urbaf2

NITRATION THEORIES 71

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XIV XV 50% XVI 30% XVII 6%

A characteristic feature of the reaction is a complete lack of meta orientation

to the nitro group. However, this is a rather complicated case of nitration accom-

panied by Bamberger rearrangement (p. 41 and Vol. III).

Holleman [47] also gave an example of an anomaly in the nitration of chloro-
2,3-dinitrobenzene (XVIII). Instead of the third nitro group entering into the 4-

or 6 positions as expected, which would be consistent with the orienting action of

Cl and of one of the NO 2 groups, he obtained compound XIX, with the third

nitro group in the 5- position:

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XVIII XIX

Among more recent investigations the experiments of Hammond, Medic and

Hedges [69] deserve special attention. They explain the influence of the medium

on orientation when nitrating 2,5-dichloro- and 2,5-dibromo-nitrobenzene. Substi-

tution into all three free positions takes place and derivatives ortho-, meta- and

TABLE 8

NITRATION OF 2,5-DICHLORONITROBENZENE

Nitration at 100°C with % ortho- % meta- % para-

Fuming sulphuric acid and sodium nitrate (6%) 14.5 53.0 32.5

96.2% sulphuric acid and sodium nitrate (6%) 21.4 50.8 27.7

96.2% sulphuric acid and sodium nitrate (15%) 35.0 45.7 19.3

Nitric acid and 60% perchloric acid (2%) 33.1 45.1 21.1

para- to the nitro group are formed. The yield of the ortho- derivative may vary

from 11.0 to 35.0% while of the yield the meta- derivative amounts to 45.7%. The

quantities of the different isomers depend on the nitrating agent. For example, in

the nitration of 2,5-dichloronitrobenzene the results obtained have been shown
in Table 8. The modern approach to substitution rules consists in molecular
orbital calculations [89].