Page 159 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements
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5A.6 DIAZONIUM SALTS 139
H 2 O, Δ
Ar OH
H PO 2
3
+ Ar H
HNO 2
ArNH 2 Ar N N (5A.28)
Kl
Ar I
Ar′H
Ar Ar′
NaOH
The structure of a diazonium cation probably provides the most valuable clue to
the mechanism of its formation. A N–N bond has to form, and for that ArNH is a
2
+
plausible nucleophile and NO the likely electrophile; there are really not many other
choices.
Ar + Ar + Ar
N O + − H
N N N N N (5A.29)
H H
H H O H O
Successive protonation of the nitroso (N=O) oxygen and elimination of water then leads to
the formation of a diazonium cation, as shown below:
Ar
Ar + N N + Ar
H + − H 3 O
N N N N
H OH
H O H OH
O
(5A.30)
H
Ar
+ − HOH +
N N H Ar N N
OH
The great synthetic utility of diazonium ions stems from the fact that they readily lose N ,
2
producing highly reactive aryl cations, which can be captured by a variety of nucleophiles
–
(Nu ):
+ − N 2 + Nu −
Ar N N Ar Ar Nu (5A.31)
Equation 5A.31 provides several examples of this type of transformation.