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CHAPTER XII

NITRO DERIVATIVES OF HALOGENOBENZENES

APART from a few exceptions nitro derivatives of halogenobenzenes are not

used as explosives. Nevertheless they are, or may be, important intermediates in

the manufacture of explosives, dyes and drugs.
Derivatives of chlorobenzene are of course the most important, as they are the

most readily available. For certain syntheses nitro derivatives of fluoro- or bromo-

benzene are used. They may be of value sometimes, since the former are less reactive

and the latter more reactive than chlorobenzene derivatives. For example, recently

2,4-dinitrofluorobenzene has been widely applied in the elucidation of the structure

of peptides (Sanger [1]).

Ingold and Bird [2] investigated the rates of nitration of fluoro-, chloro-, bromo-

and iodo-benzenes with acetyl nitrate and found that they could be arranged in the

following order: 1: 0.15: 0.033: 0.030: ca. 0.18 (taking the relative rate of nitration

of fluorobenzene as unity).

The kind of halogen atom affects the substitution in the ring with the nitro

group. Table 100 shows the results of investigations by Sandin and J. R. W. Wil-

liams [3]. J. D. Roberts and co-workers [3a] obtained similar results by using

isotope dilution analysis.

TABLE 100

Halogenonitro compound

Compound

ortho para ortho/para ratio

C 6 H 5 F 12.6 87.4 0.14

C 6 H 5 Cl 30.1 73.1 0.41

C 6 H 5 Br 37.2 62.5 0.59

C 6 H 5 I 34.2 65.8 0.52

This trend has been explained in a satisfactory manner by the rule established

by Lapworth and Robinson [4]: the increase in relative yield of the ortho-product

parallels the decrease in the negative inductive influence of the halogens. The induc-