Studies on the oxygen atom transfer reactions of peroxomonosulfate: Oxidation of lactic acid

Article abstract of DOI:10.1002/kin.20420

Kinetics of oxidation of lactic acid by peroxomonosulfate (PMS) catalyzed by Ni(II) ions has been studied in aqueous buffered (sodium acetate-acetic acid) medium. The reaction follows first order in [PMS] and [Ni(II)] and inverse first order in [H⁺]. The effect of pH on the rate suggests that both HSO- and SO₅ are the active forms of the oxidant. The intermolecular reaction between HSO₅ and nickel lactate results in hydroperoxide intermediate in the rate- limiting step. The deprotonated form of PMS, SO ₅2 , gives a lactate-nickel-peroxomonosulfate intermediate, which then undergoes intramolecular oxidation-reduction reaction. The ther- modynamic parameters also support the kinetic scheme. Comparison with (nickel) glycolate shows that the electron-donating methyl group in lactic acid enhances the nucleophilic interaction of the a-hydroxyl group. A suitable mechanistic scheme is also proposed.

Full text of DOI:10.1002/kin.20420

Studies on the Oxygen Atom
Transfer Reactions of
Peroxomonosulfate:
Oxidation of Lactic Acid
P. ANDAL, M. MURUGAVELU, S. SHAILAJA, M. S. RAMACHANDRAN
School of Chemistry, Madurai Kamaraj University, Madurai 625 021, India
Received 1 November 2008; revised 22 December 2008; accepted 30 December 2008
DOI 10.1002/kin.20420
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Kinetics of oxidation of lactic acid by peroxomonosulfate (PMS) catalyzed by Ni(II)
ions has been studied in aqueous buffered (sodium acetate-acetic acid) medium. The reaction
+
follows first order in [PMS] and [Ni(II)] and inverse first order in [H ]. The effect of pH on the
−
2−
rate suggests that both HSO5 and SO are the active forms of the oxidant. The intermolecular
5
−
reaction between HSO5 and nickel lactate results in hydroperoxide intermediate in the rate-
limiting step. The deprotonated form of PMS, SO , gives a lactate-nickel-peroxomonosulfate
2
−
5
intermediate, which then undergoes intramolecular oxidation–reduction reaction. The ther-
modynamic parameters also support the kinetic scheme. Comparison with (nickel) glycolate
shows that the electron-donating methyl group in lactic acid enhances the nucleophilic inter-
action of the α-hydroxyl group. A suitable mechanistic scheme is also proposed. ꢀC 2009 Wiley
Periodicals, Inc. Int J Chem Kinet 41: 449–454, 2009
INTRODUCTION
valuable component in athletic fluid replacement bev-
erages. Thus the importance has led to the extensive
investigations on the kinetics and mechanism of oxi-
dation of lactic acid. The oxidation of glycolic acid,
another alpha hydroxy acid, by peroxomonosulfate
(PMS) in the presence of metal ions such as Ni(II) and
Cu(II) ions at pH 4.05–5.89 has been reported from this
Aliphatic alpha hydroxy acids are compounds of in-
terest due to their occurrence and functions in human
physiology. Lactic acid, 2-hydroxy propionic acid, in
its crude form (sour milk) has been used since ancient
days as a skin rejuvenating agent. Lactic acid is pro-
duced in the human body by anaerobic metabolism of
glucose and plays a critical role in generating energy
through the process known as the Cori cycle or lactic
acid cycle [1]. Lactate is the preferred fuel source in
highly oxidative tissues such as heart muscle and slow-
twitch skeletal muscle fibers [2]. Lactic acid (LA) is a
laboratory [3]. The deprotonated form of PMS (SO2
−
)
5
reacts with nickel glycolate to give nickel glycolate
peroxomonosulfate intermediate, which rearranges to
a hydroperoxide by the oxygen atom transfer to the
hydroxyl group of the chelated glycolic acid. At the
experimental conditions, a major fraction (>99.99%)
−
of peroxomonosulfate exists as HSO5 and we could
not observe the reaction between the protonated form
of the peroxide with the metal glycolate. This is proba-
blybecausethe nucleophilic interactionof the hydroxyl
Correspondence to: M. S. Ramachandran; e-mail:
ramachandran mku@yahoo.co.in.
c 2009 Wiley Periodicals, Inc.
ꢀ

4
50
ANDAL ET AL.
group (the reaction center of the metal glycolate) with
the peroxide oxygen is made less favorable by the –I
inductive effect of the hydrogen atoms at the alpha car-
bon. If it is so, one can expect that the substitution of
the hydrogen atom by an electron-donating group may
increase the probability of the reaction between nickel-
evolution of carbon dioxide was confirmed with freshly
prepared limewater. No gas evolution was observed
when gaseous products were passed through sodium
hydroxide solution, and this clearly showed that the
gaseous product is only carbon dioxide. The forma-
tion of oxygen was also excluded by the color test
with alkaline sodium dithionite activated with indigo
carmine [6].
−
α-hydroxy carboxylate and HSO . Thus the oxidation
5
of lactic acid by PMS was carried out with an objec-
tive outlined previously, and the results are discussed
in this report.
RESULTS AND DISCUSSION
EXPERIMENTAL
The oxidation of lactic acid by PMS occurs only in the
presence of Ni(II) ions. The concentrations of Ni(II)
ions used in this study are in the range from ∼5.0 ×
Potassium salt of peroxomonosulfate, under the trade
name Oxone® (Fluka Chemie, Buchs, Switzerland),
was used as such, and no attempt was made to purify
it further because all the previous attempts have been
reported unsuccessful [4]. The purity of the sample
was checked by cerimetry using ferroin indicator, and
the absence of free hydrogen peroxide was ensured by
a test with permanganate. Fresh solution of PMS was
prepared daily and estimated by iodometry. DL-Lactic
acid (85% aqueous solution) was from Alfa Aesar
−4
−4
10
to ∼10.0 × 10 M. The kinetics was always
carried out under the conditions [LA] ꢁ [PMS] and
[
LA] ꢁ [Ni(II)]. The rates of the reactions were mea-
sured by monitoring the concentrations of PMS as ex-
plained in the Experimental section. The rate is first or-
der with respect to [PMS] as shown by the linear plots
of log[PMS]t vs. time. The plots are linear with high
2
correlation coefficient (r > 0.98) even at 80% con-
version of [PMS]. The first-order rate constant values
(Heysham, Lancashire, UK). The stock solution of
(
kobs) were calculated by the linear regression of time
lactic acid was prepared daily before starting the ex-
periments and was standardized by alkalimetric titra-
tions. Nickel(II) nitrate hexahydrate (S.D. Fine-Chem,
Mumbai, India) is the source of Ni(II) ions. All other
chemicals used were of the highest purity available.
The pH of the reaction mixture was maintained with
a high concentration of buffer, usually with 0.32 M of
vs. log[PMS]. The effect of sulfate ion, the reduction
product of PMS, on the rate is studied by calculating
the kobs values at different sulfate ion concentrations
ranging from 0.025 M to 0.10 M. The results show that
the added sulfate ion has no effect on the rate. However,
all the kinetics was carried out only in the presence of
the 0.05 M sulfate ion.
−
OAc . The kinetics was carried out in buffered medium
The effect of the metal ion concentration [Ni(II)]
on the rate is studied at different pH and temperatures.
An increase in [Ni(II)] causes a proportional increase
in kobs. The plots kobs vs. [Ni(II)] (Fig. 1) are straight
lines passing through origin or with a small negative
intercept, which can be approximated (statistically) as
through origin. The effect of the lactic acid concen-
trations on kobs is studied by calculating kobs values
at different [LA], keeping [Ni(II)] and pH at constant
values. Perusal of the results shows that the rate con-
stant kobs is independent of [LA] over the concentration
range from 0.023 to 0.14 M. Similar result is observed
at all pH values and temperatures. Thus kobs obeys
Eq. (2).
with a large excess of lactic acid over PMS in the
temperature range 298–311 K. The reaction was fol-
lowed by estimating the unreacted PMS as a function
of time by the iodometric method. The liberated iodine
was titrated against a standard sodium thiosulfate so-
lution using starch as an indicator. The stoichiometry
was determined at pH 4.05 by taking a large excess
of [PMS] (0.04 M) over [LA] (0.005 M) and [Ni(II)]
−4
(
4.0 × 10 M). The unreacted [PMS] was estimated
after 24 h. A correction for the decomposition of PMS
under identical conditions was applied. The product
analysis was also carried out with a slight excess of
stoichiometric [PMS], and the pH was adjusted to ∼4.0
with lactate/lactic acid itself (instead of acetate/acetic
acid).
1
kobs = k × [Ni(II)]
(2)
The stoichiometry of the reaction can be represented
as in Eq. (1).
1
The k values are calculated at different pH values
4.05–5.89) and temperatures from kobs vs. [Ni(II)]
Ni(II)
(
Lactic acid + 2 PMS —−→ Acetic acid + CO2 ↑ (1)
1
plots. Perusal of the k values show that they increased
with pH, and the plots log(k ) vs. pH are almost straight
1
The formation of acetic acid was confirmed by the
color test with lanthanum nitrate and iodine [5]. The
lines with a positive slope in the range of 0.7–0.8. But
International Journal of Chemical Kinetics DOI 10.1002/kin

STUDIES ON THE OXYGEN ATOM TRANSFER REACTIONS OF PEROXOMONOSULFATE
451
[
LA] = 0.048 M
pH = 4.05
Temp. = 38.0 C
o
k
0
.0002
0.0004
0.0006
0.0008
0.0010
0.0012
0.0014
[Ni(II)] M
Figure 1 Plot of kobs vs. [Ni(II)].
1
+ −1
the plots k vs. [H ] are straight lines with a positive
intercept (Fig. 2).
acid by PMS in the presence of Ni(II) ions:
+
K1 CH CH(OH)COONi
−
3
The effect of the acetate ion concentration on the
rate is also studied at pH 4.05 and 4.75. At constant
pH, the rate is not at all affected by the [OAc ] (0.08–
CH3CH(OH)COO + Ni(II) ꢀ
+
ML )
(
−
(3)
0
.32 M) and hence the reaction is not a base-catalyzed
+
−
5
CH3CH(OH)COONi + HSO
one. The added radical quenchers such as ethanol or
tert-butanol (0.025–0.05 M) have no effect on the kobs
values. The ionic strength in the range from 0.15 to
K2
−
+
ꢀ
CH3CH(OH)COONiSO + H
(4)
5
0
.30 M, adjusted with sodium nitrate, also has a negli-
k1
+
−
gible effect on the rate.
Based on the experimental results, we can propose
the following kinetic scheme for the oxidation of lactic
CH3CH(OH)COONi + HSO —−→ Products (5)
5
k2
−
CH CH(OH)COONiSO —−→ Products (6)
3
5
1
1
1
1
0
0
0
0
0
.6
.4
.2
.0
.8
.6
.4
.2
.0
o
Temp. = 25.0 C
0
2e+5
4e+5
+
6e+5
8e+5
1e+6
[
H ]
M
1
+
−1
Figure 2 Plot of k vs. [H ]
.
International Journal of Chemical Kinetics DOI 10.1002/kin

4
52
ANDAL ET AL.
+
2−
The rate equation for the kinetic scheme in reactions
CH3CH(OH)COONi + SO
5
(3)–(6) can be expressed as in Eq. (7).
K3
−
ꢀ
CH3CH(OH)COONiSO5
(12)
−
d[PMS]
+
+
=
(k1 + k2K2/[H ])[ML ][PMS]
(7)
(8)
dt
Therefore, reactions (4) and (6) represent the oxida-
2−
+
+
tion of nickel lactate by SO , whereas reaction (5)
∴
kobs = (k1 + k2K2/[H ])[ML ]
5
−
is by HSO . Results on the influence of alkali metal
ion (M ) in the oxidation reactions with PMS [14,15]
have shown that SO forms an ion pair with a metal
ion M , whereas such a reaction with HSO is not ob-
5
+
Lactic acid is a weak acid with pKa value of 3.45
2
−
◦
(
25 C) [7]. Alpha hydroxy acids react with Ni(II)ions
5
+
−
5
+
to give 1:1 complex ML in which the metal ion is
bonded to the carboxylate oxygen [8–11]. Equation (3)
represents the formation of the 1:1 complex in lactic
acid–nickel system and the reported value for K1 is
2−
served. Therefore, it is reasonable to assume that SO5
readily forms an ion pair with nickel lactate, which
may be converted into reactive peroxide intermediate.
The intramolecular electrophilic attack of the peroxide
oxygen at the hydroxyl group may result in a hydroper-
oxide, and this reaction is represented in Eqs. (4) and
−1 ◦
1
64.4 M (25 C) [8]. This value is used in our calcu-
lations. Simple calculation shows that ∼90% of Ni(II)
+
ion will be in the form of ML at our experimental
(6). Therefore, reaction (6) is the intramolecular path-
conditions. Because the concentrations of Ni(II) ions
used in this study are very small compared to [LA],
way for the oxidation and Eq. (5) is the intermolecular
mechanism.
+
we can approximate [ML ] ≈ [Ni(II)]T . Therefore,
If we substitute reaction (4) with Eqs. (11) and
Eq. (8) can be modified as in Eq. (9).
(
12), then the kinetic constant k2K2 becomes k2KdK3,
−
+
where K is the dissociation constant of HSO . There-
fore, ꢀH corresponding to the kinetic constant k2K2
d
kobs = (k1 + k2K2/[H ])[Ni(II)]T
(9)
5
ꢂ=
ꢂ
0
Comparison of Eq. (9) with Eq. (2) shows that k1
can be expressed as in Eq. (10).
given in Table I can be expressed as ꢀH = ꢀH
+
Kd
ꢂ
, where ꢀH0 is the enthalpy of dissociation for
ꢀ
H
k K
2
3
K
d
ꢂ=
the equilibrium (11). Similarly, the ꢀS can be writ-
0
K
ꢂ
1
+
ten as the sum of ꢀS and ꢀS
. Using the literature
k = k1 + k2K2/[H ]
(10)
k K
2
d
3
0
−1
[
13] values of ꢀH (20.93 kJ mol ) and ꢀS0 (−108
Kd
Kd
J K 1mol−1), ꢀH
−
ꢂ
and ꢀS
ꢂ
values are calculated
Equation (10) explains all the experimental observa-
tions. The k1 and k2K2 values and the corresponding
thermodynamic parameters are listed in Table I. The
radical quenchers ethanol and tert-butanol have no ef-
fect on the rate, and therefore the alcohol quenchable
radical intermediates such as OH and SO4 [12] are
not produced in the reaction. This clearly shows the
absence of a higher oxidation state of Ni(II) in the
mechanism.
k2K3
k2K3
−1
−1
−1
as 33.9 kJ mol and −64.1 J K mol , respectively.
Comparison of these values with those of nickel gly-
−1
−1
−1
colate (48.6 kJ mol and −18.4 J K mol ) shows
that that enthalpy of activation is lowered by ∼15 kJ
•
−•
−1
mol and the entropy of activation becomes more
−1
−1
negative by ∼35 J K mol . This means that the
formation of activated state is more favorable in nickel
lactate than nickel glycolate. Lactic acid differs from
glycolic acid only by the α-methyl group. That is, the
substitution of a hydrogen atom at the alpha carbon
by an electron-donating (+I) methyl group favors the
nucleophilic interaction of the hydroxyl group with the
peroxide oxygen.
The equilibrium (4) can be expressed as the sum of
the two equilibria (11) and (12). The literature value of
Kd is 4.0 × 10
−
10
◦
M at 25 C [13].
Kd
HSO ꢀ SO + H+
−
2−
(11)
5
5
ꢂ
ꢂ=
The ꢀH and ꢀS values for the reaction between
−
nickel lactate and HSO5 are also shown in Table I.
Table I The Kinetic and Thermodynamic Parameters
Perusal of the values shows that the enthalpy of ac-
tivation is almost three times higher than that of the
◦
−1 −1
6
−1
)
Temperature ( C)
k1 (M
s
)
10 × k2K2 (s
2−
reaction with SO . One interesting observation is that
5
ꢂ
.
.
2
3
3
ꢀ
5.0
1.0
8.0
0.13
1.61 (1.01)
2.27 (1.79)
4.42 (3.47)
54.8 (69.5)
ꢀ
S
is positive and is very close to −41 ZA ZB J
0.22
0.64
93.9
52.5
−1
−1
K
mol [16], the entropy of activation due to elec-
trostatic effects for a reaction involving two ions of
charges ZA (+1) and ZB(−1). This is in accordance
with our kinetic scheme in which the activated state
is mainly due to the (electrostatic) interaction between
ꢂ=
−1
H (kJ mol )
ꢂ
=
−1
−1
ꢀ
S (J K mol )
−172.1 (−126.4)
Values in parentheses correspond to the nickel glycolate.
International Journal of Chemical Kinetics DOI 10.1002/kin

STUDIES ON THE OXYGEN ATOM TRANSFER REACTIONS OF PEROXOMONOSULFATE
453
#
O
_
+
CH CH(OH)COONi + HSO5
CH CH-C
3
3
+
O
Ni
HO
_
H
O
O
SO3
(
Activated state)
k1
H
O
+
+
SO4 + H
CH C-C
3
+
ONi
O
O
Fast
H
+
H O + CH COCOONi
2
3
Fast
PMS
Ni + CO2 + CH COOH
2+
2+
Ni + CO +OH
3
CH CHO +
2
3
PMS
CH COOH
3
−
Figure 3 A mechanistic scheme for the reaction with HSO .
5
the two ions of opposite charge. Thus the observed en-
tropy of activation shows that the oxidation of nickel
the same active center, namely the alpha hydroxyl
group.
−
−
lactate by HSO may proceed through intermolecular
We can compare the relative reactivity of HSO
5
5
2−
interaction and is shown in Fig. 3.
and SO toward nickel lactate. Substituting the liter-
5
It has been shown that the oxidation of glycolic acid
by PMS in the presence of Ni(II) and Cu(II) proceeds
through the formation of hydroperoxide intermediate
ature value of K in k K K (=k K ), we get k K is
d
2
d
3
2
2
2
3
3
−1 −1
◦
4.03×10 M
s
at 25 C and this can be compared
with k values given in Table I. Perusal shows that the
1
2
−
[3]. When the reactions are carried out in the presence
ionized form of the oxidant (SO ) is more reactive by
5
of formaldehyde and [HCHO] > [Ni(II)], the rate is
second order with respect to [PMS] and we observed
the decomposition of PMS instead of the oxidation of
nickel lactate [17]. Similar change in the mechanism
is also observed in the nickel glycolate + HCHO +
PMS system. This may be due to the formation of low
molecular weight hemiacetals [18] as in Eq. (13).
approximately four orders of magnitude. This may be
−
2−
5
due to the different reaction of HSO and SO with
5
+
2−
metal ion M [14,15] as discussed earlier. Thus SO5
may form an ion pair with nickel ion of nickel lac-
tate, and this may be the reason for the intramolecular
oxidation–reduction mechanism (Fig. 4).
The large difference in the rate constants can also
explain why we could not observe the reaction be-
−
+
tween HSO5 and nickel glycolate. According to the
CH3CH(OH)COONi + HCHO
reactivity-selectivity principle [19–21], the more reac-
tive oxidant/substrate should be less selective than the
+
ꢀ
HOCH2OCH(CH3)COONi
(13)
2
−
4
lower reactive one. Because SO is ∼10 times more
5
Thus hemiacetal formation may protect the α-hydroxy
group, and this may the reason for the change in the
mechanism. Therefore, the oxidation of glycolate and
lactate, in the presence of Ni(II), proceeds through
reactive, it should be less selective and hence less sus-
ceptible to the substituent effect. This will explain why
the k2K2 values of nickel lactate are slightly higher
than nickel glycolate (Table I). The influence of the
International Journal of Chemical Kinetics DOI 10.1002/kin

4
54
ANDAL ET AL.
O
_
2
K
3
+
CH3CH(OH)COONi + SO5
CH3CH-C
HO
O
Ni
O
O
_
SO3
#
O
CH3CHC
ONi
O
H
k2
O
O
O
H
+
SO4
CH C-C
3
+
ONi
_
SO3
O
O
Fast
(Activated State)
H
+
H2O + CH COCOONi
3
Fast
PMS
2
+
2
+
Ni + CO2 + CH3COOH
CH CHO + Ni + CO2 + OH
3
PMS
CH COOH
3
2
−
.
Figure 4 A mechanistic scheme for the reaction with SO
5
substituent may be more effective with the less reac-
8. Evans, W. P.; Monk, C. B. J Chem Soc 1954, 550.
9. Nakamoto, K.; McCarthy, P. J.; Miniatas, B. Spec-
trochim Acta 1965, 21, 379.
−
tive HSO , and this may be the reason that we could
5
not observe k1 values in nickel glycolate–PMS system.
1
1
1
1
1
0. Harada, S.; Okuue, Y.; Kan, H.; Yasunaga, T. Bull Chem
Soc Jpn 1974, 47, 769.
1. Inoue, T.; Sugakara, K.; Kojima, K.; Shimozawa, R.
Inorg Chem 1983, 22, 3977.
The authors acknowledge the Registrar, M. K. University,
for the facilities and support. S. Shailaja thanks Council of
Scientific and Industrial Research, New Delhi, India, for the
award of senior fellowship.
2. Neta, P.; Huie, R. E.; Ross, A. B. J Phys Chem Ref Data
1
988, 17, 1027.
3. Ball, D. L.; Edwards, J. O. J Am Chem Soc 1956, 78,
125.
4. Mehrotra, M.; Soni, V.; Mehrotra, R. N. Polyhedron
008, 27, 609.
1
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International Journal of Chemical Kinetics DOI 10.1002/kin