f nitro compounds in alkaline solution (using
zinc dust and alkali, or a solution of an alkaline stannite as a reducing
agent); by oxidation of hydrazo compounds; or by the coupling of a
diazotized amine and any compound of a phenolic or aminic type, provided
that there is a free para position in the amine or phenol. They may also be
obtained by the molecular rearrangement of the diazoamines, when these are
warmed with the parent base and its hydrochloride. This latter method of
formation has been studied by H. Goldschmidt and R. U. Reinders (_Ber_.,
1896, 29, p. 1369), who found that the reaction is monomolecular, and that
the velocity constant of the reaction is proportional to the amount of the
hydrochloride of the base present and also to the temperature, but is
independent of the concentration of the diazoamine. The azo compounds are
intensely coloured, but are not capable of being used as dyestuffs unless
they contain salt-forming, acid or basic groups (see DYEING). By oxidizing
agents they are converted into azoxy compounds, and by reducing agents into
hydrazo compounds or amines.

_Azo-benzene_, C_6H_5N:NC_6H_5, discovered by E. Mitscherlich in 1834, may
be prepared by reducing nitrobenzene in alcoholic solution with zinc dust
and caustic soda; by the condensation of nitrosobenzene with aniline in hot
glacial acetic acid solution; or by the oxidation of aniline with sodium
hypobromite. It crystallizes from alcohol in orange red plates which melt
at 68deg C. and boil at 293deg C. It does not react with acids or alkalis,
but on reduction with zinc dust in acetic acid solution yields aniline.

_Amino-azo Compounds_ may be prepared as shown above. They are usually
yellowish brown or red in colour, the presence of more amino groups leading
to browner shades, whilst the introduction of alkylated amino groups gives
redder shades. They usually crystallize well and are readily reduced. When
heated with aniline and aniline hydrochloride they yield indulines
(_q.v._). Amino-azo-benzene, C_6H_5.N_2.C_6H_4NH_2, crystallizes in yellow
plates or needles and melts at 126deg C. Its constitution is determined by
the facts that it may be prepared by reducing nitro-azo-benzene by ammonium
sulphide and that by reduction with stannous chloride it yields aniline and
meta-phenylene diamine. Diamino-azo-benzene (chrysoidine),
C_6H_5.N_2.C_6H_3(NH_2)_2, first prepared by O. Witt (_Ber._, 1877, 10, p.
656), is obtained by coupling phenyl d