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GENERAL INFORMATION ON NITRO COMPOUNDS 167

The dipole moment of the nitro group alone, present in an aliphatic nitro

compound is :
in the gaseous phase 3.50-3.70 D
in solutions 3.20-3.40 D

In the case of nitrobenzene the dipole moment is:

in the gaseous phase 4.20 D

in solution 4.00 D

For this reason formulae IV are the most probable. They present the negative

charge as uniformly distributed between the two oxygen atoms:

IV

The treatment of the nitro group from the molecular orbital point of

view is based on the formulae IV. Delocalization of bonds increases its

FIG. 37. Molecular orbitals of the nitro group: (a) with the lowest energy level,

(b) with the highest energy level, (c) with the ground state energy level [88].

stability -the two oxygen atoms are equivalent. Figure 37 represents: (a)

the lowest energy level, (b) the highest energy level and (c) the ground state

energy level.

The symmetrical structure of the nitro group is also supported by X-ray ana-

lysis of nitro compounds. This is described on the pp. 179- 181.

Likewise, the constitution of, for example, nitrobenzene can be represented

as follows:

The formula contains a quinonoid ring to which the slightly yellow colour of

nitro compounds can be ascribed. The problem of the quinoid structure of aromatic

nitro compounds is discussed on pp. 176, 220.