Oxidation of threonine by the analytical reagent diperiodatocuprate(III) - An autocatalysed reaction

Article abstract of DOI:10.1016/j.molstruc.2006.05.015

The kinetics of Cu(II) autocatalysed oxidation of threonine by well-recognized analytical reagent diperiodatocuprate(III) in aqueous alkaline medium at a constant ionic strength of 0.5 mol dm^-3 was studied spectrophotometrically. The reaction between diperiodatocuprate(III) and threonine in alkaline medium exhibits 2:1 stoichiometry (DPC: threonine). The reaction is of first order in each [DPC] and [threonine] and less than unit order in [alkali]. Periodate has retarding effect on the rate of reaction. Ionic strength has negligible effect on the reaction. Increase in dielectric constant of the medium with decrease in the rate of the reaction was observed. The product, Cu(II), catalyses the reaction with a fractional order. The main products were identified by spot test and I.R. A composite mechanism involving the monoperiodatocuptrate(III) (MPC) as the reactive species of the oxidant in uncatalysed and autocatalysed reaction has been proposed. Activation parameters and the reaction constants involved in the different steps of the mechanisms are calculated.

Full text of DOI:10.1016/j.molstruc.2006.05.015

Journal of Molecular Structure 827 (2007) 137–144
www.elsevier.com/locate/molstruc
Oxidation of threonine by the analytical reagent
diperiodatocuprate(III) – An autocatalysed reaction
*
Timy P. Jose, Suresh M. Tuwar
Department of Chemistry, Karnatak Science College, Dharwad 580001, India
Received 9 March 2006; received in revised form 1 May 2006; accepted 10 May 2006
Available online 21 June 2006
Abstract
The kinetics of Cu(II) autocatalysed oxidation of threonine by well-recognized analytical reagent diperiodatocuprate(III) in aque-
ous alkaline medium at a constant ionic strength of 0.5 mol dmꢀ3 was studied spectrophotometrically. The reaction between dipe-
riodatocuprate(III) and threonine in alkaline medium exhibits 2:1 stoichiometry (DPC: threonine). The reaction is of first order in
each [DPC] and [threonine] and less than unit order in [alkali]. Periodate has retarding effect on the rate of reaction. Ionic strength
has negligible effect on the reaction. Increase in dielectric constant of the medium with decrease in the rate of the reaction was
observed. The product, Cu(II), catalyses the reaction with a fractional order. The main products were identified by spot test and
I.R. A composite mechanism involving the monoperiodatocuptrate(III) (MPC) as the reactive species of the oxidant in uncatalysed
and autocatalysed reaction has been proposed. Activation parameters and the reaction constants involved in the different steps of
the mechanisms are calculated.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Kinetics; Oxidation; Autocatalysis; Threonine
1. Introduction
one-electron oxidant for the oxidation of various organ-
ic compounds in alkaline medium and its use as an ana-
lytical reagent is now well recognized [10] and also used
in the estimation of amino acids. Cu(III) is shown to be
an intermediate in the Cu(II)-catalysed oxidation of
amino acids by peroxidisulphate [11]. The use of DPC
as an oxidant in alkaline medium may not be new
but it is restricted to a few cases [12] due to the fact
of its limited stability in aqueous medium. Moreover,
when the DPC complex is oxidant, since multiple equi-
libria between the different copper(III) species are
involved, it needs to be known which of species is the
active oxidant.
Amino acids not only act as building blocks in protein
synthesis but also play significant role in metabolism. Thre-
onine is one of the 20 different kinds of a-amino acids
found in proteins in all living organisms. It is required
for synthesis of proteins to maintain normal growth and
nitrogen balance in the body. In glycoproteins, the
Non-linear chemical phenomena that have been found
in various chemical reactions [1–6] in the catalytic
autoxidation of various substrates by different oxidants
are reported. It has also been reported that the oxidation
of certain biological substrates and polyphenols by air
oxygen catalysed by copper(II) and iron(II) coordination
compounds may exhibit transient oscillations in a batch
reactor [7,8].
The periodate and tellurate complexes of copper in
its trivalent state have been extensively used in the anal-
ysis of several organic compounds. The kinetics of self-
decomposition of these complexes were studied in some
detail. Movius [9] reported the reactivity of few alcohols
with diperiodatocuprate(III) (DPC). DPC is a versatile
*
Corresponding author.
E-mail address: sureshtuwar@yahoo.co.in (S.M. Tuwar).
0022-2860/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.05.015

138
T.P. Jose, S.M. Tuwar / Journal of Molecular Structure 827 (2007) 137–144
carbohydrates are covalently linked by O-glycoside bonds
to threonine residues of proteins.
250 cm3 with distilled water. The solution obtained
was found fairly stable in room temperature for several
months. The complex was characterized by its UV/
Visible spectrum, which exhibits a broad absorption
band at 415 nm. The aqueous solution of DPC was
standardized by iodometric titration and gravimetrically
by thiocynate [25] a method.
The Cu(II) solution was made by dissolving the
known amount of copper sulphate (BDH) in distilled
water. Periodate solution was prepared by weighing out
the required amount of sample in hot water and it was
kept for 24 h. Its concentration was ascertained iodomet-
rically [26] at neutral pH maintained by phosphate buff-
er. Since periodate is present in excess in DPC, the
possibility of oxidation of threonine by periodate in
alkaline medium at 25 ꢂC was tested, by following the
reaction iodometrically. It was found that there was no
significant reaction under the experimental conditions
employed compared to the DPC oxidation of threonine.
KOH and KNO3 (BDH, AR) were employed to
maintain the required alkalinity and ionic strength,
respectively, in reaction solutions.
Amino acids have been oxidized by a variety of
reagents [13], under different experimental conditions.
Although several types of organic [14] and inorganic
[15,16] substrates are oxidized by DPC in aqueous alka-
line medium, only a few reports exist on the oxidation of
threonine in aqueous alkaline medium. The oxidation of
amino acids is of interest as the oxidation products differ
for different oxidants [17,18]. In many cases amino acids
undergo oxidative decarboxylation and deamination.
Jayaprakash Rao et al. [19] suggested that oxidation of
ꢁ–amino acids by two electron oxidant such as diperio-
datoargentate(III) in alkaline medium involves two elec-
tron transfer, from the amino acid to the oxidant to
give iminoacid intermediate in the rate determining step.
This intermediate subsequently undergoes hydrolysis to
yield keto acids. One electron oxidants such as ceric sul-
phate [20], peroxomonosulphate [21] and hexacynofer-
rate(III) [22] also oxidize amino acids to keto acids.
But other studies with amino acids report, the oxidation
products as the corresponding aldehydes [23]. Thus, the
study of amino acids becomes important because of spec-
ificity of their oxidative products, biological significance
and selectivity towards the oxidants. There is no report
in the literature on the oxidation of threonine by DPC
autocatalysed by Cu(II). To understand more about the
autocatalysis of Cu(II) in oxidation by DPC and in order
to explore the mechanism of oxidation by Cu(III) ion in
aqueous alkaline medium, we have selected threonine as
a substrate for oxidation. The present study deals with
the title reaction to investigate the redox chemistry of
Cu(III) and to arrive at a plausible mechanism.
2.3. Kinetic measurements
The oxidation of thereonine by DPC was followed
under pseudo-first order conditions where [thereonine]
was excess over [DPC] at 25 0.1 ꢂC. The reaction was
initiated by mixing the required quantities of previously
thermostatted solution of threonine and DPC, which also
ꢀ
contained definite quantities of KOH, KNO3 and IO4
to maintain the required alkalinity, ionic strength and
periodate, respectively. Here, the total concentration of
hydroxide ion was calculated considering the KOH in
DPC as well as the KOH additionally added. Similarly,
the total metaperiodate concentration was calculated by
considering metaperiodate present in solution of DPC
and additionally added. The course of reaction was fol-
lowed by measuring the absorbance of unreacted DPC
in the reaction mixture in a 1 cm quartz cell of a thermo-
statted compartment of a Hitachi model 150–20 spectro-
photometer at its absorbance maximum, 415 nm, as a
function of time. Earlier, it was verified that there is a
negligible interference from other species present in the
reaction mixture at this wavelength. The obedience of
absorbance by DPC to Beer’s law at 415 nm was verified
earlier and the molar absorbance coefficient, ‘e’, was
found to be 6250 50 dm3 molꢀ1 cmꢀ1 at this wave-
length. The first order rate constants, kobs, were obtained
from the plots of log (a ꢀ x) vs. time, where ‘a’ and ‘x’
are the initial concentration and change in concentration
of DPC at time ‘t’, respectively. The plots were linear up
to about 25% completion of the reaction (non-linearity
above 25% was due to the autocatalysis, explained else-
where) and the rate constants were reproducible within
5%. The spectral changes during the reaction are
shown in Fig. 1(A), in which [DPC] decreases at
2. Experimental
2.1. Materials and reagents
All chemicals used were of reagent grade. Double dis-
tilled water was used throughout the work. Stock solution
of Threonine (Lobo Chemie) was prepared by dissolving
the appropriate amount of sample in water. The purity of
the sample was checked by TLC and its m.p. 253 ꢂC (Lit.
value 256 ꢂC).
2.2. Preparation of DPC
The diperiodatocuprate(III), (DPC) is prepared [24]
by oxidizing Cu(II) in the alkaline medium. Copper sul-
phate (3.54 g), potassium metaperiodate (6.8 g), potassi-
um persulphate (2.2 g) and KOH (9 g) were added in
250 cm3 water. The mixture was heated to boiling for
about 20 min on a hot plate with constant stirring.
The boiling mixture turned intensely red and the boiling
was continued for another 20 min for the completion of
the reaction. The mixture was then cooled, filtered
through sintered glass crucible (G-4) and diluted to

T.P. Jose, S.M. Tuwar / Journal of Molecular Structure 827 (2007) 137–144
139
The main reaction products were identified as aldehyde
[27] by a spot test, ammonia [25(b)] by Nessler’s reagent
and Cu(II) by its spot test [28]. The CO2 evolved is test-
ed by bubbling nitrogen gas through a tube containing
lime water [29] after acidification. The quantitative esti-
mation of aldehyde as its 2,4-DNP derivative was nearly
80% and it also was confirmed by its I.R. spectrum
which showed [30] a band at 2919 cmꢀ1 due to aldehydic
C–H stretching. It was further observed that the
aldehyde does not undergo further oxidation under the
prevailing kinetic conditions. Tests for the corresponding
acid were negative.
1.0
0.8
0.6
0.4
0.2
0.0
A
320
370
420
470
520
Wave Length (λ) (nm)
3.2. Reaction order
1.0
0.8
0.6
0.4
0.2
0.0
B
The order with respect to [threonine], [alkali] and [perio-
date] was determined by log kobs vs. log concentration plots
and the obtained orders were also confirmed by differential
method by the plot log (initial rate) vs. log concentration
using the equation log(rate) = logk + n logc; these orders
were obtained by varying the concentration of DPC, thre-
onine, periodate and alkali in turn while keeping others
constant.
The concentration of DPC was varied in the range,
2.0 · 10ꢀ5–2.0 · 10ꢀ4 mol dmꢀ3 at fixed [threonine],
[OHꢀ], ½IO4ꢀꢁ and ionic strength. The non-variation in
the pseudo-first order rate constants at various concen-
trations of DPC indicates the order in [DPC] as unity
(Table 1). This was also confirmed from the linearity
of plots of log absorbance vs. time (r > 0.9994,
S 6 0.026) up to 25% completion of the reaction. The
substrate, [threonine], was varied in the range of
5.0 · 10ꢀ4–5.0 · 10ꢀ3 mol dmꢀ3 at 25 ꢂC keeping all other
reactant concentrations constant (Table 1). The kobs val-
ues were increased with increase in concentration of thre-
onine and its order was found to be unity (Table 1). The
effect of [alkali] on the rate of reaction was studied at
(1)
(2)
(3)
0.0
1.0
2.0
3.0
4.0
Time (min.)
Fig. 1. (A) Spectroscopic changes of DPC during the reaction period in
the Cu(II) autocatalysed oxidation of threonine by diperiodatocup-
rate(III) at 25 ꢂC (scanning interval 30 s). [DPC] = 1.0 · 10ꢀ4
[TRE] = 1.0 · 10ꢀ3, [OHꢀ] = 0.1, ½IO4ꢀꢁ = 1.0 · 10ꢀ3, I = 0.5/mol dmꢀ3
,
.
(B) Autocatalysis of Cu(II) on oxidation of threonine by alkaline DPC at
25 ꢂC at various [Cu(II)]. [DPC] = 1.3 · 10ꢀ4, [TRE] = 1.0 · 10ꢀ3, [OHꢀ]
= 0.1, ½IO4ꢀꢁ = 1.0 · 10ꢀ3, I = 0.5, Cu(II) = 1 (1 · 10ꢀ6), 2 (2 · 10ꢀ6), 3
(4 · 10ꢀ6)/mol dmꢀ3
415 nm during the reaction. The autocatalysis by Cu(II)
is also shown in Fig. 1(B).
ꢀ
constant concentrations of threonine, DPC, IO4 and
3. Results and discussions
ionic strength at 0.5 mol dmꢀ3. The rate constants were
increased with increase in [alkali] and the order was
found to be less than unity (Table 1).
3.1. Stoichiometry and product analysis
Different sets of reaction mixtures containing different
concentrations of threonine and DPC at constant ionic
strength and [alkali] were kept for ca. 6 h at 25 0.1 ꢂC
in an inert atmosphere and in a closed vessel. When
[DPC] was higher than [threonine], the unreacted DPC
was estimated spectrophotometrically by measuring the
absorbance at 415 nm. The results indicated that two moles
of DPC consumed one mole of threonine as in Eq. (1).
3.3. Effect of periodate
The effect of ½IO4ꢀꢁ was observed by varying the concen-
tration from 5.0 · 10ꢀ4 to 5.0 · 10ꢀ3 mol dmꢀ3 at constant
concentrations of DPC, threonine, alkali and constant ion-
ic strength (Table 1). It was found that the added periodate
retarded the rate, and order in periodate was inverse
fractional.

140
T.P. Jose, S.M. Tuwar / Journal of Molecular Structure 827 (2007) 137–144
Table 1
Table 2
Effect of variation of [DPC], [TRE], [OHꢀ] and ½IO4ꢀꢁ on autocatalysed
*
%
D
kobs · 102 (sꢀ1
)
oxidation of threonine by [DPC] at 25 ꢂC, I = 0.5 mol dmꢀ3
(A) Effect of solvent polarity on the oxidation of threonine by DPC at 25 ꢂC
[DPC] · 104 [TRE] · 103 [OHꢀ] · 10 ½IO4ꢀꢁ · 103 kobs · 102 (sꢀ1
)
5.0
10.0
15.0
20.0
25.0
74.9
71.3
67.7
64.0
60.4
1.86
2.00
2.14
2.26
2.60
(mol dmꢀ3
)
(mol dmꢀ3
)
(mol dmꢀ3
)
(mol dmꢀ3
)
Exptl.a Calc.a
0.2
0.4
0.8
1.0
2.0
3.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
a
1.0
1.0
1.0
1.0
1.0
1.0
0.5
0.8
1.0
2.0
4.0
5.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
0.8
1.0
2.0
4.0
5.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
0.8
1.0
2.0
4.0
5.0
1.84
1.87
1.84
1.85
1.85
1.86
0.99
1.53
1.85
3.75
7.67
9.41
1.29
1.69
1.85
2.51
3.07
3.16
2.62
2.29
1.85
1.36
0.75
0.69
1.91
1.91
1.91
1.91
1.91
1.91
0.96
1.53
1.91
3.82
7.65
9.56
1.27
1.70
1.91
2.54
3.04
3.17
2.55
2.12
1.91
1.28
0.77
0.64
[Cu(II)] · 106 (mol dmꢀ3
)
kobs(autocat) · 102 (sꢀ1
)
kcal · 102 (sꢀ1
)
(B) Effect of added product, Cu(II), on oxidation of threonine by alkaline
DPC
1.0
2.0
4.0
6.0
8.0
2.01
2.26
2.66
2.95
3.21
3.47
2.07
2.23
2.55
2.87
3.19
3.51
10
[DPC] = 1.0 · 10ꢀ4
;
[TRE] = 1.0 · 10ꢀ3
;
[OHꢀ] = 0.1; ½IO4ꢀꢁ = 1.0 ·
10ꢀ3; I = 0.5/mol dmꢀ3
.
*
% of t-butyl alcohol and water.
1/[TRE] x 103 (dm3 mol-1)
0.0
0.5
1.0
1.5
2.0
2.5
1.6
1.2
0.8
0.4
0.0
1.2
0.9
0.6
0.3
0.0
Experimental and calculated.
k
obs
3.4. Effect of ionic strength and solvent polarity
osb
k
The effect of ionic strength was studied by varying the
potassium nitrate concentration in the reaction medium.
The ionic strength of the reaction medium was varied from
0.05 to 0.5 mol dmꢀ3 at constant [DPC], [threonine], ½IO4ꢀꢁ
and [alkali]. It was found that the ionic strength has negli-
gible effect on the rate of the reaction.
0.0
5.0
10.0
15.0
20.0
25.0
-
- -[IO4 ] x 10-3 (- -)1/[OH-] (dm3 mol-1)
The relative permittivity (eT) effect was studied by vary-
ing the t-butyl alcohol–water content in the reaction mix-
ture with all other conditions being constant. Attempts to
measure the relative permittivities of the mixture of t-butyl
alcohol–water were not successful. However, they were
computed from the values of pure liquids [31]. It was also
found that there was no reaction of the solvent with the
oxidant under the experimental conditions used. The kobs
values were decreased with the increase in the dielectric
constant of the medium (Table 2(A)). The plot of log kobs
vs. 1/eT was linear (r P 0.9989, S 6 0.0122) (Fig. 2) with
positive slope.
Fig. 2. Verification of rate law (Eq. (5)) in the form of Eq. (4) for Scheme
1, on oxidation of threonine by DPC at 25 ꢂC (Conditions as in Table 1).
3.6. Test for free radicals
To test the intervention of free radicals, the reaction
mixture was mixed with acrylonitrile monomer and kept
for 24 h under nitrogen atmosphere. On dilution with meth-
anol, white precipitate of polymer was formed, indicating the
intervention of free radicals in the reaction. The blank exper-
iment of either DPC or threonine in acrylonitrile alone did
not induce polymerization under the same condition as those
induced with reaction mixture. Initially added acrylonitrile
decreases the rate indicating the free radical intervention,
which is the case in earlier work [32].
3.5. Effect of initially added products
Effect of initially added product Cu(II) was studied in
the [Cu(II)] range 1.0 · 10ꢀ6–1.0 · 10ꢀ5 mol dmꢀ3, when
all other reactant concentrations were kept constant. The
initially added Cu(II) enhanced the rate of reaction (Table
2(B)) with an order ꢂ0.25. However, the other product,
2-hydroxy propanal, did not affect the rate of the reaction.
3.7. Effect of temperature
The rate of reaction was measured at different tempera-
tures at constant concentrations of reactants and other

T.P. Jose, S.M. Tuwar / Journal of Molecular Structure 827 (2007) 137–144
141
conditions being constant. The rate of reaction increased
with the increase of temperature. The activation energy
was calculated from the slope of the plot of log kobs vs.
1/T (r P 0.9948) and used to calculate other activation
parameters (Table 3).
per(III) periodate complex. However, the reports reveal
that the actual molecular formulae of DPC and MPC are
not known. Based on the active form of periodate, enhanc-
ing effect of [OHꢀ] and retarding effect of ½IO4ꢀꢁ, the plau-
sible forms of MPC and DPC are given in Eqs. (2) and (3).
The water soluble Cu(III) periodate is to be existed [33]
in the form of Na7KH(Cu(IO6)2). However, periodate
under pH >12 has not been existing as IO65ꢀ; its form
[34] is purely depending on the pH of the medium. There
K1
½CuðH3IO6Þ ꢁꢀ þ OHꢀ ꢀ ½CuðH2IO6ÞðH3IO6Þꢁ2ꢀ þ H2O
2
ð2Þ
K2
½CuðH2IO6ÞðH3IO6Þꢁ2ꢀ þ 2H2O ꢀ CuðH2IO6ÞðH2OÞ ꢁ
ꢀ
2ꢀ
are H5IO6 and H4IO6 at pH below 7, H3IO6
and
2
3ꢀ
2ꢀ
H2IO6 of pH above 7, in addition to these polymeric
þ ½H3IO6ꢁ
ð3Þ
forms at higher ½IO4ꢀꢁ. Thus, in the present study, the
In Eq. (3), bidentate ligand, H3IO62ꢀ, has been replaced by
water molecules to give diaquomonoperiodato copper(III)
complex (MPC). The similar forms of complexes have been
reported for Nickel(IV) periodate [37] and silver(III)
periodate [38] complexes.
The test for intervention of free radicals during the reac-
tion and first order each in oxidant, DPC and reductant,
threonine is a simple second order reaction, determined
from the kobs values at the initial state of reaction and by
considering the active species of Cu(III) as MPC, the
results can be accommodated as in Scheme 1.
2ꢀ
3ꢀ
periodate may be in the form of H3IO6 or H2IO6 and
may be both as pH > 11. On incorporating the periodate
species in the copper(III) periodate, it can be written
as [Cu(H3IO6)2]ꢀ. However, Lister [35] proposed
the copper(III) periodate into three forms in alkaline
medium diperiodatocuprate(III) (DPC), monoperiodato-
cuprate(III) (MPC) and tetrahydroxocuprate(III). The lat-
ter is ruled out [36] as its equilibrium constant [35] is
8.0 · 10ꢀ11 at 40 ꢂC. Hence, in the present study DPC
and MPC are to be considered as active forms of cop-
In Scheme 1, the active form of Cu(III) periodate, the
monoperiodatocuprate(III), reacts with threonine in a slow
step to give a free radical by the cleavage of a-hydrogen
atom of threonine. This free radical reacts with another
molecule of MPC in a fast step to give products. Scheme
1 leads to the following rate law (4):
Table 3
Effect of temperature on Cu(II) autocatalysed oxidation of threonine by
alkaline diperiodatocuprate(III)
Temp. (K)
kobs · 102 (sꢀ1
)
(A) Effect of temperature
298
303
308
313
1.85
2.59
3.59
4.84
d½DPCꢁ
ꢀ
¼ rate1
dt
kuK1K2½CuðIIIÞꢁ ½ OHꢀꢁ½TREꢁ
T
T
(B) Activation parameters
Ea (kJ molꢀ1
¼
½H3IO2ꢀꢁ þ K1½OHꢀꢁ½H3IO2ꢀꢁ þ K1K2½OHꢀꢁ
)
48.7 0.5
46.2 0.5
6
6
DH# (kJ molꢀ1
)
ð4Þ
DS# (J Kꢀ1 molꢀ1
)
ꢀ122
2
2
DG# (kJ molꢀ1
)
83
kuK1K2½TREꢁ½OHꢀꢁ
T
kobs
¼
ð5Þ
½H3IO2ꢀꢁ þ K1½OHꢀꢁ½H3 IO2ꢀꢁ þ K1K2½OHꢀꢁ
Log A
6.8 0.2
6
6
[DPC] = 1.0 · 10ꢀ4
;
[TRE] = 1.0 · 10ꢀ3
.
;
[OHꢀ] = 0.1; ½IO4ꢀꢁ = 1.0 ·
Eq. (5) can be verified in the following form:
10ꢀ3; I = 0.5 /mol dmꢀ3
K1
K2
[Cu(H2IO6)(H3IO6)]2-
[Cu(H3IO6)2]-
+ OH-
H2O
+
Cu(H2IO6)(H2O)2] + [H3IO6]2-
[Cu(H2IO6)(H3IO6)]2- + 2H2O
H
C
.
ku
Cu2+
+
H3C CH
OH
C
[Cu(H2IO6)(H2O)2]
H3C CH
O
+
C
+
NH3
O-
OH
NH2
3-
CO2
H2IO6
+
+
.
OH-
H3C CH
OH
C
H3C CH
[Cu(H2IO6)(H2O)2]
C
O
+
+
H
+ NH4OH
Fast
+
NH3
OH
Cu2+
3-
2H2O
+
+ H2IO6
+
Scheme 1. Mechanism with respect to reactive species.

142
T.P. Jose, S.M. Tuwar / Journal of Molecular Structure 827 (2007) 137–144
½H3IO2ꢀꢁ
½H3IO2ꢀꢁ
1
also due to -I effect of -OH group present in threonine
which induces a greater positive charge on the nitrogen
atom facilitating complex formation. Autocatalysis in
DPC oxidations has not been reported so far. It may be
a first of its kind in Cu(III) oxidations.
1
6
6
¼
þ
þ
ð6Þ
kobs kuK1K2½TREꢁ½OHꢀꢁ kuK2½TREꢁ ku½TREꢁ
The plot of 1/kobs versus [H3IO6]2ꢀ, 1/[TRE] and 1/[OHꢀ]
should be linear and found so as in Fig. 2. The slopes and
intercepts of such plots give the ku, K1 and K2 as
The diamagnetic, dsp2, square planar structure of cop-
per(III) periodate in the form of DPC, MPC and paramag-
netic dsp2 complex of Cu(II) and threonine can be shown
below.
38.1 dm3 molꢀ1 sꢀ1
,
0.2 dm3 molꢀ1 and 5.05 · 10ꢀ2
mol dmꢀ3, respectively. These data are used to reproduce
the kobs for different experimental conditions. The ‘k’
calculated and kobs are reasonably in agreement with each
other, which fortifies the mechanism (Table 1).
Rate law for Scheme 2 is derived by using Eqs. (2) and
(3) for active form of copper(III) periodate as in Eq. (7),
The autocatalysis by one of the products, Cu(II), and
first order each in DPC and threonine may be depicted as
in Scheme 2.
d½DPCꢁ
ꢀ
¼ rate2
dt
kaK1K2K3½CuðIIIÞꢁ ½TREꢁ ½OHꢀꢁ½CuðIIÞꢁ
Scheme 2 is in accordance with the fractional order in
[Cu(II)] and kinetic evidence for complex formation
between threonine and Cu2+ from the Michaelis–Menten
plot of 1/kobs vs. 1/[Cu(II)]. The complexes between mono
and bivalent states of copper and amines and amino acids
are quite common examples. The complexes of copper in
trivalent state with amines and amino acids are rare [39]
and appearing only as a transient species. Such complexes
have been proposed as intermediates in Cu(II) catalysed
oxidation of amines by hydrogen peroxide. Hence, in the
present study, the complex between Cu(II) and the amino
acid, threonine, is in expected direction and it is also evi-
denced by the hypsochromic shift of Cu(II) from 670 to
617 nm in presence of threonine. This may be more active
than the threonine alone where there is unit order each in
DPC and threonine for an outersphere mechanism for
the transfer of electrons from reductant to oxidant (Scheme
1). The higher rate observed for autocatalysis of Cu(II) is
T
T
¼
½H3IO2ꢀꢁ þ K1½OHꢀꢁ½H3IO2ꢀꢁ þ K1K2½OHꢀꢁ
6
6
ð7Þ
The composite rate law for Schemes 1 and 2 using Eqs. (4)
and (7) leads to Eq. (8).
Ratetotal ¼ rate1 þ rate2
K1K2½CuðIIIÞꢁ ½TREꢁ ½OHꢀꢁ
T
T
¼
½H3IO2ꢀꢁ þ K1½ OHꢀꢁ½H3IO2ꢀꢁ þ K1K2½OHꢀꢁ
6
6
ꢃ ½ku þ K3ka½CuðIIÞꢁꢁ
ð8Þ
K1K2½TREꢁ ½OHꢀꢁ
T
kobsðautocatÞ
¼
½H3IO2ꢀꢁ þ K1½ OHꢀꢁ½H3IO2ꢀꢁ þ K1K2½OHꢀꢁ
6
6
ꢃ ½ku þ K3ka½CuðIIÞꢁꢁ
ð9Þ
H
K3
Cu2+
H3C CH
OH
O
C
C
+
Complex(C)
O-
NH2
.
ka
3-
2Cu2+
Complex(C)
H3C CH
OH
C
[Cu(H2IO6)(H2O)2]
[Cu(H2IO6)(H2O)2]
+
H2IO6
+
+
Slow
+
NH3
2H2O + CO2
.
H3C CH
OH
C
H3C CH
OH
C
O
H
NH4OH
+
+
+ 2OH-
Fast
+
NH3
3-
+ Cu2+
H2IO6
+
+ 2H2O
Scheme 2. Mechanism with species including autocatalysis.

T.P. Jose, S.M. Tuwar / Journal of Molecular Structure 827 (2007) 137–144
143
[3] I.R. Epstein, K. Showalter, J. Phys. Chem. 100 (1996) 13132.
Eq. (9) is verified (Fig. 2) and the kaK3 calculated from the
graph of kobs(autocat) vs. [Cu(II)]. kaK3 value is found to be
[4] F.W. Schneider, A.F. Nichtlineare, Dynamik in der Chemie, Spek-
trum Akademischer Verlag, Heidelberg, Germany, 1996.
[5] I.R. Epstein, J.A. Pojman, An Introduction to Nonlinear Chemical
Dynamics: Oscillations, Waves, Patterns and Chaos, Oxford Univer-
sity Press, Oxford, 1998.
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World Scientific, Singapore, 1993.
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Khim. 27 (1990) 175.
3.17 · 106 dm6 molꢀ2 ꢀ1. Using the data K1, K2, kaK3 and
s
ku, the experimental rate constants are regenerated for
[Cu(II)] variation, compared with the kobs(autocat), and were
found to be in good agreement with each other (Table
2(B)).
Non-variation of rate of reaction with increase in ionic
strength [40] is due to the involvement of non-ionic species
or ionic and neutral species in the rate determining step. It
supports the mechanisms of Schemes 1 and 2 as non-ionic
species like MPC and negatively charged threonine are
involved.
[8] K.B. Yatsimirskii, L.N. Zakrevskaya, P.E. Strizhak, E.V. Rybak-
Akimova, Chem. Phys. Lett. 186 (1991) 15.
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Z. Kovat, Acta. Chim. hung. 21 (1959) 247;
Z. Kovat, Acta. Chim. hung. 22 (1960) 313.
Increase in rate with decreasing of the dielectric constant
[41] of the media is due to the interaction of a negatively
charged threonine and a polar molecule, monoperiodato-
cuprate(III), to give a less solvating activated complex in
a polar solvent than the reactants alone. It may be due to
the loss of degree of freedom in the activated complex to
give a rigid sphere. Dealing with the activation parameters
for the reaction is complicated as it involves autocatalysis
by Cu(II). However, the large negative value of entropy
activation [42] (Table 3) and high positive value of DG#
explain the loss of degree of freedom in the activated com-
plex which might be a rigid sphere.
The large negative value of entropy of activation,
enhancing the rate in lower dielectric medium and non-var-
iation of rate with ionic strength variation, clearly envisag-
es the transfer of electron from substrate to oxidant in an
outer sphere mechanism. It is also supported by the fact
that the derived small rate constant, ku = 38.1
dm3 molꢀ1 sꢀ1, entrenches the outer sphere mechanism.
[11] M.G. Ram Reddy, B. Sethuram, T. Navaneeth Rao, Indian J. Chem.
A 16 (1978) 31.
[12] K. Bal Reddy, B. Sethuram, T. Navaneeth Rao, Indian J. Chem. A 20
(1981) 395;
J. Padmavati, K. Yusiff, Transit. Metal Chem. 6 (2001) 315.
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uda, Int. J. Chem. Kinet. 14 (1982) 1183;
K. Balreddy, B. Sethuram, T. Navaneeth Rao, Indian J. Chem. A 20
(1981) 395.
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Soc. 107 (1985) 4220.
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[17] M.K. Mahanti, D. Laloo, J. Chem. Soc. Dalton Trans. (1990) 311;
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(1988) 1341.
[18] K. Balreddy, B. Sethuram, T. Navaneethrao, Indian J. Chem. A 20
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Soc. 53 (1976) 887.
4. Summary
[21] R.M. Shanmugam, T.V. Sabburamiyar, J. Chem. Soc. [Perkin II]
(1988) 1341.
[22] M.K. Mahanti, D. Laloo, J. Chem. Soc. Dalton Trans. (1990) 311.
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(1981) 395.
[24] P.K. Jaiswal, K.L. Yadava, Indian J. Chem. 11 (1973) 837;
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[25] G.H. Jeffery, J.M. Bassett, J. Mendham, R.C. Denny, Vogels’ Text
book of Quantitative Chemical Analysis, Fifth ed., ELBS, Longman,
UK, 1996 (a) p. 455, (b) p. 371.
[26] G.P. Panigrahi, P.K. Misro, Indian J. Chem. A 16 (1978) 201.
[27] F. Feigl, Spot Tests in Organic Analysis, New York, 1975, p.195.
[28] G. Svehla, A.I. Vogel’s Qualitative Inorganic Analysis, Seventh ed.,
Longman Ltd, London, 1998, p.85.
Among various species of DPC in alkaline medium,
monoperiodatocuprate (MPC) is considered as active spe-
cies for the title reaction. One of the products Cu(II) catal-
yses the reaction. The reaction proceeds through the
intervention free radicals. Equilibrium constants involved
in the mechanism and activation parameters of the reaction
were evaluated. The overall mechanistic sequence described
here is consistent with product studies, mechanistic studies
and kinetic studies.
Acknowledgements
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Methuen and Co, London, 1958, p.425.
[31] D.R. Lide, CRC Hand Book of Chemistry and Physics, Seventy-third
ed., CRC press, London, 1992, p. 8.
One of the authors S.M. Tuwar thanks the UGC,
Regional Office Bangalore, for providing financial
assistance.
[32] I.M. Kolthoff, E.J. Meehan, E.M. Carr, J. Am. Chem. Soc. 75 (1953)
1439;
S. Bhattacharya, P. Benerjee, Bull. Chem. Soc. Jpn. 69 (1996) 3475.
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