Page 1052 - Advanced Organic Chemistry Part B - Reactions & Synthesis

1028 In alkaline solution, diazonium ions are converted to diazoate anions, which are in
equilibrium with diazo oxides. 84
CHAPTER 11
Aromatic Substitution + – –
2
Reactions ArN N + 2 OH ArN N O + H O
diazoate anion
+
–
ArN N O + ArN N ArN N O N NAr
diazo oxide

In addition to the aqueous method for diazotization, diazonium ions can be generated
in organic solvents by reaction with alkyl nitrites.

H
RO N O + ArNH 2 ArN N O + ROH
H
H + +
ArN N O ArN N OH ArN N + H O
2
Diazonium ions form stable adducts with certain nucleophiles such as secondary amines
and sulfide anions. 85 These compounds can be used as precursors of diazonium ion
intermediates.

+
ArN N + HNR 2 ArN NNR 2
+
–
ArN N + SR ArN NSR
The wide utility of aryl diazonium ions as synthetic intermediates results from
the excellence of N as a leaving group. There are several general mechanisms by
2
which substitution can occur. One involves unimolecular thermal decomposition of
the diazonium ion, followed by capture of the resulting aryl cation by a nucleophile.
The phenyl cation is very unstable (see Part A, Section 3.4.1.1) and therefore highly
unselective. 86 Either the solvent or an anion can act as the nucleophile.
+ +
N N + N 2

+ –
+ X X

Another general mechanism for substitution is adduct formation followed by collapse
of the adduct with loss of nitrogen.

+
N N + X – N N X X + N 2

84 E. S. Lewis and M. P. Hanson, J. Am. Chem. Soc., 89, 6268 (1967).
85 M. L. Gross, D. H. Blank, and W. M. Welch, J. Org. Chem., 58, 2104 (1993); S. A. Haroutounian,
J. P. DiZio, and J. A. Katzenellenbogen, J. Org. Chem., 56, 4993 (1991).
86
C. G. Swain, J. E. Sheats, and K. G. Harbison, J. Am. Chem. Soc., 97, 783 (1975).