rystallizes in long silky needles. The
sulphate and chloride are similar, but they are not quite so unstable as
the nitrate. The bromide may be prepared by the addition of bromine to
an ethereal solution of diazo-amino-benzene (tribromaniline remaining in
solution). By the addition of potassium bromide and bromine water to
diazonium salts they are converted into a _perbromide_, e.g. C6H5N2Br3,
which crystallizes in yellow plates.

The diazonium salts are characterized by their great reactivity and
consequently are important reagents in synthetical processes, since by
their agency the amino group in a primary amine may be exchanged for
other elements or radicals. The chief reactions are as follows:--

1. _Replacement of -NH2 by -OH_:--The amine is diazotized and the
aqueous solution of the diazonium salt is heated, nitrogen being
eliminated and a phenol formed.

2. _Replacement of -NH2 by halogens and by the -CN and -CNO
groups_:--The diazonium salt is warmed with an acid solution of the
corresponding cuprous salt (T. Sandmeyer, _Ber._, 1884, 17, p. 2650),
or with copper powder (L. Gattermann, Ber., 1890, 23, p. 1218; 1892,
25, p. 1074). In the case of iodine, the substitution is effected by
adding a warm solution of potassium iodide to the diazonium solution,
no copper or cuprous salt being necessary; whilst for the production
of nitriles a solution of potassium cuprous cyanide is used. This
reaction (the so-called "Sandmeyer" reaction) has been investigated by
A. Hantzsch and J. W. Blagden (_Ber._, 1900, 33, p. 2544), who
consider that three simultaneous reactions occur, namely, the
formation of labile double salts which decompose in such a fashion
that the radical attached to the copper atom wanders to the aromatic
nucleus; a catalytic action, in which nitrogen is eliminated and the
acid radical attaches itself to the aromatic nucleus; and finally, the
formation of azo compounds.

3. _Replacement of -NH2 by -NO2_:--A well-cooled concentrated solution
of potassium mercuric nitrate is added to a cooled solution of benzene
diazonium nitrate, when the crystalline salt 2C6H5N2.NO3, Hg(NO2)2 is
precipitated. On warming this with copper powder, it gives a
quantitative yield of nitrobenzene (A. Hantzsch, _Ber._, 1900, 33, p.
2551).

4. _Replacement of -NH2 by hydrogen_:--This exchange is brought about,
in some cases, by boiling the diazonium salt with alcohol;