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NITRO DERIVATIVES OF HALOGENOBENZENES^451

The o- and p- isomers can be obtained during the nitration of chlorobenzene.

The higher the nitration temperature, the more o- isomer is formed.

As Holleman and de Bruyn [8] reported:

At 0°C 30.1% of the ortho- and 69.9% of the para- isomer is formed, at 30°C

26.9% of the ortho-, and 73.1% of the para-isomer is formed. Besides, higher

temperatures favour the formation of some small quantities of m- chloronitrobenzene.

The product is prepared commercially mainly by chlorination of nitrobenzene

in the presence of ferric chloride.

According to other authors (McCormack [13]; Dey and co-workers [14]), the

para- to ortho- isomer ratio varies from 66/34 to 64/36, depending on the nitration-

conditions.

Bieber and Schuring [15] carried out extensive investigations into the mono-

nitration of chlorobenzene. Mixtures of nitric and sulphuric acids were used and

variables included the nitric acid content in the nitrating acid, the ratio of nitric acid

to chlorobenzene, the sulphuric acid concentration and the nitration temperature.

In the range of conditions investigated, concentration of nitric acid and the

ratio of nitric acid to chlorobenzene had little influence on the final distribution

of the chloronitrobenzene isomers. The ratio between p- and o- isomers varied be-

tween 63/37 to 70/30. The temperature effect can be seen from the figures for runs

with a final concentration of H 2 SO 4 of 80% and 70%:

80% 70%

70.5

at -5°C the ratio was -

29.5

65.6 61.1.

at 50°C ,, ,,

34.4 32.3

at 90°C ,, ,, ,,

63.4 65.6

36.6 34.4

The formation of p- chloronitrobenzene could be related to the sulphuric acid

concentration and nitration temperature (t) by the equation:

% p- ChloroNB = [0.13 - 0.0027 S] t + 70.6 (1)

where S is % of H 2 SO 4.

The content of m- chloronitrobenzene varied from 0 to 1.5% apparently increas-

ing with temperature.

According to Ingold and Bird [2] the rate of nitration of each nuclear position

can be illustrated by the diagram:

in which unity represents rate of attack on one nuclear position of benzene.