration plant.
The chlorobenzene from the extraction stage is used for the next nitration batch.
Chloronitrobenzene is sent to a tile-lined, 10 m3
washing tank. Here it is agitated
three times with 3000 1. of water at 60-70°C by an air stream until neutral to Congo
red. Then it is washed with 20 kg of sodium carbonate in 3000 1. of water, and
finally with hot water to neutrality. The product is dried at 90-100°C under reduced
pressure in a 7 m^3 drying vessel heated by a heating coil. The yield is 98% of the
theoretical.
The product has a m. p. of 53-54°C with a dinitro compounds content below
0.1% and is free from nitrophenols. The spent acid contains 70% of H 2 SO 4.
Separation of isomers. To separate o- and p- chloronitrobenzenes the Leverku-
sen Works crystallize large quantities (14,000-15,000 1. batches) of crude chloro-
nitrobenzene in crystallizers of 15 m3
capacity, equipped with two cooling coils
(cooling area of 600 m2). The two coils are separated from each other by a horizon-
tal, perforated plate. The crystallizer is charged with hot (80-100°C) crude chloro-
nitrobenzene. The product is cooled down to 20°C during 25 hr, then the liquid
eutectic mixture (6000-7000 1.) is drawn off through the tap at the bottom of the
crystallizer, which takes about 8 hr. The crystalline residue in the crystallizer is
washed three times with methyl alcohol (2000-4000 1. each time). The alcohol used
for the first washing has already served twice for the same purpose. The second
washing is done with alcohol which was used only once, while for the third one
freshly distilled methyl alcohol is used. After being used three times the methyl
alcohol is sent to a distillation unit, where it is re-distilled and then returned to the
washing operation. The residue left after the alcohol is distilled off is added to the
liquid eutectic.
The washed product left in the crystallizer is pure commercial p- chloronitroben-
zene and should have a freezing point of 82-82.5% If not, an additional wash
should be given.
The liquid eutectic goes to a distillation column where it is separated into frac-
tions at a pressure in the receiver of about 10 mm Hg and at 180°C at the bottom
of the column; the temperature in the dephlegmator is 100-120°C. The first fraction.
I, is a mixture of chlorobenzene and chloronitrobenzene isomers (mainly m- chloro-
nitrobenzene). The second, II, is rich in p- chloronitrobenzene. Finally the main
fraction is distilled off, melting initially at 35°C and towards the end of distillation
at 28°C. The last, III, is o- chloronitrobenzene of 90% purity.
From fraction I a mixture containing 72% of m- chloronitrobenzene and 28%
of p- chloronitrobenzene is drawn off by distillation.
By treating this mixture with sodium hydroxide solution the p- isomer is con-
verted to p- nitrophenol, while the m- isomer remains unchanged.