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Russ.Chem.Bull., Int.Ed., Vol. 55, No. 10, October, 2006
Antonov et al.
and treated with a saturated solution of KMnO4 in 20% aqueous
NaOH until resistance to oxidation was attained (unchanged
color after 30 min of shaking), again washed with water, dried
with CaCl2, and distilled. Tetraphenylhydrazine (~1 g L–1) was
added, argon was purged through the mixture for 15 min, and
the mixture was heated to 70—80 °C and kept for 2 h at this
temperature. Then the temperature was slowly raised, and the
mixture was distilled under argon. The bottoms were orangeꢀ
colored due to the formation of quinoneimine and diphenylꢀ
nitroxide radical.12 Repeated distillation was carried out in the
presence of tetraphenylhydrazine (0.2—0.3 g L–1) under the same
conditions. Now the bottoms were not orange but weakly
greenꢀyellowish due to the formation of semidines.13—15 After
distillation chlorobenzene was passed through a glass column
(70×2.5 cm) packed in layers with the following powders (layer
length from the inlet of the column/cm): concentrated H2SO4
on SiO2 (3), SiO2 (2), KMnO4 on Al2O3 (7), SiO2 (2), CaCl2 (8),
KOH (10), and activated Al2O3 was the rest. Chlorobenzene
purified using this procedure behaves as quite inert solvent. It
should be mentioned that upon prolong storage (for 1 year at
~20 °C under Ar) small amounts of admixtures are accumuꢀ
lated, which accept, in particular, stable 2,4,6ꢀtriꢀtertꢀbutylꢀ
phenoxyl radicals. To remove the admixtures, it is enough to
pass this chlorobenzene through a column packed as described
above.
ementary reactions involving this diimine are rateꢀdeterꢀ
mining for the whole process.
We have recently9 attempted to determine DNH(2).
The experimental estimate of DNH(2) (274.7 3.3
kJ mol–1) is lower than the value calculated by the quanꢀ
tum chemical methods (290.6 kJ mol–1). In the present
work, we again attempted to determine experimentally
the DNH(2) value. For this purpose we proposed a new
approach based on the use of temperature dependences of
the equilibrium constants of two auxiliary reversible chain
reactions in quinonenimine—hydroquinone systems.
Experimental
In experiments special attention was given to the purity of
reagents and solvent. Diamine 1 and 4ꢀhydroxydiphenylamine
(4) were purified as follows. To remove mechanical and strongly
polar admixtures, a solution (close to saturation) of comꢀ
pound 4 (~5 g) in benzene (or compound 1 (~5 g) in a benꢀ
zene (95%)—chloroform (5%) mixture) was passed through a
glass column (10×2.5 cm) packed with silica gel L (Chemapol,
40—100 micron). The eluate was evaporated by ~95% at ~20 °C,
the mother liquor was rejected, and the crystals that precipitated
were washed with small portions of benzene and dried in vacuo.
Then compounds 4 and 1 were recrystallized from methanol or a
methanol—chloroform (3—5%) mixture, respectively, and then
recrystallized additionally from pure toluene.
Equilibrium constants of quinoneimines 3 and 5 with hydroꢀ
quinone 7 were determined by spectrophotometry. Quinoneꢀ
imines are orangeꢀcolored (λ
≈ 450 nm), while the other
max
species are colorless or very weakly absorb in the visible region
(for quinone 7 ε ≈ 20 L mol–1 cm–1). Experiments were carꢀ
450
ried out in temperatureꢀcontrolled quartz cells, which served as
bubble reactors, 8.5 and 6.0 cm3 in volume and 2.0 cm optical
path incorporated into a Specord UV VIS spectrophotometer.
Experiments were started at 298 K. At this temperature the
consumption of quinoneimines were continuously monitored by
the absorption of the latter at a chosen wavelength in the visible
region. The experiments were continued until equilibration. Then
the controlling thermometer in the thermostat was changed and,
continuing the experiment, the temperature in the thermostat
and reactor was raised to a new specified value, the experiment
was continued at a new specified temperature until equilibraꢀ
tion, etc. After measurements at all chosen temperatures, the
reactor was cooled to 298 K and the solution was equilibrated at
this temperature to compare the result with that obtained at the
onset of the experiment. Taking into account that at prolong
duration of entries the results can be distorted because of
evaporation of the solvent, relatively vigorous argon bubbling
(4—5 bubbles per second) was carried out only during the first
~15 min at 298 K, after which the argon flow was sharply deꢀ
creased to 1—2 bubbles per minute, remaining at this level within
the whole experiment.
Quinonediimine 3 and Nꢀphenylꢀ1,4ꢀbenzoquinonemonoꢀ
imine (5) were synthesized from reduced forms of compounds 1
and 4, respectively, by the oxidation of the latter with PbO2 and
purified by liquid chromatography on SiO2 and recrystallization
from methanol.7
2,5ꢀDichloroquinone (6) (Aldrich) was purified by double
sublimation in vacuo. 2,5ꢀDichlorohydroquinone (7) was synꢀ
thesized by the reduction of purified quinone 6 using Na2S2O4
by a known procedure10 and recrystallized two times from
methanol.
Chlorobenzene was used as the solvent. Chlorobenzene puꢀ
rified by a standard procedure11 (treatment with concentrated
H2SO4, washing with water and soda solution, drying with CaCl2,
and distillation) turned out to be inappropriate for work, beꢀ
cause solutions of compounds 1 and 4 in this solvent after sevꢀ
eral hours at ~20 °C gained a noticeable orange tint due to the
formation of quinoneimines. The following purification proceꢀ
dure was used. Initial chlorobenzene (highꢀpurity grade) was
distilled, treated with concentrated H2SO4, washed with water,
Results and Discussion
The sum of the dissociation energies of the N—H
bonds in diamine 1 and its radical 2 can be determined
from the enthalpy ∆H of the reaction of quinonediimine 3
with 4ꢀhydroxydiphenylamine (4), because the dissociaꢀ
tion enthalpies of the N—H and O—H bonds in amine 4
Antonov
