urbaf2

168 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

VISIBLE AND ULTRA-VIOLET ABSORPTION SPECTRA

Changes occurring on the conversion of nitrobenzene to trinitrobenzene have

been the subject of investigations of a number of authors (Radulescu and Elexa

[3], Kortüm [4], Fielding and Le Févre [5]). All of them obtained similar results.

Fielding and Le Févre give the following data for compounds in an alcoholic solu-

tion (Table 19).

TABLE 19

Substance

Nitrobenzene

m- Dinitrobenzene

sym-Trinitrobenzene

Trinitromesitylene

(^2608130)
(^235) 17,400
(^225) 25,700
below (^220) over15,800
The data for trinitromesitylene illustrate the steric influence due to the accumu-
lation of substituents around the benzene ring.
Steric influence on the absorption spectra of aromatic nitro compounds has
been studied in detail by Brown and Reagan [6]. They investigated the influence
of alkyl groups on the absorption spectra curves. As may be seen from the curves,
substitution with an alkyl group in the orrho position results in a decrease of the
intensity of the nitro group band (compare the absorption spectrum of nitroben-
zene with those of o- nitrotoluene, o- nitrocumene, o- nitro-tert-butylbenzene). The
larger the alkyl group the stronger its influence. Thus, the band of the nitro group
present in o- nitro-tert-butylbenzene disappears entirely. The presence of three
methyl groups in a molecule gives a similar effect, as may be seen from the nitro-
mesitylene spectrum (Fig. 38).
The nitro group is a chromophor. It produces an absorption band in the ultra-
violet region of the spectrum. The position and the intensity of the band depend
on several factors which will be discussed later.
In the case of simplest nitro compounds the absorption band produced by
this group corresponds to the following values. For aliphatic nitro compounds,
for example nitromethane, nitroethane, 1- and 2-nitropropanes, they are: 260-270
mμ at extinctions ε = 40-120. (According to Ungnade and Smiley [l] the values for
higher nitroparaffins are 274-278 mμ, ε = 24-41.) For aromatic nitro compounds,
such as nitrobenzene the bands are: 250-260 mμ at extinctions ε = 9000-10,000.
When a primary or secondary nitro group takes an aci-form, the maximum
disappears, as shown by earlier research work of Hantzsch and Voigt [2].
The absorption maximum of a nitro group can be shifted and its intensity may
change under the influence of a larger number of nitro groups on the aromatic
nucleus.
λ max
mμ ε max