Mechanism of oxidation of alanine by chloroaurate(III) complexes in acid medium: Kinetics of the rate processes

Article abstract of DOI:10.1002/kin.20422

The kinetics of the oxidation of alanine by chloroaurate(III) complexes in acetate buffer medium has been investigated. The major oxidation product of alanine has been identified as acetaldehyde by 1H NMR spectroscopy. Under the experimental conditions, AuCl- and AuCl₃(OH)- are the effective oxidizing species of gold(III). The reaction is first order with respect to Au(III) as well as alanine. The effects of H⁺ and Cl- on the second- order rate constant k₃ have been analyzed, and accordingly the rate law has been deduced: k₂ = (k₁[H⁺][Cl-] + k₃K₄K₅)/(K₄K₅ + [H ⁺ ][Cl-]). Increasing dielectric constant of the medium has an accelerating effect on the reaction rate. Activation parameters associated with the overall reaction have been calculated. A mechanism involving the two effective oxidizing species of gold(III) and zwitterionic species of alanine, consistent with the rate law, has been proposed.

Full text of DOI:10.1002/kin.20422

Mechanism of Oxidation of
Alanine by Chloroaurate(III)
Complexes in Acid Medium:
Kinetics of the Rate
Processes
PRATIK K. SEN,1 NASIMUL GANI,1 JAYANTA K. MIDYA,1 BISWAJIT PAL2
1Department of Chemistry, Jadavpur University, Kolkata 700 032, India
2Department of Chemistry, St. Paul’s C.M. College, 33/1 Raja Rammohan Roy Sarani, Kolkata 700 009, India
Received 1 October 2008; revised 12 January 2009; accepted 13 January 2009
DOI 10.1002/kin.20422
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The kinetics of the oxidation of alanine by chloroaurate(III) complexes in
acetate buffer medium has been investigated. The major oxidation product of alanine has
been identified as acetaldehyde by 1H NMR spectroscopy. Under the experimental conditions,
AuCl−4 and AuCl3(OH)− are the effective oxidizing species of gold(III). The reaction is first
order with respect to Au(III) as well as alanine. The effects of H+ and Cl− on the second-
order rate constant k2ꢀ have been analyzed, and accordingly the rate law has been deduced:
k2ꢀ = (k1[H+][Cl−] + k3 K4 K5)/(K4K5 + [H+][Cl−]). Increasing dielectric constant of the
medium has an accelerating effect on the reaction rate. Activation parameters associated
with the overall reaction have been calculated. A mechanism involving the two effective
oxidizing species of gold(III) and zwitterionic species of alanine, consistent with the rate law,
has been proposed. ꢁ 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 473–482, 2009
C
importance. A number of kinetic studies have been
reported for the oxidation of some amino acids by dif-
ferent metal ion oxidants such as FeIII [1, 2], AgIII [3,
4], VV [5, 6], CeIV [7, 8], BiV [9], and MnVII [10–
14] in aqueous acidic or alkaline medium. Over the
past few years studies in the chemistry of gold(I) and
gold(III) compounds have received much attention. A
few gold(I) compounds are biologically active and are
used as anti-inflammatory drugs in the treatment of
rheumatoid arthritis [15–18]. There are reports on the
kinetic study of the reduction of gold(III) complexes
by sulfurIV [19], dimethylsulfide [20], and H2O2 [21].
Some recent studies for using gold(III) complexes as
INTRODUCTION
Amino acids are the chemical units or building blocks
of protein molecules and enzymes and play a key role in
life processes. Amino acids found within protein con-
vey a vast array of chemical versatility. The chemical
properties of the amino acids determine the biological
activity of the protein. The mechanism of redox reac-
tions involving amino acids is therefore of immense
Correspondence to: Pratik K. Sen; e-mail: senpk@yahoo.com.
2009 Wiley Periodicals, Inc.
c
ꢀ

474
SEN ET AL.
antitumor and anti-HIV agents have been reported [22–
24]. Investigations on redox and ligand exchange reac-
tions of potential gold(I) and gold(III)-cyanide metabo-
lites under biomimetic conditions [25] and those on a
metal-centered photo-induced electron transfer reduc-
tion of a Au(III)-porphyrin cation in nonpolar solvents
[26] have recently been reported. Recently, the reac-
tions of a few gold(III) complexes with serum albu-
min [27] were investigated using spectroscopic meth-
ods and separation techniques. The presence of both
weak and strong metal–protein interactions and adduct
formation for such gold(III) complexes through co-
ordination at the level of surface histidines has been
proposed. In a recent kinetic study in the reduction of
gold(III) complexes with calyx(4)areness [28] having
thioether groups on their upper rim, both intramolec-
ular and intermolecular mechanisms have been pro-
posed. There is now a growing interest in the produc-
tion of gold nanoparticles [29], and the oxidation of
gold nanoparticles by Au(III)–CTAB complexes has
been studied [30]. The mechanism of oxidation de-
pends on whether Au(III) is attached to the micelles.
It has been proposed that the CTAB micelles approach
the nanoparticles preferentially at the tips, leading to
spatially directed oxidation. The kinetics of oxidation
of glycolaldehyde [31], a few alkanols [32], aryl al-
cohols [32], and α-hydroxy acids [33] by Au(III) has
already been reported from our laboratory. We have
recently studied the kinetics of oxidation of glycine by
chloroaurate(III) complexes in acetate buffer [34]. An
attempt has therefore been made in the same direction
to study whether the oxidation of alanine follows the
same mechanistic path. For reasons mentioned earlier
[34], the kinetics of the reaction has been studied in
the sodium acetate–acetic acid buffer medium.
Kinetics and Measurements
All the kinetic investigations were studied under
pseudo-first-order conditions, where [alanine] and
[Cl−] were present in large excess compared to [AuIII].
Also [H+] was kept constant using sodium–acetate–
acetic acid buffer. The kinetic progress of the reaction
was followed by measuring the absorbance of the re-
action mixture at 400 nm for reasons discussed ear-
lier [34]. Alanine and the products are transparent at
this wavelength. The kinetic measurements were per-
formed in a Systronics (Ahmedabad, India) visible
spectrophotometer (model 106) with a thermostated
cell compartment. The plots of log A (A = absorbance)
vs. time were linear at least up to two half lives, and the
pseudo-first-order rate constants (kobs), evaluated from
these plots, were reproducible to within 5%.
Product Analysis
The reaction mixture (concentration of each reactant
being 20 times that under kinetic condition) was al-
lowed to stand for 2 h in a closed vessel and then
distilled. The distillate was collected in a closed con-
tainer, and the following tests were performed with the
distillate:
To the first part, five drops of piperidine was added,
followed by a few drops of 5% sodium nitroprus-
side solution and then made alkaline with Na2CO3
solution—a blue color developed [37], indicating the
presence of CH3CHO. The second part of the solu-
tion was treated with a few drops of sodium nitroprus-
side solution followed by a bead of NaOH—the devel-
opment of a red color [38] indicated the presence of
CH3CHO in the reaction product. Another part of the
distillate was treated with 2,4-dinitrophenylhydrazine
(DNP) solution in 4 N H2SO4 when an orange DNP
derivative of the product was precipitated. It was fil-
tered, washed thoroughly, and dried, and its melting
point was found to be 145◦C [39]. 1H NMR (300 MHz,
Bruker DPX 300) of the 2,4-dinitrophenylhydrazone
derivative of the reaction product in CDCl3 displayed
a broad signal at δ 11.17 due to N H; two doublets,
one at δ 9.15 (J = 2.3 Hz) and the other at δ 9.1 (J
= 2.5 Hz) [total 1H] due to aromatic-H flanked be-
tween two NO2 groups; doublet of doublets at δ 8.34
(J1 = 9.9 Hz, J2 = 2.2 Hz) and at δ 8.30 (J1 = 9.9 Hz,
J2 = 2.4 Hz) due to aromatic-H ortho to one NO2
group (total 1H) and doublets at δ 7.97 (J = 9.1 Hz)
and δ 7.94 (J = 9.3 Hz) (total 1H) due to aromatic-
H meta to both the NO2 groups. The NMR picture
clearly showed the existence of two isomeric DNP
derivatives. Most interestingly, the 1H NMR spectrum
of the said compound furnished a pair of quartets at
EXPERIMENTAL
Reagents
All chemicals were of the highest available purity. The
solution of DL-alanine (extra pure, AR; SRL, Mumbai,
India) was estimated by the formol titration method
[35] in the presence of excess neutralized formalin
with standard NaOH. Chloroauric acid trihydrate (GR;
E. Merck, Darmstadt, Germany) was dissolved in 0.01
mol dm−3 HCl, and the strength of the Au(III) stock
solution was determined by measurement of the ab-
sorbance of the solution (after proper dilution in 1 mol
dm−3 NaCl and 0.01 mol dm−3 HCl) at the absorbance
maximum [36] (313 nm, ε = 4860 dm3 mol−1 cm−1) of
AuCl−4 . All the solutions were made in doubly distilled
water.
International Journal of Chemical Kinetics DOI 10.1002/kin

MECHANISM OF OXIDATION OF ALANINE BY CHLOROAURATE(III) COMPLEXES
475
δ 7.56 (J = 5.3 Hz) and at δ 7.11 (J = 5.5 Hz) along
with two doublets at δ 2.14 (J = 5.4 Hz) and the
other at δ 2.08 (J = 5.6 Hz). Therefore, it is conclu-
sively proved that two stereoisomeric (syn and anti)
2,4-dinitrophenylhydrazones were present in the DNP
derivative of the reaction mixture. For one stereoisomer
(presumably anti) quartet at δ 7.56 (1H, J = 5.3 Hz)
due to olefinic-H and the doublet at δ 2.14 (3H,
J = 5.4 Hz) due to the CH3 group attached with
olefinic carbon were furnished. Similarly, for other
stereoisomer (presumably syn) quartet at δ 7.11 (1H,
J = 5.5 Hz) due to olefinic-H and the doublet at
δ 2.08 (3H, J = 5.3 Hz) due to CH3 attached with the
olefinic group were observed. The relative population
of the anti (more stable E configuration) and the syn
(less stable Z configuration) diastereomers can also be
determined from the relative intensities of two quartets
at δ 7.56 and 7.11, and it comes out E:Z = 2.6:1. These
observations clearly indicate the presence of two di-
astereomeric 2,4-dinitrophenylhydrazone derivatives
of acetaldehyde. This conclusively proves the gener-
ation of acetaldehyde in the reaction mixture as the
reaction product. The presence of ammonia in the re-
action product was tested by Nessler’s reagent. The
product CO2 was qualitatively detected by bubbling
nitrogen gas through the reaction mixture and passing
the liberated gas through limewater. The stoichiometry
of the reaction involving one of the gold(III) species
may thus be written as
(20% v/v) in the reaction mixture. Neither any hazi-
ness nor any precipitate appeared even after 3 h,
thereby indicating the absence of involvement of one-
electron transfer process in the reduction of Au(III)
to Au(I).
RESULTS
Effect of Reactant Concentrations
The oxidation of alanine by gold(III) was studied at
different initial [AuIII] ((1.82–4.25) × 10−3 mol dm−3),
keeping [alanine], [Cl−], pH, and temperature constant
at 4.0 × 10−2 mol dm−3, 5.0 × 10−2 mol dm−3, 4.45,
and 303 K, respectively. In all the kinetic experiments,
[alanine] and [Cl−] were present in a large excess com-
pared to [AuIII]. Acetate buffer was used to keep the
[H+] at a constant value. The kinetic experiments were
thus followed under pseudo-first-order conditions. The
value of kobs was found to be (10.06 0.50) × 10−4
s
−1, thereby indicating that the rate is of first order with
respect to Au(III). The influence of variation of the ala-
nine concentration on the reaction rate was also stud-
ied at constant [Au(III)], [Cl−], and pH of 3.04 × 10−3
mol dm−3, 5.0 × 10−2 mol dm−3, and 4.45, respec-
tively, and at four different temperatures. The rate was
found to increase with an increase in the alanine con-
centration (Table I). At each temperature, the plot of
kobs vs. [alanine] was linear passing through the origin
(Fig. 1), indicating a simple first-order dependence on
the alanine concentration. The reaction thus appeared
to follow the rate law:
AuCl−4 + +H3N(CH3)CHCOO− + H2O →
AuCl−2 + CH3CHO + CO2 + NH+4 + H+ + 2Cl− (1)
(1/[AuIII])d[AuIII]
= k2ꢀ [Alanine]
(2)
Polymerization Test
kobs = −
dt
Participation of free radicals, if any, was examined
by the polymerization of initially added acrylonitrile
where kꢀ2 is the second-order rate constant.
Table I Variation of Pseudo-First-Order Rate Constants with Alanine Concentrations at Different Temperatures
104 kobs (s−1
)
102 [Alanine]
(mol dm−3
)
298 K
303 K
308 K
313 K
1.0
2.0
3.0
4.0
6.0
2.07 (2.07)
4.30 (2.15)
6.49 (2.16)
8.60 (2.15)
13.4 (2.24)
16.2 (2.03)
22.07 (2.21)
2.54 (2.54)
5.33 (2.67)
7.68 (2.56)
10.1 (2.52)
15.2 (2.53)
20.1 (2.51)
26.0 (2.60)
3.25 (3.25)
6.45 (3.23)
9.65 (3.22)
13.0 (3.26)
19.6 (3.27)
25.3 (3.16)
32.5 (3.25)
4.02 (4.02)
8.18 (4.09)
12.3 (4.10)
16.5 (4.13)
24.4 (4.06)
33.0 (4.13)
40.8 (4.08)
8.0
10.0
[Au(III)] = 3.04 × 10−3 mol dm−3, [H+] = 3.55 × 10−5 mol dm−3, and [Cl−] = 0.05 mol dm−3
.
Figures in the parentheses represent the values of the second-order rate constants (kꢀ2 = kobs/[alanine], dm3 mol−1 s−1).
k2ꢀ values are of the order of 10−2
.
International Journal of Chemical Kinetics DOI 10.1002/kin

476
SEN ET AL.
Figure 1 Observed rate constants kobs as a function of [alanine] at four different temperatures. [Au(III)] = 3.04 × 10−3 mol
dm−3, [H+] = 3.55 × 10−5 mol dm−3, and [Cl−] = 0.05 mol dm−3
.
The rate was found to be inhibited by an increase in
[H+]. At a constant [Cl−], the plot of k2ꢀ vs. [H+]−1
(Fig. 2) was initially linear but showed a curvature at
higher [H+]−1. The relation between k2ꢀ and [H+] may
be expressed as
Effect of the Hydrogen Ion Concentration
The effect of variation of [H+] on the pseudo-first-
order rate constant was studied by changing the pH of
the buffer solution from 3.72 to 4.77, keeping [Au(III)],
[alanine], [Cl−], and temperature constant (Table II).
No attempt was made to keep the ionic strength con-
stant since the values of kobs remained unchanged at dif-
ferent ionic strengths varied by the addition of NaClO4.
kb + ka[H+]−1
k2ꢀ =
(3)
1 + kc[H+]−1
Table II Second-Order Rate Constants (k2ꢀ ) and Different Empirical Constants (ka, kb, and kc) Obtained from the Acid
Effect
[Cl−] (mol dm−3
0.05
)
pH
102k2ꢀ (dm3 mol−1 s−1
)
107 k (s−1
)
103 kb (dm3 mol−1 s−1
)
105 k (mol dm−3
)
a
c
3.72
3.85
3.94
4.05
4.27
4.45
4.77
1.30
1.46
1.55
1.62
1.90
2.15
2.50
14.4 1.9
8.88 0.68
5.00 0.76
0.20
3.72
3.85
3.94
4.05
4.27
4.45
4.77
0.99
1.05
1.10
1.14
1.31
1.43
1.85
3.54 0.56
8.76 0.25
1.00 0.29
[Au(III)] = 3.04 × 10−3 mol dm−3, [alanine] = 4.0 × 10−2 mol dm−3, and 298 K.
International Journal of Chemical Kinetics DOI 10.1002/kin

MECHANISM OF OXIDATION OF ALANINE BY CHLOROAURATE(III) COMPLEXES
477
Figure 2 Variation of the second-order rate constant with [H+]. Nonlinear plot of k2ꢀ vs. [H+]−1 at 298 K.
[Au(III)] = 3.04 × 10−3 mol dm−3, [alanine] = 4.0 × 10−2 mol dm−3, and [Cl−] = 0.05 mol dm−3
.
where ka, kb, and kc are empirical constants. The curve-
fitting procedure has been employed to find out the
empirical constants. The values of ka, kb, and kc at two
different [Cl−] are presented in Table II. It is evident
that ka and kc are dependent on [Cl−], whereas kb is
independent of [Cl−].
the addition of NaCl, keeping [Au(III)], [alanine], and
temperature constant at two different pH values, 4.05
and 4.45 (Table III). Chloride ion was found to have an
inhibiting effect on the reaction rate. The plot of k2ꢀ vs.
[Cl−]−1 was again found to be of similar nature as in
the case of [H+] variation (Fig. 3). Thus at a constant
[H+], the dependence of the second-order rate constant
(k2ꢀ ) on [Cl−] may be expressed as
Effect of the Chloride Ion Concentration
ke + kd[Cl−]−1
The influence of varying [Cl−] (0.05–0.4 mol dm−3
on the pseudo-first-order rate constant was studied by
)
k2ꢀ =
(4)
1 + kf[Cl−]−1
Table III Second-Order Rate Constants and Different Empirical Constants Obtained from the Cl− Effect
pH
10 [Cl−] (mol dm−3
)
102 k2ꢀ (dm3 mol−1 s−1
)
103 kd (s−1
)
103 k (dm3 mol−1 s−1
)
102 kf (mol dm−3
)
e
4.45
0.5
1.0
1.5
2.0
2.5
3.0
4.0
2.12
1.74
1.60
1.44
1.39
1.35
1.24
1.77 0.34
9.92 0.53
5.73 1.5
4.05
0.5
1.0
1.5
2.0
2.5
3.0
4.0
1.69
1.38
1.26
1.20
1.14
1.12
1.07
0.74 0.07
9.50 0.11
.
2.20 0.4
[Au(III)] = 3.04 × 10−3 mol dm−3, [alanine] = 4.0 × 10−2 mol dm−3, and 298 K.
International Journal of Chemical Kinetics DOI 10.1002/kin

478
SEN ET AL.
Figure 3 Variation of the second-order rate constant with [Cl−]. Nonlinear plot of k2ꢀ vs. [Cl−]−1 at 298 K.
[Au(III)] = 3.04 × 10−3 mol dm−3, [alanine] = 4.0 × 10−2 mol dm−3, and pH = 4.45.
where kd and kf are [H+]-dependent constants, whereas
ke is independent of [H+] (Table III). It has been ob-
served that at comparatively low pH and higher [Cl−]
the plot of k2ꢀ vs. [Cl−]−1 becomes linear (Fig. 4). It is
plausible that at lower pH and [Cl−] ꢂ kf the rate law
(Eq. (4)) simplifies to
k2ꢀ = ke + kd[Cl−]−1
(5)
This limiting form of the rate law is also applicable to
the initial linear part of the plot (Fig. 3) where [Cl−] is
Figure 4 Variation of the second-order rate constant with [Cl−]. Linear plot of k2ꢀ vs. [Cl−]−1 at 298 K. [Au(III)] = 3.04 × 10−3
mol dm−3, [alanine] = 4.0 × 10−2 mol dm−3, and pH = 3.72.
International Journal of Chemical Kinetics DOI 10.1002/kin

MECHANISM OF OXIDATION OF ALANINE BY CHLOROAURATE(III) COMPLEXES
479
comparatively high. It has been observed that the H+
independent constant, ke, is almost identical at all the
three studied pH values (3.72, 4.05, and 4.45).
and also contains the attacking nucleophile carboxy-
late anion [34]. The oxidant tetrachloroauric(III) acid
is known to involve the following equilibria in an aque-
ous solution [41,42]:
Effect of Solvent
K3
ꢀ
+
−
HAuCl4 ꢁ H + AuCl4
(9)
The effect of solvent composition on the rate of
the reaction was studied by varying 1,4-dioxane (0–
25% v/v), keeping [Au(III)], [alanine], pH, [Cl−],
K4
ꢀ
AuCl−4 + H2O
AuCl3(OH2) + Cl (10)
AuCl3(OH ) + H
−
ꢁ
K5
ꢀ
−
+
and temperature constant at 3.04 × 10−3 mol dm−3
,
ꢁ
AuCl3(OH2)
(11)
4.0 × 10−2 mol dm−3, 4.45, 0.05 mol dm−3, and 305 K,
respectively. The reaction rate was found to be retarded
by the addition of dioxane. Thus, the rate constant de-
creases with the decreasing dielectric constant of the
medium.
where the values of the equilibrium constants K3, K4,
and K5 are 1.0, 9.5 × 10−6, and 0.25, respectively,
at 298 K. Under the conditions of the experiment
(pH 3.72–4.77 and [Cl−]: 0.05–0.30 mol dm−3), us-
ing the values of K3, K4, and K5, it can be shown
that both HAuCl4 and AuCl3(OH2) will be present
in a negligible concentration. Hence, it becomes ev-
ident that the predominant and effective oxidizing
species of gold(III) in the present system are AuCl4−
and AuCl3(OH)−, as observed in our recent commu-
nication [34]. Therefore, it may be proposed that the
reaction takes place via two parallel paths involving
the zwitterions and the oxidizing species, AuCl−4 and
AuCl3(OH)−, as follows:
Effect of Temperature
Variation of pseudo-first-order rate constants for the
oxidation of alanine by Au(III) was studied at different
[alanine] at four different temperatures. The values are
presented in Table I. The linear plot of log (k2ꢀ /T ) vs.
T −1 enables one to evaluate the enthalpy of activation
(ꢀH=) using Eq. (6), followed by the determination
of the entropy of activation (ꢀS=).
log(k2ꢀ /T ) = [log(kB/h) + ꢀS=//2.303 R]
k1
AuCl−4 + +H3NCH(CH3)COO− −→ CH3CH NH+2
− ꢀH=//2.303 RT
(6)
slow
+CO2 + AuCl−2 + H+ + 2Cl−
(12)
The values of ꢀH= and ꢀS= are found to be (31 2)
kJ mol−1 and −(174 7) J K−1 mol−1, respectively.
k1
AuCl3(OH)− + +H3NCH(CH3)COO− −→
slow
CH3CH NH+2 + CO2 + AuCl2− + H2O + Cl−
DISCUSSION
(13)
It is well known that in an aqueous solution the follow-
ing equilibria between the cationic, zwitterionic, and
anionic forms of alanine exist:
This mechanism is in agreement with the oxidation
of glycine [34] and other amino acids [14]. It may be
suggested that an unstable intermediate is formed in a
slow step between the zwitterion and gold(III) species
followed by its decomposition to produce the iminic
cation (Eq. (14)).
CH CH(+H )COOH
N
K1
3
3
+
CH CH( H )COO− + H+
(7)
(8)
ꢀ
N
ꢁ
3
3
CH CH(+H )COO−
K2
N
3
3
-
-
−
+
ꢀ
H
H
CH3CH(NH2)COO + H
ꢁ
+
........
N
H
H
O
Cl
The values of pK1 and pK2 are 2.348 and 9.866, re-
spectively, at 298 K [40]. Since the experiments were
carried out in the pH range 3.72–4.77, it is quite ev-
ident that the concentration of the anionic species is
negligible compared to the other two species of ala-
nine. Among the zwitterionic and the cationic species,
the former is expected to be the effective reducing
agent since it is present in a large excess over the latter
C
-
Au
Cl
C
O
Cl
CH3
-
-
(X)
H
+
+ CO2 + AuCl2- + H+ + Cl-
(14)
NH2
C
CH3
International Journal of Chemical Kinetics DOI 10.1002/kin

480
SEN ET AL.
In several previous studies, the involvement of simi-
lar short-lived gold(III) intermediate complex has been
reported in the oxidation of amino acids [34, 43] as well
as other organic substrates [33]. In the final step, the
iminic cation hydrolyzes rapidly to give acetaldehyde
and NH+4 [44].
(Tables II and III). The values of the rate constants
(k1 and k3) for the rate-determining steps (12) and (13),
respectively, have been evaluated for different sets of
experiments. The average values of k1 and k3 are found
to be (0.882 0.046) × 10−2 and (3.01 0.43) × 10−2
dm3 mol−1 s−1 obtained from acid effect, whereas re-
spective values obtained from the chloride effect are
(0.97 0.024) × 10−2 and (2.79 0.29) × 10−2 dm3
+
H2O
CH3CH
H −→ CH CHO + NH+
(15)
N
2
3
mol−1 −1. The values are in good agreement with
s
4
fast
each other. The value of k3 has been found to be higher
than that of k1, which indicates the greater reactivity of
AuCl3(OH)− over AuCl−4 . This fact agrees well with
earlier reports [31, 34, 36]. Owing to nonavailability
of the values of dissociation constants (K3, K4, K5) at
303, 308, and 313 K, the values of k1 and k3 could not
be determined at these temperatures. Consequently, the
calculated values of ꢀH= and ꢀS= correspond to the
activation parameters for the overall reaction. The re-
action system involves several equilibria as well as the
rate-determining steps and hence is very much com-
plicated. Therefore, it is very difficult to state clearly
which step plays the key role to contribute to the high
negative value of ꢀS=. However, the rate-limiting step
may have significant contribution to this ꢀS=. Here
the transition state in the rate-limiting step becomes
highly solvated due to polarization developed in it (as
it is going to break down into a number of oppositely
charged ions). This is associated with an immobiliza-
tion of an appreciable number of solvent molecules
(i.e., increase in electrostriction) [49], which results in
a loss of entropy.
The involvement of similar iminic cation intermediate
and its hydrolysis to give the corresponding aldehyde
has been proposed in the oxidation of amino acids
by Mn(VII) [14], Bi(V) [9], and N-bromoacetamide
[45]. In the oxidation of glycine [3], DL-alanine [46],
β-phenylalanine [46, 47], and DL-leucine [48], the re-
ported products were the respective aldehydes along
with NH+4 and CO2.
Based on the proposed mechanism, the rate of con-
sumption of Au(III) may be expressed as
d[Au(III)]
−
= {k1[AuCl−4 ] + k3[AuCl3(OH)−]}[Ala]
dt
(16)
If C0 = [Au(III)] and x = [AuCl3(OH−)], Eq. (16)
may be written as
d[Au(III)]
−
= {k1(C0 − x) + k3x}[Ala]
(17)
dt
Expressing the value of x in terms of K4 and K5, the
above Eq. (17) is transformed to
Considering the equilibrium steps (9–11) involv-
ing the different Au(III) species, it is evident that an
increase in either [H+] or [Cl−] has an adverse ef-
fect on the concentration of the most reactive species
of Au(III), namely, AuCl3(OH)−. Consequently, the
rate of the reaction diminishes with increasing [H+]
or [Cl−]. The rate has been found to decrease with an
increase in the proportion of dioxane in the reaction
medium (Table IV). A logical explanation is that with
1
d[Au(III)]
−
[Au(III)]
dt
k1[H+][Cl−] + k3K4K5
= kobs
=
[Ala] (18)
K4K5 + [H+][Cl−]
Therefore, the second-order rate constant, k2ꢀ , is
kobs
k1[H+][Cl−] + k3K4K5
K4K5 + [H+][Cl−]
k2ꢀ =
=
(19)
[Ala]
Table IV Effect of Solvent on the Pseudo-First-Order
Rate Constant at 305 K
Equation (19) is equivalent to the empirical rate ex-
pressions (3)–(5) provided one assumes that ka =
Dioxane (%, v/v)
ε
104kobs (s−1
)
k3K4K5/[Cl−], kb = k1, kc = K4K5/[Cl−], kd
=
k3K4K5/[H+], ke = k1, and kf = K4K5/[H+]. This
equivalent nature suggests that the mechanism de-
picted in steps (12)–(15) is reasonable, at least math-
ematically. It is clearly evident from the above rela-
tions that ka and kc are [Cl−]-dependent constants, kd
and kf are [H+]-dependent constants, whereas kb and
ke are independent of both [H+] and [Cl−] and this
is in agreement with the experimental observations
0
5
10
15
20
25
78.6
74.2
69.7
65.3
60.8
56.4
12.3
9.98
7.98
6.91
6.58
5.76
[Au(III)] = 3.04 × 10−3 mol dm−3, [alanine] = 4.0 × 10−2 mol dm−3
,
[H+] = 3.55 × 10−5 mol dm−3, and [Cl−] = 0.05 mol dm−3
.
International Journal of Chemical Kinetics DOI 10.1002/kin

MECHANISM OF OXIDATION OF ALANINE BY CHLOROAURATE(III) COMPLEXES
481
a decrease in the dielectric constant of the medium, the
electrostatic attraction between the oppositely charged
species increases and consequently the concentration
of the effective oxidizing Au(III) species as well as
the reactive zwitterionic form of alanine diminishes,
thereby decreasing the reaction rate. It is interesting to
note that the intermediate cyclic complex in the rate-
limiting step decomposes to a number of oppositely
charged ions, and hence it is quite expected that the
transition state will be very much polar. Consequently,
a decrease in the dielectric constant of the medium will
disfavor the transition state and hence will decrease the
reaction rate.
The reaction mechanism in this study has been
found to be very much similar to that for the Au(III)-
glycine reaction. However, the reaction rate is slower
than that for the oxidation of glycine. This may be ac-
counted for by the fact that introduction of a CH3
group to the α-carbon of glycine increases the steric
crowding in the transition state, and this is possibly
reflected in the higher value of overall enthalpy of acti-
vation of Au(III)–alanine reaction over that of Au(III)–
glycine reaction.
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