Briggs-Rauscher reaction with 1,4-cyclohexanedione substrate

Article abstract of DOI:10.1524/zpch.2006.220.8.1071

A new organic substrate has been used to promote oscillations under batch conditions in the Briggs-Rauscher oscillating system. The new substrate, 1,4-cyclohexanedione (CHD), reacts with aqueous iodine via an enol mechanism. We discuss the effect of the initial concentrations, the temperature and chemical perturbations. In a definite range of concentrations long-lived oscillations with two significantly different frequency periods were observed. The low frequency parts are temperature-dependent while the high frequency oscillations do not show temperature dependence. The inhibitory effects of 1,4-hydroquinone and 1,4-benzoquinone on the oscillations and the kinetics of some important component reactions were studied to develop a model for the simulation of the observed oscillations. by Oldenbourg Wissenschaftsverlag.

Full text of DOI:10.1524/zpch.2006.220.8.1071

Z. Phys. Chem. 220 (2006) 1071–1082 / DOI 10.1524/zpch.2006.220.8.1071
by Oldenbourg Wissenschaftsverlag, München
Briggs–Rauscher Reaction with
1,4-Cyclohexanedione Substrate
∗
By Kla´ra Keresztu´ri and Istva´n Szalai
Institute of Chemistry, L. Eötvös University, P.O. Box 32, H-1518 Budapest 112,
Hungary
(Received February 22, 2006; accepted May 8, 2006)
Nonlinear Dynamics / Oscillatory Reactions / Briggs–Rauscher Reaction /
1,4-Cyclohexanedione / Temperature Compensation
A new organic substrate has been used to promote oscillations under batch conditions
in the Briggs–Rauscher oscillating system. The new substrate, 1,4-cyclohexanedione
(CHD), reacts with aqueous iodine via an enol mechanism. We discuss the effect
of the initial concentrations, the temperature and chemical perturbations. In a definite
range of concentrations long-lived oscillations with two significantly different frequency
periods were observed. The low frequency parts are temperature-dependent while the
high frequency oscillations do not show temperature dependence. The inhibitory effects
of 1,4-hydroquinone and 1,4-benzoquinone on the oscillations and the kinetics of some
important component reactions were studied to develop a model for the simulation of the
observed oscillations.
1. Introduction
The Briggs–Rauscher reaction [1], referred here as BR reaction, is one of the
most fascinating oscillatory reaction. This reaction is widely used as a class-
room demonstration of the chemical oscillators due to the observable spectac-
ular periodical colour changes. The “classic” BR system consists of hydrogen-
peroxide, iodate, Mn(II) catalyst, malonic acid as organic substrate and sulfuric
acid. In the variants of the BR reaction the malonic acid is replaced with mal-
onic acid derivatives [2], acetone [3], crotonic acid, acrylic acid, anisole and
p-nitrophenol [4].
Recently BR reaction has received growing attention due to the inhibitory
effects of antioxidants on the oscillatory regime [5]. These effects make the BR
reaction capable to measure the antioxidant capacity of radical scavengers. An-
other interesting application of the BR reaction was published by Grzybowski
*
Corresponding author. E-mail: pisti@chem.elte.hu

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K. Keresztu´ri and I. Szalai
and coworkers. They coupled the BR reaction with a new patterning technique
(wet stamping). Large variety of novel wave phenomena, like twist-symmetry
breaking and kinetic autofocusing were observed by this method [6].
Here we present a novel variant of the BR reaction with 1,4-cyclohexane-
dione (CHD). During the last years CHD became one of the most popu-
lar organic substrate of the Belousov–Zhabotinsky (BZ) reaction, due to its
bubble-free and long-lived oscillatory behaviour [7]. The reactions of CHD
with bromate and bromine were studied in detail [8]. According to the analogy
between the BR and BZ reactions, we investigated CHD as a substrate of the
BR reaction.
2. Experimental section
H2SO4 (Chemolab 96%), 1,4-cyclohexanedione (Aldrich 98%), KIO3 (Aldrich
99 5%), MnSO4·H2O (Reanal a.r.), Ce NH4 SO4 4 ·2H2O (Reanal a.r.),
4
H2O2 (Reanal purris), I2 (Reanal a.lt.), 1,4-hydroquinone (Fluka purris), 1,4-
benzoquinone (Merck 98%) and bidistilled water were used to prepare the
working solutions.
Spectra for the kinetic measurements were taken on Milton Roy 3000
and Agilent 8452 diode array spectrophotometers equipped with quartz cells
(path length 1 cm, volume 2 mL). Redox potentials were monitored with
a smooth Pt electrode (Radelkis) and a Hg Hg2SO4 K2SO4(sat.) reference
electrode (Radiometer). Analog signals were digitized with a 12 bit AD con-
verter (Labtech PCL-711 S) and processed on a PC. The temperature was kept
at 25 0 0 1 ◦C, and the solutions in the cell were mixed with a magnetic
stirrer.
The simulations were done with the program XPPAUT [9].
3. Results and discussion
3.1 Iodonation of 1,4-cyclohexanedione
The reaction between the organic substrate and iodine plays an essential role
in the mechanism of the BR reaction. Noyes and Furrow pointed out, that the
efficiency of the organic substrate for scavenging iodine must be in a spe-
cific range to give rise oscillations [10]. The iodination of CHD was followed
at different CHD (0 03–0 09 mol dm3), I2 (2 5×10−4 –5×10−4 ) and H2SO4
(0 05–1 0 mol dm3) concentrations. The consumption of iodine was moni-
tored at 465 nm. The reaction is expected to proceed trough an enol mechanism
according to the scheme
CHDketo ꢀ CHDenol
(1)
(2)
I2 +CHDenol −→ ICHD+I− +H+

Briggs–Rauscher Reaction with 1,4-Cyclohexanedione Substrate
1073
Fig. 1. (a) Absorbance changes during the iodonation of 1,4-cyclohexanedione at 465 nm.
[CHD]0 = 9 0×10−3 mol dm3, [I2]0 = 2 5×10−4 mol dm3, (ꢀ)[H2SO4]0 = 0 05 mol dm3
and (ꢁ) [H2SO4]0 = 0 15 mol dm3, (b) absorption spectra of a reacting mixture at the
starting (solid line) of the reaction and after 24 hours (dashed line). Initial composition:
[CHD]0 = 9 0×10−3 mol dm3, [I2]0 = 9 0×10−4 mol dm3, [H2SO4]0 = 0 2 mol dm3.
The dotted line shows the spectrum of an 2 5×10−4 mol dm3 1,4-hydroquinone solution.
In these experiments CHD was in high excess (6–36 ×) over the iodine. The
obtained absorbance vs. time curves show pseudo zero order kinetics. This
observation supports that the rate determining step is the acid catalized enoliza-
tion of CHD (Fig. 1a). From the measured data we evaluated a second order
rate constant according to the rate expression
d[I2] dt = k1[CHD][H+]
(3)
In the range of 0 05–1 0 mol dm3 sulfuric acid concentrations, the value of
the rate constant observed is 5 1 0 2 ×10−4 mol−1 dm3 s−1. This value is
close to the other one obtained in the bromine–CHD reaction (2 13 ×10−4
mol−1 dm3 s−1) [8].

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K. Keresztu´ri and I. Szalai
Fig. 2. Formation of H2Q in the CHD–halogen reactions. Abbreveations are: CHD ≡ 1,4-
cyclohexanedione, CHDE≡ enol form of 1,4-cyclohexanedione, CHDX ≡ halogeno-1,4-
cyclohexanedione, H2Q ≡ 1,4-hydroquinone.
The ICHD is not stable, it decomposes similarly to the BrCHD (Fig. 2). The
appearance of 1,4-hydroquinone (H2Q), that is the endproduct of the decom-
position of ICHD, is visible on the spectrum, which was taken 24 hours later
(Fig. 1b).
ICHD −→ H2Q+I− +H+
The rate of this reaction depends on the acidity:
= k6[ICHD][H+]
(4)
(5)
The measured value of the rate constant is 4 6 0 2 ×10−5 mol−1 dm3 s−1.
There is no significant difference between the rate constants measured here
for the decomposition of ICHD and previously for BrCHD (5 0 × 10−5
mol−1 dm3 s−1).
3.2 Oscillatory behavior
Under batch conditions oscillations appear in wide range of initial reagent-
concentrations. The typical compositions of our experiments were: io-
date (0 05–0 1 mol dm3) – hydrogen-peroxide (0 4–0 6 mol dm3) – Mn(II)
(2 5×10−4 –6 0×10−2 mol dm3) – CHD (0 05–0 4 mol dm3) – H2SO4 (0 03–
0 075 mol dm3). The temporal evolution of the reactions can be followed by
measuring the redox potential, the potential of an iodide selective electrode
and the absorbance changes. In Fig. 3 we show a characteristic redox potential
vs. time curve. After a relatively short induction time long lasting, low fre-
quency oscillations (section A in Fig. 3) appear in the system. At the end of
the oscillatory period the frequency suddenly switches to a significantly higher
value (section B in Fig. 3). These high frequency oscillations appear in a certain
range of initial Mn(II) concentrations (1×10−2 –5×10−2 mol dm3).
The period time of the low frequency oscillations is proportional to
1 [CHD] over a wide range of the concentrations (Fig. 4). This relationship

Briggs–Rauscher Reaction with 1,4-Cyclohexanedione Substrate
1075
Fig. 3. Oscillations in the redox potential during the iodate–hydrogen-peroxide–
Mn(II)–CHD reaction. After a long lasting, low frequency oscillatiory period (A)
suddenly high frequency oscillations (B) appear. Initial concentrations: [KIO3] =
0 083 mol dm3, [MnSO4] = 0 00815 mol dm3, [H2SO4] = 0 053 mol dm3, [CHD] =
0 09 mol dm3, [H2O2] = 0 5 mol dm3.
Fig. 4. Effect of changing the initial concentration of CHD on the period time of the os-
cillations. Initial concentrations: [KIO3] = 0 083 mol dm3, [MnSO4] = 0 00815 mol dm3,
[H2SO4] = 0 053 mol dm3, [H2O2] = 0 5 mol dm3.
supports that the oscillation period is mainly effected by the iodine consump-
tion. This is in agreement with earlier observations with other substrates like
aceton [3], malonic acid and derivatives of malonic acid [2].
3.3 Effect of the temperature
To characterize the temperature dependence of the oscillatory reactions Ko˝rös
suggested to use the Arrhenius equation for calculating the “apparent acti-
vation energy”, by plotting the logarithm of the frequency as a function of

1076
K. Keresztu´ri and I. Szalai
Fig. 5. The frequency of the low frequency oscillations (ꢀ) and the high fre-
quency oscillations (ꢁ) as a function of 1 T. Initial concentrations: [KIO3] =
0 083 mol dm3, [MnSO4] = 0 00815 mol dm3, [H2SO4] = 0 053 mol dm3, [CHD] =
0 09 mol dm3, [H2O2] = 0 5 mol dm3.
1 T [11]. Here we obtained 89 5 kJ mol for the long lasting, low frequency
oscillations (Fig. 5). This value is higher than the value of the activation en-
ergy measured in the “classical” Briggs–Rauscher reaction with malonic acid
(68 2 kJ mol) [5] or with benzylmalonic acid substrate (57 8 kJ mol) [12].
Cervellati and coworkers obtained such high activation energy after adding
radical scavengers (antioxidants), diphenol derivatives into the “classical” BR
reaction. They assumed that in this phase of the reaction the substrate is
one of the degradation product of the antioxidants, instead of the malonic
acid.
However, the activation energy of the high frequency oscillations (sec-
tion B on Fig. 3) seems to be zero within the experimental error (Fig. 5).
Temperature-independent period length or temperature compensation is an es-
sential feature of some biological rhythms. Ra´bai and coworkers studied this
phenomenon in different oscillatory reactions [13]. Recently, they observed
temperature compensation in the iodate–hydrogen–peroxide reaction, a sub-
system of the BR reaction [14]. They suggested to use the Q10 value to charac-
terize the temperature dependence of the oscillatory reactions, which describes
the relative change in the period length when the temperature is increased by
10 ◦C. It is defined as follows:
10 T −T
2
1
Q10 = P1 P2
(6)
where P1 and P2 are the period length in seconds at T1 and T2 absolute tempera-
tures, respectively. In the low frequency part we evaluated Q10 = 3 4, while
in the high frequency part Q10 appears close to 1.0 (Q10 = 0 94). The latter
value indicates temperature compensation. In Fig. 6 we show the redox poten-

Briggs–Rauscher Reaction with 1,4-Cyclohexanedione Substrate
1077
Fig. 6. High frequency oscillations at (a) 20 ◦C, (b) 25 ◦C and (c) 30 ◦C. Initial concentra-
tions: [KIO3] = 0 083 mol dm3, [MnSO4] = 0 00815 mol dm3, [H2SO4] = 0 053 mol dm3,
[CHD] = 0 09 mol dm3, [H2O2] = 0 5 mol dm3.
tial vs. time curves at three different temperatures. It is clearly visible that there
is no significant change in the period-time. Note that above 35 ◦C these high
frequency oscillations disappear.
3.4 Effect of 1,4-hydroquinone and 1,4-benzoquinone
As we discussed previously the decomposition of ICHD produces slowly H2Q,
which is known to be an inhibitor of the oscillations in the BR reaction [5].
Cervelatti and coworkers found out that adding antioxidants (e.g. diphenols)
to the oscillating BR mixture (with malonic acid substrate) stops the oscil-
lations immediately. However, after a definite inhibition time, depending on
the antioxidant concentration, oscillations restart. They assumed that the in-
hibitory effects are due to the HOO· scavenging action of the diphenols and the
quinones.
The effect of H2Q on the iodate–hydrogen–peroxide–Mn(II)–CHD reac-
tion is shown on Fig. 7. In this system oscillations are not terminated below
a critical concentration of H2Q, that is 5×10−4 mol dm3. The main effect is the
total oscillatory period-shortening. Above this limit oscillations are stopped,
and we did not observed any subsequent regeneration of oscillations like in the
“classical” BR reaction. In a typical BR mixture, the lifetime of H2Q is short,
due to its fast reactions with iodate and iodine [5]. However, the time scale of
the effect of adding H2Q is quite long. Seemingly H2Q affects the oscillatory
behaviour after hours of the addition (Fig. 7).
Cervelatti and coworkers assumed that 1,4-benzoquinone (Q), which is the
oxidation product of H2Q, has scavenging effect according to reaction (7):
Q+2HOO· −→ H2Q+2O2
(7)

1078
K. Keresztu´ri and I. Szalai
Fig. 7. Effect of 1,4-hydroquinone on the oscillatory behaviour. The arrows indicate the
injection-time of 1,4-hydroquinone into the oscillatory system. Initial concentrations:
[KIO3] = 0 083 mol dm3, [MnSO4] = 0 00815 mol dm3, [H2SO4] = 0 053 mol dm3,
[CHD] = 0 09 mol dm3, [H2O2] = 0 5 mol dm3, (a) [H2Q] = 1 0×10−4 mol dm3,
(b) [H2Q] = 1 5×10−4 mol dm3 and (c) [H2Q] = 2 5×10−4 mol dm3.
It means that both H2Q and Q act as an inhibitor in the system. The effect of Q
on the oscillations is presented in Fig. 8. Similar to H2Q, Q shortens the length
of the oscillatory period and stops the oscillations above a certain concentration
limit. Contrary to H2Q, Q increases the oscillations frequency significantly. We
think that, besides the reaction of Q with HOO· radicals, the reaction of Q with
iodide ions must be taken into account. We have tested spectrophotometrically
that Q can oxidize iodide ions to iodine at [H2SO4] = 0 053 mol dm3.
Q+2I− +2H+ ꢀ H2Q+I2
(8)
This reaction may have important role in the mechanism, since iodine and
iodide are key intermediates of the BR reaction. In the unperturbed system, be-
fore termination of the oscillations, the frequency suddenly increases (Fig. 3).
We assume that slow production of H2Q and Q from CHD leads to this phe-
nomenon.
3.5 Simulations
For the simulations we used an extended version of the FCA model of the
BR reaction [4, 5]. The mechanism and the rate laws are presented−in Ta-
bles 1 and 2. This model contains 13 variables (HOI, I−, I2, HOIO, IO3 , IO2·,
H2O2, HOO·, CHD, CHDE, ICHD, H2Q, Q). The concentrations of Mn(II)
and H+ are parameters. The role of mangenese ion in the mechanism of the
BR reaction is not understood completely [4]. We used the rate constant re-
ported by Furrow et al. [4, 5]. The reaction of CHD and iodine is studied

Briggs–Rauscher Reaction with 1,4-Cyclohexanedione Substrate
1079
Fig. 8. Effect of 1,4-benzoquinone on the oscillatory behaviour. The arrows indicate the
injection-time of 1,4-benzoquinone into the oscillatory system. Initial concentrations:
[KIO3] = 0 083 mol dm3, [MnSO4] = 0 00815 mol dm3, [H2SO4] = 0 053 mol dm3,
[CHD] = 0 09 mol dm3, [H2O2] = 0 5 mol dm3, (a) [Q] = 1 0×10−4 mol dm3, (b) [Q] =
1 77×10−4 mol dm3 and (c) [Q] = 2 17×10−4 mol dm3.
Table 1. Extended FCA model.
step
R1
R1r
R2
R3
R4
R5
HOI + I− + H+ −→ I2 + H2O
I2 + H2O −→ HOI + I− + H+
I− + HOIO + H+ −→ 2HOI
I− + IO3− + 2H+ −→ HOI + HOIO
2IO2· + H2O −→ HOIO + IO − + H+
HOIO + 2H2O2 + Mn(II) −→ 3
2HOO· + HOI + Mn(II) + H2O
H+ + IO3− + HOO· −→ IO2· + H2O + O2
IO2· + H2O2 −→ HOIO + HOO·
HOI + H2O2 −→ I− + O2 + H+ + H2O
2HOO· −→ H2O2 + O2
R6
R7
R8
R9
R10
R11
R12
R13
R14
CHD ꢀ CHDE
CHDE + I2 −→ ICHD
ICHD −→ H2Q + I− + H+
2IO3− + 5H2Q + 2H+ −→ I2 + 5Q + 6H2O
H2Q + I2 −→ Q + 2I− + 2H+
here. The backward rate constant of the reaction R10 is calculated by using
the enolization equilibrium constant reported by Keeffe and coworkers [15].
The reaction of the enol form with halogens are reported as diffusion con-
trolled reactions [16], but the FCA model does not show oscillations with

1080
K. Keresztu´ri and I. Szalai
Table 2. Rate laws used for simulation.
step
rate law
ref.
R1
R1r
R2
R3
R4
R5
R6
R7
R8
R9
R10
3 67×109 [HOI][I−]
1 98×10−3 [I2] [H+]
5×109[I−][HOIO]
1430[I− ][IO3−][H+]2
5×109[IO2]2
[4]
[17]
[4]
[18]
[4]
[4]
[4]
[4]
[2]
3 1×104 [HOIO][H2O2][Mn(II)]
1 3×104 [H+][IO3−][HOO·]
20[IO2][H2O2]
5[HOI][H2O2]
7 5×105 [HOO·]2
[19]
this work
5 1×10−4 [CHD]−
(5 1×10−4 4 07×10−7 )[CHDE]
7×105[CHDE][I2]
4 6×10−5 [ICHD][H+]
120[IO3−][H2Q][H+]
10[H2Q][I2]
R11
R12
R13
R14
this work
this work
[5]
[5]
Fig. 9. Comparison of the simulated and the experimentally observed oscillations. Initial
concentrations in the experiment: [KIO3] = 0 083 mol dm3, [MnSO4] = 0 00815 mol dm3,
[H2SO4] = 0 053 mol dm3, [CHD] = 0 09 mol dm3, [H2O2] = 0 5 mol dm3. In the simula-
tion the initial concentrations of KIO3, CHD, H2O2, Mn(II) are the same like in the experi-
ment and for [H+] we used 0 058 mol dm3.
k11 = 2 8×109 mol−1 dm3 s−1. We adjusted k11 to get the appropriate period
time. The obtained model is capable to simulate the long lasting low frequency
oscillations (Fig. 9).

Briggs–Rauscher Reaction with 1,4-Cyclohexanedione Substrate
1081
The simulation of the high frequency oscillations appear at the end of the
oscillatory period requires more kinetic informations. We assume that Q plays
an essential role in this part of the reaction. It can react with the key intermedi-
ates, HOO· and iodide ion, of the BR reaction. We discussed here that Q forms
as the oxidation product of CHD by iodine. However, the oxidation of CHD by
Mn(III) produces Q also [7]:
2Mn(III) +CHD −→ 2Mn(II) +H2Q+2H+
(9)
2Mn(III) +H2Q −→ 2Mn(II) +Q+2H+
(10)
Since the high frequency oscillations appear in a definite range of the initial
Mn(II) concentration, these reactions must be taken into account in the sim-
ulations. The extension of the model with the reactions related to the Q is in
progress.
4. Conclusion
Here we present a novel variant of the BR reaction. The BR reaction with
CHD shows long lasting oscillations under batch condition in wide range of the
initial reagent-concentrations. Remarkably a high frequency, temperature com-
pensated oscillations appear at the end of oscillatory period. The iodine–CHD
reaction was found to be similar to the bromine–CHD reaction, the primary
products (BrCHD and ICHD) in both cases are unstable in acidic solutions. The
halogeno derivatives of CHD decompose to H2Q. The experiments presented
here support that the reactions of the oxidation products of CHD, H2Q and Q
have a significant effect on the observed behavior.
Acknowledgement
We thank for the support of the Hungarian Academy of Sciences (HAS)
(F034976 and T0437473).
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