Kinetic study of the fast step of the alkaline hydrolysis of p-chloranil using stopped flow technique

Article abstract of DOI:10.1002/(SICI)1097-4601(1997)29:5<385::AID-KIN9>3.0.CO;2-Q

The alkaline hydrolysis of p-chloranil or 2,3,5,6-tetrachloro-1,4-benzoquinone (C₆Cl₄O₂,Q) was studied, using stopped flow spectrophotometry and Electron Spin Resonance techniques (E.S.R.). In the present study it was shown for the first time, that a free radical is produced chemically and that it can account for the propagation of the reaction. It was found that in alkaline conditions chloranil in a `Michael' fashion undergoes 1,2 addition being hydrolyzed and in turn produces a chloranil free radical (Q·). The hydrolysis then proceeds via a number of intermediates yielded by this radical and a number of different products is formed. The formation of these products, both quantitatively and qualitatively has a strong dependence on the concentration of the OH^- species and chloranil. The various possible routes of the hydrolysis are studied either spectrophotometrically or by E.S.R. Two different intermediates are observed absorbing at 426 nm and at 540 nm, respectively. Each species was formed and destroyed within 10 s to 30 min depending on the exact conditions. The reaction rate constants for the formation and the decay of the intermediates was estimated using the Guggenheim method. At both wavelengths the rate constants seem to have a complex relation to the concentration of the anion.

Full text of DOI:10.1002/(SICI)1097-4601(1997)29:5<385::AID-KIN9>3.0.CO;2-Q

Kinetic Study of the Fast
Step of the Alkaline
Hydrolysis of p-Chloranil
Using Stopped Flow
Technique
S. LAFIS, C. N. KONIDARI, P. G. VELTSISTAS, N. TZERPOS, M. I. KARAYANNIS
University of Ioannina, Department of Chemistry, Laboratory of Analytical Chemistry, Ioannina, 45110, Greece
Received 14 February 1996; accepted 30 September 1996
ABSTRACT: The alkaline hydrolysis of p-chloranil or 2,3,5,6-tetrachloro-1,4-benzoquinone
(
C Cl O , Q) was studied, using stopped flow spectrophotometry and Electron Spin Reso-
6 4 2
nance techniques (E.S.R.). In the present study it was shown for the first time, that a free rad-
ical is produced chemically and that it can account for the propagation of the reaction. It was
found that in alkaline conditions chloranil in a “Michael” fashion undergoes 1,2 addition be-
ing hydrolyzed and in turn produces a chloranil free radical (Q ). The hydrolysis then pro-
и
ceeds via a number of intermediates yielded by this radical and a number of different prod-
ucts is formed. The formation of these products, both quantitatively and qualitatively has a
strong dependence on the concentration of the OHϪ species and chloranil. The various possi-
ble routes of the hydrolysis are studied either spectrophotometrically or by E.S.R. Two differ-
ent intermediates are observed absorbing at 426 nm and at 540 nm, respectively. Each
species was formed and destroyed within 10 s to 30 min depending on the exact conditions.
The reaction rate constants for the formation and the decay of the intermediates was esti-
mated using the Guggenheim method. At both wavelengths the rate constants seem to have
a complex relation to the concentration of the anion. © 1997 John Wiley & Sons, Inc. Int J Chem
Kinet 29: 385–391, 1997.
INTRODUCTION
substitution, losing one chloride ion, and producing
monohydroxy chloranilic acid or 2,3,5-trichloro-6-
hydroxy-1,4 benzoquinone (C Cl O H) and subse-
According to the literature [1–5] p-Chloranil
6
3
3
or 2,3,5,6-tetrachloro-1,4-benzoquinone (C Cl O ,
quently in the second step another chlorine ion is lost
to get eventually chloranilic acid. In this work there is
no concluding evidence of chloranilic acid being pro-
duced within the time scales measured, and at 25ЊC.
There is experimental evidence that the reaction pro-
ceeds through a different mechanism, than previously
suggested, involving more reactive intermediates.
The presence of intermediates, other than the cor-
responding acids, has been suggested in the literature
6
4
2
Q), in alkaline conditions yields chloranilic acid or
,5-dichloro-3,6-dihydroxy-1,4-benzoquinone
C Cl O H ). The exact mechanism of the process is
2
(
6
2
4
2
not clearly understood. It has been suggested that in
excess of alkali the reaction proceeds via two steps.
In the first step p-chloranil undergoes nucleophilic
©
1997 John Wiley & Sons, Inc.
CCC 0538-8066/97/050385-07

3
86
LAFIS ET AL.
although their identity has not been revealed. Slifkin
1] has indicated that the reaction proceeds through a
transit state” that could not be identified, while
[
“
Bishop [2] could not account for a compound formed
when p-chloranil was dissolved in cold KOH.
p-chloranil is a very good electron acceptor that
can draw electrons forming free radicals [6]. Rich lit-
erature exists [6–9] on the formation of free radicals,
using laser “flush photolysis,” by benzoquinones that
under appropriate conditions produce the ben-
zosemiquinone free radical in the single, triplet, the
neutral, and the anionic form.
In order to monitor this reaction the stopped-flow
technique was chosen. According to this method two
solutions flow separately and are mixed at the reac-
tion vessel, where the flow is abruptly interrupted. At
the point of interruption the reaction between the two
solutions is monitored spectrophotometrically. All
reagents were made in a 1:1 dioxane to water mix-
ture.
Figure 1 Spectrum of 1.25 ϫ EϪ4M p-chloranil (a) and
after treatment with 1.25 ϫ EϪ2M NaOH (b).
apparent that all reactants, products, and intermedi-
ates absorb in that region, and so distinguishing be-
tween the different absorption bands would be very
difficult. Thus, all efforts were concentrated in the
visible area of the spectrum. Another important ob-
servation made was that small changes in the reactant
concentration resulted in significant changes in the
reaction pattern, that could not be explained by exper-
imental error. This is in accordance with the literature
EXPERIMENTAL
All chemicals were of analytical grade obtained ei-
ther from Sigma or from Merck. The stopped-flow
was the model “The bio sequential SX17MV” made
by Applied Photophysics Ltd, England and the E.S.R.
was “VARIAN E-109 Spectrometer.” Working solu-
tions of p-chloranil in dioxane were made up at equal
volumes with water. The resulting solution was intro-
duced into one of the syringes of the instrument. In
the other syringe was a solution containing an alka-
line solution made up of 1/1 by volume with dioxane.
So both syringes contained solutions of the same
mixture of solvents (v/v water to dioxane). Spectra
were obtained approximately 1 min after mixing un-
less otherwise stated. Temperature was controlled at
[7]. Figure 2 illustrates this observation where varia-
tions on the concentration of NaOH result in signifi-
cant differences in the form of the spectra. This lead
us to believe that the intermediates formed (very
2
5ЊC.
RESULTS AND DISCUSSION
The spectrum of p-chloranil in dioxane shows no
peaks as seen in Figure 1(a). Upon addition of NaOH,
two new peaks appear, Figure 1(b). Both these peaks
show the same decay with respect to time while a fur-
ther peak is formed later at 332 nm. Under specific
conditions (low NaOH concentration) another peak is
formed at 362 nm. Attempts were made to study the
spectra in the UV region. However, it soon became
Figure 2 Spectra of 1.25 ϫ EϪ4M p-chloranil after treat-
ment with six successive solutions of different NaOH con-
centration.

ALKALINE HYDROLYSIS OF p-CHLORANIL
387
rapidly) have a complex dependence on the concen-
tration of the reactants.
which was expected since the alkaline strength of the
group is much smaller.
Analysis of the Kinetic Curves
KINETIC STUDY
In order to describe the curves shown on Figure 3 the
Guggenheim method was applied [10,11]. This is a
method for estimating the rate constant of a first-or-
der reaction when the absorbance at infinite time is
not known. According to this method a plot of
ln(A Ϫ A ) vs. t, where ␶ is an arbitrary time con-
The kinetic behavior of the species resulting in the
two peaks (Fig. 1), is shown in Figures 3(a) and 3(b).
Under different experimental conditions similarly
shaped curves were obtained. If the reaction was a
simple hydrolysis then the two peaks at 426 nm and
t
tϩ␶
5
40 nm would be attributed to intermediates (e.g., the
stant, yields a straight line of slope equal to the rate
of the reaction (kobs1). Tables I and II show the rates
obtained by this method. Equation (1) is true for two
consecutive first-order reactions [10–12].
monohydroxy acid) and the peak at 332 nm to the fi-
nal product (chloranilic acid). According to the litera-
ture these peaks (with the exception of the peak at
3
32 nm, which was identified in the literature as chlo-
All k’s refer to k observed:
ranilic acid) correspond either to photochemically
produced radicals [13] or to complexes of the acid
Tmax ϭ ln(k /k )/(k Ϫ k )
(1)
2
1
2
1
[15]. Since the formation of a chloranilic acid com-
plex was unlikely we tested the hypothesis that free
radicals were present, produced chemically.
Since the rate of the second reaction is much
slower than that of the first k ϾϾ k then eq. (1) be-
1
2
The same experimental procedure was followed
but substituting NaOH with KOH and Na CO . In the
comes:
2
3
case of KOH the experimental observations were
similar to those of NaOH except that the rate of de-
composition of the peak at 426 nm was reduced,
while the formation of that peak accelerated. That led
us to believe that there is some process of stabiliza-
tion involving the cation Na or K . In the case of
the carbonate group the intermediate was much more
stable and the behavior of the system was reminis-
cent of that observed at low concentrations of NaOH
Tmax Х ln(k /k )/(Ϫk ) "Q
2 1 1
k1 ϫ Tmax Х Ϫln(k /k ) "Q
2
1
Ϫk1Tmax
k /k Х e
"Q
2
1
Ϫk1Tmax
k Х k xe
(2)
ϩ
ϩ
2
1
Therefore, from eq. (2) the values of k can be calcu-
2
lated knowing k and T
.
1
max
The complexity of the reaction is obvious from Ta-
bles I and II. However, by studying the Guggenheim
method it can be seen that the rate constant of all the
reactions decreases with increasing OHϪ concentra-
tion. Also it can be seen that at both wavelengths
the rate constant remains unchanged with respect to
p-chloranil concentration when KOH was used. The
values for the decay of the intermediate differ from
those given in the literature [13,17] probably because
the conditions are not identical. Taking into account
the spreading of these values this should only be ex-
pected.
E.S.R. STUDY
Proof for the presence of the free radical was ob-
tained from the E.S.R. studies. A solution containing
1 ϫ 10 p-chloranil in dioxane and 0.05 M NaOH
Figure 3 Kinetic curve of the production and decay of the
intermediate at 426 nm (a) and 540 nm (b). Initial concen-
Ϫ4
Ϫ3
trations: p-chloranil 1.25 ϫ E M and NaOH 1.25 ϫ
Ϫ2
E
M.
was introduced into the instrument and within 1 min

3
88
LAFIS ET AL.
Table I Calculations of Kobs Values by Guggenheim’s Method using KOH as the
Anion Donor
4
26 nm
,
540 nm
,
KOH ϫ EϪ2 Chl ϫ EϪ4 Tmax
,
k
s
k
s
,
Tmax
s
,
k
s
k
s
,
obs1
Ϫ1
obs2
Ϫ1
obs1
obs2
Ϫ1
Ϫ1
M
M
s
1
.25
.25
.25
.25
.25
.62
0.5
1.25
2.5
5.0
0.5
0.5
0.5
0.7
0.7
0.6
0.6
3.2*
3.2*
1.2
6.9 5.5 ϫ 10Ϫ2
1.6
1.7
1.7
1.5
0.7
0.8
1.7
4.1
4.5
4.5
4.4
9.2
9.2
4.5
5.8 ϫ 10Ϫ3
2.1 ϫ 10
2.1 ϫ 10Ϫ3
Ϫ2
Ϫ3
1
1
1
0
0
6.9 5.5 ϫ 10
6.9 1.1 ϫ 10Ϫ1
6.9 1.1 ϫ 10Ϫ1
6.0 ϫ 10
Ϫ3
Ϫ19
1.5 ϫ 10Ϫ2
5.8 ϫ 10Ϫ3
2.1 ϫ 10Ϫ3
14.0 4.9 ϫ 10
12.1 1.8 ϫ 10
Ϫ16
2
.5
8.9 2.0 ϫ 10Ϫ4
*
At these values it was very difficult to read Tmax since kobs2 was very small.
the spectrum obtained. g-values were measured with
reference to diphenylpicrylhydrazyl (DPPH) (g ϭ
had similar g-values of 2.00585 and 2.00578, respec-
tively. The proximity of these values leads to the con-
clusion that it is the same free radical present in all
these systems. The decay of the radical as studied by
E.S.R. is shown on Figure 4, which shows consecu-
tive spectra obtained every 0.3 min. Using the infinite
2
.0037). The g-value for the radical produced in solu-
tion was calculated as 2.0059. This value is very
close to both the neutral radical of the semibenzo-
quinone (g ϭ 2.0055) [13] and to the anionic radical
Ϫ1
(g ϭ 2.0057) [16]. From this information it cannot be
time method kobs was found to be 0.005 s . This
concluded which radical form was produced since
g-values from different laboratories are difficult to
compare [13,14]. However, this radical is charged
since when produced with KOH instead of NaOH its
kinetic behavior was altered.
The data obtained from the E.S.R. study, together
with the proximity of the visible spectrum, compared
to that of the laser-flash photolysis experiments
value was comparable to the kobs values found from
the Guggenheim method for the intermediate decay-
ing at 426 nm, thus providing further evidence that
the free radical is absorbed at this wavelength.
In order to study further the formation of this radi-
cal a stopped-flow/E.S.R. instrumental arrangement
would be necessary.
[6,7,15,16], made us believe that this intermediate is
the same free radical as that produced by the laser ex-
periments. The wavelengths where the radicals
showed maximum absorbance almost coincided with
those of the chemical production of the free radical.
The free radicals produced by KOH and NaOH
MECHANISM
A mechanism can be drawn for the alkaline hydroly-
sis of chloranil involving three steps:
Table II Calculations of Kobs Values by Guggenheim’s Method using NaOH as the
Anion Donor
4
26 nm
,
540 nm
NaOH ϫ EϪ2 Chl ϫ EϪ4 Tmax
,
k
s
k
s
,
Tmax
s
,
k
,
k
s
,
obs1
Ϫ1
obs2
Ϫ1
obs1
obs2
Ϫ1
Ϫ1
M
M
s
s
0
.1
.25
.5
.62
.25
.5
5.0
5.0
5.0
5.0
5.0
5.0
5.0
1.6
2.7
6.0*
5.1*
1.8
13.3 7.6 ϫ 10Ϫ9
4.6 1.9 ϫ 10Ϫ5
0.4
1.2
1.6
1.9
2.2
1.5
1.3
7.3
9.1
7.7
4.2
2.4
2.3
2.3
3.9 ϫ 10Ϫ1
1.6 ϫ 10Ϫ4
0
0
0
1
2
5
Ϫ11
Ϫ5
4.4 1.5 ϫ 10
7.6 1.1 ϫ 10
3.4 ϫ 10
Ϫ16
1.4 ϫ 10Ϫ3
6.3 7.5 ϫ 10Ϫ5
1.2 ϫ 10Ϫ2
Ϫ2
Ϫ2
0.9
0.6
7.3 1.0 ϫ 10
7.3 ϫ 10
.0
7.3 9.1 ϫ 10Ϫ2
1.2 ϫ 10Ϫ1
*
At these values it was very difficult to read Tmax since kobs2 was very small.

ALKALINE HYDROLYSIS OF p-CHLORANIL
389
Figure 4 Decay of the intermediate as observed by the E.S.R. Initial concentrations: p-chloranil
Ϫ4
Ϫ2
1
.25 ϫ E M and NaOH 1.25 ϫ E M.
Step 1
Step 2
The first step is the alkaline hydrolysis of chloranil in
a “Michael” fashion, i.e., 1,2 addition (Fig. 5). The
anion (I) produced reacts with another chloranil mol-
ecule yielding neutral radical (II) and a radical anion
When chloranil is in abundance then either self dis-
proportionation, yielding compound (VII), or self
substitution (compounds VIII and IX) can occur.
(
III). The neutral radical has a very short life time
and probably does not take any further part in the re-
action. The radical anion was observed in the spec-
trophotometric experiments giving rise to the double
peak at 426 and 454 nm and in the E.S.R. spectrum
with a g-value of 2.00585. This radical can exist in
the three resonance forms shown in Figure 6 (IV,
V, VI).
Step 3
When the OHϪ is in excess then the reaction pro-
ceeds as shown in Figure 7. The OH attacks radical
VI and produces the two resonance forms noted, X
and XI. Form X reacts with another chloranil mole-
cule yielding monohydroxy acid (XII) and the origi-
Ϫ
Figure 5 Suggestive mechanism of the first step of the alkaline hydrolysis of p-chloranil.

3
90
LAFIS ET AL.
Figure 6 Suggestive mechanism of the second step of the alkaline hydrolysis of p-chloranil.
nal radical anion (III). Form XI is attacked by an-
other OHϪ molecule producing compound XIII,
which, in turn with another chloranil molecule, can
yield chloranilic acid (XIV) and the original radical
anion III.
ied this was not the case due to the reactivity of the
radicals.
The scheme shown could be summarized as follows:
This mechanism considers the various possible
steps of the alkaline hydrolysis of p-chloranil when a
free radical is formed. Other possibilities exist when
no radical is present mainly the 1,4 addition yielding
chloranilic acid. However, under the conditions stud-
Step 1 (1,2 addition)
Q ϩ OH 9: QϪ
Ϫ
Ϫ
Ϫ
Q ϩ Q 9: Q ϩ Q
и и
Figure 7 Suggestive mechanism of the third step of the alkaline hydrolysis of p-chloranil.

ALKALINE HYDROLYSIS OF p-CHLORANIL
391
Step 2
BIBLIOGRAPHY
Qи 9: QиϪ(IV) ;: Q
Ϫ
Ϫ(V) ;: Q Ϫ(VI)
и
и
1
2
3
4
. M. A. Slifkin, R. A. Sumner, and J. G. Heathcote,
Ϫ
Spectrochim. Acta, 23A, 1751 (1967).
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3043 (1964).
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trochim. Acta, 25A, 1479 (1969).
2Qи
2Qи
9: Q ϩ QH (Self Disproportionation)
2
Ϫ
9: Polymer (IX) (Self Substitution)
. P. G. Veltsistas, M. I. Karayannis, and M. A. Kouparis,
Talanta, 41, 1725 (1994).
Step 3
Qи
Ϫ
(VI) ϩ OH 9: Q(OH) Ϫ(X) ;:
Ϫ
5. J. W. Hancock, E. Morrell, and D. Rhum, Tetrahedron
Lett., 22, 987 (1962).
6
и
Q(OH)иϪ(XI)
. G. Jones and N. Mouli, J. Phys. Chem., 92, 7174
(1988).
Q(OH) Ϫ(X) ϩ Q 9: Monohydroxy
и
chloranilic acid ϩ Q Ϫ
7. G. Jones and B. Huang, Tetrahedron Lett., 34, 269
1993).
8. M. A. Elsayed and S. P. Agarwal, Talanta, 29, 535
1982).
9. R. Gschwind and E. Haselbach, Helv. Chim. Acta, 62,
41 (1979).
и
(
Q(OH) Ϫ(XI) ϩ OH 9: Q(OH) Ϫ(XIII)
Ϫ
и
2
и
(
Q(OH)2и
Ϫ(XIII) ϩ Q 9: Chloranilic acid ϩ Q Ϫ
и
9
1
1
1
1
1
1
0. A. A. Frost and R. G. Pearson, Kinetics and Mecha-
nism, Wiley, Japan, 1961.
1. G. Fleck, Chemical Reactions and Mechanisms, Holt,
Rinehart and Winston, Inc., 1971.
No conclusive experimental evidence can be provided
for all the different steps shown in this mechanism.
However, this scheme can explain the complex de-
pendence of the rate of the reaction to the concentra-
tion of the reactants and the complicated appearance
of Tables I and II.
2. M. I. Karayannis, Habil. Thesis University of Athens,
1
973.
3. S. K. Wong, L. Fabes, W. J. Green, and J. K. S. Wan, J.
Chem. Soc. Faraday Trans., 68, 2211 (1972).
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Phys., 43, 4191 (1965).
5. S. P. Agarwal and M. A. Elsayed, Analyst, 106, 1157
(1981).
The authors would like to acknowledge Assist. Prof. A.
Zarkadis for his valuable assistance in the E.S.R. measure-
ments, Assist. Prof. G. Pilidis for his valuable comments on
the mechanism, and Mr. I. Kokkinos for mathematical as-
sistance.
16. M. Zhang, Z. Liu, L. Yang, and Y. Liu, J. Chem. Soc.,
Chem. Comm., 1054 (1991).
17. G. Jones and A. W. Haney, J. Phys. Chem., 90,
5410–5414 (1986).

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