Palladium(II)-catalyzed oxidation of l-tryptophan by hexacyanoferrate(III) in perchloric acid medium: A kinetic and mechanistic approach

Article abstract of DOI:10.1007/s12039-015-1015-7

The catalytic effect of palladium(II) on the oxidation of l-tryptophan by potassium hexacyanoferrate(III) has been investigated spectrophotometrically in aqueous perchloric acid medium. A first order dependence in [hexacyanoferrate(III)] and fractional-first order dependences in both [l-tryptophan] and [palladium(II)] were obtained. The reaction exhibits fractional-second order kinetics with respect to [H ⁺]. Reaction rate increased with increase in ionic strength and dielectric constant of the medium. The effect of temperature on the reaction rate has also been studied and activation parameters have been evaluated and discussed. Initial addition of the reaction product, hexacyanoferrate(II), does not affect the rate significantly. A plausible mechanistic scheme explaining all the observed kinetic results has been proposed. The final oxidation products are identified as indole-3-acetaldehyde, ammonium ion and carbon dioxide. The rate law associated with the reaction mechanism is derived.

Full text of DOI:10.1007/s12039-015-1015-7

c
J. Chem. Sci. Vol. 128, No. 2, February 2016, pp. 247–256. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-015-1015-7
Palladium(II)-catalyzed oxidation of L-tryptophan by
hexacyanoferrate(III) in perchloric acid medium: a kinetic and
mechanistic approach
AHMED FAWZYa,b,∗
aChemistry Department, Faculty of Applied Sciences, Umm Al-Qura University, Makkah, Saudi Arabia
bChemistry Department, Faculty of Science, Assiut University, Assiut, Egypt
e-mail: afsaad13@yahoo.com
MS received 9 July 2015; revised 27 October 2015; accepted 21 November 2015
Abstract. The catalytic effect of palladium(II) on the oxidation of L-tryptophan by potassium hexacyano-
ferrate(III) has been investigated spectrophotometrically in aqueous perchloric acid medium. A first order
dependence in [hexacyanoferrate(III)] and fractional-first order dependences in both [L-tryptophan] and [palla-
dium(II)] were obtained. The reaction exhibits fractional-second order kinetics with respect to [H+]. Reaction
rate increased with increase in ionic strength and dielectric constant of the medium. The effect of tempera-
ture on the reaction rate has also been studied and activation parameters have been evaluated and discussed.
Initial addition of the reaction product, hexacyanoferrate(II), does not affect the rate significantly. A plausible
mechanistic scheme explaining all the observed kinetic results has been proposed. The final oxidation products
are identified as indole-3-acetaldehyde, ammonium ion and carbon dioxide. The rate law associated with the
reaction mechanism is derived.
Keywords. Kinetics; oxidation; L-tryptophan; hexacyanoferrate(III); palladium(II).
1. Introduction
solubility, single equivalent change and its moderate
reduction potential of 0.45 V, leading to its reduction
to hexacyanoferrate(II), a stable product. In addition,
it adds less error to the experimental results, and the
data can be analyzed meticulously to establish the reac-
tion path. On the other hand, studies involving HCF as
A large number of kinetic investigations on the oxida-
tion of amino acids are being carried out using various
oxidants under different experimental conditions1–16
because of their biological significance, selectivity
towards the oxidants and to understand the mechanism an oxidant in acid media are limited.28–33 Furthermore,
of such biological redox reactions. In many cases,5–14 it
was reported that amino acids undergo oxidative decar-
boxylation and deamination. L-Tryptophan (Trp) is one
of the essential amino acids that is required in human
diet. It is a reducing agent in chemical and biochemical
systems. The rate of reduction depends on the oxidant
and pH of the medium. Our literature survey reveals that
the kinetic studies on the oxidation of L-tryptophan by
different oxidants have received limited attention.10–16
The most important iron(III) cyanide complex is
the hexacyanoferrate(III) anion (HCF), [Fe(CN)6]3−, in
which the Fe3+ center is bound in octahedral geometry
to six cyanide ligands. Hexacyanoferrate(III) is an effi-
cient one-electron oxidant and has been observed to be
a “substitution inert-transition metal complex”.17 Oxi-
dation by HCF in alkaline medium is well understood,
for organic17–23 and inorganic24–30 compounds. Its use-
fulness may be due to its unequivocal stability, water
hexacyanoferrate(III) is very sensitive to the solvent and
acidity and forms different intermediate species in acid
media.28
It was shown by the previous workers that the oxida-
tion of amino acids by hexacyanoferrate(III) proceeds
very slowly in the absence of a catalyst while it follows
a complex kinetics in the presence of it.34 Transition
metals are known to catalyze many oxidation–reduction
reactions because they involve multiple oxidation states.
In recent years, the use of transition metal ions such as
ruthenium, osmium, palladium, manganese, chromium,
and iridium, either alone or as binary mixtures, as cata-
lysts in various redox processes has attracted consider-
able interest.35 Although the mechanism of the catalysis
depends on the nature of the substrates, oxidant and
experimental conditions, it has been shown36 that metal
ions acts as catalysts by one of several different
paths such as formation of complexes with reactants,
oxidation of substrates themselves or through formation
of free radicals.
∗For correspondence
247

248
Ahmed Fawzy
No work, however, has been reported on the oxida- length 1 cm, and 3-4 experimental readings were taken
tion of L-tryptophan by hexacyanoferrate(III) in acid in each run. It was observed that the oxidation reaction
medium in either absence or presence of a catalyst. The did not proceed in the absence of palladium(II) catalyst.
present report deals with the title reaction in order to Good straight lines for ln(absorbance) versus time plots
understand the active species of oxidant and catalyst, were obtained for about 85% of the reaction comple-
to check the reactivity of L-tryptophan towards hexa- tion and the pseudo-first order rate constant values (kC)
cyanoferrate(III) in perchloric acid medium, to examine were calculated as the gradients of such plots. The rate
the catalytic activity of the catalyst and to propose an constants were reproducible to within 4%.
appropriate reaction mechanism.
3. Results and Discussion
2. Experimental
3.1 Stoichiometry and product analysis
2.1 Materials
Reaction mixtures containing varying ratios of hexa-
All the chemicals employed in the present work were cyanoferrate(III) to L-tryptophan in 2.0 mol dm−3 per-
of reagent grade and their solutions were prepared by chloric acid at a constant ionic strength of 3.0 mol
dissolving the requisite amounts of the samples in dou- dm−3 were kept for over 24 h at 25◦C in a closed
bly distilled water. A stock solution of L-tryptophan vessel for completion of the reaction. The unreacted
(E. Merck, US) was prepared afresh by dissolving the [HCF] was analyzed spectrophotometrically by measur-
amino acid sample in bidistilled water. A fresh solu- ing its absorption at 420 nm. The results indicated that
tion of hexacyanoferrate(III) was prepared by dissolv- two moles of HCF are consumed by one mole of L-
ing potassium hexacyanoferrate(III) (BDH, USA) in tryptophan to yield the oxidation products as shown in
water and its concentration was ascertained by iodo- the following stoichiometric equation (1),
metric titration.37a Hexacyanoferrate(II) solution was
O
PdII
+ 2H[Fe(CN)6]3-
H
-
OH
+ + H2O
2H[Fe(CN)6]2
prepared by dissolving potassium hexacyanoferrate(II)
(SD Fine Chemicals, India; Aldrich, USA) in water and
standardizing it with cerium(IV) solution.37b The Pal-
ladium(II) chloride solution was prepared by dissolv-
ing a known weight of the sample (SD Fine Chemi-
cals) in hydrochloric acid and stored in a black coated
bottle to prevent any photochemical deterioration. Per-
chloric acid and sodium perchlorate were used to vary
the acid concentration and ionic strength in the reaction
medium, respectively.
+ CO2 + NH4 + H+
+
O
NH2
N
H
N
H
Tryptophan
Indole-3-acetaldehyde
(1)
The product obtained was tested for the aldehyde,
indole-3-acetaldehyde by TLC. One spot was obtained
when the chromatogram was assayed with 2,4-dinitro-
phenylhydrazine. An aliquot (5 mL) was pipetted into
50 mL of 2.0 N HCl saturated with 2,4-dinitrophe-
nylhydrazine which was filtered, washed, dried and
weighed. The yield was calculated from the amount of
2,4-dinitrophenylhydrazone formed and was found to
about 84%. Further proof of the formation of the corre-
sponding aldehyde was obtained by isolating the alde-
hyde using the standard method.38 The byproducts were
identified as ammonia by Nessler’s reagent39 and car-
bon dioxide by limewater. Similar oxidation products
with different experimental conditions have been also
reported earlier.10–16
2.2 Kinetic measurements
The kinetic runs were carried out under pseudo-first
order conditions keeping L-tryptophan always in excess
over hexacyanoferrate(III) concentration at a constant
ionic strength of 3.0 mol dm−3 and at a constant tem-
perature of (25 0.1)◦C unless otherwise stated. The
progress of the reaction was followed spectrophotomet-
rically by monitoring the decrease in the absorbance
of HCF at its absorption maximum, λmax = 420 nm,
as a function of time using Shimadzu UV-VIS-NIR-
3600 double-beam spectrophotometer keeping the cell
3.2 Spectroscopic changes
compartment at constant temperature. Solutions of the The spectroscopic changes associated with the
oxidant and the mixture containing L-tryptophan sub- palladium(II)-catalyzed oxidation of L-tryptophan
strate, perchloric acid and palladium(II) chloride cat- by hexacyanoferrate(III) in perchloric acid medium
alyst were separately thermostated for nearly 30 min. are shown in figure 1. The scanned spectra shown in
The oxidant was then added to the mixture, the over- figure 1a indicate gradual disappearance of HCF(III)
all reaction mixture was transferred to the cell of path band located at λ = 420 nm with time as a result

Palladium(II)-catalyzed oxidation of L-tryptophan
249
1.0
0.8
0.6
0.4
0.2
0.0
HCF
Trp
200
300
400
500
600
Wavelength, nm
(a)
0.80
0.78
0.76
0.74
0.72
0.70
0.68
1.0
0.8
0.6
0.4
0.2
0.0
350
360
370
380
390
400
420
440
460
480
500
(b)
Wavelength, nm
(c)
Wavelength, nm
Figure 1. (a) Spectroscopic changes associated with palladium(II)-catalyzed oxidation of
L-tryptophan by hexacyanoferrate(III) in perchloric acid medium. (b, c) Careful examination
of the spectroscopic changes around the two isosbestic points. [Trp] = 0.01, [HCF] = 7.0
× 10−4, [H+] = 2.0, [PdII] = 5.0 × 10−5 and I = 3.0 mol dm−3 at 25◦C. Scan time intervals
= 2 min.
of its reduction to HCF(II). A careful examination and ionic strength and temperature. The order with
of the spectroscopic changes, figure 1b and c, mani- respect to [HCF] was found to be unity, as plots of ln
fests a simultaneous growing of an absorption band [absorbance] against time were linear with no variation
located above 445 nm and exhibit two isosbestic in the slope for different [HCF] as shown in table 1.
points centered at wavelengths 370 and 444 nm. These
The rate constant was determined at different initial
spectroscopic features are consistent with the forma- concentrations of the reductant L-tryptophan keeping
tion of an adduct between palladium(II) catalyst and all other reactant concentrations constant including pal-
L-tryptophan substrate.
ladium(II) catalyst. It was found that the rate of reac-
tion increased with increasing the concentration of L-
tryptophan as listed in table 1. The plot of kC versus
[Trp], figure 2, gave a good straight line with a positive
3.3 Reaction orders
The reaction orders with respect to the reactants were intercept on kC axis suggesting that the order of reaction
determined from plots of log kC versus log (conc.) by with respect to [Trp] is less than unity.
varying the concentrations of L-tryptophan substrate,
perchloric acid and palladium(II) chloride catalyst, in conditions remaining constant, perchloric acid was var-
At constant ionic strength of 3.0 mol dm−3 and other
turn, while keeping the others constant.
ied from 1.0 to 3.0 mol dm−3. It was noticed that as
The effect of [HCF] on the rate of reaction was stud- [H+] increased the rate of reaction increased (table 1)
ied by varying its concentration in the range 3.0 × 10−4 with a fractional-second order as indicated by the plot
to 11.0 × 10−4 mol dm−3 at fixed [Trp], [H+], [PdII] of kC versus [H+]2 (figure 3).

250
Ahmed Fawzy
Table 1. Effect of variation of [HCF], [Trp], [H+], [PdII] and ionic strength, I,on the pseudo-first order rate constant (kC)
in the paladium(II)-catalyzed oxidation of L-tryptophan by hexacyanoferrate(III) in perchloric acid medium at 25◦C.
104[HCF]
(mol dm−3
102[Trp]
[H+]
105[PdII]
I
104 kC
(s−1
)
(mol dm−3
)
(mol dm−3
)
(mol dm−3
)
(mol dm−3
)
)
3.0
5.0
7.0
9.0
11.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
1.0
1.0
1.0
1.0
1.0
0.3
0.5
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
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.0
1.5
2.0
2.5
3.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
1.0
3.0
5.0
7.0
9.0
5.0
5.0
5.0
5.0
5.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.3
3.6
4.0
4.5
22.1
23.2
22.8
23.5
22.7
9.6
14.1
22.8
39.8
53.7
6.6
13.8
22.8
34.5
47.7
7.7
15.5
22.8
30.8
40.0
22.8
23.3
24.1
25.0
25.9
Experimental error 4%.
60
50
40
30
20
10
0
50
40
30
20
10
0
0
2
4
6
8
10
0.0
0.5
1.0
1.5
2.0
2.5
3.0
102 [Trp], mol dm-3
[H+]2, mol2 dm-6
Figure 2. Plot of kC vs [Trp] in the paladium(II)-catalyzed Figure 3. Plot of kC versus [H+]2 in the paladium(II)-
oxidation of L-tryptophan by hexacyanoferrate(III) in per- catalyzed oxidation of L-tryptophan by hexacyanoferrate(III)
chloric acid medium. [HCF] = 7.0 × 10−4, [H+] = 2.0, [PdII] in perchloric acid medium. [Trp] = 0.01, [HCF] = 7.0
= 5.0 × 10−5 and I = 3.0 mol dm−3 at 25◦C.
× 10−4, [PdII] = 5.0 × 10−5 and I = 3.0 mol dm−3 at 25◦C.
The palladium(II) catalyst concentration was varied
from 1.0 × 10−5 to 9.0 × 10−5 mol dm−3 at constant
concentrations of Trp, HCF, H+ and at constant ionic
strength and temperature. Reaction rate was found to
increase with increasing [PdII] (table 1). The order was
found to be less than unity as shown in figure 4.

Palladium(II)-catalyzed oxidation of L-tryptophan
251
3.4 Effect of ionic strength and dielectric constant
acetic acid (78.5 and 6.15 at 25◦C, respectively). The
data clearly reveals that the rate constant kC decreased
with the increase in acetic acid content, i.e., decrease
in dielectric constant of the solvent mixture. The plot
of log kCversus 1/D was linear with a negative slope as
shown in figure 5b.
The effect of ionic strength was studied in the con-
centration range 3.0-4.5 mol dm−3 at constant [HCF],
[Trp], [PdII], [H+] and temperature by varying the con-
centration of NaClO4. The results are presented in table
1. These results show that the pseudo-first order rate
constant kC increased with increasing ionic strength of
the medium and the Debye-Hückel plot was found to be
linear with a positive slope, figure 5a.
In order to determine the effect of dielectric con-
stant (D) of the medium on the rate, the oxidation of
L-tryptophan by HCF was studied at different solvent
compositions (v/v) of acetic acid and water. The dielec-
tric constant of the medium at different compositions
was calculated using dielectric constants of water and
3.5 Effect of initially added product [HCF(II)]
The effect of added hexacyanoferrate(II) product was
studied also in the concentration range 3.0 × 10−4 to
11.0 × 10−4 mol dm−3 at fixed concentrations of the
oxidant, reductant, perchloric acid and catalyst. It was
found that HCF(II) did not have any significant effect
on the rate of reaction.
3.6 Test for free radicals
50
40
30
20
10
0
It is well known that hexacyanoferrate(III) is a genera-
tor of free radicals due to its single equivalent change
in redox reactions. Hence, the involvement of free rad-
ical species in the organic substrate, L-tryptophan, was
assayed by a polymerization test. A known quantity of
acrylonitrile monomer was added initially to a reaction
mixture in an inert atmosphere, with the result of pro-
gressive formation of a white precipitate in the whole
reaction mixture, indicating presence of free radicals
during the reaction. When the experiment was repeated
in the absence of L-tryptophan under similar condition,
the test was negative. This indicates that the reaction
was routed through a free radical path.
0
2
4
6
8
10
105 [PdII], mol dm-3
Figure 4. Plot of kC versus [PdII] in the paladium(II)-
catalyzed oxidation of L-tryptophan by hexacyanoferrate(III)
in perchloric acid medium. [Trp] = 0.01, [HCF] = 7.0 ×
10−4, [H+] = 2.0 and I = 3.0 mol dm−3 at 25◦C.
3.7 Effect of temperature
The kinetics was studied at different temperatures, 288,
293, 298, 303 and 308 K, at constant concentrations of
-2.59
-2.60
-2.61
-2.62
-2.63
-2.64
-2.65
-2.64
-2.68
-2.72
-2.76
0.62
0.64
0.66
0.68
0.70
0.013
0.014
0.015
0.016
0.017
I1/2 / (1+I1/2
)
(b)
1/D
(a)
Figure 5. Effect of, a) ionic strength (I) and, b) dielectric constant (D) of the medium on the rate of
paladium(II)-catalyzed oxidation of L-tryptophan by hexacyanoferrate(III) in perchloric acid medium. [Trp]
= 0.01, [HCF] = 7.0 × 10−4, [H+] = 2.0 and [PdII] = 5.0 × 10−5 mol dm−3 at 25◦C.

252
Ahmed Fawzy
-30.4
-1.0
-1.2
-1.4
-1.6
-1.8
-2.0
-2.2
-30.6
-30.8
-31.0
-31.2
-31.4
-31.6
3.25
3.30
3.35
3.40
3.45
3.50
3.25
3.30
3.35
3.40
3.45
3.50
103 (1/T) K-1
103 (1/T) K-1
(a)
(b)
Figure 6. (a) Arrhenius and, (b) Eyring plots in the palladium(II)-catalyzed oxidation of L-tryptophan by
hexacyanoferrate(III) in perchloric acid medium. [Trp] = 0.01, [HCF] = 7.0 × 10−4, [H+] = 2.0, [PdII] =
5.0 × 10−5 and I = 3.0 mol dm−3
.
Table 2. Activation parameters of the second order rate constant k2 in the
palladium(II)-catalyzed oxidation of L-tryptophan by hexacyanoferrate(III) in per-
chloric acid medium. [Trp] = 0.01, [HCF] = 7.0 × 10−4, [H+] = 2.0, [PdII] = 5.0
× 10−5 and I = 3.0 mol dm−3
.
ꢀS=, J mol−1K−1
ꢀH=, kJ mol−1
ꢀG2=98, kJ mol−1
Ea=, kJ mol−1
−152.98
31.04
76.63
33.54
Experimental error 4%
the reactants and ionic strength. The obtained results K1K2 and β3 = K1K2K3 belonging to the species with
indicate that the pseudo-first order rate constant kC one, two and three protons, respectively.41
was increased with raising temperature. The activation
K1
parameters of the second order rate constant, k2(k2 =
kC/ [Trp]), are calculated using Arrhenius, (figure 6a)
and Eyring (figure 6b) plots and are listed in table 2.
[Fe(CN)6]3- + H+
H[Fe(CN)6]2-
H2[Fe(CN)6]-
H3[Fe(CN)6]
(2)
(3)
(4)
K2
K3
H[Fe(CN) ]2- + H+
6
H2[Fe(CN)6]- + H+
3.8 Behavior of L-tryptophan in acid media
Martell and Smith40 reported that amino acids exist as
zwitterions in neutral media and predominantly tend
to protonate in acid media (Eq. (5) in scheme 1). The
high concentration of hydrogen ion employed in the
present reaction as well as the observed enhancement
of the reaction rate upon increasing acid concentra-
tion suggesting protonation of L-tryptophan in a pre-
equilibrium step and its protonated form (Trp+) appears
to be the reactive species in the rate-determining
step.
where K3 < K2 < K1 < 10.
The existence of higher protonated complexes is
ruled out as they exist at relatively higher [H+].42 Thus,
a single protonated ferricyanide, H[Fe(CN)6]2−, may be
the reactive species in the present investigation.
3.10 Behavior of palladium(II) species in acid media
In most of the studies using palladium(II) as a homoge-
nous catalyst, it has been employed in the form of
palladium(II) chloride. It is important to know the prob-
able species of palladium(II) chloride in acid media.
3.9 Behavior of hexacyanoferrate(III) in acid media
In perchloric acid media, hexacyanoferrate(III) forms The different possible mononuclear complexes of palla-
different protonated species as shown in the equilibria dium(II) such as [PdCl3L]−, [PdCl2L2], [PdClL3]+and
(2)–(4) with the stability constants: β1 = K1, β2 = [PdL4]2+ (where L represents a ligand like amine,

Palladium(II)-catalyzed oxidation of L-tryptophan
253
phosphine, sulphide) are reported.43 It is dissolved in
the presence of Cl−. Elding44 reported that in the pres-
ence of chloride ion, palladium(II) chloride exists as
[PdCl4]2− and in the aqueous solution, it may be further
hydrolyzed to [PdCl3(H2O)]−.
i.e., two moles of hexacyanoferrate(III) requires one
mole of L-tryptophan. The order with respect to [HCF]
was found to be unity and less than unity each with
respect to both [Trp] and [PdII]. As [H+] increases,
the reaction rate also increases with fractional-second
order dependence suggesting protonation of both L-
tryptophan and hexacyanoferrate(III). The rate is not
considerably affected by HCF(II) suggesting that the
probability of any fast equilibrium with the product pre-
ceding the rate-determining step was ruled out. The
rate-determining step should be irreversible as is gen-
erally the case for one electron oxidants45 and the oxi-
dation takes place through generation of a free radical
3.11 Reaction mechanism
The hexacyanoferrate(III) oxidation of L-tryptophan
occurs in measurable quantities in the presence of
small amounts of palladium(II) catalyst in perchloric
acid medium. The reaction has a stoichiometry of 2:1,
O
O
K
OH
+ H+
OH
(5)
(6)
(7)
NH2
NH3
+
N
H
N
H
(Trp)
+ H+
(Trp+ )
K1
[Fe(CN)6]3-
H[Fe(CN)6]2-
O
O
KF
+ [PdIICl4]2
-
PdIICl4
2
-
OH
O
H
NH3
NH3
+
+
N
H
N
H
(Trp+)
(C1)
O
O
3-
k1
slow
PdIICl4 + H[Fe(CN)6]2-
2
-
.
+ H[Fe(CN)6]
O
H
(8)
(9)
O
II
2
-
+
NH3
+ [Pd Cl4] + H
+
NH3
+
N
H
N
H
.
(C1)
(Trp )
O
.
.
H
fast
+ CO2 + H+
O
NH3
NH2
+
N
H
N
H
.
.
(Trp )
(X )
.
H
fast
+ H[Fe(CN)6]2-
H
+ H[Fe(CN)6]3-
(10)
(11)
NH2
NH2
+
N
H
N
H
.
(X )
fast
H
H
+
+ H2O
+ NH4
O
NH2
+
N
H
N
H
Scheme 1. Mechanism of palladium(II)-catalyzed oxidation of L-tryptophan by hexacyanoferrate(III) in perchloric acid
medium.

254
Ahmed Fawzy
1/[H+]2, dm6 mol-2
as confirmed experimentally. The effect of ionic
strength and dielectric constant on the reaction rate sug-
gests that similarly charged species are interacting in
the rate-determining step.46
0
1.0
1.2
0.6
0.2
0.4
0.8
80
60
40
20
0
Based on the experimental results, the proposed
mechanism (scheme 1) involves protonation of both
L-tryptophan and hexacyanoferrate(III) followed by
complexation of the protonated L-tryptophan with PdII
species, [PdCl4]2−, to form an adduct C1 prior to the
rate-determining step. It is reported5–7 that amino acids
form adducts with some transition metal ions owing
to the availability of electron pair on oxygen atom of
the carboxylate group. Within protolytic amino acid
system, there are carboxylate and amine groups which
may act as nucleophiles, depending on the pH of the
medium. The protolytic group with the highest basicity
interacts with the transition metal ion. Thus, at low pH,
where the amine group is protonated, the carboxylate
group should be able to attack the metal ion. Complex
formation was proved spectrophotometrically as
shown in figure 1a-c as well as kinetically by the non-
zero intercept of the plot [PdII]/kC versus 1/[Trp] (figure
7). Such a complex between amino acid and palla-
dium(II) catalyst has been reported in earlier studies.47
Then, the active species of the oxidant, H[Fe(CN)6]2−,
attacks the formed intermediate complex in a slow (rate-
determining) step to yield L-tryptophan free radical
and HCF(II) with regeneration of the catalyst PdII. This
is followed by decarboxylation of L-tryptophan free
0
60
120
180
240
300
360
1/[Trp], dm3 mol-1
Figure 7. Verification of the rate law (17) in the form
of Eq. (19) in the palladium(II)-catalyzed oxidation of
L-tryptophan by hexacyanoferrate(III) in perchloric acid
medium. [HCF] = 7.0 × 10−4 and I = 3.0 mol dm−3 at 25◦C.
radical forming a new radical intermediate (X.). The
latter reacts with another H[Fe(CN)6]2− species in a
subsequent fast step to give rise to the final oxidation
products.
The suggested mechanism leads to the following rate
law (see Appendix),
k1KK1KF[Trp][Fe(CN)36−][H+]2[PdII]
Rate =
(12)
1+ K[H+] + K1[H+] + KK1[H+]2 + KKF[Trp][H+] + KK1KF[Trp][H+]2
Rate
[Fe(CN)36−]
k1KK1KF[Trp][H+]2[PdII]
kC =
=
(13)
1+ K[H+] + K1[H+] + KK1[H+]2 + KKF[Trp][H+] + KK1KF[Trp][H+]2
and ꢀS= were both favorable for electron transfer
process. Again, the positive values of both ꢀH = and
ꢀG= indicate the endothermic formation of adduct and
its non-spontaneity, respectively.
Equation (13) can be rearranged to the following
form, which is suitable for verification:
ꢀ
ꢁ
[PdII]
1 + m
1
1
1
=
+
+
kC
k1KK1KF[H+]2 [Trp] k1K1[H+] k1
(14)
4. Conclusions
where, m = [H+](K + K1+ KK1[H+]).
According to Eq. (14), other conditions being con- The reaction between L-tryptophan and hexacyanofer-
stant, plots of [PdII]/kC versus 1/[Trp] and [PdII]/kC ver- rate(III) in perchloric acid medium does not proceed
sus 1/[H+]2 should be linear which were found to be so in the absence of a catalyst and occurs in a signif-
(figure 7).
icant rate in the presence of small amount of palla-
Activation parameters listed in table 2 may be dis- dium(II) catalyst. The mode of electron transfer has
cussed as follows. The observed large negative val- been indicated through an adduct between L-tryptophan
ues of ꢀS= confirms the compactness of the formed and palladium(II) catalyst via the carboxyl group rather
L-tryptophan-palladium(II) adduct and such activated than the amine group. The overall mechanistic sequence
adduct is more ordered than the reactants due to loss of described here is consistent with the experimental
degree of freedom.48 The experimental values of ꢀH= results and product analysis.

Palladium(II)-catalyzed oxidation of L-tryptophan
255
Appendix
Therefore,
[Trp]T
Derivation of rate law:
According to the suggested mechanism and regard-
ing to reaction (8),
[Trp]F =
(23)
1+ K[H+] + KKF[H+][PdII]
In view of the low concentration of PdII, the third
denominator term KKF[H+][PdII] in the above equation
is neglected. Therefore,
−d[Fe(CN)36−]
Rate =
= k1[C][HFe(CN)26−] (15)
dt
[Trp]T
from reactions (6)–(8)
[Trp]F =
(24)
1+K[H+]
[Trp+]
K =
, [Trp+] = K[Trp][H+]
(16)
Also,
[Trp][H+]
[Fe(CN)36−]T = [Fe(CN)63−]F + [HFe(CN)26−] (25)
Substituting Eq. (17) into Eq. (25),
and
[HFe(CN)26−]
K1
=
, [HFe(CN)26−]
[Fe(CN)36−][H+]
[Fe(CN)36−]T = [Fe(CN)63−]F(1 + K1][H+])
(26)
= K1[Fe(CN)36−][H+]
(17)
(18)
Therefore,
[Fe(CN)36−]T
[Fe(CN)36−]F =
(27)
(28)
[C]
[Trp+][PdII]
1+ K1[H+]
KF
=
, [C] = KF[Trp+][PdII]
Similarly,
= KKF[Trp][H+][PdII]
[PdII]T = [PdII]F + [C]
Substituting Eq. (18) into Eq. (28) and rearrangement
Substituting Eqs. (17) and (18) into Eq. (15) leads to,
Rate = k1KK1KF[Trp][Fe(CN)36−][H+]2[PdII] (19)
The total concentration of Trp is given by:
gives,
[PdII]T = [PdII]F(1 + KKF[Trp][H+])
(29)
(30)
[PdII]T
[Trp]T = [Trp]F + [Trp+] + [C]
(20)
[PdII]F =
1+KKF[Trp][H+]
where ‘T’ and ‘F’ stand for total and free concentra-
tions.
Substituting Eqs. (16) and (18) into Eq. (20) and
rearrangement gives,
In view of the high concentrations of [H+],
[H+]T = [H+]F
Substituting Eqs. (24), (27), (30) and (31) into Eq.
(31)
[Trp]T = [Trp]F + K[Trp][H+]
(19) (and omitting ‘T’ and ‘F’ subscripts) leads to,
k1KK1KF[Trp][Fe(CN)36−][H+]2[PdII]
Rate =
+KKF[Trp][H+][PdII]
(21)
(1+K[H+])(1+K1[H+])(1+KKF[Trp][H+])
[Trp]T = [Trp]F(1 + K[H+] + KKF[H+][PdII]) (22)
(32)
k1KK1KF[Trp][Fe(CN)36−][H+]2[PdII]
Rate =
(33)
+
2
1 + K[H+] + K1[H+] + KK1[H ] + KKF[Trp][H+] + KK1KF[Trp][H+]2 + K2KF[Trp][H+]2
+ K2K1KF[Trp][H+]3
The terms K2KF[Trp][H+]2 and K2K1KF[Trp][H+]3
in the denominator of Eq. (33) are negligibly small
compared to unity in view of the low concentration of
Trp used. Therefore, Eq. (33) can be written as:
k1KK1KF[Trp][Fe(CN)36−][H+]2[PdII]
Rate =
(34)
1+ K[H+]+ K1[H+]+ KK1[H+]2 + KKF[Trp][H+] + KK1KF[Trp][H+]2
−d[Fe(CN)36−]
Under pseudo-first order condition, the rate-law can
be expressed by Eq. (35),
Rate =
= kC[Fe(CN)36−]
(35)
dt

256
Ahmed Fawzy
Comparing Eqs. (34) and (35), the following relation-
ship is obtained,
k1KK1KF[Trp][H+]2[PdII]
kC =
(36)
1+K[H+] + K1[H+] + KK1[H+]2 + KKF[Trp][H+] + KK1KF[Trp][H+]2
and with rearrangement,
24. Tuwar S M, Nandibewoor S T and Raju J R 1991
Transition Met. Chem. 16 335
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Singh S B 2007 Transition Met. Chem. 32 74
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S T 1998 React. Kinet Cat. Lett. 63 403
28. Chimatadar S A, Salunke M S and Nandibewoor S T
2004 Transition Met. Chem. 29 743
ꢀ
ꢁ
[PdII]
1 + m
1
1
1
=
+
+
kC
k1KK1KF[H+]2 [Trp] k1K1[H+] k1
(37)
where,
m = [H+](K + K1 + KK1[H+]).
29. Chimatadar S A, Koujalagi S B and Nandibewoor S T
2001 Transition Met. Chem. 27 62
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