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72 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

A number of authors have studied substitution with the nitro group in benzene

derivatives containing ortho-para directing substituents, when nitric acid with acetic

anhydride was used. The experiments led to the conclusion that replacement of

water in the nitrating mixtures by acetic anhydride produces an increase of the

ratio of ortho- to para- isomers.

This was observed in the nitration of acetanilide (Holleman, Hertogs and van

der Linden [53]; Arnall [70]), propionanilide (Arnall [70]), anisol (Griffiths, Wal-

key and Watson [71]; Halverson and Melander [72]). The effect was much less

pronounced when toluene was nitrated (Hollernan, Vermeulen and de Mooy [54]).

Recently Paul [73] nitrated chloro- and bromobenzene with a solution of nitric

acid in acetic anhydride with the addition of a catalytic quantity of sulphuric acid.

The result was contrary to that observed earlier: the proportion of para- isomer

was much higher than in the instance of nitration with 90% nitric acid, e.g. acetic

anhydride containing 2 M HNO 3 and 0.04 M H 2 SO 4 nitrated chlorobenzene to

yield o- and p-chloronitrobenzenes in the proportion of 10:90. When 90 per cent

nitric acid was used alone Holleman and Bruyn [50] obtained the isomers in the

proportion 30:70. Bromobenzene gave the figures 25:75 and 38:62 respectively.

According to Paul, the result depends on dipole moments: (1) of substituents

attached to the ring (prior to nitration) and (2) of the solvent (such as acetic an-

hydride).

TABLE 9

RELATIVE REACTIVITY TOWARDS NITRATION

Relative reactivity

Compound

ortho meta para

C 6 H 5 .H^111

C 6 H 5 .CH 3 43 3 55

C 6 H 5 .COOC 2 H 5 0.0026 0.0079 0.0009

C 6 H 5 .Cl 0.030 0.000 0.139

C 6 H 5 .Br 0.037 0.000 0.106

Among other researches, those carried out by Ingold and his co-workers [74,75]

are of considerable importance. They have determined the relative reactivities of

the various ortho, meta and para positions in several substituted benzenes by

competitive nitration. Relative rates of nitration were determined in experiments

in which benzene and a substituted benzene derivative were nitrated together, an

insufficient quantity of nitric acid being used. The relative quantities of the products:

C 6 H 5 NO 2 and X.C 6 H 4 .NO 2 gave the relative rates of nitration of benzene and

of C 6 H 5 .X. When these results were combined with the relative quantities of o-,

m- and p- isomers formed, it was possible to arrive at figures representing the re-

lative reactivity of each of the possible substitution positions (Table 9).

The results show that the methyl group is a typical ortho-para directing group.

The relative reactivity of all three positions is greater than that of benzene. This