Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

PHENYLDIAZONIUM PBKBKOMIDB 289
Phenyldiazonium Perbromide .•—To a fresh ice-cold solution of one
of the solid diazonium salts, prepared as described above, or to the
diazo-solution from 2 g. of aniline, there is added the solution of
1-5 c.c. of bromine in 15 c.c. of potassium bromide solution (25 per
cent), with ice cooling, until precipitation of dark-coloured oil ceases.
The aqueous solution is then decanted ; when the residual perbromide
is washed a few times with ice-water it crystallises.
In order to convert it into phenyl azide (" diazobenzeneimide ")
the perbromide is added in three or four portions to about 10 c.c. of
well-cooled concentrated ammonia solution. A vigorous reaction
takes place, resulting in the formation of the pungent smelling
phenyl azide, which is purified by steam distillation. It can also
be distilled in a vacuum without decomposition. Since it explodes
when rapidly heated it must be handled cautiously.
The bromides of organic bases form with bromine insoluble perbromides,
in the present case the addition compound C6H5.N=N .
Br.Br2
The mechanism of the reaction with ammonia is as follows. The
perbromide bromine is converted into hypobromite and at the same
time the diazonium salt undergoes rearrangement to syw-diazohydroxide,
which immediately couples with NH3 to give phenyltriazene (" diazobenzeneamide
"). The latter is then dehydrogenated by the hypobromite
yielding phenyl azide (Dimroth) :
C6H5.N==N 3NH3 C6H5.N=NOH + 2 NH4Br + NH40Br ,
Br.Br2 2H,o
C6HS.N:NOH + NH3 —>- C6H5.N:N.NH2 + H2O ,
C6HB.N:N.NH2 + NH4OBr > C6H5.N< < | || + NH4
By very careful hydrogenation (with stannous chloride in ethereal
hydrogen chloride) phenyl azide has been converted into the exceedingly
sensitive phenyltriazene (Dimroth), which, as has been shown, can
be reconverted into the former by denydrogenation. As in the case
of the aliphatic diazo-compounds, an open chain structural formula
has lately also been assigned to hydrazoic acid and its esters, so that
the changes just mentioned may be formulated as follows :
+ 2H
C6HB.N=N=N C6H5.N=N—NH2 .
-2H
Phenylazide is best prepared from phenylhydrazine (p. 299).