Autocatalytic oxidation of β-alanine by peroxomonosulfate in the presence of copper(II) ion

Article abstract of DOI:10.1002/kin.20288

Kinetics and mechanism of oxidation of β-alanine by peroxomonosulfate (PMS) in the presence of Cu(II) ion at pH 4.2 (acetic acid/sodium acetate) has been studied. Autocatalysis was observed only in the presence of copper(II) ion, and this was explained due to the formation of hydroperoxide intermediate. The rate constant for the catalyzed (k₂^obs) and uncatalyzed (K₁^obs) reaction has been calculated. The kinetic data obtained reveal that both the reactions are first order with respect to IPMS]. K₁^obs values initially increase with the increase in [β-alanine] and reach a limiting value, but k₂^obs values decrease with the increase in [β-alanine]. k₁ ^obs values increase linearly with the increase in [Cu(II)], whereas k₂^obs values increase with [Cu(II)]². Furthermore, k₁^obsvalues are independent of [acetate], but K₂^obs values decrease with the increase in acetate. A suitable mechanism has been proposed to explain the experimental observation. The reaction has been studied at different temperatures, and the activation parameters are calculated.

Full text of DOI:10.1002/kin.20288

Autocatalytic Oxidation of
β-Alanine by
Peroxomonosulfate in the
Presence of Copper(II) Ion
R. SAYEE KANNAN,1 D. EASWARAMOORTHY,2 K. VIJAYA,3 M. S. RAMACHANDRAN4
1Department of Chemistry, Thiagarajar College, Madurai 625 009, India
2Department of Chemistry, B. S. A. Crescent Engineering College, Vandalur, Chennai 600 048, India
3Department of Chemistry, PSNA College of Engineering and Technology, Dindigul 624 622, India
4School of Chemistry, Madurai Kamaraj University, Madurai 625 021, India
Received 21 February 2007; revised 5 August 2007; accepted 11 August 2007
DOI 10.1002/kin.20288
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Kinetics and mechanism of oxidation of β-alanine by peroxomonosulfate (PMS)
in the presence of Cu(II) ion at pH 4.2 (acetic acid/sodium acetate) has been studied. Auto-
catalysis was observed only in the presence of copper(II) ion, and this was explained due to
the formation of hydroperoxide intermediate. The rate constant for the catalyzed (k2obs) and
uncatalyzed (k1obs) reaction has been calculated. The kinetic data obtained reveal that both
the reactions are first order with respect to [PMS]. k1obs values initially increase with the in-
crease in [β-alanine] and reach a limiting value, but k2obs values decrease with the increase
in [β-alanine]. k1obs values increase linearly with the increase in [Cu(II)], whereas k2obs values
increase with [Cu(II)]2. Furthermore, k1obs values are independent of [acetate], but k2obs values
decrease with the increase in acetate. A suitable mechanism has been proposed to explain the
experimental observation. The reaction has been studied at different temperatures, and the
activation parameters are calculated. ꢀ 2007 Wiley Periodicals, Inc. Int J Chem Kinet 40: 44–49,
C
2008
proposed involves the interaction between aldehyde
and peroxomonosulfate (PMS) to produce hydroper-
oxide, which in turn reacts with amino acids to give
the product. The kinetics of oxidation of α-amino acids
in buffered medium and also in the presence of metal
ions, such as Cu(II) and Ni(II), has also been studied
recently [6]. Here, the presence of metal ion inhibits
the autocatalysis because the PMS was not free to form
hydroperoxide, which is responsible for the autocatal-
ysis. PMS reacts with metal ions, such as Cu(II) [7]
and Ni(II) [8], to form an intermediate that reacts with
a faster step with amino acids to give the product.
INTRODUCTION
The kinetics of oxidation of α-amino acids by perox-
omonosulfate has been studied extensively in buffered
medium [1–4] (acetic acid/sodium acetate) and also
in highly alkaline medium [5] (NaOH > 0.1 M). In
buffered medium, the observation was the exhibition of
autocatalysis of the product aldehyde. The mechanism
Correspondence
to:
D.
Easwaramoorthy;
e-mail:
easwaramoorthi@yahoo.com.
c
ꢀ
2007 Wiley Periodicals, Inc.

AUTOCATALYTIC OXIDATION OF β-ALANINE BY PEROXOMONOSULFATE
45
Although the kinetics of α-amino acids with PMS
has been studied extensively, but little information
is available in the literature about the oxidation of
β-amino acids, such as β-alanine, with PMS. More-
over, an α-amino acid undergoes oxidative decar-
boxylation and deamination, whereas a β-amino acid
undergoes oxidation by deamination only, because
the removal of direct conjugation between NH+3 and
COO− groups by the intervening CH2 groups is re-
sponsible for the difficulty in decarboxylative deami-
nation. Hence, it is proposed to study the kinetics of
the oxidation of β-alanine in buffered medium and the
results obtained are discussed in this paper.
Figure 1 Plot of log [PMS]t versus time [β-alanine] = 0.05
mol dm−3, [NaOAc] = 0.08 mol dm−3 [Cu(II)] = 2.5 × 10−3
mol dm−3, pH 4.2 0.1.
EXPERIMENTAL
is, [peroxomonosulfate]/[β-alanine] = 2.5 with [Cu2+
]
Potassium peroxomonosulfate was obtained from
Dupont (Lancaster, PA, USA) under the trade name
“Oxone.” The purity of the triple salt 2KHSO4·KHSO4·
K2SO4 was estimated by iodometry and was found to
be 98%. However, the presence of H2O2 in an oxone
sample was tested and it showed negative results, thus
eliminating the chances of the hydrolysis of oxone. A
fresh solution of peroxomonosulfate was prepared be-
fore starting the experiments. β-alanine was purchased
from Loba-Chemie Indo Austranal Co. (Mumbai,
India) and was used as such. All other chemicals used
in this study were of the Analar grade and were used
as such without any further treatment.
2.5 × 10−3 mol and pH 4.2 allowed to stand for 48 h.
The experiment was performed with different initial
concentrations of the oxidant and β-alanine by main-
taining the ratio at 2.5. Then, the unreacted oxidant
in each case was estimated by iodometry. After mak-
ing corrections for self-decomposition of the oxidant,
which was carried out under the same kinetic con-
ditions, stoichiometry was calculated as β-alanine:
PMS = 1:1. Thus, the stoichiometric ratio evident from
the data corresponds to the reaction and can be written
as
Doubly distilled water was employed throughout
the study; the second distillation was from alkaline
permanganate solution in an all glass still.
H2N CH2 CH2 COOH + PMS
Cu2+
−→ OHC CH2 COOH + Cu2+
pH4.0
The reaction mixtures containing all reactants
except peroxomonosulfate in glass-stoppered Erlen-
meyer flasks were immersed in the water bath ther-
mostated at 35 0.1◦C. Peroxomonosulfate was taken
in another flask to attain the same temperature of 35
0.1◦C. The required volume of the oxidant solution
was pipetted out into the reaction mixture. Experi-
ments were carried out in acetic acid/sodium acetate
buffered medium at various pHs ranging from 3.6 to
5.2. The rate of oxidation of β-alanine was followed
by monitoring the concentration of unreacted oxone by
iodometry. Autocatalysis was observed from the plot
of log10 (volume of thio) at various time intervals (Vt)
versus time (Fig. 1). The values of catalyzed and un-
catalyzed rate constants were calculated. The relative
standard errors of the above-mentioned rate constants
for a single run and the relative standard errors of the
mean were about 2%.
+ HSO−4 + NH+4
(1)
The product analysis was done under kinetic con-
ditions as described below. The reaction mixture con-
taining β-alanine at pH 4.2 and PMS in the presence
of Cu(II) was allowed to stand for 48 h for the comple-
tion of the reaction. After completion of the reaction,
the excess oxidant was destroyed by adding NaHSO3
and then extracted with dichloromethane. The organic
layer obtained was analyzed to identify the product.
FT-IR and H1 NMR data confirm the product as mal-
onic semialdehyde, and the retention time at 9.423 min
and 99.1% purity was confirmed by gas chromatog-
raphy/mass spectrometry. The chromatographic con-
ditions used for the oxidation kinetics to identify the
product were EC5, 30 m × 0.2 mm, 0.25-µm column
with nitrogen as a carrier gas with a flow of 1.0 mL/min.
Injection and detection temperatures for the study were
280◦C and 290◦C, respectively. The oven temperature
was initially started from 50◦C and held for 2 min, and
The stoichiometry of the reaction was determined
under the kinetic conditions by taking a known ex-
cess concentration of the oxidant over β-alanine, that
International Journal of Chemical Kinetics DOI 10.1002/kin

46
SAYEE KANNAN, EASWARAMOORTHY, AND RAMACHANDRAN
the temperature was raised to 280◦C with increments
of 15◦C/min and held at 280◦C for 30 min.
RESULTS AND DISCUSSION
The rate of oxidation of β-alanine by PMS is very
slow. However, the added Cu(II) ions even at a con-
centration of 2.5 mM enhance the rate of reaction ap-
preciably; hence, the reaction has been studied only in
the presence of Cu(II) ions at 35◦C. pH of the reac-
tion was maintained with acetic acid/sodium acetate at
3.6–5.2. The kinetics was investigated under pseudo-
first-order conditions, that is, [β-alanine] ≥ [PMS].
The rate of the reaction was measured by monitoring
the concentration of unreacted PMS at various time
points iodometrically.
a)
The rate of oxidation of β-alanine follows first-
order kinetics with respect to [PMS] only at lower
concentrations of β-alanine < 0.05 M, as evidenced by
log [PMS] versus time plots with r ≥ 0.99. However,
as the concentration of β-alanine is increased, the first-
order plot shows curvature toward the time axis, which
may be due to the product that catalyzes the reaction.
The oxidation of β-alanine can be represented by the
autocatalysis reaction as
b)
Figure 2 (a) Plot of 1/k1obs versus [β-alanine]−1 at 294
K, [NaOAc] = 0.08 mol dm−3, [Cu(II)] = 2.5 × 10−3 mol
dm−3, pH 4.2 0.1, r2 = 0.9948. (b) Plot of k2obs versus [β-
alanine]−1 at 294 K, [NaOAc] = 0.08 mol dm−3, [Cu(II)] =
2.5 × 10−3 mol dm−3, pH 4.2 0.1, r2 = 0.9953.
β-Alanine + Cu(II) + PMS → I1 + Product (2)
I1 + PMS → I2 + Product
(3)
I2 + β-alanine · · · Cu(II) → 2I1 + Product (4)
The effect of Cu(II) on the rate constant values is
studied by keeping other parameters constant. k1obs and
k2obs values are found to increase linearly with the in-
crease in the Cu(II) ion concentration, and the plots
of k1obs versus [Cu(II)] and k2obs versus [Cu(II)]2 give
straight lines (Fig. 3).
Under the experimental condition, where [AA] >
[PMS], the rate equation can be written with the as-
sumption that [I1] = [PMS]0 − [PMS]t as
−d[PMS]/dt = k1obs[PMS]t + k2obs[PMS]t
+ k2obs[PMS]t ([PMS]0 − [PMS]t ) (5)
The effect of pH on the rate is studied by keeping
other parameters constant. The rate of disappearance
of [PMS] at pH 3.6 becomes simple first-order kinetics
as shown by a good correlation between log [PMS] and
time (r = 0.999). At all other pH values, the interme-
diate catalyzed oxidation also contributes to the rate
as evidenced by the nonlinear regression with higher
correlation coefficient (r = 0.99). Perusal of the results
in β-alanine shows that the k1obs value first decreases
and then increases with the increase in the pH values.
The effect of the acetate ion is studied by keeping
other parameters constant at pH 4.2 (Fig. 4). Perusal
of the results shows that k1obs values are independent
of [acetate], but k2obs value decreases with the increase
in acetate. The reaction has been studied at different
temperatures, and the thermodynamic parameters are
calculated and given in Table I
The best values of k1obs and k2obs can be obtained from
the nonlinear region. The kinetic constants are calcu-
lated at different PMS concentrations. Perusal of the
results shows that the kinetic constants remain constant
even after a threefold increase in [PMS]. This shows
that the reaction is first order with respect to [PMS].
The effect of β-alanine on the rate constant values is
studied at constant pH and by keeping other parameters
at a constant value. k1obs value initially increases with
the increase in [β-alanine] and then reaches a limiting
value. Moreover, the plot at 1/k1obs versus 1/(β-alanine)
is a good straight line (r = 0.99) with a positive in-
tercept (Fig. 2a). But k2obs values decrease with the
increase in [β-alanine] and the plot of k2obs versus [β-
alanine]−1 is a straight line (Fig. 2b).
International Journal of Chemical Kinetics DOI 10.1002/kin

AUTOCATALYTIC OXIDATION OF β-ALANINE BY PEROXOMONOSULFATE
47
Peroxomonosulfate ion HSO−5 is a weak acid with
pKa value of 9.4 [9,10] at moderate acidic pH, and
PMS exists predominantly as HSO5−
kd
HSO−5 −→ SO25− + H+
(6)
β-Amino acids similar to α-amino acids exist in the
following equilibrium:
a)
(7)
The pKa values [11] of β-alanine suggest that at the
experimental conditions, that is, at pH 4.2, a zwitter
ionic form is the active form of β-alanine. In aqueous
solution, the zwitter ions lose a proton from the +NH3
group and chelate with a metal ion of the transition
metal ions; Cu(II) is one of the most kinetically labile
ions [12]. The equilibrium of the copper–β-alanine
complex in aqueous medium can be represented as
b)
Figure 3 (a) Plot of k1obs versus [Cu(II)] at 294 K,
[β-alanine] = 0.05 mol dm−3, [NaOAc] = 0.08 mol dm−3
pH 4.2 0.1, r2 = 0.9941. (b) Plot of k2obs versus [Cu(II)]2 at
294 K, [β-alanine] = 0.05 mol dm−3, [NaOAc] = 0.08 mol
dm−3, pH 4.2 0.1, r2 = 0.9995.
K1
ꢀ
ꢁ
+
Cu2+H2N CH2 CH2 COO−
Cu(A)
(8)
K2
ꢀ
ꢁ
Cu(A)2+ + H2N CH2 CH2 COO−
Cu(A)2
(9)
Cu(A)+ represents copper–mono-β-alanine complex,
and Cu(A)2 is the bis complex. A review of literature
on Cu–β-alanine complexes shows that the formation
of the bis complex is insubstantial and hence may be
neglected; furthermore, the percentage of the mono
complex increases with the increase in pH [13].
On the basis of the experimental results, the follow-
ing reaction scheme for the oxidation of β-alanine is
proposed:
Figure 4 Plot of 1/k2obs versus [acetate] at 294 K,
[β-alanine] = 0.05 mol dm−3, [Cu(II)] = 2.5 × 10−3 mol
dm−3, pH 4.2 0.1, r2 = 0.9929.
Table I Temperature Variation and Thermodynamic
Parameters for the Cu2+-Catalyzed Oxidation of
β-Alanine by Peroxomonosulfate
k1
Cu(II) + PMS −→ Intermediate
(10)
β-Alanine
fast
Parameters
105 × k1obs
102 × k2obs
Intermediate −→ Cu(II) + SO24− + O2 ↑ (11)
Temperature (K)
fast
Intermediate + β-alanine −→ Product (12)
294
301
308
2.16
5.12
15.51
45.8
44.0
12.43
39.96
110.20
46.4
49.0
−289.27
Ka
ꢀ
ꢁ
Cu(II) + β-alanine
Cu(A)
(13)
(14)
(15)
E (kJ mol−1
)
a
k2
Cu(A) + Intermediate −→ Product
ꢀH= (kJ mol−1
ꢀS= (J mol−1 deg−1
)
)
−362.056
k3
PMS −→ Intermediate 1
International Journal of Chemical Kinetics DOI 10.1002/kin

48
SAYEE KANNAN, EASWARAMOORTHY, AND RAMACHANDRAN
Table II Kinetic Constants for the Cu2+ Catalyzed Oxidation of β-Alanine by Peroxomonosulfate
Amino Acid
β-Alanine
105 × k mol−1 dm3 s1
105 × k mol−1 dm3 s1
105 × k mol−1 dm3 s1
1
2
3
320.0
3650.0
1.6
fast
salts [17,18]. Available information on the catalyzed
self-decomposition of PMS suggests that the reaction
proceeds through H2O2 as an intermediate in strong
acidic medium [19] and HO−2 in alkaline medium [10];
moreover, in the recent literature formation of nickel
peroxide in acetic acid/sodium acetate buffer [8] is re-
ported. Therefore, we can propose the mechanism as
Intermediate 1 + β-alanine −→ Product (16)
k4
β-alanine + PMS −→ Product
The rate equation for the above scheme at constant pH
can be written as
−d/dt[PMS]/[PMS] = k1obs = k1[Cu(II)]
rds
+ k2[Cu]2[β-alanine] + k3 + k4[β-alanine] (17)
Cu(II) + HSO−5 −→ CuO2 + SO42− + H+ (18)
H2O
fast
The values of k1, k2, and k3 are calculated from the
different plots and are tabulated in Table II.
CuO2 + β-alanine −→ product
(19)
The experiment carried out in the absence at β-
alanine shows that PMS is quite stable in the presence
of Cu(II) at pH 4.2, and only in the presence of β-
alanine the oxidation is observed. This can be explained
by the fact that the reaction between Cu(II) and PMS
may be the rate-limiting step in which the reactive
intermediate is formed, which in turn reacts with β-
alanine rapidly. This means the rate-limiting step is the
formation of reactive copper or PMS intermediate.
Available information shows that in acidic medium
the mode of reaction of PMS with a metal ion is through
the production of radicals, such as HSO−5 • [14–16]
SO−4 • [17], or OH• [18].
The experimental results suggest that the oxidation
by PMS proceeds through deamination of β-alanine
to the corresponding oxo acids. The other intermedi-
ate from PMS, such as CuO2, also follows the same
mechanism of the intermediate-catalyzed oxidation of
β-alanine in the presence of Cu(II) ion.
The oxidative decarboxylation of α-amino acids by
PMS at pH 4.2 shows that aldehyde catalyzes the re-
action, and the catalytic effect is attributed to the for-
mation of hydroperoxide, which in turn reacts with
α-amino acids to give the product. The observation
of k2obs showing an inverse dependence on [β-alanine]
can be explained by the fact that the formation of hy-
droperoxide from the oxo intermediate may be the rate-
determining step.
M
n+ + HSO5− → M(n−1)+ + H+ + SO−5 •
M
n+ + HSO5− → M(n−1)+ + OH− + SO−4 •
Cu(I) + HSO−5 → Cu(II) + OH• + SO24−
OHC CH2 COOH + Cu2+
K
ꢀ
ꢁ
Extension of such metal ion initiated radical mech-
anism to this system can be ruled out, because the
Cu(II)-catalyzed oxidation is observed only in the pres-
ence of β-alanine. Furthermore, the presence of radical
mechanism was ruled out completely on the basis of
the experimental observations that the rate of oxidation
is not affected by (i) the introduction of O2 into the re-
action mixture, (ii) the modification of the surface area
of the reaction vessel, and (iii) nonpolymerization of
acrylonitrile added to the reaction mixture.
The foregoing discussion suggests that some reac-
tive intermediates from Cu(II) and PMS are responsi-
ble for the reaction.The hydroperoxo Cu-(II) ion [16],
CuOOH+, is formed by the action of H2O2 on Cu(II)
ions at neutral pH. Cu-(II) peroxide can be prepared
by the action of H2O2 on alkaline solution of Cu(II)
OHC CH2 COO− · · · Cu+ + H+ (20)
OHC CH2 COO− · · · Cu+ + HSO−5
k2
−→ Hyderoxide(I)
(21)
Hydroperoxide(I) + H3+N CH2 CH2 COOH
OHC CH2 COO− · · · Cu+
fast
−→
+
(22)
(23)
OHC CH2 COO−
OHC CH2 COO− + H+
Therefore, the kinetic constant, k2obs = Kk2 [Cu(II)]f,
by substituting the concentration of [Cu(II)f] and by
International Journal of Chemical Kinetics DOI 10.1002/kin

AUTOCATALYTIC OXIDATION OF β-ALANINE BY PEROXOMONOSULFATE
49
approximation
2. Ramachandran, M. S.; Vivekanandam, T. S.; Malimmani
Raj, R. P. J Chem Soc, Perkin Trans 2 1984, 1345.
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5. Ramachandran, M. S.; Easwaramoorthy, D.; Suresh
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9. Ball, D. L.; Edwards, J. O. J Am Chem Soc 1956, 78,
1125.
10. Goodman J. F.; Robson, P. J Chem Soc 1963, 2871.
11. Lide, D. R.; Frederikse, H. P. R. (Eds.). CRC Hand
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2238.
13. Nair, M. S.; Santappa, M. Indian J Chem A 1981, 20,
990.
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1990, 1281.
15. Gilbert, B. C.; Stell, J. K. J Chem Soc, Faraday Trans
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In Rodd’s Chemistry of Carbon Compounds, 2nd ed.;
Coffey, S. (Ed.); Elsevier: New York, 1965; Vol. 1,
Part-D, Ch. 16.
k2K[Cu(II)]2T[H+]{K1 + [H+]}
k2obs
=
(24)
[H+]{K1 + [H+]}KaK1K2[β-alanine]T
The above equation explains all the experimental ob-
servations. The inhibitory effect of the acetate ion on
k2obs can be explained by the following reaction:
OHC (CH2) CH2 COO · · · Cu+ + OAc−
ꢁ
OHC CH2 COO + Cu(OAc)+
(25)
ꢀ
The effect of acetate on k2obs also supports the fact
that the copper complex of the resultant oxo acid is
involved in this reaction. The structure of aldehyde
obtained from β-alanine involves a carboxyl group and
a hydrogen ion. Furthermore, keto-enol tautomerism is
well known in oxo carboxylic acid [20] as
ꢀ
ꢁ
OHC CH2 COOH
HO CH CH COOH
(26)
The presence of Cu2+ may inhibit the equilibrium. This
may be the reason for observation of autocatalysis only
in the presence of Cu2+ in the oxidation of β-alanine.
The outcome from the oxidation of β-alanine is that
the aldehyde group of oxo acid also reacts with PMS
resulting in a more reactive intermediate.
BIBLIOGRAPHY
1. Ramachandran, M. S.; Vivekanandam, T. S. J Chem Soc,
Perkin Trans 2 1984, 1341.
International Journal of Chemical Kinetics DOI 10.1002/kin