Autocatalytic Oxidation of Thiamine Hydrochloride (Vitamin B1) by Permanganate in Aqueous Sulfuric Acid Medium: A Kinetic and Mechanistic Study

Article abstract of DOI:10.1002/kin.20991

The reaction kinetics of autocatalytic oxidation of thiamine hydrochloride (vitamin B₁) by the permanganate ion in aqueous sulfuric acid medium has been investigated spectrophotometrically under the pseudo-first-order condition at 25°C. The observed stoichiometry is 6:5 in terms of the mole ratio of permanganate ions and thiamine hydrochloride. Formation of products was confirmed by UV-vis, IR, GC-MS, and NMR spectral data. Usually in the permanganate oxidation-reduction reactions, one of the products, Mn²⁺ autocatalyzes the reaction, but in the present investigation the autocatalytic effect is due to the (4-methyl-thiazol-5-yl) acetic acid, a product formed from the oxidation of vitamin B₁, which is a rare case. The added Mn²⁺ does not have any significant effect on the rate of reaction. The reaction is first order with respect to both permanganate and thiamine hydrochloride concentrations. An increase in the sulfuric acid concentration decreases the rate of reaction. A composite scheme and rate laws were proposed. The activation parameters with respect to the slow step and reaction constants involved in the mechanism were determined and discussed.

Full text of DOI:10.1002/kin.20991

Autocatalytic Oxidation of
Thiamine Hydrochloride
(
Vitamin B ) by
1
Permanganate in Aqueous
Sulfuric Acid Medium: A
Kinetic and Mechanistic
Study
1
1
2
2
ANITA SAVANUR, AMIT TERADALE, SHEKAPPA LAMANI, SHIVAMURTI CHIMATADAR
1P. G. Department of Studies in Chemistry, Karnatak University, Pavate Nagar, Dharwad, 580 003, India
2P. G. Department of Studies in Chemistry, S. B. Arts & K. C.P. Science College, Bijapur, 586 103, India
Received 10 August 2015; revised 12 February 2016; accepted 28 February 2016
DOI 10.1002/kin.20991
Published online 22 March 2016 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: The reaction kinetics of autocatalytic oxidation of thiamine hydrochloride (vitamin
B1) by the permanganate ion in aqueous sulfuric acid medium has been investigated spec-
trophotometrically under the pseudo–first-order condition at 25°C. The observed stoichiometry
is 6:5 in terms of the mole ratio of permanganate ions and thiamine hydrochloride. Formation
of products was confirmed by UV–vis, IR, GC-MS, and NMR spectral data. Usually in the perman-
2
+
ganate oxidation–reduction reactions, one of the products, Mn autocatalyzes the reaction,
but in the present investigation the autocatalytic effect is due to the (4-methyl–thiazol-5-yl)
acetic acid, a product formed from the oxidation of vitamin B1, which is a rare case. The added
2+
Mn does not have any significant effect on the rate of reaction. The reaction is first order with
respect to both permanganate and thiamine hydrochloride concentrations. An increase in the
sulfuric acid concentration decreases the rate of reaction. A composite scheme and rate laws
were proposed. The activation parameters with respect to the slow step and reaction constants
involved in the mechanism were determined and discussed. ꢀC 2016 Wiley Periodicals, Inc. Int
J Chem Kinet 48: 281–291, 2016
Correspondence to: S. Chimatadar; e-mail: shekar.63@
rediffmail.com, shekar.62@gmail.com.
ꢀ
2016 Wiley Periodicals, Inc.
C

2
82
SAVANUR ET AL.
INTRODUCTION
oxidation of vitamin B1 by permanganate to arrive at
suitable mechanisms.
Thiamine hydrochloride, known as vitamin B1, occurs
in the outer coats of the seeds of many plants includ-
ing cereal grains. Thiamine hydrochloride is funda-
mentally associated with carbohydrate metabolism [1].
Vitamin B1 might also serve as a modulator of neuro-
muscular transmission [2]. The requirement of vitamin
B1 is related to the metabolic rate and is greater when
carbohydrate is a source of energy [3]. Thiamine is syn-
thesized in bacteria, fungi, and plants. Animals must
cover all their needs from their food, and insufficient
intake results in a disease called beriberi, affecting the
peripheral nervous system (polyneuritis) or the cardio-
vascular system, with fatal outcome if not cured by
thiamine administration [4]. As most feedstuffs used
in poultry diets contain enough quantities of vitamins
to meet the requirements in this species, deficiencies in
this vitamin does not occur with commercial diets [5].
Today, there is still a lot of work devoted to elucidating
the exact mechanisms by which thiamine deficiency
leads to the specific symptoms observed. Finally, new
thiamine phosphate derivatives have recently been dis-
covered [6]. The oxidation kinetics and mechanism
of vitamin B1 are thus important to the process in
vitro.
EXPERIMENTAL
Materials and Methods
Materials. The solutions were prepared in water
which had been twice distilled in an all-glass unit
in the presence of potassium permanganate. Reagent-
grade chemicals were used. A stock solution of vi-
1
tamin B was prepared by dissolving in water. Per-
−
manganate (MnO4 ) stock solution was obtained by
dissolving potassium permanganate (Glaxo; analar,
India) in water and standardized by titrating against
oxalic acid [13]. Always freshly prepared and standard-
−
ized MnO4 solutions were used in the kinetics. The
manganese(II) solution was made by dissolving man-
ganese sulfate (AR) in water. The (4-methyl–thiazol-
5-yl) acetic acid (CDH) solution was prepared by dis-
solving it in water. Na SO (AR) and H SO (AR)
2
4
2
4
were used to provide the required ionic strength and
acidity, respectively.
Kinetic Measurements. All kinetic measurements
were performed under pseudo–first-order conditions
The permanganate ion is widely used as an oxidiz-
ing agent in synthetic as well as in analytical chem-
istry [7] and according to Insauti et al., it has several
advantages as an analytical reagent [8]. In general,
reduction of the permanganate, in acid media goes
to either Mn(IV) or Mn(II) having the reduction po-
tential [9] of the couple Mn(VII)/Mn(IV): 1.695 V
and Mn(VII)/Mn(II): 1.51V. In acid medium, it exists
with vitamin B concentration greater than perman-
1
ganate concentration at constant ionic strength of
−
3
1.60 mol dm except in the variation of acid, in
−
3
which I = 3.10 mol dm . The reaction was initi-
ated by mixing thermally equilibrated (25.0 ± 0.1°C)
solutions of vitamin B and permanganate, which has
1
also contained the required amounts of sodium sul-
+
in different forms, viz., HMnO4, H2MnO4 , HMnO3,
fate and sulfuric acid. The reaction was followed by
measuring the absorbance of the permanganate con-
centration in the reaction mixture at 525 nm in a 1-cm
cell placed in the thermostated compartment of a Varian
Cary 50 Bio UV–vis spectrophotometer. Applicationof
Mn2O7, and depending on the nature of the reductant
the oxidant has been assigned both the inner sphere
and outer sphere mechanism pathways in their redox
reactions [10,11].
−
4
It is usual in the case of permanganate oxidation–
Beer’s law was verified between 1.0 × 10 and 1.0 ×
reduction reactions, the product, Mn2 autocatalyzes
the reaction. But, in the present investigation one of the
products (4-methyl–thiazol-5-yl) acetic acid, formed
+
−3
−3
10 mol dm of permanganate concentration at
525 nm under the reaction conditions, and the mo-
lar extinction coefficient was found to be ε = 2200 ±
−
3
−1
−1
from the oxidation of vitamin B1 by MnO4 , auto-
50 dm mol cm . The kinetic runs were followed
more than 85% completion of the reaction. Since one of
the products, (4-methyl–thiazol-5-yl) acetic acid auto-
catalyzes the reaction, the pseudo–first-order rate con-
stants, kobs, were calculated from the plots of log (ab-
sorbance) vs. time, for about 70% completion of the
reaction after which sigmoid curves were obtained
(Fig. 1). The kobs values were reproducible within ± 5%
and are the average of at least three independent kinetic
runs (Table I) [14,15].
catalyzes the rate of reaction, which is a rare case.
Owing to this, the mechanism may be quite com-
plicated and interesting one. Permanganate in acid
−
+
solution exists as MnO4 , H2MnO4 , HMnO3, and
−
Mn2O7. Among all the species, MnO4 is the most
active oxidizing species [12]. The literature survey
indicates that there are no reports on the oxidation
of vitamin B1 by permanganate in an acid medium.
Hence, we have investigated the autocatalyzed
International Journal of Chemical Kinetics DOI 10.1002/kin.20991

AUTOCATALYTIC OXIDATION OF THIAMINE HYDROCHLORIDE (VITAMIN B1) BY PERMANGANATE
283
−
2.0
1.6
1.2
0.8
0.4
0.0
Table I Effect of Variation of MnO4 and Vitamin B1
Concentrations on the Autocatalyzed Oxidation of
−
Vitamin B1 by MnO4 in Aqueous Sulfuric Acid Medium
+
−3
at 25°C. [H ] = 0.28; I = 1.60/mol dm
−
4
[
(
MnO4 ] × 10
[Vitamin B1] ×
kobs ×
kcal ×
−
3
3
−3
3
−1
3
−1
)
mol dm
)
10 (mol dm
)
10 (s
)
10 (s
(3)
0
1
2
3
4
5
2
2
2
2
.50
.0
.0
.0
.0
.0
.0
.0
.0
.0
3.0
3.0
3.0
3.0
3.0
3.0
1.0
2.0
3.0
4.0
6.0
8.0
10.0
5.19
5.22
5.30
5.30
5.18
5.15
1.69
3.39
5.30
6.91
9.91
13.6
17.0
5.19
5.22
5.10
5.41
5.18
5.15
1.80
3.60
5.40
7.01
10.4
14.4
18.0
(
2)
(
1)
2.0
2
2
0.0
5.0
10.0
15.0
20.0
.0
.0
Time (min)
Figure 1 Plot of log (absorbance) vs. time [vitamin B1]
=
1.0 × 10− ; [H2SO4] = 0.50; I = 1.60, [MnO4 ] ×
3
−
the reaction mixture was treated with the 5% Na2CO3
and then it was extracted by ether. The ether layer was
separated and dried well on CaCO3. On evaporation
of the ether, the obtained product was found to be (4-
amino- 2-methyl–pyrimidine-5-carbaldehyde), which
was confirmed by GC-MS, where a molecular ion peak
was observed at m/z = 137 (Fig. 2), and it was also
4
−3
1
0 /mol dm : (1) 0.5, (2) 1.0, and (3) 2.0.
RESULTS
Stoichiometry and Product Analysis
Different sets of concentrations of reactants in
1
confirmed by the H NMR (CDCl3) in which δ 9.80 (s,
−
−
3
0
1
.50 mol dm sulfuric acid at constant ionic strength,
.60 mol dm were kept for over 5 h at 25°C in
1H, for –C–CHO), δ 9.01 singlet proton(s, 2H, NH2),
3
δ 8.34 singlet proton (S, 1H, Ar-H), and δ 2.33 sin-
glet (s, 3H CH3) (Fig. 3). The remaining aqueous layer
was acidified with 5% HCl and extracted with ethyl ac-
etate. After removing ethyl acetate, the formed residue
was recrystallized by using ethyl acetate and hexane
mixture to get yellow needles of (4-methyl–thiazol-5-
yl) acetic acid. The (4-methyl–thiazol-5-yl) acetic acid
was further confirmed by the IR (KBr) where the OH
a closed container. When [permanganate] > [vita-
min B1], the remaining permanganate concentration
was assayed by measuring the absorbance at 525
2
+
,
nm. The reaction products were identified as Mn
4-methyl–thiazol-5-yl) acetic acid, and (4-amino-2-
(
methyl–pyrimidine-5-carbaldehyde). The results indi-
cate that 5 mol of vitamin B1 consumed 6 mol of per-
manganate according to Eq. (1).
Cl
+
CH3
CHO
NH2
CH2
9
H SO4
2
N
N
N
6
KMnO4
+
5
6MnSO4
+
5
S
CH CH OH
3C
HN
3C
HN
NH2
2
2
(
1)
CH3
N
+
14 H O
2
+
3
K SO
2
4
+
5
S
CH COOH
2
The products were isolated by using the TLC sepa-
band shows stretching at 3400 cm−1, for CO the broad
−
1
ration technique and characterized by physicochemical
spectral studies. The reaction products were confirmed
band at 1730 cm . In GC–MS, a molecular ion peak
1
was observed at m/z = 112. (Fig. 4), where as in H
1
by UV–vis, IR, GC-MS, and H NMR spectral stud-
NMR, δ 11.43 singlet (s, 1H, COOH), δ 8.35 singlet
(s,1H, Ar-H), δ 5.57 (s, 2H, CH2), and 2.57 singlet (s,
3H, CH3) (Fig. 5).
2
+
ies. Mn was confirmed by UV–vis spectra and spot
test [16]. Two hours after completion of the reaction,
International Journal of Chemical Kinetics DOI 10.1002/kin.20991

2
84
SAVANUR ET AL.
Figure 2 GC–MS spectra of the product (4-amino-2-methyl–pyrimidine-5-carbaldehyde).
Figure 3 1H NMR of the product (4-amino-2-methyl–pyrimidine-5-carbaldehyde).
Reaction Orders
Effect of [Permanganate]
−
The oxidant, permanganate (MnO4 ), concentration
The reaction orders were determined from the slope
of logkobs vs. log (concentrations) plots by varying
the concentration of vitamin B1 and sulfuric acid in
turn, keeping all other concentrations and conditions
constant.
−5
was varied in the range of 5.0 × 10 to 5.0 ×
−4 −3
10
mol dm . The observed pseudo–first-order rate
constants, kobs, were almost constant (Table I), indicat-
ing first-order dependence with respect to the perman-
ganate concentration.
International Journal of Chemical Kinetics DOI 10.1002/kin.20991

AUTOCATALYTIC OXIDATION OF THIAMINE HYDROCHLORIDE (VITAMIN B1) BY PERMANGANATE
285
Figure 4 GC–MS spectra of the product (4-methyl–thiazol-5-yl) acetic acid.
Figure 5 1H NMR spectra of the product (4-methyl–thiazol-5-yl) acetic acid.
Effect of [Vitamin B1]
relative coefficient (r = 0.9996), which indicates first
order with respect to the vitamin B1 concentration.
The effect of variation of vitamin B1 on the rate of reac-
−
3
tion was studied in the concentration range, 1.0 × 10
−
2
−3
Effect of [Acid]
to 1.0 × 10 mol dm , at constant concentrations of
−
−
MnO4 , H2SO4, and constant ionic strength. It was ob-
At a fixed concentration of oxidant, [MnO ] = 2.0 ×
4
−3
−
4
−3
−3
,
served that as the vitamin B1 concentration increased,
kobs also increased (Table I). The value of the slope
of the plot of log kobs vs. log [vitamin B1] was found
to be unity, and it shows a linear relationship with the
10 mol dm , vitamin B = 3.0 × 10 mol dm
1
and by keeping other conditions constant, the k was
obs
found to be decreased with increasing sulfuric acid
concentration (Table II). The in situ H ion concentra-
tion in the sulfuric acid–sulfate media was calculated
+
−
5
linear regression equation y = 1.688x + 5 × 10 and
International Journal of Chemical Kinetics DOI 10.1002/kin.20991

2
86
SAVANUR ET AL.
−
Table II Effect of the Sulfuric Acid Concentration on the Autocatalyzed Oxidation of Vitamin B1 by MnO4 at 25°C.
[
MnO4 ] = 2.0 × 10 ; [vitamin B1] = 3.0 × 10−3, I = 3.1 mol dm
−
−4
−3
+
2−
−
H2SO4
(
[H ]
[SO4
]
[HSO4 ]
mol dm 3)
−
(mol dm
−3
)
(mol dm
−3
)
(mol dm
−3
)
kobs × 10 (s
3
−1
)
kcalcd × 10 (s
3
−1
)
0
0
0
0
0
0
0
1
.10
.20
.30
.40
.50
.60
.80
.00
0.025
0.059
0.126
0.182
0.280
0.414
0.719
1.074
0.855
0.689
0.556
0.412
0.310
0.244
0.149
0.101
0.175
0.340
0.474
0.618
0.720
0.786
0.881
0.926
7.93
7.03
6.53
6.07
5.30
4.30
3.66
3.00
7.25
6.93
6.38
5.96
5.40
4.70
3.75
2.99
Table III Effect of Initially Added Product,
4-Methyl–thiazol-5-yl) acetic acid, on the Oxidation of
Vitamin B1 in Aqueous Sulfuric Acid Medium at 25°C.
Time (min)
(
0.0
2.0
4.0
6.0
8.0
10.0
0.0
−
−4
−3
;
[
MnO4 ] = 2.0 × 10 ; [vitamin B1] = 3.0 × 10
+
−3
[H ] = 0.28, I = 1.60 /mol dm
-0.2
[
4-Methyl–thiazol-5-yl) acetic
-
-
-
-
-
-
-
0.4
0.6
0.8
1.0
1.2
1.4
1.6
4
−3
3
−1
)
acid] × 10 (mol dm
)
kobs × 10 (s
0
0
1
1
2
2
.25
.50
.00
.50
.00
.50
5.21
8.31
22.9
38.0
44.0
51.0
(1)
(2)
(3)
(4)
by using the known ionization constant [17] of acid
sulfate as in the earlier study [18]. The value of the
slope of the plot of log kobs vs. log [acid] was found
to be order with respect to the H ion concentration
was negative and less than unity, and it shows a linear
-1.8
Figure 6 Effect of the added product, (4-methyl–thiazol-
-yl acetic acid), on the oxidation of vitamin B1 by MnO4 .
+
−
5
[
0
−4
4-methyl–thiazol-5-yl) acetic acid]: (1) 0.25 × 10 , (2)
relationship with the linear regression equation, y =
−4
−4
−4
−3
.
.50 × 10 , (3) 1.0 × 10 , and (4) 1.5 × 10 mol dm
–
5.577x + 8.196 and relative coefficient (r = –0.9863).
Effect of Initially Added Products
indicate the autocatalytic nature of the product, (4-
methyl–thiazol-5-yl) acetic acid. The plots of log
The initially added products, manganese(II), (4-
amino-2-methyl–pyrimidine-5-carbaldehyde), and (4-
methyl–thiazol-5-yl) acetic acid, were studied in the
(absorbance) vs. time for initially added product,
(4-methyl–thiazol-5-yl) acetic acid at different con-
−
5
−4
−3
centrations, showed almost linearity up to 70% com-
pletion of the reaction (Fig. 6), after which sigmoid
curves were obtained (not shown).
concentration range, 5.0 × 10 to 5.0 × 10 mol
−3
−5
−4
dm and 2.5 × 10 to 2.5 × 10 mol dm con-
centration ranges, respectively, while keeping the reac-
tant concentrations and all other conditions constant. It
was observed that added (4-methyl–thiazol-5-yl) acetic
acid enhances the rate of reaction (Table III) with an
unit order, whereas the added manganese(II) and (4-
amino-2-methyl–pyrimidine-5-carbaldehyde) did not
change the rate of reaction appreciably. The results
Effect of Ionic Strength and Dielectric
Constant
At constant concentrations of reactants and at other
conditions kept constant, the ionic strength was varied
International Journal of Chemical Kinetics DOI 10.1002/kin.20991

AUTOCATALYTIC OXIDATION OF THIAMINE HYDROCHLORIDE (VITAMIN B1) BY PERMANGANATE
287
Scheme 1
between 1.6 and 3.1 mol dm 3 by varying the con-
centrations of sodium sulfate. At constant acidity and
other constant conditions, the dielectric constant of the
medium was varied by varying acetic acid–water (v/v)
content in the reaction mixture from 0 to 50%. Both
ionic strength and dielectric constant did not have any
significant effect on the rate of reaction.
−
an increase in temperature. The rate constant, k1, of the
slow step of Scheme 1 was obtained from the intercept
+
of the plots of [vitamin B1]/kobs vs. [H ] at four differ-
ent temperatures (Table IV). The energy of activation,
Ea, was evaluated from the slope of the plot of log k
−
1
vs. 1/T and the value is 54 ± 1 k J mol . The enthalpy
ࣔ
of activation, ꢀH , and the entropy of activation,
S , were obtained by the Eyring equation [19].
ࣔ
ꢀ
Test for Free Radicals
kBT
#
(
−ꢀG /RT)
k =
e
e
The intervention of free radicals was examined as fol-
h
3
3
lows: To a 10-cm of reaction mixture, 2 cm of acry-
lonitrile scavenger was added and kept in the inert
atmosphere for 5 h. Diluting with methanol, no pre-
cipitate resulted, indicating the absence of free radical
intervention.
kBT
#
#
−(ꢀH +TꢀS )/RT
=
(2)
h
where k is the second-order rate constant, k is the
B
Boltzmann’s constant, R is the gas constant, T is the
absolute temperature, and ꢀGࣔ is the free energy of
activation. The linear form of Eq. (2) is
Effect of Temperature
The rate of reaction was studied at four different tem-
peratures 15, 25, 35, and 45°C by varying the sulfuric
acid concentration. The rate of reaction increased with
k
ꢀH #
RT
ꢀS#
R
kB
+ In h
In = −
+
(3)
T
International Journal of Chemical Kinetics DOI 10.1002/kin.20991

2
88
SAVANUR ET AL.
−
Table IV Effect of Temperature on the Autocatalyzed
Oxidation of Vitamin B1 by MnO4 , in Aqueous Sulfuric
formation of MnO4 species decreased in the reac-
−
tion mixture. The reaction between vitamin B1 and
permanganate in sulfuric acid has a stoichiometry 5:6
with first order each in permanganate and vitamin
B1 concentrations. The oxidation products were man-
ganese(II), (4-methyl–thiazol-5-yl) acetic acid, and (4-
amino-2-methyl–pyrimidine-5-carbaldehyde). It was
observed that, one of the products, (4-methyl–thiazol-
5-yl) acetic acid, autocatalyzed the reaction rate, where
as other products, manganese(II) and (4-amino-2-
methyl–pyrimidine-5-carbaldehyde), did not have any
significant effect on the rate of reaction. In view of
the decreasing rate with increasing H ion, HMnO4
decomposes to give MnO4 and H ion. Such a
type of active species of MnO4 is evidenced by
the literature [12]. Furthermore, MnO4 species re-
acts with vitamin B1 in a rate-determining step to
give the intermediate HMnO4 and carbocation(II).
In a fast step, intermediate(II) hydrolyzes to give (4-
amino-2-methyl–pyrimidine-5-carbaldehyde)(III) and
4-methyl–thiazol-5-yl)-ethanol(IV). In further fast
Acid Medium and with Respect to the Slow Step of
3
−1 −1
Scheme 1 Temperature (K) k1 (dm mol
s ) and
Effect of Temperature on the First Equilibrium Step of
Scheme 1
Temperature (K)
K1 (mol dm 3)
−
Effect of temperature on the autocatalyzed oxidation of
vitamin B1 by MnO4–
2
2
3
3
88
0.97
2.53
5.78
8.23
98
08
13
+
+
−
−
Activation parameter
−
Parameter
Values*
−
1
2−
Ea
54 ± 1 kJ mol
#
−1
ꢀ
ꢀ
ꢀ
H
S
G
52 ± 1 kJ mol
#
−1
−1
−48 ± 2 J K mol
#
−1
52 ± 2 kJ mol
(
log A
9.9 ± 0.2
step, (4-methyl–thiazol-5-yl) ethanol(IV) reacts with
2
−
1
mol of HMnO4
to give (4-methyl–thiazol-5-yl)
Effect of temperature on the first equilibrium step of
Scheme 1
−
acetaldehyde(V) and MnO2 . The formed intermedi-
ate (V) which in turn reacts with two moles of MnO2
to give the final product (4-methyl–thiazol-5-yl) acetic
acid(VI) and Mn . The results are accommodated in
the following mechanism (Scheme 1).
From Scheme 1, the following rate law (6) can be
derived as follows:
−
–
3
Temperature (K)
K1 (mol dm )
2
+
2
2
3
3
88
98
08
13
0.74
0.67
0.60
0.55
Thermodynamic quantities with respect to K1
−
−
d[MnO ]
4
−
Rate =
= k1[MnO ] [vit. B1]
H (kJ mol 1)
−
4
ꢀ
ꢀ
ꢀ
−7 ± 2
−28 ± 5
1.0 ± 0.02
dt
−
1
−1
S (J K mol
)
−
1
1 1 4 f 1
k K [HMnO ] [vit. B ]
G (kJ mol
)
=
(4)
+
H
*
Error against each parameter is average of three independent
kinetic measurements.
The total concentration of permanganate ion is given
by
The slope of the plot of log k/T vs. 1/T gives the value
ࣔ
−1
of enthalpy of activation, ꢀH = 52 ± k J mol . By
ࣔ
using this value ꢀH , and the rate constant at a par-
−
−
[
MnO ]t = [HMnO4]f + [MnO ]
4
4
ࣔ
ticular temperature T, the value of ꢀS was obtained
ꢀ
ꢁ
K1
by simple rearrangement of Eq. (2) and the value is
=
[HMnO4]f 1 +
ࣔ
−1
−1
+
ꢀ
ꢀ
ꢀ
S = –48.8 ± 2 J K mol . Using these values of
[H ]
ࣔ
ࣔ
H and ꢀS , the obtained free energy of activation,
−
+
[
MnO ]t [H ]
ࣔ
−1
4
G298 is 52 ± 2 k J mol
.
∴
[HMnO4]f =
(5)
K1 + [H+]
Substituting Eq. (5) in Eq. (4) and omitting the sub-
scripts,
DISCUSSION
In the present investigation as the sulfuric acid con-
centration increased the rate of reaction decreased. It
indicates that as the H concentration increased the
Rate
k1K1 [vit B1]
=
−
4
kobs =
(6)
+
+
[MnO ]
K + [H ]
1
International Journal of Chemical Kinetics DOI 10.1002/kin.20991

AUTOCATALYTIC OXIDATION OF THIAMINE HYDROCHLORIDE (VITAMIN B1) BY PERMANGANATE
289
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.7
0.6
0.5
0.4
0.3
0.2
o
1
5 C
o
25 C
o
35 C
0.1
0
o
4
5 C
0
1000
2000
3000
3
4000
5000
0.0
0.2
0.4
0.6
0.8
1.0
1.2
-1
1/ [A ] dm mol
+
-3
[H ] mol dm
Figure 8 Verification rate law (13) (conditions are as in
Figure 7 Verification rate law (6) in the form of Eq. (7)
Table III).
(conditions are as in Table II).
The rate law (6) accommodates all the experi-
mental results except the autocatalytic effect of
The steps shown in Scheme 2 will form the part of
Scheme 1.
(
4-methyl–thiazol-5-yl) acetic acid. The rate law (6)
The evidence for adduct formation was obtained
from UV–vis spectra of both MnO4 and (4-methyl–
thiazol-5-yl) acetic acid and permanganate mixture,
in which a hypsochromic shift of 6 nm from 236 to
30 nm and hyperchromicity at 230 nm, occurred. This
−
may be rearranged to Eq. (7), which is suitable for
verification.
+
[vit. B1]
[H ]
1
2
=
+
(7)
kobs
K1 k1
k1
was also confirmed from the Michaelis–Menten plot
(
Fig. 8). The zero intercept of the plot indicates the
+
−
According to Eq. (7), a plot of [vit B1]/kobs vs. [H ]
should be linear and is found to be so at different tem-
peratures (Fig. 7). The intercept of the plot gave k1 =
weak complex formed between MnO4 and product,
A. Thus formed weak complex is short lived and reac-
tive. Hence, it may be considered as an adduct. Such
adduct formation is observed in the literature [20].
From Scheme 2, the following rate law (10) can be
obtained as follows:
3
−1 −1
s at 25°C. From the values of
2
.53 ± 0.10 dm mol
slope and k1, K1 was calculated, K1 = 0.67 ± 0.05 mol
−3
dm at 25°C. The obtained value of K1 agrees reason-
ably well with the literature [10]. Using these values,
the rates under different experimental conditions were
calculated and compared with experimental data. The
experimental and calculated values agreed reasonably
well (Tables I and II) which confirms Scheme 1.
−
d[MnO ]
4
Rate =
= k2 [adduct] [vit. B1]
dt
k2K1K2 [HMnO4] [A] [vit. B1]
=
(8)
+
[H ]
Autocatalysis
But, the total concentration of permanganate ion is
given
Autocatalysis by one of the products (4-methyl
–
thiazol-5-yl) acetic acid [A] is interesting. The order
of unity in [(4-methyl–thiazol-5-yl) acetic acid] may
be attributed to the weak complex formation between
the product, (4-methyl–thiazol-5-yl) acetic acid, and
oxidant, MnO4 . This is followed by the interaction of
weak complex with vitamin B1 as shown in Scheme 2.
−
−
[
MnO ]t = [HMnO4]f + [MnO ]
4
4
ꢀ
ꢁ
−
K1
=
[HMnO4]f 1 + [
+
H ]
International Journal of Chemical Kinetics DOI 10.1002/kin.20991

2
90
SAVANUR ET AL.
Scheme 2
−
+
k1 K1 [vit. B1]
k2 K1 K2 [A] [vit. B1]
[
MnO ] [H ]
4
kgross =
+
∴
[HMnO4]f =
(9)
+
+
+
(
K + [H ])
(K + [H ])
[
H ] + K1
1
1
(12)
By substituting Eq. (9) in Eq. (10) and omitting the
subscripts, we get
ꢀ
ꢁ
+
[
vit. B1]
1
[H ]
1
=
+
(13)
kautocat
[A] k2 K1 K2
k2 K2
Rate
k2K1K2 [A] [vit B1]
=
kautocat =
(10)
−
+
[
MnO ]
[H ] + K1
At constant concentration of reductant, a plot of LHS
Left hand side) vs.1/[4-methyl-thiazol-5-yl acetic
4
(
acid] (A) of equation [13] should be linear and was
found to be so (Fig. 8).
Thus, when (4-methyl–thiazol-5-yl) acetic acid is
present initially, a composite scheme involving all the
steps of Schemes 1 and 2 operates and the rate law is
given by
The effect of ionic strength and solvent polarity
suggests the reaction between anion and a neutral
molecule, which is in the right direction as given in
Scheme 1. The negative value of ꢀSࣔ indicates the
adduct formed is more ordered than reactants [21].
kgross = kobs + kautocat
(11)
International Journal of Chemical Kinetics DOI 10.1002/kin.20991

AUTOCATALYTIC OXIDATION OF THIAMINE HYDROCHLORIDE (VITAMIN B1) BY PERMANGANATE
291
The observed modest entropy of activation and higher
rate constant of the slow step indicates that the ox-
idation presumably occurs by an inner-sphere mech-
anism. This conclusion is supported by the literature
6. Bettendorff, L.; Wirtzfeld, B.; Makarchikov, A. F.;
Mazzucchelli, G.; Fr e´ d e´ rich, M.; Gigliobianco, T.;
Gangolf, M.; De Pauw, E.; Angenot, L.; Wins, P. Nat
Chem Biol 2007, 3, 211–212.
7
8
. Hiremath, G. A.; Timmangoudar, P. L.; Nandibewoor,
S. T. Transition Met Chem 1996, 21, 560–564.
. Insauti, M. J.; Meta-Perez, F.; Alvaez Machs, M. P. Int
J Chem Kinet 1995, 27, 507–515.
[22].
CONCLUSION
9. Day, M. C.; Selbin, J. Theoretical Inorganic Chemistry;
East West: New Delhi; 1985.
1
1
0. Hassan, R. M. Can J Chem 1991, 69, 2018–2023.
1. Sen, P. K.; Saniyan, A.; Gupta, K. S. Int J Chem Kinet
The reaction between permanganate and thiamine hy-
drochloride, (vitamin B1) is an autocatalytic reaction
in sulfuric acid at room temperature. The autocat-
alytic reaction is due to one of the products, (4-methyl
1
995, 27, 379–389.
1
1
2. Wiberg, K. B. Part A, Oxidation in Organic Chemistry;
Academic Press: New York, 1965.
3. Jeffery, G. H.; Bassett, J.; Mendham, J.; Denney, R. C.
Vogel’s Textbook of Quantitative Chemical Analysis;
ELBS, Longman: Essex, UK, 1996; p 370.
–
thiazol-5-yl) acetic acid, formed in the reaction from
the vitamin B1. The main active species of perman-
−
ganate is MnO4 . The role of hydrogen ion is crucial
to the reaction. The proposed mechanism is consistent
with all the experimental evidences.
14. Joaquin, F.; Perez, B. J Phys Chem A 2011, 115, 9876–
9885.
1
5. Desai, S. M.; Halligudi, N. N.; Nandibewoor, S. T. Tran-
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1
6. Vogel, A. I. Text Book of Macro and Semi Micro Qual-
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International Journal of Chemical Kinetics DOI 10.1002/kin.20991