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NITRO DERIVATIVES OF BENZENE^233

However, the nitrobenzene obtained still contains some acid. To remove it

the product is washed with water, then with a 3.0-3.3% solution of sodium

hydroxide. The quantity of NaOH used for washing comprises 0.5% of the nitro-

benzene weight. In this way not only the remaining acid (which of course is harm-

less if the nitrobenzene is to be used for further nitration) can be removed, but

also any nitrophenols formed in the nitration process. The latter are particularly

undesirable impurities in the product. Finally the nitrobenzene is washed once

more with water and dried under reduced pressure.

The Leverkusen (I. G. Farbenindustrie) process differs in that it uses a slightly

different mixed acid and a rather lower nitration temperature.

The nitrating acid had the composition:

HNO 3 35.5%

H 2 SO 4 52.5%

H 2 O 12.0%

The nitrator (7 m^3 capacity) is fed with 2200 kg of benzene. Mixed acid (4900 kg)

is run in for 10 hr, allowing the temperature to rise gradually from 20 to 45°C, which

is maintained for further 1 hr with stirring. A sample is taken for a quick test.

The density of a crude unwashed sample of nitrobenzene should be 1.200 g/cm

3

at

15°C The waste acid should have the following characteristics: density at 15°C

1.610-1.620 g/cm^3 , HNO 3 below 0.2%, HNO 2 below 0.5%. Generally the last two

figures are 0.0% and 0.3% respectively.

If the test is satisfactory the contents of the nitrator are transferred into an-

other vessel where separation is performed. Three batches are collected in a

washing tank, washed with water until neutral to Congo Red and then with water

containing ca 0.5% of Na 2 CO 3 until strongly alkaline to phenolphthalein. The

product is stream distilled until the density of the oil coming over is 1.2. The oil

with density < 1.0 is collected and nitrated when a sufficient quantity of it is available

for a batch. The oil with a density over 1.0 and below 1.2 is steam distilled with

the next batch. The product, freed from unnitrated material, is washed with 3000 1.

of water to which 10 kg of a 30% NaOH solution has been added to remove

phenols. Then all is washed until neutral and dried at 90°C under reduced pressure.

The yield is 98% of the theoretical. The waste acid contains ca. 70% H 2 SO 4.

DINITROBENZENES

There are three known isomers of dinitrobenzene - meta, ortho and para: