Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

BASICITY OF THE NITEANILINBS 173
On the mobilising effect of nitro-groups on halogens, cf. p. 106.
The basic character of the amino-group in aniline, which is in itself
feeble, is already greatly reduced by the entrance of a single nitro-group.
Hence the three nitranilines are very weak bases which dissolve only in
excess of acid to form salts. Whilst the nitranilines themselves are of a
deep orange-yellow colour, their salts, when pure, are colourless. The
powerful colour-deepening (bathochromic) influence which the free
amino-group exerts on nitrobenzene (itself almost colourless when perfectly
pure) is completely abolished by salt formation, i.e. by conversion
of the unsaturated trivalent nitrogen into that of an ammonium compound.
Of the three nitranilines the o-compound is the most feebly basic,
then comes the p- and finally the m-compound.
These facts illustrate relationships which are of great importance
throughout the whole chemistry of aromatic compounds, and have
already been mentioned above in connexion with the mobilisation of
halogens by nitro-groups in the o- and y-positions. Altogether two
substituents in ortho- or in para-positions have a much stronger influence
on each other than is exerted mutually by two substituents which are
meta to each other. A satisfactory explanation of these facts has not
yet been found although the close relationships between the 1 and 2 and
the 1 and 4 positions (Thiele) throw some light on the question.
B
The unequal basicities of the three nitranilines can be illustrated by
the following experiment. It is a general property of the salts of weak
bases—as well as of weak acids—that in aqueous solution they are stable
only if an excess of acid (or alkali) is present. When such solutions are
diluted with water hydrolysis occurs as a result of the operation of the
law of mass action. In the present case this phenomenon shows itself
in the appearance of the yellow colour characteristic of the bases and
finally, since the nitranilines are sparingly soluble in water, in their precipitation
in crystalline form. The weaker the base the smaller is the
amount of water which must be added in order to make the hydrolysis
perceptible.
Experiment.—Of each of the three nitranilines—they can be
procured in any laboratory—0-5 g. is separately dissolved by stirring
with glass rods in three test tubes, each containing 3 c.c. of concentrated
sulphuric acid. The colourless solutions thus prepared