Kinetic studies of hydroxyquinone formation from water soluble benzoquinones

Article abstract of DOI:10.1039/c3nj01274c

The kinetics and mechanisms of the redox reactions between hydrogen peroxide and 1,4-benzoquinone, 2-methyl-1,4-benzoquinone, 2,6-dimethyl-1,4- benzoquinone, 2-chloro-1,4-benzoquinone and 2,6-dichloro-1,4-benzoquinone were studied in aqueous media using spectrophotometric monitoring. The formation and decay of a hydroxylated 1,4-benzoquinone was detected. The formation of the intermediate was first order with respect to the parent 1,4-benzoquinone and hydrogen peroxide, whereas inverse first order dependence was revealed with respect to the hydrogen ion. The decomposition reaction had two parallel pathways: one was first order with respect to the intermediate, while the other showed second-order dependence. The values of the rate constant measured for the formation step were successfully correlated with both the redox potentials of the substituted quinone-hydroquinone systems and the pK_a values of the hydroxylated quinone derivatives. Therefore, electronic effects govern the reactivity of the quinones in this process. NMR and GC-MS measurements were carried out to identify the products in the system. Quantum mechanical calculations were also carried out in these systems.

Full text of DOI:10.1039/c3nj01274c

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Kinetic studies of hydroxyquinone formation from
water soluble benzoquinones†
Cite this:DOI: 10.1039/c3nj01274c
a
ab
a
a
a
E´ va Jozsa, Mihaly Purgel, Marianna Bihari, Peter Pal Feher, Gabor Sustyak,
´
´
´
´
´
´
´
a
a
a
a
+
Balazs Varnagy, Virag Kiss, Eszter Lado and Katalin Osz*
´
´
´
´
The kinetics and mechanisms of the redox reactions between hydrogen peroxide and 1,4-benzoquinone,
-methyl-1,4-benzoquinone, 2,6-dimethyl-1,4-benzoquinone, 2-chloro-1,4-benzoquinone and 2,6-dichloro-
,4-benzoquinone were studied in aqueous media using spectrophotometric monitoring. The formation and
2
1
decay of a hydroxylated 1,4-benzoquinone was detected. The formation of the intermediate was first order
with respect to the parent 1,4-benzoquinone and hydrogen peroxide, whereas inverse first order dependence
was revealed with respect to the hydrogen ion. The decomposition reaction had two parallel pathways: one
was first order with respect to the intermediate, while the other showed second-order dependence. The
values of the rate constant measured for the formation step were successfully correlated with both the redox
Received (in Montpellier, France)
15th October 2013,
Accepted 13th November 2013
a
potentials of the substituted quinone–hydroquinone systems and the pK values of the hydroxylated quinone
derivatives. Therefore, electronic effects govern the reactivity of the quinones in this process. NMR and
GC-MS measurements were carried out to identify the products in the system. Quantum mechanical
calculations were also carried out in these systems.
DOI: 10.1039/c3nj01274c
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oxidizing agents are required that do not react with water. Our
choices were hydrogen peroxide and the periodate ion.
1
. Introduction
Benzoquinones play an important role in the living organisms as
electron acceptors. Bioelectrochemical cells were constructed to was reported in the literature. Hydroxy-para-benzoquinone,
The oxidation of numerous quinones to hydroxyquinones
1,2
10
0
0
0
0
0
utilize solar energy, mimicking the photosynthetic system using a 2-hydroxy-5-methoxy-3-(8 ,Z,11 Z)-pentadeca-8 ,11 ,14 -trienyl-1,4-
3,4
quinone as a mediator. Upon the UV irradiation of benzoquinone benzoquinone, metachromin-A, and 2-hydroxyphenanthrene-1,4-
11–17
derivatives in aqueous media, hydroquinone, hydroxyquinone and quinone were prepared through the Thiele–Winter acetoxylation.
5–7
oxygen are formed. In photoelectrochemical energy conversion Hydroxy-benzoquinone was prepared by the oxidation of hydroxy-
1
8,19
systems, hydroxylation of quinones is considered a side reaction hydroquinone (benzene-1,2,4-triol).
6-Hydroxy-1,2,3,4-tetra-
because it decreases the amount of the reducible compound thus hydronaphthalene-5,8-dione was obtained by the demethylation of
20
causing the decay of such cells. The effect of substituents on the the trimethoxy derivatives. The synthesis of naturally occurring
hydroxyquinone formation and the quinone reduction is not well maesanin, (ꢀ)-illimaquinone, and (ꢁ)-mamanuthaquinone was
7
understood. The direct formation of hydroxyquinone in the carried out by the oxidative demethylation of tetramethoxybenzene
2
1–23
photochemical process is unlikely because the triplet excited states with cerium(IV) ammonium nitrate.
8,9
of benzoquinones are strong oxidizing agents. Thus, it is more
likely that this triplet quinone oxidizes another quinone molecule benzoquinone was studied in detail in the oxidation reaction
to hydroxyquinone. of 2,6-dichloro-1,4-benzoquinone with hydrogen peroxide as a
The kinetics of the formation of 3,5-dichloro-2-hydroxy-1,4-
Triplet quinone is highly unstable in aqueous solution. To function of reactant concentrations and pH. The process was
understand the reaction mechanism of hydroxyquinone oxida- found to be first order with respect to both reagents and inverse
tion and the substituent effects in this reaction, more stable first order with respect to hydrogen ion concentration. The latter
finding was explained by the assumption that the deprotonated
ꢀ
form of hydrogen peroxide, HO
2
, is the actual reactive form of
a
Department of Physical Chemistry, University of Debrecen, H-4032 Debrecen,
Egyetem t ´e r 1, Hungary. E-mail: osz.katalin@science.unideb.hu;
Tel: +36 52-512-900, Ext.: 22387
the oxidizing agent even though the pH range of the studies was
far from the known first pK of H O . This interpretation was in
a
2 2
good qualitative agreement with the fact that extremely fast
reaction rates were found at higher pH, but evidence for the
occurrence of numerous side reactions was also obtained in this
b
Research Group of Homogeneous Catalysis and Reaction Mechanism,
H-4032 Debrecen, Egyetem t ´e r 1, Hungary
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
2
4
c3nj01274c
more basic medium.
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In this paper, our aim was to study the formation kinetics of and the decomposition of hydroxyquinones. After measuring
some hydroxyquinones containing electron donating or electron the spectrum of the hydroxyquinone product, only one wave-
withdrawing substituents. The hydroxyquinone derivatives were length at the maximum absorbance in the visible range was
formed by oxidizing the corresponding 1,4-benzoquinones with used to follow the reaction. In contrast to a diode array spectro-
hydrogen peroxide or potassium periodate in aqueous solution. photometer and full-scan mode, the light intensity passing
Hydrogen peroxide is a strong oxidizing agent that has high through the samples in this way is low enough not to initiate
oxidation potential in a wide pH range: E = 1.763 V at pH 0 photochemical decomposition of 1,4-benzoquinones. This was
2
5
and E = 0.878 V at pH 14. It is used extensively in chemical also proved experimentally. Most reactions (except for the
syntheses since its decomposition products, namely H O and O temperature dependent kinetic curves) were measured in 1.00 cm
2
2
,
are environmentally friendly, but its reactivity is often moderate: path length quartz cuvettes at 25.0 ꢁ 0.1 1C.
catalysts are regularly needed in its reactions. In addition, stock
Temperature dependent kinetic measurements were per-
solutions of hydrogen peroxide are not very stable and cannot be formed after a 10 min pre-thermostation period of the samples
stored for extended times. Potassium periodate was the other and in the 10–40 1C temperature range.
oxidant used here. Due to its high oxidizing power and high
For the oxidation reactions of unsubstituted 1,4-benzoquinone
reactivity, it is often used in organic oxidation reactions. In its and at higher hydrogen peroxide concentrations, rapid kinetic
solid form, no stability problems are encountered and the experiments were carried out in an Applied Photophysics DX-17
oxidation by-product, iodide ion, does not pose a major known MV stopped-flow instrument with a photomultiplier tube as the
environmental risk. There seems to be a literature contradiction detector. Absorbance traces were collected using an optical cell of
ꢀ
about the dominating aqueous form of the periodate ion (IO
1.00 cm path length. The dead time of the stopped-flow instrument
4
ꢀ
26–30
38
or H IO ),
but this is seldom important from the view point was determined to be 1.51 ꢁ 0.03 ms using a standard method.
4
6
of redox chemistry. The least squares fitting of the measured data was carried
3
9
out using the general fitting software MicroMath SCIENTIST.
2
.3. GC-MS measurements
2
. Experimental
For the GC-MS measurements, a sample containing the benzoqui-
2.1. Materials
ꢀ
3
ꢀ3
ꢀ3
none derivative at 1 ꢂ 10 mol dm and H
2 2
O at 0.576 mol dm
Solid 1,4-benzoquinone (Q) and its derivatives (QR) were pur-
chased from Alfa Aesar and used as received. The concentra-
tions of the stock solutions in our experiments were typically
concentration was prepared. The reaction was followed spectro-
photometrically. One GC-MS measurement was performed when
the concentration of the hydroxyquinone was at the maximum, and
another after 1 day, when the decomposition of the hydroxyquinone
was complete. A GC-MS measurement of a sample containing only
the benzoquinone was also performed.
ꢀ
3
ꢀ3
1
ꢂ 10
compounds in solution.
prepared for all experiments in brown glass volumetric flasks.
0% (w/w) hydrogen peroxide solution (analytical grade,
mol dm . 1,4-Benzoquinones are photosensitive
7
,8,31–37
Therefore, fresh solutions were
3
The measurements were carried out using a Shimadzu
GCMS-QP2010plus system. Compounds were separated on a
Zebron ZB-5MS capillary column (30 m ꢂ 0.25 mm i.d. ꢂ 0.25 mm.).
The temperature of the column was 50 1C for 1.5 min after injection.
Later, it was programmed to increase at 20 1C min to 100 1C then
at 10 1C min to 280 1C, and finally maintained at that temperature
for 4 min. Helium was used as the carrier gas and the injector
temperature was 300 1C. A split ratio of 10:1 and a column flow rate
of 1.1 cm min were applied. The MS parameters were as follows:
interface temperature 300 1C; ionization mode EI+; electron energy
purissimum) was purchased from VWR. The accurate concentra-
tions of the solutions used for the oxidation experiments were
determined by permanganometric titrations.
Solid potassium periodate was purchased from Reanal, Hungary
and used without further purification to prepare aqueous stock
solutions.
ꢀ1
ꢀ1
Oxidation reactions were proved to be pH sensitive. Therefore,
NaH
2
PO
4
–Na
2
HPO
4
buffer solutions at pH B 6.5 were prepared
3
ꢀ1
using sodium dihydrogen phosphate dihydrate (analytical grade,
normapur) and di-sodium hydrogen phosphate dodecahydrate
70 eV. Compounds were identified by using NIST05 library spectra.
(analytical grade, normapur), both purchased from VWR. For
further experiments at pH B 4.5, CH COOH–CH COONa buffer 2.4. NMR measurements
3
3
solutions were prepared using the two buffer components
purchased from VWR.
1
H-NMR spectra were measured on a BRUKER DRX 400 instrument
operating at 400 MHz and at 25 1C. As a solvent, 99.99% D O (for
the 2-methyl-1,4-benzoquinone only) or H O:D O = 70:15 (after
addition of the oxidant) was used. The pH of the solution was
2
2
2
2
.2. Instruments and software for kinetic measurements
ꢀ
3
pH measurements of the buffered benzoquinone solutions were kept constant using a 0.50 mol dm phosphate buffer (pH 6.3).
carried out using a Metrohm hydrogen ion selective glass electrode Chemical shifts were referenced to the water signal. For the evalua-
binding with an ABU 93 Triburette potentiometric titrator. The tion of the spectra, the MestReNova NMR software was used.
stability of the pH was also followed during some of the kinetic
2
.5. Cyclic voltammetric measurements
A Shimadzu UV-1601 double beam spectrophotometer with The cyclic voltammograms of the benzoquinone(QR)/hydro-
UV-Probe software was used for the kinetic study of the formation quinone(QR–H ) redox systems were obtained by means of a
runs for a total of 8 hour measurement time.
2
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Metrohm VA 746 Trace Analyzer equipped with 747 VA Stand program was used to evaluate the data. The fitted mathematical
controlled by a personal computer. All the measurements were equation took also the dilution of the samples into account. For
carried out in aqueous solution at slightly acidic or physiological a given derivative, 8–14 carefully chosen wavelengths were used
ꢀ
3
pH (0.20 mol dm Na SO was used as the supporting electro- for the calculations.
2
4
lyte). The solutions were degassed by using argon. The systems
Since the ratio of the two products of the photoreaction was not
were analyzed at 25.0 1C with a three electrode assembly. During known, the molar absorption coefficient values for the hydroxy-
the experiments, platinum (Metrohm 6.1204.120) was used as quinone derivatives could not be determined from these data,
ꢀ
the working electrode. The counter electrode was also a platinum only the e(QR–OH)/e(QR–O ) ratio could be determined.
electrode (distributed by ALS Co. Japan), while the reference
ꢀ3
electrode was a Vycor tip Ag/AgCl electrode stored in 3.0 mol dm
NaCl (BASI Instr. RE-5B, MF-2079). The concentration of benzo-
2.7. Theoretical calculations
We performed DFT calculations with the B3LYP exchange–
ꢀ3
ꢀ3
quinone was 1 ꢂ 10 mol dm . The pH of the sample was
45–47
correlation functional.
6-31G(d), 6-31+G(d), 6-311+G(d) and
ꢀ
3
adjusted using 0.05 and 0.005 mol dm H SO or KOH solutions.
2
4
6-311+G(d,p) were the standard split-valence basis sets. In the
3
The volume of the sample was 20.0 cm . Before each scan, the
working electrode was treated with alumina paste (0.03 micron
ordered from Buehler Company) and the surface was cleaned with
the help of sandpaper.
final results shown in this article, the 6-311+G(d,p) basis set is
used exclusively, whereas calculations with all others are
included in the ESI† for comparison. The M06 functional with
the 6-31G(d) basis set was also used to calibrate the method for
the structural parameters. Geometry optimizations were carried
The electrochemical measuring system was calibrated with
3ꢀ
4ꢀ
40
the [Fe(CN)6] /[Fe(CN)6] redox system. The potential range was
out both in vacuo and using the polarizable continuum model
changed between +800 mV and ꢀ800 mV. The voltammograms
48,49
(PCM) to account for solvent effects.
For PCM calculations,
ꢀ
1
were recorded at 100 mV s sweep rates. For the analysis of the
voltammograms, the CACYVO program was used. The half-wave
potential (E1/2) values were calculated based on the following
equation:
we used Gaussian 09’s default Integral Equation Formalism variant
50,51
(IEFPCM) PCM model.
The previous versions of Gaussian had
convergence problems for geometry optimization using PCM, but a
different algorithm was introduced in Gaussian 09, which can be
useful for geometry optimization in the polarizable cavity. Unlike in
previous versions of Gaussian, Gibbs energies can be obtained
directly from these calculations without the need for zero point
energy and entropy corrections from the in vacuo results. All
Gibbs energies were calculated for 298 K. The transition states
Epc þ Epa
E1=2
¼
(1)
2
where Epc and Epa are the cathodic and anodic peak potentials,
0
0
respectively. Considering that E1/2 = E , E values throughout this
0
work were referenced to NHE taking into account that E
versus ENHE(water) is +0.209 V at 25.0 1C.
Ag/AgCl
were optimized by means of the synchronous transit-guided
0
41
52,53
quasi-Newton method.
The lack of imaginary frequencies
0
0
E = E1/2 + EAg/AgCl
(2)
in vibrational spectral calculations were taken to verify that the
calculated stationary points on the potential energy surfaces
(
(
PES) represented true minima as opposed to transition states
TS). The reactants and products are obtained from intrinsic
2
a
.6. Spectrophotometric determination of pK values
For the determination of the pK values of the hydroxyquinone
derivatives, they were prepared by illuminating 50.0 cm of
a
reaction coordinate (IRC) calculations. The relative energies are
Gibbs free energies obtained by frequency analysis.
3
ꢀ
3
ꢀ3
the appropriate 1 ꢂ 10
mol dm
aqueous solutions of
In addition to DFT calculations, in vacuo geometry optimiza-
1,4-benzoquinone and its derivatives with a high intensity Spectro-
54–58
tions were also done using the MP2 method
basis set. These calculations were carried out for the minima of
the PES but not for the transition states.
with the 6-31G(d)
42
line FC-100/F UV-A lamp emitting at 365 nm. The photon flux of
17
ꢀ1
this lamp is 2.7 ꢂ 10
s
in the geometric arrangement used in
43
our experiments. Photoreactions of these samples result in
the formation of colored hydroxyquinones and colorless hydro-
quinones, the mixture of which seems to be stable for several days
All these calculations were carried out using the Gaussian 09
59
software package.
44
even under normal laboratory light conditions. The formation
reaction was followed using a pH-stat titrating system (ABU 91
AUTOBURETTA RADIOMETER COPENHAGER) and was completed
3
. Results and discussion
for all substituted derivatives after a maximum illumination The reaction between benzoquinone derivatives and hydrogen
time of 1 hour. peroxide was measured for different substituted benzoquinones.
Then, spectrophotometric titrations of 2.50 cm samples pre- For all measurements, a high excess of H ensured the constant
pared in this way were performed, that is well-known volumes of concentration of the oxidant. At the same time, the pH of the
3
2 2
O
ꢀ3
ꢀ3
0
.010 mol dm HCl or 0.042 mol dm KOH solutions were solution was kept constant by using a phosphate buffer in high
added to the samples to adjust the pH of the solution and both excess compared to the benzoquinone concentration. So, under
the pH and the absorption spectrum were measured. For every our experimental conditions, the shape of the kinetic curve only
derivative, 15–20 data pairs were measured mainly in the pH depends on the reaction order with respect to benzoquinone
39
range of the deprotonation process. The MicroMath SCIENTIST
and/or hydroxyquinone concentrations. In the absence of any
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buffer, the pH of the solution becomes acidic during the
Attempts were made to react 1,4-benzoquinone with periodate
measurements because of the acidic proton in the product ion. Unfortunately, the oxidation reaction did not stop at the
hydroxyquinone. E.g., for 1,4-benzoquinone, the pH drops from hydroxyquinone, so this oxidant was not investigated further.
6.5 to 3.4 during the reaction.
The formation of hydroxyquinone, a colored product, was
3.1. Formation kinetics of the hydroxyquinone
shown in all measurements, followed by the decomposition of
the hydroxyquinone indicated by the fading of the color formed.
The benzoquinones are almost colorless in the visible wavelength
range, and the same is true for the final solutions after the
decomposition of the hydroxyquinones. So, absorbance–time
curves measured in the visible wavelength range are selective for
the hydroxyquinone (see: Fig. 1). Selected wavelength values for the
different derivatives are as follows: 480 nm for 1,4-benzoquinone,
3
.1.1. Dependence of kW1 on the hydrogen peroxide
concentration. From fitting of the kinetic curves, kC1 apparent
rate constants were calculated at many different hydrogen peroxide
concentrations for all substituted benzoquinone derivatives. These
k
C1 values were used to determine the rate equation of the benzo-
quinone oxidation process by observing the dependence of the kC1
values on the hydrogen peroxide concentration. For all derivatives,
the data showed a linear relationship with an almost 0 intercept. The
process was studied at c(H O ) = 0 as well, and some formation of
490 nm for 2-methy-1,4-benzoquinone, 530 nm for 2,6-dimethy-
2
2
1,4-benzoquinone, 510 nm for 2-chloro-1,4-benzoquinone and
the hydroxyquinone was detected because of the photosensitivity of
the aqueous benzoquinone solutions. This was at least three orders
of magnitude lower than the initial rate of the oxidation at the lowest
hydrogen peroxide concentration studied. To avoid the photo-
reaction as much as possible, a scanning spectrophotometer
was used in single wavelength measurement mode.
5
20 nm for 2,6-dichloro-1,4-benzoquinone.
Kinetic curves could be well fitted for all derivatives and under
all measured conditions using the following kinetic scheme:
H2O2
ꢀ
QR ƒƒ! QR ꢀ OH ðor QR ꢀ O Þ v1 ¼ kC1½QRꢃ
(3)
H2O2
The kC1 values are shown in Fig. 2 as a function of the
concentration of hydrogen peroxide for the 1,4-benzoquinone–
hydrogen peroxide system at pH 6.56–6.59. Molar absorption
coefficients for the same pH value are as follows: e480(Q) = 24 ꢁ
QR ꢀ OH ƒƒ ! Decomposition productðsÞ v2 ¼ kC2½QR ꢀ OHꢃ
(4)
2
3
v = kC3[QR–OH]
(5)
3
ꢀ1
ꢀ1
ꢀ
3
ꢀ1
ꢀ1
1
dm mol cm and e480(Q–O ) = 1703 ꢁ 88 dm mol cm
.
where QR is an R-substituted 1,4-benzoquinone and QR–OH
The same kinds of experiments were performed for 2-methyl-
,4-benzoquinone and 2-chloro-1,4-benzoquinone. In all cases,
ꢀ
(or QR–O ) is the corresponding substituted hydroxyquinone.
1
From least squares fitting with MicroMath SCIENTIST, molar the same linear hydrogen peroxide concentration dependence of
ꢀ
absorption coefficients of QR, QR–OH (or QR–O ) and the the kC1 values was found (see: Tables S1–S3 and Fig. S1–S3 in the
decomposition product(s) and
constants were calculated. The molar absorbance values are
pH-dependent in the pH range around the pK of the hydroxy-
quinones. The dependence of the apparent rate constants on
the concentration of the oxidant and the pH were studied in
detail and used to deduct the full rate equations of the
k
C1
–
k
C3 apparent rate ESI†). 2,6-Dimethyl-1,4-benzoquinone was also reacted with
, but there was no measurable change after 20 min even
and at the highest hydrogen peroxide concentrations, c(H ) =
2 2
H O
a
2 2
O
ꢀ
3
ꢀ3
0
.58 mol dm and c(H
.1.2. Dependence of kW1 on the hydrogen ion concentration.
In the case of 1,4-benzoquinone and 2-methyl-1,4-benzoquinone,
2 2
O ) = 2.3 mol dm .
3
processes. Temperature dependence was also studied to calculate pH dependent kinetic curves were also measured. For the experi-
ꢀ
3
activation parameters.
ments using the methyl derivative, c(H O ) was 0.58 mol dm
2 2
and c(2-methyl-1,4-benzoquinone) was 0.95 mmol dm
ꢀ3
.
Fig. 1 Kinetic curves measured for the reaction between 2-methyl-1,4-
benzoquinone (QMe) and hydrogen peroxide at 25.0 1C. c(QMe) =
Fig. 2
H
2
O
2
concentration dependence of the kC1 apparent rate con-
system at pH 6.56–6.59 and
stants in the 1,4-benzoquinone (Q)–H
2
O
2
ꢀ4
ꢀ3
ꢀ3
9
ꢂ 10 mol dm , pH = 6.6, c(H
2
O
2
) values in mol dm unit are given
25.0 1C. Small dots (ꢄ) are from spectrophotometric and large triangles (D)
in the legends.
are from stopped-flow experiments.
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The apparent rate constant decreased by an order of magnitude
when the pH was decreased by 1 unit (at pH 6.59, kC1 was
ꢀ
3
ꢀ1
(
1
8.03 ꢁ 0.06) ꢂ 10
s
and at pH 5.69, kC1 was (9.5 ꢁ 0.2) ꢂ
ꢀ
4
ꢀ1
0
s ). A similar pH dependence was found for the unsubstituted
1,4-benzoquinone oxidation reaction.
Based on these data, both the 2-methyl and the unsubstituted
derivative show the same inverse first order hydrogen ion concen-
24
tration dependence as the 2,6-dichloro derivative. The pH depen-
dence of the 2-chloro derivative was not checked but assumed
to be the same:
þ
½
QRꢃ½H2O2ꢃ
½H ꢃ
v1 ¼ kC1½QRꢃ ¼ k1
that is k1 ¼ kC1
(6)
þ
½
H ꢃ
½H2O2ꢃ
Fig. 3 Kinetic curves measured at different temperatures for the reaction
Based on this equation, k
derivatives were calculated by multiplying the slopes of the kC1
versus c(H ) values with the hydrogen ion concentration of
the solutions (when the pH is constant), or, by finding the
values for the different substituted between 2-methyl-1,4-benzoquinone and hydrogen peroxide. c(QMe) =
1
4
ꢀ3
ꢀ3
2 2
O ) = 0.58 mol dm , pH = 6.39.
9
.5 ꢂ 10ꢀ mol dm , c(H
2 2
O
+
range for these measurements was 10–40 1C. In the case
of the temperature dependent experiments, c(QMe) = 9.5 ꢂ
slopes of the kC1 ꢂ [H ] versus c(H
2 2
O ) fittings (Fig. S2 and S3 in
the ESI†). Calculated k values for different derivatives are
1
ꢀ4
ꢀ3
ꢀ3
1
0
mol dm , c(H O ) = 0.58 mol dm and pH = 6.39 were
2 2
shown in Table 1 together with some other measured para-
meters for the different derivatives.
used. Kinetic curves measured at different temperatures are
shown in Fig. 3. It is clear that both the formation and the
decomposition reaction of the hydroxyquinone become faster
with increasing temperature.
All curves were fitted as described earlier to determine
temperature dependent kC1 values. Using these, k
The rate equation described by eqn (6) can be explained by
the mechanism that was proposed for the 2,6-dichloro-
2
4
benzoquinone–hydrogen peroxide reaction:
KaðH2O2Þ
þ
H þ HO2ꢀ fast pre-equilibrium;
H2O2
,
b
values were
calculated using eqn (6) and (9) together with the pK (H O )
(7)
a
2 2
þ
ꢀ
½
H ꢃ½HO2 ꢃ
60
values known from the literature.
Using these temperature dependent k
and Eyring plots were drawn to determine the activation para-
meters, namely the E activation energy, DG Gibbs energy
a
(or free energy) of activation, DH enthalpy of activation and
KaðH2O2Þ ¼
½
H2O2ꢃ
b
values, Arrhenius
kb
‡
QR þ HO2ꢀ ! QR ꢀ OH þ OH ðor QR ꢀ O þ H2OÞ
ꢀ
ꢀ
(8)
‡
rate determining step
‡
DS entropy of activation. The data found are as follows for the
ꢀ
1
‡
Based on this mechanism, the rate constant of the rate 2-methyl-1,4-benzoquinone: E = 18.4 ꢁ 0.4 kJ mol , DH =
a
ꢀ
1
‡
ꢀ1 ꢀ1
determining step, k can be calculated as follows:
15.9 ꢁ 0.4 kJ mol and DS = ꢀ129 ꢁ 1 J mol
K . Activation
b
parameters were determined using the non-linearized forms of
the Arrhenius and Eyring equations and using a relative weigh-
k1
k1 ¼ KaðH2O2Þkb that is kb ¼
(9)
KaðH2O2Þ
2
ing (1/R ) in SCIENTIST. These parameters are in good agree-
To support this assumed mechanism further, activation ment with the activation parameters calculated by theoretical
parameters for the rate determining step (8) of the 2-methyl- methods supporting the assumed mechanism.
‡
ꢀ1
1
,4-benzoquinone–hydrogen peroxide reaction were determined
The ratio of the standard deviations for DH (ꢁ420 J mol
)
‡
ꢀ1 ꢀ1
61
both experimentally and by DFT calculations.
and DS (ꢁ1.41 J mol
K ), as expected, gives the average
.1.3. Temperature dependence of hydroxyquinone formation. temperature of the measurements. DG values for the 10–40 1C
To determine the activation parameters for the rate determining temperature range are between 52.4 and 56.3 kJ mol (all
‡
3
ꢀ
1
step of the mechanism described by eqn (7) and (8), temperature values are listed in the ESI† section in Table S4). The reaction
‡
dependent kinetic curves were measured in the 2-methyl-1,4- has a large entropy contribution to the DG values. A linearized
benzoquinone–hydrogen peroxide system. The temperature Eyring plot of the data is shown in Fig. 4.
0
Table 1
k
1
, pK
a
(QR–OH) and E (QR/QR–H
2
) values for different 1,4-benzoquinone derivatives at 25.0 1C
ꢀ1
0
Derivative
k
1
(s
)
pK
a
(QR–OH)
E (QR/QR–H
2
) at pH 5.1 (V)
ꢀ9
ꢀ8
ꢀ7
2
1
2
2
-Methyl-1,4-benzoquinone (QMe)
,4-Benzoquinone (Q)
-Chloro-1,4-benzoquinone
,6-Dichloro-1,4-benzoquinone
(3.46 ꢁ 0.01) ꢂ 10
(1.23 ꢁ 0.02) ꢂ 10
(1.49 ꢁ 0.03) ꢂ 10
4.5 ꢁ 0.4
3.4 ꢁ 0.1
2.8 ꢁ 0.6
1.8 ꢁ 0.6
0.059
0.120
0.188
0.244
ꢀ7 a
2.7 ꢂ 10
a
From ref. 24.
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1
2
3
Fig. 4 Plot of the linearized Eyring equation for the QMe þ
Fig. 5 The structures of Q–OOH , Q–OOH , Q–OOH and EQ. The
k
b
ꢀ
ꢀ
ꢀ4
ꢀ3
mol dm ,
inserted pictures show the bent ring of EQ from a different view.
HO2 ƒ! QMe-O þ H2O reaction. c(QMe) = 9.5 ꢂ 10
ꢀ3
2 2
c(H O ) = 0.58 mol dm , pH = 6.39.
1
2
ꢀ1
of the Q–OOH - Q–OOH realignment is 21.8 kJ mol , as
Also, the position of the hydroxyl group in the hydroxy- shown by TS in Fig. 6.
quinone is a question in the case of the methyl (or any mono-
From both forms (Q–OOH and Q–OOH ), we investigated
substituted derivative) because the three available positions three possibilities of chemical reactions: the O–O peroxo bond
numbered 3, 5 and 6 on the quinone ring) are different. To breaking, the protonation of the outer oxygen atom (O4 in Fig. 5),
find out the structure of the hydroxyquinone formed during the and the deprotonation of the C2 atom. We found that there is no
1
1
2
(
1
1
reaction, H-NMR measurements were performed. Based on the minimum starting from Q–OOH , while the calculations starting
spectra, it seems more likely that the hydroxylation happens in from Q–OOH show different minima, i.e. different pathways,
2
the 5th or 6th position and not in the neighboring position to which are shown in Fig. 6. It is a bit surprising that C2 can be
ꢀ
the methyl group. This finding is in good agreement with the spontaneously deprotonated by an OH ion, which results in a
3
results of the theoretical calculations as will be discussed later. planar aromatic ring (Q–OOH in Fig. 5). However, this pathway
3
.1.4. Mechanism of the formation of hydroxyquinone. We seems to be unrealistic (and so, drawn with grey in Fig. 6) when
have studied the mechanism of the formation of the hydroxy- the size of the calculated system is increased by adding two water
ꢀ
quinones using DFT and MP2 methods. B3LYP and M06 func- molecules to the OH . We found that, in this case, the deprotona-
2
tionals were also tested to find out the structural parameters of tion of QꢀOOH is not a spontaneous process because of the
ꢀ
ꢀ
2
the starting reagent benzoquinone. We concluded that there is stabilization of the OH ion (OH + 2H O cluster) i.e. decrease of
ꢀ
no significant difference between the structural parameters the hard character of the OH . In this case, the energy barrier is
ꢀ
1
calculated by the two functionals, so, in our further calculations 34.4 kJ mol , in contrast to the barrierless reaction in the
3
for investigating the possible mechanisms, only the B3LYP absence of water molecules. From Q–OOH , the deprotonated
ꢀ
functional was used. The B3LYP functional was earlier proved to hydroxyquinone (Q–O ) can be formed through a small energy
‡
ꢀ1
be a useful level for the calculations in studying the reactions barrier (DG = 8.3 kJ mol ), as shown by TS in Fig. 6.
between variously substituted quinones and hydrogen peroxide.
3
62,63
We used the 6-311+G(d,p) basis set in our calculations to obtain
the mechanism of the reaction between benzoquinone (Q) and
H O . Geometry optimizations were carried out using the default
2 2
polarizable continuum model (PCM) of the Gaussian 09 software.
The stationary points were identified by frequency analysis and
with IRC calculations, too. (In vacuo results obtained with smaller
basis sets, such as 6-31G(d) and 6-31+G(d) at the B3LYP level, and
also the results of the MP2 calculations with the 6-31G(d) basis set
are collected in the ESI.†)
The reaction of Q with H
2
O
2
did not give any product, while
ꢀ
the coordination of OOH to the C1 carbon atom of Q resulted
1
in an adduct (Q–OOH ). (Note that this structure could not
be found as an intermediate using smaller basis sets.) From
1
2
the Q–OOH structure, we found another form (Q–OOH ),
ꢀ
where the OOH is coordinated to C2 of Q: there is an
intramolecular hydrogen bond in this case. The structures of
Q–OOH and Q–OOH can be seen in Fig. 5. The energy barrier hydroxyquinone.
Fig. 6 Gibbs energy profile of the possible pathways of formation of
1
2
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2
In the other pathway, the peroxo bond breaking of Q–OOH
results in the formation of an epoxide (EQ, see Fig. 5), where
ꢀ1
the activation barrier is 19.3 kJ mol , as shown by TS in Fig. 6.
2
However, when the size of the calculated system is increased by
2
2
adding two water molecules to QꢀOOH (QꢀOOH + 2H O
2
coordinated to the O4 oxygen atom), the peroxo bond breaking
shows a much lower energy barrier (4.0 kJ mol ). It indicates
ꢀ1
2
that, in reality, the peroxo bond breaking inside the QꢀOOH
molecule is more probable than the deprotonation of C2. This
phenomenon supports the pathway through EQ. EQ is formed
when the remaining oxygen (O3) coordinates to the C3 after the
peroxo bond breaking. The quinone ring of EQ is strongly bent
Fig. 8 The structures and the relative energies of the possible isomers of
the QMe–OOH complex.
and this might be called a boat conformation because of the of the possible isomers were checked (Fig. 8). The intermediates
similarity of cyclohexane conformations. It is worth mentioning for all possible pathways can be found in the ESI† as Fig. S8.
that a protonation of the outer oxygen atom (O4) can indicate a
It can be seen that the cis isomer is not favored at all.
barrierless peroxo bond breaking in Q–OOH yielding EQ again. Therefore, no further calculations were run for that. We found
2
The significance of this pathway depends on the experimental that the mechanism is the same for the two trans-QMe isomers
1
2
conditions (pH of the solution). The main structural para- (tr-QMe and tr-QMe ) as for unsubstituted Q. We were mainly
x
meters of Q, Q–OOH (x = 1–3) and EQ are summarized in interested in the activation barrier of the rate-determining step
Table S6 of the ESI.†
EQ can then be deprotonated on C2 or C3 by an OH ligand calculations for Q and with the experimental data. It can be seen
(epoxide ring opening) to make a comparison with the theoretical
ꢀ
ꢀ
1
with an activation barrier of 20.8 kJ mol , as shown by TS
4
in that the activation barrier of the ring opening is higher in the case
Fig. 6. From this deprotonated EQ derivative (EQd), Q–O is of methyl derivatives than for Q (see Table 2). It is in good
ꢀ
‡
ꢀ1
obtained when the epoxide ring opens (DG = 63.7 kJ mol ), as agreement with the experimental data (see k
shown by TS
in Fig. 6. Using smaller basis sets, the calculated which show that the process is a bit slower for the methyl
activation barriers were found to be somewhat smaller (see: derivative. Experimentally, we found that the DG values for the
values in Table 1),
1
5
‡
ꢀ
1
ꢀ1
Table S9 in the ESI†). The other possibility from EQ would be methyl derivative are between 52.4 kJ mol and 56.3 kJ mol for
the protonation of EQ on the oxygen of the epoxide unit (O3). the 10–40 1C temperature range (Table S4 in the ESI†). These values
However, based on our calculations, this cannot happen: there are in acceptable agreement with the calculated one (Table 2).
was no minimum found.
The unfavored formation of the cis isomer may also explain
In the energy profile of the full mechanism (Fig. 6), it can be why no hydroxylated product was detected for 2,6-dimethyl-1,4-
ꢀ
seen that the formations of EQ and Q–O are extremely favored benzoquinone experimentally: in this case, both available posi-
thermodynamically. The stationary points of the rate determining tions are cis to one of the methyl substituents.
step, i.e. the epoxide ring opening, can be seen in Fig. 7. All
structures (xyz coordinates and pictures) of the full mechanism quinones chosen for the kinetic investigation include unsub-
and IRC pathways can be seen in the ESI.†
stituted 1,4-benzoquinone (Q), its derivatives with electron
3.1.5. The effect of substituents on the reaction rate. The
The EQ intermediate is a stable form, so there is a straight- donating (methyl) substituents and derivatives with electron
forward deprotonation of EQ and (finally) ring opening from withdrawing (chloro) groups. As can be seen from the data in
this point. The very high negative value determined experimen- Table 1, the electron donating or -withdrawing properties of the
tally for the entropy of activation is also in conceptual agree- substituents significantly change the rate constant of the
ment with the formation of an epoxide structure in the benzoquinone–hydrogen peroxide reaction. Moreover, the two
transition state, as this process certainly means a major loss other measured parameters, namely the pK
a
values of the
of translational degrees of freedom.
hydroxyquinone and the redox potentials of the benzoquinone/
We also carried out calculations for the 2-methyl-1,4- hydroquinone systems also correlate with the electron donating
benzoquinone (QMe) derivative. At first, the relative energies or -withdrawing properties of the substituents. Hammett sub-
stituent constants for substituted benzoquinones are not known
from the literature yet. However, the pK values for hydroxyquinones,
a
Table 2 The activation Gibbs free energies of the Q and trans-QMe complexes
‡
ꢀ1
DG (kJ mol
)
Peroxo bond
breaking
Deprotonation of the
epoxide form
Epoxide ring
opening
Q
19.3
18.9
14.8
20.8
25.6
25.5
63.7
74.8
78.4
1
Fig. 7 The reactant (EQd + H
2
O), transition state (TS
5
) and product tr-QMe2
tr-QMe
ꢀ
(
Q–O + H
2
O) of the rate determining step (epoxide ring opening).
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Fig. 11
H
2
O
2
concentration dependence of the kC2 and kC3 apparent rate
Fig. 9 Linear correlation between log10
hydrogen peroxide reactions (primary y axes,
k
1
values for the benzoquinone–
), log10 values of the
constants in the 2-methyl-1,4-benzoquinone (QMe)–H
pH 6.5–6.7 and 25.0 1C.
2 2
O system at
~
K
a
hydroxyquinones (secondary y axes, &) and formal redox potentials of the
benzoquinone–hydroquinone systems (x axes).
for the hydroxyquinone decomposition. These values are shown
in Fig. 11 for the 2-methyl derivative and in Table S5 of the ESI.†
Both kC2 and kC3 values show a linear dependence when fitted
against c(H O ) under certain conditions. The pH dependence
2 2
for the hydroxyquinone decomposition reaction is more
complex and will not be discussed in this paper.
The main importance of the whole curve measurement and
1
fitting for the determination of k values is demonstrated in
Fig. 1 for 2-methyl-1,4-benzoquinone (and in the ESI† Fig. S5
for 1,4-benzoquinone). In Fig. 1, the concentration of 2-methyl-
1,4-benzoquinone is the same in all samples. However, the
maximum absorbance measured strongly depends on the
concentration of the hydrogen peroxide (Amax = 0.36 for
ꢀ
3
c(H
2
O
2
) = 1.081 mol dm
2 2
and Amax = 0.21 for c(H O ) =
Fig. 10 Pourbaix-diagram of different 1,4-benzoquinone derivatives: & =
ꢀ
3
1,4-benzoquinone (Q),
~
= 2-methyl-1,4-benzoquinone (QMe), K = 2-chloro- 0.054 mol dm ). This is even more characteristic when not
).
1
,4-benzoquinone (QCl) and D = 2,6-dichloro-1,4-benzoquinone (QCl
2
2 2
only the c(H O ) but also the pH of the two solutions are
different. In many cases, the increasing part of the curves can
be fitted using a smaller molar absorption coefficient value
the redox potential values for the quinone/hydroquinone redox than the real one and resulting in the calculation of a larger kC1
systems and the logarithms of the k rate constant values for than the real. To avoid this pitfall, a careful analysis of the
1
the benzoquinone–hydrogen peroxide reactions show a linear calculated molar absorption coefficients was used, which
correlation indicating that these values are influenced by a involved an inspection of the effect of hydrogen peroxide
common structural factor, which most probably arises from the concentration and the results were accepted only if there was
electron donating or -withdrawing properties of the substitu- no trend in the values. The other method to avoid this problem
ents. The linear correlation is demonstrated in Fig. 9.
was a simultaneous fitting of all measured curves for a
ꢀ
Redox potential values as a function of pH were also measured given derivative together with only one e(QR–O ) parameter.
(Fig. 10). High cathodic and anodic peak separations were Additionally, full kinetic curves were measured in many cases
obtained at pH 4 for all derivatives. This can be explained by and fitted to obtain even more reliable data for the first step.
dimeric–polymeric quinone compound formation on the surface
Another reason for the pH-dependent hydroxyquinone
of the working electrode that blocks free flow of electrons. There- molar absorption coefficient values can be the deprotonation
fore, the virtual number of electrons is less than 2 (the expected process of the hydroxyquinone derivative, which can be
64,65
value for quinone reduction–oxidation).
a
described by its pK value. In the case of our measurements,
however, this was never the case since all measurements were
carried out at least 1 pH unit above the pKa value of the
3.2. Decomposition kinetics of the hydroxyquinone
3.2.1. Dependence of kW2 and kW3 on the hydrogen peroxide appropriate 2-hydroxy-1,4-benzoquinone, so, the absorbing
and hydrogen ion concentrations. Fitting the kinetic curves species was the deprotonated hydroxyquinone in all measure-
yielded not only apparent rate constants for the hydroxyquinone ments and the determined molar absorption coefficient was the
ꢀ
formation, but also pseudo-first and -second order rate constants e(QR–O ), not the e(QR–OH).
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3
.2.2. Attempts to identify the decomposition products. To The authors wish to thank Sarolta Tim ´a ri, Lajos Nagy, Zolt ´a n
identify the decomposition products, both NMR and GC-MS Nagy, G ´a bor Lente and M o´ nika Galajda for helpful discussions.
measurements were performed. In the GC-MS spectra, we could
identify the starting 2-methyl-1,4-benzoquinone before adding
hydrogen peroxide to the sample. After adding hydrogen peroxide,
dominating MS peaks with 44 and 32 m/z values were found
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final product and decomposition of the hydrogen peroxide excess
to O (32 m/z) under GC-MS conditions. NMR measurements
show that the hydroxyquinone decomposes (the intensity of the
aromatic region decreases) but in the aliphatic region, there are
new signals indicating the decomposition of the hydroxyquinone
to smaller aliphatic fragments.
2
(44 m/z) as the
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DFT calculations are in good agreement with the experi-
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step process where the rate determining step is the epoxide ring
opening. The calculated energies are similar to the activation
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prefers the trans position for the formation of the 2-methyl-
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