Kinetics and mechanism of the oxidation of DL-methionine by tetrakis(pyridine)silver dichromate

Article abstract of DOI:10.14233/ajchem.2013.13887

The oxidation of methionine by tetrakis(pyridine)silver dichromate in dimethyl sulphoxide leads to the formation of corresponding sulphoxide. The reaction is of first order with respect to tetrakis(pyridine)silver dichromate. Michaelis-Menten type kinetic

Full text of DOI:10.14233/ajchem.2013.13887

Asian Journal of Chemistry; Vol. 25, No. 5 (2013), 2779-2782
http://dx.doi.org/10.14233/ajchem.2013.13887
Kinetics and Mechanism of the Oxidation of DL-Methionine by Tetrakis(Pyridine)silver Dichromate
1
1
2
1
2
1,*
M. PATEL , L. MATHUR , K. JHA , A. KOTHARI , I. SHASTRI and PRADEEP K. SHARMA
1Department of Chemistry, Jai Narain Vyas University, Jodhpur-342 005, India
2Department of Chemistry, R.D. National College, Mumbai-400 050, India
*Corresponding author: E-mail: drpkvs27@yahoo.com
(Received: 11 April 2012;
Accepted: 23 November 2012)
AJC-12453
The oxidation of methionine by tetrakis(pyridine)silver dichromate in dimethyl sulphoxide leads to the formation of corresponding
sulphoxide. The reaction is of first order with respect to tetrakis(pyridine)silver dichromate. Michaelis-Menten type kinetics was observed
with respect to methionine. The reaction is catalyzed by hydrogen ions. The hydrogen-ion dependence has the form: kobs = a + b [H+]. The
oxidation of methionine was studied in 19 different organic solvents. The solvent effect was analyzed by Kamlet's and Swain's multiparametric
equations. Solvent effect indicated the importance of the cation-solvating power of the solvent. A suitable mechanism has also been
postulated.
Key Words: Dichromate, Kinetics, Mechanism, Methionine, Oxidation.
method. Methionine (Merck) was used as supplied. Due to
non-aqueous nature of the solvent, toluene-p-sulphonic acid
(TsOH) was used as a source of hydrogen ions. Other solvents
were purified by the usual methods9.
INTRODUCTION
Cr(VI) salts have long been used as oxidizing reagents in
synthetic organic chemistry. However these salts are rather
drastic in nature and non-selective oxidants. Further, they are
insoluble in most of the organic solvents. Thus miscibility is a
problem. To overcome these limitations, a large number of
organic derivatives of Cr(VI) have been prepared and used in
organic synthesis as mild and selective oxidants in non-aqueous
solvents1. One of such compounds is tetrakis(pyridine)silver
dichromate reported by Firozabadi et al.2. We have been
interested in the kinetic and mechanistic aspects of the oxida-
tion by complex salts of Cr(VI) and several studies have already
been reported from our laboratory3-6. It is, known however,
that mode of oxidation depends upon the nature of counter-ion
attached to the chromium anion. Methionine (Met), a sulphur-
containing optically active essential amino acid, is reported to
behave differently from other amino acids, towards many oxi-
dants7,8, due to electron-rich sulphur center which is easily
oxidizable. There seems to be no report on the oxidation aspects
of tetrakis(pyridine)silver dichromate (TPSD). Therefore, in
continuation of our earlier work by halochromates, we report
here the kinetics of oxidation of DL-methionine (Met) by
tetrakis(pyridine)silver dichromate in dimethyl sulphoxide
(DMSO) as solvent.A suitable mechanism has also been proposed.
Product analysis: Product analysis was carried out under
kinetic conditions. The oxidation of methionine by tetrakis-
(pyridine)silver dichromate resulted in the formation of corres-
ponding sulphoxide, which was determined by the reported
method10. The yield of sulphoxide was 94 3 %. The oxidation
state of chromium in completely reduced reaction mixtures,
as determined iodometrically, was + 4.
Kinetic measurements: The pseudo-first order conditions
were attained by maintaining a large excess (× 15 or more) of
the methionine over tetrakis(pyridine)silver dichromate. The
solvent was DMSO, unless specified otherwise. The reactions
were followed, at constant temperatures ( 0.1 K), by monito-
ring the decrease in [TPSD] spectrophotometrically at 370 nm.
No other reactant or product has any significant absorption at
this wavelength. The pseudo-first order rate constant, kobs, was
evaluated from the linear (r = 0.990 - 0.999) plots of log [TPSD]
against time for up to 80 % reaction. Duplicate kinetic runs
showed that the rate constants were reproducible to with in
3 %. All experiments, other than those for studying the effect
of hydrogen ions, were carried out in the absence of TsOH.
The second order rate constant, k2, was evaluated from the
relation k2 = kobs/[Met]. Simple and multivariate linear regre-
ssion analyses were carried out by the least-squares method
on a personal computer.
EXPERIMENTAL
Tetrakis(pyridine)silver dichromate was prepared by the
reported method2 and its purity was checked by an iodometric

2780 Patel et al.
RESULTS AND DISCUSSION
Asian J. Chem.
TABLE-1
RATE CONSTANTS FOR THE OXIDATION
OF METHIONINE BY TPSD AT 298 K
Stoichiometry: The oxidation of methionine by tetrakis-
(pyridine)silver dichromate resulted in the formation of the
corresponding sulfoxides. The overall reaction may therefore,
be represented as eqn. (1).
103[TPSD] (mol dm-3)
[Met] (mol dm-3)
−
104kobs (s )
1
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
2.0
4.0
6.0
8.0
1.0
0.10
0.20
0.40
0.60
0.80
1.00
1.50
3.00
0.20
0.20
0.20
0.20
0.20*
3.36
4.98
6.56
3Me-S-R + Cr2O7-2 + 8H+ → 3Me-S-R + 4H2O + 2Cr3+ (1)
7.34
||
O
7.81
8.11
where, R is CH2CH2CH(NH2)COOH
8.56
Rate laws: The reactions were found to be first order with
respect to tetrakis(pyridine)silver dichromate (Fig. 1). In
individual kinetic runs, plots of log [TPSD] versus time were
linear (r2 > 0.995). Further, it was found that the observed rate
constant, kobs, does not depend on the initial concentration of
tetrakis (pyridine) silver dichromate. The order with respect
to methionine was less than one (Table-1). A plot of 1/kobs
versus 1/[Met] was linear with an intercept on the rate ordinate
(Fig. 2). Thus Michaelis-Menten type kinetics were observed
with respect to Met. This leads to the postulation of following
9.06
4.77
5.31
4.05
5.22
5.76*
*contained 0.001 mol dm-3 acrylonitrile
0.35
0.3
overall mechanism (eqns. 2 and 3) and the rate law (4).
K
0.25
0.2
MeSR + TPSD
[Complex]
(2)
0.15
0.1
k2
[Complex]
Rate = k2K [MeSR] [TPSD]/(1 + K [MeSR])
Products
(3)
(4)
0.05
0
The dependence of kobs on the concentration of methionine
was studied at different temperatures and the values of K and
k2 were evaluated from the double reciprocal plots. The thermo-
dynamic parameters for the complex formation and activation
parameters of the disproportionation of the complexes, at 298
K, were calculated from the values of K and k2 respectively at
different temperatures (Tables 2 and 3).
0
5
10
15
1/[Met]
Fig. 2. Oxidation of methionine by tetrakis(pyridine)silver dichromate: A
double reciprocal plot
TABLE-2
FORMATION CONSTANTS AND THERMODYNAMIC
PARAMETERS OF THE OXIDATION OF
TPSD-MET COMPLEXES
2.5
2
K/(dm3 mol−
)
∆H*
∆S*
-∆G*
1
−
1
−
1
−
1
(kJ mol ) (J mol1 K ) (kJ mol )
288 K 298 K 308 K 318 K
1.5
1
6.03
5.35 4.50
3.78 -14.4 0.6
-27
2
6.59 0.5
TABLE-3
RATE CONSTANTS AND ACTIVATION PARAMETERS OF THE
DECOMPOSITION OF TPSD-MET COMPLEXES
0.5
0
−
−
1
1
104k2/(dm3 mol s )
∆H*
∆S*
∆G*
−
−
−
1
1
1
(kJ mol ) (J mol1 K ) (kJ mol )
288 K 298 K 308 K 318 K
0
500
1000
Time (sec.)
1500
4.41
9.63 21.6
45.9 57.1 0.7
-111 2 90.1 0.5
Fig. 1. Oxidation of methionine by tetrakis(pyridine)silver dichromate: A
typical kinetic run
Effect of acidity: The reaction was studied at different
acidities by adding varying amount of toluene-p-sulphonic acid
(TsOH) to the reaction mixtures. The reaction is catalyzed by
hydrogen ions (Table-4). The hydrogen-ion dependence has
the form kobs = a + b [H+]. The values of a and b are 3.39
0.04 × 10-4 s-1 and 6.18 0.06 × 10-4 mol-1 dm3 s-1 respectively
(r2 = 0.9997).
Induced polymerization of acrylonitrile/test for free
radicals: The oxidation of methionine, in an atmosphere of
nitrogen, failed to induce the polymerization of acrylonitrile.
Further, the addition of acrylonitrile had no effect on the rate
of oxidation (Table-1). Therefore, a one-electron oxidation,
giving rise to free radicals, is unlikely. To further confirm the
absence of free radicals in the reaction pathway, the reaction
was carried out in the presence of 0.05 mol dm-3 of 2,6-di-t-
butyl-4-methylphenol (butylated hydroxytoluene or BHT). It
was observed that BHT was recovered unchanged, almost
quantitatively.
Solvent effect: The oxidation of methionine was studied
in 19 organic solvents. The choice of solvents was limited due
to the solubility of tetrakis(pyridine)silver dichromate and its
reaction with primary and secondary alcohols. There was no
reaction with the chosen solvents. The kinetics are similar in
the all the solvents. The values of K and k2 are recorded in

Vol. 25, No. 5 (2013)
Kinetics and Mechanism of the Oxidation of DL-Methionine by Tetrakis(Pyridine)silver Dichromate 2781
Table-5. A perusal of the data shows that the formation constants
do not vary much with the nature of the solvents. However,
the rate constants, k2 varied considerably with the solvents. The
rate constants for oxidation, k2, in eighteen solvents (CS2 was
not considered, as the complete range of the solvent parameters
was not available) were correlated in terms of the linear solvation
energy relationship eqn. (6) of Kamlet et al.11
The data on the solvent effect were analyzed in terms of
Swain's equation13 of cation- and anion-solvating concept of
the solvents as well.
log k2 = aA + bB + C
(11)
HereA represents the anion-solvating power of the solvent
and B the cation- solvating power. C is the intercept term. (A
+ B) is postulated to represent the solvent polarity. The rates
in different solvents were analyzed in terms of eqn. (11), sepa-
rately with A and B and with (A + B).
log k2 = A0 + pπ* + bβ + aα
(6)
In this equation, π* represents the solvent polarity, β the
hydrogen bond acceptor basicities and α is the hydrogen bond
donor acidity. A0 is the intercept term. It may be mentioned
here that out of the 18 solvents, 12 have a value of zero for α.
The results of correlation analyses in terms of eqn. (6), a
biparametric equation involving π* and β and separately with
π* and β are given below.
log k2 = 1.19 ( 0.04) A + 1.45 ( 0.03) B - 3.65 (12)
R2 = 0.9960; sd = 0.03; n = 19; ψ = 0.07
log k2 = 0.99 ( 0.48) A - 2.65
r2 = 0.2013; sd = 0.50; n = 19; ψ = 0.92
log k2 = 1.36 ( 0.21) B - 3.26
r2 = 0.7054; sd = 0.24; n = 19; ψ = 0.56
log k2 = 1.37 0.04 (A + B) - 3.64
r2 = 0.9867; sd = 0.50; n = 19; y = 0.12
(13)
(14)
(15)
log k2 = -4.48 + 1.59 ( 0.16)π* + 0.15 ( 0.13)β + 0.29 ( 0.12)α (7)
R2 = 0.8966; sd = 0.14; n = 18; ψ = 0.35
log k2 = - 4.41 + 1.49 ( 0.17)π* + 0.25 ( 0.14)β (8)
R2 = 0.8568; sd = 0.16; n = 18; ψ = 0.40
The rates of oxidation of methionine in different solvents
show an excellent correlation with Swain's equation with both
the cation- and anion-solvating powers playing significant
roles, though the contribution of the cation-solvation is slightly
more than that of the anion-solvation. The solvent polarity,
represented by (A + B), also accounted for ca. 99 % of the
data. However, the correlations individually withA and B were
poor. In view of the fact that solvent polarity is able to account
for ca. 98 % of the data, an attempt was made to correlate the
rate with the relative permittivity of the solvent. However, a
plot of log k2 against the inverse of the relative permittivity is
not linear (r2 = 0.4958; sd = 0.31; ψ = 0.73).
The observed solvent effect points to a transition state
more polar than the reactant state. Further, the formation of a
dipolar transition state, similar to those of SN2 reactions, is
indicated by the major role of both anion- and cation-solvating
powers. However, the solvent effect may also be explained
assuming that the oxidant and the intermediate complex exist
as ion-pair in non-polar solvent like cyclohexane and be
considerably dissociated in more polar solvents.
log k2 = - 4.35 + 1.57 ( 0.18)π*
r2 = 0.8260; sd = 0.65; n = 18; ψ = 0.43
log k2 = - 3.54 + 0.52 ( 0.33)β
(9)
(10)
r2 = 0.1367; sd = 0.39; n = 18; ψ = 0.96
Here n is the number of data points and ψ is the Exner's
statistical parameter12.
TABLE-4
DEPENDENCE OF THE REACTION RATE ON
HYDROGEN-ION CONCENTRATION
−
−
3
3
Temp. 298 K
[Met] 0.10 mol dm
[TsOH]/ mol dm−3
104 kobs/s-1
[TPSD] 0.001 mol dm
0.10 0.20 0.40 0.60 0.80
3.96 4.68 5.85 7.11 8.37
1.00
9.54
TABLE-5
EFFECT OF SOLVENTS ON THE OXIDATION
OF METIONINE BY TPSD AT 298 K
Solvents
K (dm3mol-1)
5.42
5.49
6.03
5.35
4.47
6.12
5.27
3.89
5.49
5.58
4.65
5.29
5.51
4.67
5.85
4.89
5.04
4.95
5.36
105 k2 (s-1)
Chloroform
1,2-Dichloroethane
Dichloromethane
DMSO
77.6
87.1
72.4
216
Mechanism
In view of the absence of any effect of radical scavenger,
acrylonitrile, on the reaction rate and recovery of unchanged
BHT, it is unlikely that a one-electron oxidation giving rise to
free radicals, is operative in this oxidation reaction. The
observed Michaelis-Menten type of kinetics observed with
respect to methionine, suggests the formation of 1:1 complex
of tetrakis(pyridine)silver dichromate and Met in a rapid
pre-equilibrium (Scheme-I). With present set of data, it is
difficult to state the definite nature of the intermediate complex.
The experimental results can be accounted for in terms of electro-
philic attack of methionine-sulphur at the metal via an inter-
mediate complex. Transfer of an unshared pair of electrons to
an empty d orbital of the metal resulted in the formation of a
coordinate bond. The formation of intermediate is likely to
undergo a further rapid reaction in which the incipient.
It is of interest to compare here the mode of oxidation of
methionine by pyridinium fluorochromate (PFC)14, pyridinium
chlorochromate (PCC)15, pyridinium bromochromate (PBC)16
and tetrakis(pyridine)silver dichromate. The oxidation by PFC
and PBC presented a similar kinetic picture, i.e. the reactions
Acetone
63.1
117
DMF
Butanone
51.3
95.5
22.4
2.81
18.6
83.2
33.9
41.7
38.9
22.9
11.2
45.7
30.9
Nitrobenzene
Benzene
Cyclohexane
Toluene
Acetophenone
THF
t-butylalcohol
1,4-Dioxane
1,2-Dimethoxyethane
CS2
Acetic acid
Ethyl acetate
Kamlet's11 triparametric equation explains ca. 89 % of the
effect of solvent on the oxidation. However, by Exner's13 criterion
the correlation is not even satisfactory (cf. eqn. 7). The major
contribution is of solvent polarity. It alone accounted for ca. 83
% of the data. Both β and α play relatively minor roles.

2782 Patel et al.
Asian J. Chem.
O
Me
O
K
Me
R
CrO2OAgPy2
+
[AgPy2]2Cr2O7
S
S
Cr
O
OAgPy2
R
O
O
Me
CrO2OAgPy2
Cr
O
S
R
OAgPy2
Slow
+
[CrO2AgPy2]
[CrO3O(AgPy2)]
Me
S
R
+
O
+
[AgPy2]2Cr2O7
Me
S
R
••
R
Me
Me
••
S
O
S
O
R
O
CrO2AgPy2
O
CrO2AgPy2
Product
OR
O
Cr
O
Cr
OAgPy2
OAgPy2
Scheme-I
3. P. Swami, D.Yajurvedi, P. Mishra and P.K. Sharma, Int. J. Chem. Kinet.,
42, 50 (2010).
are of first order with respect to the reductants. While in the
oxidation by PCC and tetrakis(pyridine)silver dichromate,
Michaelis-Menten type kinetics was observed with respect to
the reductants. It is possible that the values of the formation
constants for the reductant-PFC/PBC complexes are very low.
This resulted in the observation of second-order kinetics. No
explanation of the difference is available presently. Solvent
effects and the dependence of the hydrogen ions are of similar
nature in all these reactions, for which essentially similar
mechanisms have been proposed.
4. K. Vadera, D. Yajurvedi, P. Purohit, P. Mishra and P.K. Sharma, Prog.
React. Kinet. Mech., 35, 265 (2010).
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32, 315 (2011).
6. D. Sharma, P. Panchariya, S. Vyas, L. Kotai and P.K. Sharma, Int. J.
Chem., 1, 29 (2012).
7. D.S. Mahadevappa, S. Ananda, N.M.M. Gouda and K.S. Rangappa, J.
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8. S. Mittal, V. Sharma and K.K. Banerji, J. Chem. Res. (S), 264 (1986).
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ACKNOWLEDGEMENTS
Thanks are due to the UGC, New Delhi for financial support
as BSR, One Time Grant No. F.4-10/2010(BSR) dated
07.03.2012 and to Prof. K.K. Banerji for critical suggestions.
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