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NITROGEN
140
REVIEW PROBLEM 5A.8
Diazonium salt chemistry provides a classic synthetic route to fluorinated arenes, the
Balz–Schiemann reaction:
+
Ar F
HNO 2 Ar N N Δ
Ar NH 2 + +
HBF 4 − − N 2
BF 4 BF 3
Write out the mechanism for the last step.
5A.7 AZO COMPOUNDS AND DIAZENE
2
1
2
1
Azo compounds have the general formula R –N=N–R , where R and R are alkyl or
aryl groups. Aromatic azo compounds are generally strongly colored and many are easily
prepared via a so-called diazo coupling reaction, in which a diazonium cation is coupled
with a relatively “active” (i.e., highly nucleophilic) arene such as a phenol or aniline. The
dye called aniline yellow, for example, is prepared as follows:
NH 2
− +
N +
Cl N NH 2 − H
+ N
− N
− Cl
Aniline Aniline yellow
(5A.32)
REVIEW PROBLEM 5A.9
Suggest a mechanism for the above reaction.
Aliphatic azo compounds are also important, most notably as initiators of radical reac-
tions, including radical polymerizations. The most common reagent used for this purpose
is azobisisobutyronitrile (AIBN), which decomposes on gentle heating to N and a pair of
2
2-cyanoprop-2-yl radicals: