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NITRATION THEORIES^73

is in accordance with the known electron-repelling effect of the methyl group.
The ester group lowers the reactivity of all three positions, but especially that
of the ortho and para positions, in accordance with the known electron-attracting

effect of the group.

On the basis of these experiments, the conclusion could be drawn that nitration

is not only an electrophilic reaction, but that the orientation of the product is con-

trolled by the selective activation and deactivation of various substitution positions.
Ingold also examined the directing influence of a positively charged group
-N(CH 3 ) 3

+

. In nitration this group directs exclusively into the meta position.
When, however, the positive charge is separated by (CH 2 )n groups, the amount
of meta substituted product rapidly diminishes as n increases. Thus:

C 6 H 5 N(CH 3 ) 3 100% meta-

C 6 H 5 CH 2 N(CH 3 ) 3 88%

C 6 H 5 CH 2 N(CH 3 ) 3 19%

C 6 H 5 CH 2 CH 2 CH 2 N(CH 3 ) 3 5%

Similarly the meta- directing force of the nitro group is rapidly diminished when

it is separated from the ring:

C 6 H 5 NO 2 93% meta-

C 6 H 5 CH 2 NO 2 67%

C 6 H 5 CH 2 CH 2 NO 2 13%

Ogata and Tsuchida [76] found in 1956 that the orienting activity of the nitro

group may be partly changed in the presence of mercury ions. Thus, from nitro-

benzene fairly considerable quantities of o- dinitrobenzene are obtained Blong with

m- dinitrobenzene :

(34)

Other instances of anomalous substitution under the influence of substituents

already present on the ring are also known.

Thus, for example, Kym and Ratner [77] found that benzimidazolone (XX)

is readily nitrated to the 5,6-dinitro derivative (XXI). According to the experiments

of Efros and Yeltsov [78] the compound obtained may undergo further nitration

to the tetranitro derivative (XXII) having all nitro groups placed adjacent to

one another (see also p. 552):