Chemistry Reference
In-Depth Information
The diazonium salts were obtained through diazotization in acidic solution with
sodium nitrite, the nitrous acid excess was destroyed with ammonium sulphamate.
The concentrations and the preparation technique were those described in literature
(Ciuhandu and Mracec
1968
).
Solid sulphanilic acid can also be treated with gaseous NOCl, to give solid
diazonium chloride in 100 % yield. The water produced by the reaction is removed
in situ reaction with excess of NOCl (Kaupp et al.
2002
).
Diazonium salts both as diazotization products and electrophilic reactants in
azocoupling processes are also involved in thermal decompositions (Simon and
Badilescu
1967
).
The kinetics of thermal decompositions in solution of diazonium salts of
sulphanilic acid and sulphanilamide were followed spectrophotometrically. The
following activation energies (E) and preexponent (A) were obtained for ther-
mal decompositions of benzenediazonium salts: for the sulphanilic acid derivative
E
127.19 kJ mol
−
1
,A
6.0·10
15
s
−
1
, for the sulphanilamide derivates E
=
=
=
124.20
kJ mol
−
1
,A
4.0·10
15
s
−
1
(Isac et al.
1981b
).
For these quick reactions of breaking the C-N bonds, A
>
10
14
s
−
1
indicates that
within the activated complex the interaction of
=
electrons between the nitrogen
atoms and those of the molecule residue is weaken or even cancelled (Simon
1966a
).
The correlation diagram of molecular orbitals and the correlation diagram of
electronic states for the decomposition reaction of the benzenediazonium cation
were established, using Hückel molecular orbital calculations (Isac et al.
1985
). The
activation energy of the thermal decomposition must be of purely thermodynamic
nature, without a certain barrier, as the diazonium cation is more stable than the
decomposition products (Simon and Badilescu
1967
).
The following activation enthalpies (
H
=
) and activation entropies (
S
=
) were
obtained for thermal decompositions of benzenediazonium salts: for the sulphanilic
acid derivative
H
=
=
π
124.683 kJ mol
−
1
,
S
=
=
48.116 J K
−
1
mol
−
1
for the
sulphanilamide derivates
H
=
=
44.350 J K
−
1
mol
−
1
(Isac et al.
1981b
). The activation enthalpies were quite high, and in contrast, the
corresponding activation entropies have high positive values.
The experimental data indicate that the stability of substituted diazonium ions,
precipited as diazoniumfluoroborates, can be increased by complexation with crown
ethers (Bräse et al.
2004
). Diazonium cations are not strong electrophilic reac-
tants because of their positive charge delocalization. Increased reactivity is found in
those diazonium cations containing electron attractive substituents in para position
(Nenitescu
1966
, Part II, Chap. 5, pp. 590-593).
The azocoupling of these compounds may occur only with strong nucleophiles
such as phenolates, naphtholates, and free aromatic amines. In this case the reaction
selectivity is high (Nenitzescu
1968
, Part IV, Chap. 5, pp. 430-436). This behavior
makes the azocoupling processes to occur almost unitary into positions of highest
electron density (
121.336 kJ mol
−
1
,
S
=
=
ρ
r
) of the coupling agents (Simon
1973
, Chap. 2, pp. 35-50). The
reactivity of coupling components variously substituted in the same position falls
within the series -O
−
>
-NH
2
>
-OH.
