Anomalous control mechanism in a B-Z type oscillator involving fructose as organic substance

Article abstract of DOI:10.1016/S0009-2614(01)01487-7

In order to elucidate the mechanism of Ce⁴⁺ + BrO^-₃ + H₂SO₄ + fructose [F] oscillatory reaction which has no apparent bromine removal mechanism, detailed experimental studies have been undertaken to investigate (i) [Br^-] at different time intervals during oscillations, (ii) effect of addition of (a) Br^- during oscillations, (b) fructose and BrO^-₃ after stoppage of oscillations and (c) tartaric acid [TA], below the lower critical limit of [F] for oscillations. A reaction mechanism has been suggested which involves active participation of free radicals and which also correctly predicts most of the observed oscillatory features.

Full text of DOI:10.1016/S0009-2614(01)01487-7

13 February 2002
Chemical Physics Letters 353 (2002) 40–48
www.elsevier.com/locate/cplett
Anomalous control mechanism in a B–Z type
oscillator involving fructose as organic substrate
a,
a
a
b
c
R.P. Rastogi *, M.M. Husain , Prem Chand , G.P. Misra , Mukul Das
a
Chemistry Department, Lucknow University, Lucknow 226 007, India
b
Chemistry Department, Panjab University, Chandigarh, India
Industrial Toxicology Research Centre, Lucknow 226 007, India
c
Abstract
In order to elucidate the mechanism of Ce4þ þ BrOÀ þ H2SO4 þ fructose [F] oscillatory reaction which has no
3
apparent bromine removal mechanism, detailed experimental studies have been undertaken to investigate (i) ½BrÀŠ at
different time intervals during oscillations, (ii) effect of addition of (a) BrÀ during oscillations, (b) fructose and BrOÀ3
after stoppage of oscillations and(c) tartaric acid[TA], below the lower critical limit of [F] for oscillations. A reaction
mechanism has been suggested which involves active participation of free radicals and which also correctly predicts
most of the observedoscillatory features. Ó 2002 Publishedby Elsevier Science B.V.
1. Introduction
system [15] where free-radical control mechanism
has been invoked.
Oscillations in a number of different variants of
Belousov–Zhabotinskii (B–Z) reaction are found
to be bromide-ion controlled [1–6] involving dif-
ferent types of bromine-removal mechanisms [7–
9]. However, in some cases anomalous situations
may arise such as in case of Racz’s oscillator
It has also been suggestedthat both types of
mechanisms may be simultaneously operative in
specific systems [16]. Even switching from one type
to other may occur depending on certain condi-
tions.
Recently oscillations in F þ Ce4þ þ BrO3Àþ
H2SO4 have been reported[17,18] which belong to
the above class of anomalous oscillators. Consid-
erable amount of bromine is evolvedduring the
course of oscillatory reaction. In the case of batch
reactor, oscillations occur within definite range of
[F]. In this Letter, we report additional evidences,
which show that occurrence of oscillations cannot
be explainedby Br À-controlledmechanism andit
wouldbe necessary to propose some alternate/
modified mechanism. Results indicate significant
role of free radicals in the mechanism, which is
also supportedby stuides on oscillator where
(malonic acid þ BrOÀ3 þ Ce4þ þ H2SO4),
where
[MA] ꢀ 0.6 M. In order to account for oscillations
in such a case, an alternative feedback loop in-
volving autocatalysis of BrOÅ2 (positive feedback)
Å
andscavenging of BrO
by other organic free
radicals [10–14] produced from malonic acid has
2
been proposed. A recent example is the fRu
2þ
ðbpyÞ g þ BrOÀ3 þ bromomalonic acidþ Ce4þþ
3
* Corresponding author.
E-mail address: rprastogi@yahoo.com (R.P. Rastogi).
0009-2614/02/$ - see front matter Ó 2002 Publishedby Elsevier Science B.V.
PII: S0009-2614(01)01487-7

R.P. Rastogi et al. / Chemical Physics Letters 353 (2002) 40–48
41
À
Fig. 1. Oscillations in (i) BrÀ potential and(ii) reodx potential for the system: BrO
ð0:06 MÞ þ Ce4þ ð1:45 Â 10À3 MÞþ
3
H2SO4 ð1:5 MÞ þ TA ð0:05 MÞþ: (a) F (0.005 M), (b) F (0.01 M), (c) F (0.015 M), (d) F (0.02 M), (e) F (0.025 M), (f) F (0.03 M).
Temp.: 30 Æ 1 °C.

42
R.P. Rastogi et al. / Chemical Physics Letters 353 (2002) 40–48
fructose [F] + tartaric acid[TA] is usedas double
substrate. For testing the mechanism computer
studies have also been undertaken and the results
are reportedin this Letter.
360 L (60 MHz) was employed. Mass spectra were
studied using Jeol-JMS-D 300. One of the prod-
ucts, which couldbe unambiguously confirmed,
was tartaric acid.
It may be notedthat such Saccharide oscilla-
tors, where free radicals play an important role
have potential interest in the context of physio-
logical studies on diabetes [19].
For detection of free radicals in (a) F ð0:075
MÞ þ Ce4þ ð1:45 Â 10À3 MÞ þ H2SO4 (1.5 M);
(b) TA ð0:006 MÞ þ Ce4þ ð1:45 Â 10À3Þ þ H2SO4
(1.5 M), a saturatedsolution of acrylamide in 1.5
M H2SO4 was added to the reaction mixture. Gel
was formedin each case confirming the presence of
free radicals. Time taken for the formation of
acrylamide gel was found to be less than about 1 h
in both the cases.
2. Experimental
2.1. Monitoringof oscillations
Chemicals andexperimental techniques were
similar to that described in earlier communications
[18,20]. Oscillations in B–Z systems containing (i)
F and(ii) F + TA as organic substrate were stud-
ied. In the latter case, experiments were performed,
below the lower critical limit of [F]. ½BrÀŠ during
different stages of oscillations for the two types of
oscillators was also recorded. For this purpose,
voltage was measuredwith a idgital voltmeter
(DVM). The corresponding ½BrÀŠ was readfrom
the voltage–BrÀ curve obtainedearlier. The results
are recorded in Figs. 1 and 2.
2.3. Stoppage and revival of oscillations
In order to test wÀhether oscillations are quen-
chedby higher Br
concentration, calculated
amount of solutions of KBr in 1.5 M H2SO4 was
added to the reaction mixture during oscillations.
Typical results are recorded in Fig. 4a–d. The ½BrÀŠ
refers to the effective concentration of BrÀ in the
total reaction mixture of 60 ml.
After cessation, oscillations were revivedFig. 4e
when 3.3 ml of 1.39 M of fructose (ꢀ0.075 M) in
F-oscillator was added to the reaction mixture.
The regeneratedoscillations were recoeddas
usual.
2.2. Kinetics and mechanism of reactions involved
Kinetics of Br2 evolution was studied spectro-
photometrically. The absorbance of the reaction
mixtures was measuredat 530 nm at different in-
tervals of time using Perkin Elmer Spectropho-
tometer Lambda Bio 20 using cuvette having
cross-sectional area equal to 1 Â 1 cm2. The solu-
tion in the cell was continuously stirredwith the
help of a magnetic stirrer suppliedwith the in-
strument. A microprocessor was attachedto
spectrophotometer. The results are recorded in
Fig. 3.
2.4. Computer simulation studies
Numerical solution of the resulting differential
equations on the basis of proposedreaction
mechanism was obtainedusing subroutine
STIFF3. The results are recorded in Figs. 4e and f
and5.
3. Discussion
The kinetics of reduction of F (0.075 M) by
Ce4þ ð1:45 Â 10À3 MÞ in H2SO4 was also moni-
toredby noting the absorbance spectrophotomet-
Results on BrÀ potential oscillations, recorded
in Figs. 1 and2 show that for F + TA oscillator
oscillations occur even though the concentration
of F is below the lower critical limit. Results re-
corded in Fig. 2 show that concentration of BrÀ
during different stages estimated with help of
rically at 316.9 nm for Ce4þ
.
Efforts were made to isolate and identify the
reaction products by TLC, IR, NMR and mass
spectroscopy. For IR spectra, Perkin Elmer Model
577 was usedwhile for NMR; Varian Model EM-
DVM ranges between 10À5 and10
types of oscillators.
M for both
À7

R.P. Rastogi et al. / Chemical Physics Letters 353 (2002) 40–48
43
Fig. 2. ½BrÀŠ at various stages during oscillations for the systems BrOÀ3 ð0:06 MÞ þ Ce4þ ð1:45  10À3 MÞ þ H2SO4 ð1:5 MÞþ: (a) F
(0.075 M), (b) F (0.065 M), (c) F ð0:025 MÞ þ TA ð0:05 MÞ, (d ) Fð0:01 MÞ þ TA ð0:05 MÞ. Temp.: 30 Æ 1 °C.
It is clear from Fig. 3b that ½Br2Š in the reaction
mixture rapidly increases during the course of the
reaction. The absorbance A attains a maximum
value after ꢀ5 min, the maximum values of bro-
mine was estimatedto be ꢀ 10À1 M. Beer’s law
holds good within this range. A question arises as
to how bromine is removedfrom the system after
reaching the maximum value. It may be due to the
formation of glycollide (a diketone) produced
from glycolic acid[21] which is a product of os-
cillatory reaction andwhich can easily remove
bromine from the system.
Effect of addition of BrÀ=fructose during oscil-
lations was also investigatedfor the two systems.
The results are recorded in Fig. 4. Oscillations are
À
stoppedby high Br concentration. The nature of
bifurcation viz; supercritical Hopf bifurcation re-
mains the same in general, when oscillations stop.

44
R.P. Rastogi et al. / Chemical Physics Letters 353 (2002) 40–48
Fig. 3. Absorption A due to Br2 evolution at 530 nm in the system: (a) F ð0:075 MÞ þ BrOÀ3 ð0:06 MÞ þ H2SO4 (1.5 M); (b) F
ð0:075 MÞ þ BrOÀ3 ð0:06 MÞ þ Ce4þ ð1:45 Â 10À3 MÞ þ H2SO4 (1.5 M). Temp.: 30 Æ 1 °C.
One observes only a sudden overshoot on addi-
tion. However, the oscillations are revivedby
fructose. This indicates that fructose is rapidly
consumedby series of reactions so that [F] goes
below the desired level for oscillations to occur.
The polymerization studies show that acryla-
mide can be polymerized by both fructose þ Ce4þ
andtartaric acid þ Ce4þ systems. It may be noted
that aqueous polymerization of acylamide in sim-
ilar redox systems such as glycollic acid þ Ce4þ are
well known [22]. The ESR spectra for TA þ Ce4þ
suggest that the free radical produced could be [18]
On the basis of experimental data, at the first
instance it seems reasonable to speculate that the
oscillations are free-radical controlled. The exis-
tence of induction period is, however, not neces-
sarily an evidence against free-radical controlled
mechanism [14]. The induction time includes (i)
time taken for reduction of Ce4þ–Ce3þ in adequate
amount and(ii) production of sufficient amount of
BrÀ (in case of BrÀ control mechanism) or the
organic free radicals (capable of destroying BrO2Å
in case of free-radical control).
Basedon the above results, one can postulate a
tentative mechanism of the oscillatory reaction as
follows:
ÅCðOHÞCOOH
j
HCðOHÞCOOH
BrÀ þ HOBr þ Hþ ꢀ Br2 þ H2O
BrÀ þ HBrO2 þ Hþ ꢀ 2HOBr
ð1Þ
ð2Þ
since a doublet is obtained. ESR spectra of fruc-
tose after gamma-radiolysis suggest the formation
of –CH(OH)–CÅ(OH)–CH2–O– free radical
[19,23].
BrÀ þ BrOÀ þ 2Hþ ꢀ HOBr þ HBrO2
ð3Þ
3

R.P. Rastogi et al. / Chemical Physics Letters 353 (2002) 40–48
45
Fig. 4. Effect of addition of KBr/fructose on BrÀ potential for the systems: (I) BrOÀ3 ð0:06 MÞ þ Ce4þ ð1:45 Â 10À3 MÞþ
H2SO4 ð1:5 MÞ þ F ð0:075 MÞ: (a) effect of addition of ½BrÀŠ ꢀ 2:0  10À2 M; (b) effect of addition of ½BrÀŠ ꢀ 2:0  10À3 M. (II)
BrOÀ3 ð0:06 MÞ þ Ce4þ ð1:45  10À3Þ þ H2SO4 þ F ð0:025 MÞ þ TA ð0:05 MÞ: (c) effect of addition of ½BrÀŠ ꢀ 2:0  10À2 M; (d) ef-
fect of addition of ½BrÀŠ ꢀ 2:0  10À3 M. (III) Effect of addition: (e) of fructose (0.075 M) for the system (IÀ). (IV) Computer simulation
results: (f) effect of addition of fructose (0.075 M) after stoppage of oscillations; (g) effect of addition of ½Br Š ðꢀ 1:0  10À3 MÞ. Arrow
indicates the instant of addition of bromide ion/fructose in the reaction mixture.
2HBrO2 ꢀ HOBr þ BrOÀ þ Hþ
ð4Þ
ð5Þ
ð6Þ
Ce3þ þ BrO2Å þ Hþ ꢀ Ce4þ þ HBrO2
ð7Þ
ð8Þ
ð9Þ
3
HBrO2 þ BrOÀ þ Hþ ꢀ Br2O4 þ H2O
À
þ
1
2
3F þ BrO3 þ H ! Br2 þ 3P
3
Br2O4 ꢀ 2BrOÅ
2
P þ Ce4þ ꢁPÅ þ Ce3þ þ Hþ

46
R.P. Rastogi et al. / Chemical Physics Letters 353 (2002) 40–48
HOBr þ P ! Products þ Hþ þ BrÀ
2PÅ ! Inert products
PÅ þ BrOÅ2 ! Products þ Hþ þ BrÀ
ð10Þ
ð11Þ
ð12Þ
F þ Ce4þ ! PÅ þ Ce3þ þ Hþ
where steps (8)–(13) are in addition to inorganic
ð13Þ
component of FKN mechanism.
For BrOÅ2 removal, the free radical is essentially
obtainedfrom P (5-keto glutaric acid) when it re-
acts with Ce4þ. However, the other oxidation
products of fructose, could be saccharic acid, tar-
taric acidandglycollic acid[21], can also con-
tribute to generation of free radicals.
Using the above model and using the rate
constants andthe initial values given in Tables 1
and 2, numerical modeling was attempted. The
Table 1
Rate constants of different steps
Step Forwardreaction
Back reaction
1
2
3
4
5
6
7
8
9
8:0E09 MÀ2 sÀ1
2:5E06 MÀ2 sÀ1
1:6 MÀ3 sÀ1
80:0 sÀ1
2:0E À 05 MÀ1 sÀ1
3:2 MÀ1 sÀ1
3:0E03 MÀ1 sÀ1
33:0 MÀ2 sÀ1
1:0E À 08 MÀ1 sÀ1
2:2103 sÀ1
7:5E04 sÀ1
1:4E09 MÀ1 sÀ1
8:4E03 MÀ1 sÀ1
6:2E04 MÀ1 sÀ1
0:03 MÀ4 sÀ1 (assigned)
3:0 MÀ1 sÀ1 (assigned)
2:2E04 MÀ2 sÀ1
10 3:0 MÀ1 sÀ1 (assigned)
11 5:0E09 MÀ1 sÀ1 (assigned)
12 1:0E09 MÀ1 sÀ1 (assigned)
13 5:0E À 03 MÀ1 sÀ1 (assigned)
Table 2
Initial values of the reactants
Reactant
Value
½BrÀŠ
1:0E À 20 M
1:0E À 20 M
7:0E À 07 M
1:0E À 20 M
0.06 M
[HOBr]
½HBrO2Š
½Br2Š
½BrOÀ3 Š
½Br2O4Š
1:0E À 20 M
1:45E À 03 M
1:0E À 20 M
1:0E À 20 M
0.075 M
Fig. 5. Computedresults for bromide-ion oscillations for the
systems: BrO3À ð0:06 MÞ þ Ce4þ ð1:45 Â 10À3 MÞ þ H2SO4 (1.5
M) and(a) F (0.05 M), (b) F (0.06 M), (c) F (0.075 M), (d) F
(0.1 M), (e) F (0.15 M), (f) F (0.2 M), (g) F (0.3 M).
½Ce4þ
[P]
Š
½PŊ
[F]

R.P. Rastogi et al. / Chemical Physics Letters 353 (2002) 40–48
47
mechanism yields non-linear equations involving
11 variables. It may be notedthat most of the rate
constants refer to a temperature of 20 °C whereas
the experiments have been performedat 30 °C. But
it is not so relevant, since the order of magnitude
of the rate constants wouldbe unaffected.
The computational studies lead to following
conclusions:
again. This is actually foundto be the case when
TA is added (Fig. 1) which provides evidence in
support of free-radical mechanism. In the case of
F + TA, the control on production of BrOÅ2 is ex-
ercisedby the reaction,
BrOÅ2 þ HOOC–CÅðOHÞ–CHðOHÞ–COOHðTAÅÞ
! 2CO2 þ 2HCOOH þ Hþ þ BrÀ
ð14Þ
(i) ½Ce4þŠ during oscillations is of correct order of
where TAÅ is the free radical produced by
TA þ Ce4þ reaction. It may be notedthat oscilla-
tions are not observedwhen TA alone is usedas
organic substrate [24] in the B–Z system.
magnitude.
(ii) The induction period is found to be ꢀ11 min
whereas experimental value is ꢀ7 min when
[F] ¼ 0.075 M.
The present mechanism shouldbe consideredas
skeleton mechanism. The oscillatory reaction in-
volving fructose is a highly complex reaction that
may involve number of electron transfer reactions,
free-radical reactions involving free radicals or
organic/inorganic species andreactions between
organic andinorganic species as presentedin GTF
model [16]. It is likely that in the present case both
the free radical and BrÀ control may be simulta-
neously operative. This is also indicated by the fact
that under certain circumstances, analogous B–Z
systems containing double organic substrates such
as F + TA, F + oxalic acid[25] also exhibit oscil-
lations below the lower critical limit of [F], al-
though individually TA and oxalic acid as organic
substrates do not exhibit oscillations.
(iii) The computedlife time of oscillation is ꢀ9
min as comparedto experimental value of
ꢀ9 min for the same [F].
(iv) Computational results predict critical limits of
[F] (0.05–0.3 M) between which oscillations
occur whereas the experimental value is
0.04–0.6 M.
(v) Both computedandexperimental results
show that oscillations are not stoppedeven
at high ½BrÀŠ.
(vi) Computedresults as well as the experiments
show that oscillations are revived by the addi-
tion of fructose after the stoppage of oscilla-
tions.
In spite of the above support in favour of the
proposedreaction mechanism one rdawback is
that the predicted (i) ½BrÀŠ during oscillations and
(ii) the amplitude of oscillations in BrÀ are much
smaller as comparedto the experimental values.
This indicates that the model still requires refining.
As an alternative, simulation was attemptedusing
the above mechanism but without step (2). How-
ever, no oscillations were predicted which indicates
equally significant role of BrÀ.
Acknowledgements
Thanks are due to Indian National Science
Academy andDepartment of Science andTech-
nology for supporting the investigation. One of the
authors (PC) thanks the Council of Scientific and
Industrial Research for the award of a Senior
Research Fellowship.
Steps (5)–(7) can leadto autocatalysis by
HBrO2 or BrOÅ2. This woulddependon (i) the
situation when consumption of BrOÅ2 is not so fast,
autocatalysis of HBrO2 is possible, and(ii) the
situation when production of BrOÅ2 is counterbal-
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