Chemistry Reference
In-Depth Information
Fig. 11.13
The graphical plot for the bondonic features of Table
11.22
in respecting the rate of
reaction (LogK) at 25
◦
C for molecules of Table
11.21
and N substituted 4-aminophenyldiazonium salts at room temperature is relatively
high and the photochemical effects can be easily distinguished (Isac et al.
1981b
).
The sulphanilic acid, the sulphanilamide and 4-N, N'-dimethylaminoaniline were
diazotized according to an analytical method (Ciuhandu and Mracec
1968
).
The thermal and photochemical stability of the obtained diazonium salts was
verified by a spectrometric method. The diazonium salts of sulphanilic acid and
sulphanilamide are photochemically stable up to 353 K, but thermally instable; that
is why the kinetics of their decomposition could be followed in the UV domain.
The diazonium salt of 4-N, N'-dimethylaminoaniline is thermally stable, but very
sensitive photochemically, which allows the qualitative study of its photodecompo-
sition in the UV and visible domain. There is a surprising effect of substituents upon
kinetics of thermal and photochemical decomposition of aromatic diazonium salts
(Table
11.21
) (Isac et al.
1981b
; Simon
1966a
).
The substituents with
I effect in meta or para positions decrease the
reaction rates. Electron donor substituents in meta position increase the reaction
rates, but the same substituents in para position strongly decrease the reaction rates.
It is supposed that this stabilization is due to a resonance effect that increases the
C-N bond order (Barraclough et al.
1972
).
Table
11.23
shows a satisfactory correlation between the positions of experimental
absorbtion maxima and the transition energies calculated by the HMO method (Isac
et al.
1981a
).
Although the first calculated transition energies for the diazonium salts are too low,
the relative order of the calculated transitions corresponds to the experimental one.
Based on experimental data and reactivity indices calculated by HMO method we
proposed structure-reactivity correlations for some diazonium cations (Table
11.24
)
(Isac et al.
1981a
,
1985
).
−
M and
−
