220 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES
toluene becomes more intense under the influence of sulphuric acid. That is ascribed
to the formation of the quinonoid ion:
in the protonizing medium of sulphuric acid.
ADDITION COMPOUNDS OF AROMATIC POLYNITRO COMPOUNDS
The ability to form addition compounds, especially with aromatic hydrocarbons
composed of condensed rings, is one of the specific properties of aromatic poly-
nitro compounds. For example, compounds of trinitrobenzene or picric acid with
naphthalene, as well as with other hydrocarbons with condensed rings, are very
characteristic. Generally they are intensely coloured.
The nature of the bonds forming molecular addition compounds has been
investigated by several workers, but up to now it has not been made sufficiently
clear. Pfeiffer [112] held that complex formation was due to the mutual saturation of
“residual valencies”, but Briegleb [113,114] advanced the theory that addition
compounds should be regarded as “polarization aggregates” which owed their
stability to electrostatic interactions, possibly due to polarization of one component
by the other. Thus, in the case of polynitro compounds, their strongly polar mole
cules influence the non polar molecules of the hydrocarbon. Further, no covalent
bonds exist between these two kinds of molecules. This hypothesis has received
considerable support and a new development.
The experimental observations which support this point of view are as follows:
(1) The interatomic distances between the two components were found byX-ray analysis to be of the order 3.0-3.5 Å (Powell and Huse [115]; Rapson, Saunder
and Stewart [116]). This is too long for covalent bonds.
(2) Molecules of 1,3,5-trinitrobcnzene or p- dinitrobenzene have no electric
dipole moments but they have moments in solutions where molecular com-
pounds are formed. For example they have no moments in carbon tetrachlo-
ride or chloroform, but they do have moments in benzene, naphthalene, or
dioxane.
(3) Molecular addition compounds are readily formed and decomposed, e.g.
by acting with a suitable solvent, which would dissolve one of the components and
extract it. For example addition compound formed between picric acid and a
hydrocarbon can be split into its components by extracting the picric acid with
alcohol while the hydrocarbon remains undissolved. Another fact also indicative
of the instability of the addition products is that those of picric acid may be de-
composed by treating their solutions with saturated solution of potassium chloride.
Potassium picrate is then precipitated, while the other component remains in solu-
tion (Taben and Kosak [117]). Generally speaking, the organic addition compounds