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166 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

-CH 2 NO 2 -> -CH=NO(OH) (2a)

-CH 2 CO- -> -CH=C(OH)- (2b)

Formula II presents a pentacovalent nitrogen atom surrounded by 10 electrons.

All five valences are the ordinary, nonpolar ones. Compounds of pentacovalent

nitrogen such as the type N(CH 3 ) 5 are not known. On the other hand formulae

III present nitrogen as a tetracovalent atom, surrounded by an electron octet. Here,

one of the nitrogen atoms is bonded with an oxygen atom by a coordinate

(semi-polar) link.

Confirmation of formulae III is provided by data on the molecular refractivity of

nitro compounds. The refractivity for the bonds present in formulae III (6.708)

is consistent with the experimental data (6.761).
At present formulae III are generally accepted. Nevertheless, they offer some
difficulties. Thus, measurements of dipole moments of nitro compounds have shown
the nitro group symmetry along the central nitrogen bond:

symmetry axis

For example, the dipole moment of p-dinitrobenzene is close to zero, like that

of p- dichlorobenzene

contrary to, for example, hydroquinone or tetramethyl-p-phenylenediamine esters,
which have a relatively high dipole moment, since the substituents introduce
asymmetry into molecules: