Mechanistic studies of ruthenium(III)-catalyzed oxidation of DL-methionine by hexacyanoferrate(III) in an alkaline medium

Article abstract of DOI:10.1080/17415993.2011.573558

The kinetics and mechanism of ruthenium(III) catalyzed oxidation of dl-methionine by alkaline hexacyanoferrate(III) (HCF(III)) in an alkaline medium were studied spectrophotometrically at 300.1C. The reaction was first-order-dependent each on [HCF(III)] and [ruthenium(III)] and fractional-order-dependent on [alkali]. The rate of the reaction was found to be decreased with the increase in [methionine]. The main product of oxidation was methionine sulfone nitrile (3-(methylsulfonyl)propanenitrile) and it was identified and confirmed by FT-IR and mass spectral studies. Further, no effect of added reaction product was observed. A plausible mechanism was proposed involving complexation between methionine and ruthenium(III) species, [Ru(H ₂O)₅OH]²⁺. Thermodynamic parameters for the reaction, E_a and ΔS#, were computed using linear least squares method and are found to be 65.83±1.03kJ/mol and -249.58±3.35J/ Kmol, respectively.

Full text of DOI:10.1080/17415993.2011.573558

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Mechanistic studies of ruthenium(III)-
catalyzed oxidation of DL-methionine
by hexacyanoferrate(III) in an alkaline
medium
B. Dharma Rao a , T.V.N. Partha Sarathi b , N. Annapurna a & P.
Vani a
a Department of Inorganic and Analytical Chemistry, School of
Chemistry , Andhra University , Visakhapatnam , 530 003 , India
b M.V.G.R. College of Engineering , Vizianagaram , 535 005 , India
Published online: 18 Apr 2011.
To cite this article: B. Dharma Rao , T.V.N. Partha Sarathi , N. Annapurna & P. Vani
(2011) Mechanistic studies of ruthenium(III)-catalyzed oxidation of DL-methionine by
hexacyanoferrate(III) in an alkaline medium, Journal of Sulfur Chemistry, 32:3, 263-272, DOI:
10.1080/17415993.2011.573558
To link to this article: http://dx.doi.org/10.1080/17415993.2011.573558
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Journal of Sulfur Chemistry
Vol. 32, No. 3, June 2011, 263–272
Mechanistic studies of ruthenium(III)-catalyzed oxidation of
DL-methionine by hexacyanoferrate(III) in an alkaline medium
B. Dharma Raoa, T.V.N. Partha Sarathib, N. Annapurnaa and P. Vania*
aDepartmentofInorganicandAnalyticalChemistry, SchoolofChemistry,AndhraUniversity,Visakhapatnam
530 003, India; bM.V.G.R. College of Engineering, Vizianagaram 535 005, India
(Received 1 February 2011; final version received 16 March 2011)
The kinetics and mechanism of ruthenium(III) catalyzed oxidation of dl-methionine by alkaline hexa-
cyanoferrate(III) (HCF(III)) in an alkaline medium were studied spectrophotometrically at 30 0.1◦C.
The reaction was first-order-dependent each on [HCF(III)] and [ruthenium(III)] and fractional-order-
dependent on [alkali]. The rate of the reaction was found to be decreased with the increase in [methionine].
The main product of oxidation was methionine sulfone nitrile (3-(methylsulfonyl)propanenitrile) and it
was identified and confirmed by FT-IR and mass spectral studies. Further, no effect of added reaction
product was observed. A plausible mechanism was proposed involving complexation between methion-
ine and ruthenium(III) species, [Ru(H2O)5OH]2+. Thermodynamic parameters for the reaction, Ea and
ꢀS#, were computed using linear least squares method and are found to be 65.83 1.03 kJ/mol and
−249.58 3.35 J/K mol, respectively.
III
Ru
CH3 − S − (CH2)2 − CH (NH2) − COO− + 8HCF (III) −→ CH3 − SO2 − (CH2)2 − CN + CO2 + 8HCF (II)
H O
2
Keywords: oxidation of dl-methionine; hexacyanoferrate(III); ruthenium(III); mechanism
1. Introduction
Methionine is a sulfur-containing essential amino acid which acts as a powerful reductant in both
acid and alkaline medium. It plays an important role in human metabolism and several in-born
diseases result from disturbances in its biochemistry. Individuals with ‘methionine malabsorption
syndrome’are unable to absorb the compound from the gut and its deficiency leads to the formation
of a fatty liver, mainly due to the accumulation of triacyl glycerols.
Several studies have been reported on the oxidation of methionine by various oxidants such as
hexacyanoferrate(III) (HCF(III)) (uncatalyzed and osmium(VIII) catalyzed) (1, 2), quinolinium-
chlorochromate (3), ditelluratocuprate(III) (4), sodium N-bromobenzene-sulphonamide (5) and
dichloramine-T(6). HCF(III)actsasanelectron-abstractingagentandhasbeenwidelyusedtooxi-
dize numerous organic and inorganic compounds in alkaline medium. Ruthenium(III) is reported
*Corresponding author. Email: vani_chem@rediffmail.com
ISSN 1741-5993 print/ISSN 1741-6000 online
© 2011 Taylor & Francis
DOI: 10.1080/17415993.2011.573558
http://www.informaworld.com

264 B.D. Rao et al.
to be an efficient catalyst for such oxidations in alkaline medium (7–10). The uncatalyzed reac-
tion between dl-methionine and HCF(III) in alkaline medium was reported previously by Laloo
and Mahendra (2) at 55◦C. Since ruthenium(III) was found to catalyze the reaction considerably
at 30◦C and since no kinetic studies were reported earlier on the oxidation of dl-methionine
by HCF(III) using ruthenium(III) as a catalyst in alkaline medium, we have undertaken the title
reaction to explore the mechanism of oxidation of dl-methionine.
2. Results
2.1. Stoichiometry and product analysis
The reaction mixture containing excess [HCF(III)] over [methionine] in the presence of ruthe-
nium(III) at 0.2 mol/dm3 alkali was allowed to react at 30 0.1◦C. After 24 h, the residual
[HCF(III)] in each case was determined spectrophotometrically at 420 nm. The results obtained
indicate that 8 mol of HCF(III) was consumed by 1 mol of methionine. The stoichiometry of the
reaction was found to correspond to the equation
RuIII
CH3 − S − (CH2)2 − CH (NH2) − COO− + 8HCF (III) −→ CH3 − SO2
+H2O
− (CH2)2 − CN + CO2 + 8HCF(II)
The reaction product was isolated by a solvent extraction method using an organic solvent
diethyl ether and was identified by its IR spectrum (Figure 1), in which the absorption ban−d1s
(ν) at 1159 and 1322 cm−1 correspond to O S O stretching and the band (ν) at 2244 cm
= =
≡
corresponds to −C N stretching. The positions of the absorption bands for sulfone were found
to be in good agreement with the literature values of symmetric and asymmetric SO2 stretching
frequencies.Also the absorption band for nitrile was found to be close to the triple bond stretching
region of nitrile.
Further, the reaction product was confirmed by mass spectral studies (Figure 2); the com-
pound displayed a peak at m/z 133.1 which was obtained after the loss of one carbon and
three hydrogens from its molecular ion mass (m/z 148.02). In line with these observations,
from the IR and mass spectral studies, the product of oxidation of methionine by alkaline
HCF(III) in the presence of ruthenium(III) was confirmed to be methionine sulfone nitrile
(3-(methylsulfonyl)propanenitrile).
Figure 1. IR spectrum of methionine sulfone nitrile.

Journal of Sulfur Chemistry 265
Figure 2. The mass spectrum of methionine sulfone nitrile.
2.2. Reaction orders
2.2.1. Effect of [oxidant],[substrate],[alkali] and [ruthenium(III)]
To study the effect of [HCF(III)], it was varied in the range of 2.0–7.0 × 10−4 mol/dm3 and
the plot of log(absorbance) versus time was found to be linear, indicating that the order with
respect to [HCF(III)] was unity. The concentration of methionine was varied in the range of 0.5–
2.5 × 10−4 mol/dm3 at 30 0.1◦C keeping the concentrations of all other reactants constant.
The pseudo-first-order rate constants kꢀ were found to decrease with the increase in [methionine]
(Table 1). The effect of [alkali] on the reaction rate was studied, and the pseudo-first-order rate
constants kꢀ obtained wer−e found to increase with the increase in [OH−] (Table 1). Further, the
plot of 1/kꢀ versus 1/[OH ] was found to be a straight line with a positive intercept on the Y-axis,
indicating fractional-order dependence on [OH−] (Figure 3). To know the order with respect to
[ruthenium(III)] on the reaction rate, [ruthenium(III)] was varied by keeping the concentrations
of all other reactants constant. The plot of kꢀ versus [RuIII] was found to be a straight line passing
through the origin indicating unit order dependence on [RuIII].
2.2.2. Effect of hexacyanoferrate(II)
To study the effect of hexacyanoferrate(II), one of the products on the rate of reaction, kinetic
runs were carried out keeping the concentration of all other reactants constant and varying the
concentration of hexacyanoferrate(II). The constancy of rate constants obtained from the slopes
of pseudo-first-order plots showed that hexacyanoferrate(II), one of the reaction products, did not
have any significant effect on the reaction rate.
2.2.3. Effect of ionic strength
The effect of ionic strength was studied by varying the concentration of NaClO4 from 0.30 to
0.80 mol/dm3. The pseudo-first-order rate constants obtained from log(absorbance) versus time
plots showed that increasing the ionic strength (μ) increased the rate of the reaction (Table 1).
2.2.4. Test for free radicals
The test for free radicals was carried out by taking methionine, sodium hydroxide and ruthe-
nium(III) in a thumberg tube and acrylonitrile and HCF(III) in the bent tube. After evacuating the

266 B.D. Rao et al.
Table 1. Effect of [HCF(III)], [Met], [RuIII], [OH−] and ionic strength (μ) on the pseudo-first order rate constant (kꢀ)
at 30 0.1◦C.
[HCF(III)]×104
(mol/dm3)
[Met]×102
(mol/dm3)
[RuIII] × 106
(mol/dm3)
[OH−]
(mol/dm3)
μ (mol/dm3)
kꢀ × 104(s−1
)
2.0
3.0
4.0
5.0
6.0
7.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
1.00
1.00
1.00
1.00
1.00
1.00
0.50
0.75
1.00
1.50
2.00
2.50
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.5
5.0
5.5
6.0
7.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
0.20
0.20
0.20
0.20
0.20
0.20
0.20
0.20
0.20
0.20
0.20
0.20
0.20
0.20
0.20
0.20
0.20
0.20
0.20
0.10
0.15
0.20
0.25
0.30
0.35
0.20
0.20
0.20
0.20
0.20
0.20
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
0.4
0.4
0.4
0.4
0.4
0.4
0.3
0.4
0.5
0.6
0.7
0.8
8.43
8.44
8.44
8.38
8.40
8.40
11.89
10.75
8.44
7.68
6.91
5.76
8.44
12.57
14.60
16.03
17.65
21.11
23.00
5.37
6.50
8.50
9.20
11.10
13.01
8.44
10.75
13.05
16.00
17.02
19.06
2.0
1.6
1.2
0.8
0.4
0.0
[Met]=1.0 X10–2 mol dm–3
[HCF(III)]=4.0 X10–4 mol dm–3
[RuIII ]=4.0 X10–6 mol dm–3
µ= 0.4 mol dm–3
t=30 0.1ºC
0
2
4
6
8
10
12
1/ [OH– ], mol–1 dm3
Figure 3. Plot of 1/kꢀ versus 1/[OH−].

Journal of Sulfur Chemistry 267
system, the solutions were mixed by tilting the tube. The reaction mixture was kept aside, and
even after 24 h no precipitate was observed indicating the absence of free radicals.
2.2.5. Effect of temperature (T)
To study the effect of temperature on the reaction, the kinetic runs were carried out at four different
temperatures (25◦C, 30◦C, 35◦C and 40◦C). The plot of log kꢀ versus 1/T was found to be linear,
indicating that the reaction obeys Arrhenius temperature dependence. The energy of activation,
Ea, and entropy of activation, ꢀS#, were computed using a linear least squares method and found
to be 65.83 1.03 kJ/mol and −249.58 3.35 J/K mol respectively. A high negative entropy
value ꢀS# indicates that the transition state was rigid in nature.
3. Discussion
Methionine is a sulfur-containing amino acid and it has two pKa values, one correspond-
ing to the carboxylic acid group (pK1 = 2.28) and the other to the amino group (pK2 =
9.21). Under the experimental conditions employed in the present kinetic study ([OH−] =
0.2 mol/dm3), 99% of+methionine existed as H3CSCH2CH2CH(NH2)COO−(Met) and the rest
as H CSCH CH CH( H )COO−(Met ). Therefore, the major component, the anionic form of
N
3
2
2
3
Z
methionine (Met) was presumed to be the active species in alkaline medium.
HCF(III) is an octahedral inner orbital complex and acts as one electron oxidant. Trivalent
ruthenium is highly stable, and the following equilibrium exists in alkaline medium (11, 12):
[Ru(H2O)6]3+ + OH− ꢀ [Ru(H2O)5OH]2+ + H2O
Since the rate of the reaction increases with the increase in [OH−], the active form of
ruthenium(III) was considered to be [Ru(H2O)5OH]2+
.
Basedonthesefactsandtheexperimentalobservations, thefollowingmechanismwasproposed:
Mechanism
K1
[Ru(H2O)6]3+ + OH− ꢀ[Ru(H2O)5OH]2+ + H2O
(1)
(2)
(3)
(4)
K2
R − S − CH3((Met) + [Ru(H2O)5OH]2+ ꢀ Complex (C1)
K1
Met + C1 ꢀ Complex (C2) (less active)
K
C1 + HCF(III) −→ intermediate(I)
slow
I −→ R − + −CH + [Ru(H O) OH]2+ + HCF(II)
(5)
(6)
S
3
2
5
fast
R − + −CH + HCF(III)
−→
Products
H2O
S
3
(in several fast steps)
where R = −CH2CH2CH(NH2)COO−
−d[Fe(CN)36−
]
∴ Rate =
= k[C1][HCF(III)]
= kK2 [Met]e [Ru(H2O)5OH2+]e[HCF(III)]
(7)
(8)
dt

268 B.D. Rao et al.
where [Met]e = [Met]t and since
[RuIII]t = [Ru(H2O)63+]e + [Ru(H2O)5OH2+]e + [C1]e + [C2]e
(9)
and K1 = [Ru(H2O)5OH2+]e/[Ru(H2O)63+]e[OH−].
Therefore,
K1[OH−][RuIII]t
[Ru(H2O)5OH2+]e =
.
(10)
(11)
−
−
−
2
1 + K1[OH ] + K1K2[Met]t[OH ] + K1K2K3[Met]t [OH ]
Substituting for [Ru(H2O)5OH2+]e from Equation (10) into Equation (8) gives
kK1K2[Met]t[RuIII]t[HCF(III)][OH−]
Rate =
.
−
−
−
2
1 + K1[OH ] + K1K2[Met]t[OH ] + K1K2K3[Met]t [OH ]
The above rate equation explains the first order dependence on [HCF(III)] and [RuIII], fractional
order with respect to [OH−] and decrease in the rate with the increase in [methionine].
Since
Rate
kK1K2[Met]t [RuIII]t[OH−]
= kꢀ =
,
(12)
−
−
−
2
[HCF(III)]
1 + K1[OH ] + K1K2[Met]t[OH ] + K1K2K3[Met]t [OH ]
Equation (12) may be transformed to
1
kꢀ
1
1
1
K3[Met]t
k[RuIII]t
=
+
+
+
.
(13)
kK1K2[Met]t[RuIII]t[OH−] kK2[Met]t[RuIII]t
k[RuIII]t
The above equation predicts the plot of 1/kꢀ versus 1/[OH−] to be a straight line with a positive
intercept on the Y-axis. A similar plot was obtained experimentally, thus supporting the proposed
mechanism.
−
−
−
2
t
Further assuming 1 + K1[OH ] + K1K2[Met]t[OH ] << K1K2K3[Met] [OH ], expres-
sion (12) becomes
k[RuIII]t
kꢀ =
.
(14)
K3[Met]t
From this expression, one can expect the plot of kꢀ versus 1/[Met]t (Figure 4) at a constant
concentration of RuIII and OH− to be a straight line with a positive slope passing through the
origin. However, a straight line plot with a positive intercept on the Y-axis was obtained, which
indicates the contribution of the term (1 + K1[OH−] + K1K2[Met]t[OH−]) in the denominator
of expression (12). Hence, from the above treatment, it may be concluded that the rate expression
explains the observed kinetic results.Therefore, the rate law for ruthenium(III) catalyzed oxidation
of methionine by HCF(III) was given by
−d[Fe(CN)36−
dt
]
kK1K2[Met]t[RuIII]t[OH−]
Rate =
=
.
−
−
−
2
1 + K1[OH ] + K1K2[Met]t[OH ] + K1K2K3[Met]t [OH ]
(15)
Further, from a critical examination of the nature and the conditions under which the reaction
was investigated, the following arguments point towards these most likely conclusions.

Journal of Sulfur Chemistry 269
–4
–3
12
10
8
[HCF(III)]=4.0 X10 mol dm
III
–6
–3
[Ru ]=4.0 X10 mol dm
–
–3
[OH ] = 0.2 mol dm
–3
µ = 0.3 mol dm
t=30 0.1ºC
6
4
2
0
0
5
10
15
20
25
1
–1
3
1/ [Met ] X 10 , mol dm
Figure 4. Plot of kꢀ versus 1/[Met].
• [Ru(H2O)6]3+ isknowntoundergoligandreplacementthroughadissociativemechanismwhich
is favored by a decrease in the overall charge on the complex. Hence [Ru(H2O)5OH]2+ with
a lower overall charge on the complex is supposed to undergo ligand replacement faster than
[Ru(H2O)6]3+, thus facilitating attack by the substrate molecule.
• Further, OH− is a negatively charged ligand comparatively less labile than neutral H2O. Hence,
dissociation of the hydroxo species through loss of a water molecule makes a coordination site
on the metal accessible for attack by the substrate.
• The substrate is most likely attached through the sulfur atom rather than through oxygen.
Comparatively, the ruthenium ion in the complex with a dipositive charge can be considered to
be nearer to class ‘b’ metal ions compared with the ruthenium in the complex with tripositive
charge. As such, the decrease in the overall charge on ruthenium complex due to the formation
of hydroxyl species increases its tendency for complexation through sulfur which is also a class
‘b’ base.
• The attack by the substrate on [Ru(H2O)5OH]2+ species is facilitated because of less steric
hindrance compared with the attack on [Ru(H2O)6]3+. As such, the steric effect also facilitates
a faster reaction with [Ru(H2O)5OH]2+
.
• Further replacement of the water molecule from the hydroxyl ruthenium(III)–substrate complex
(C1) can attach to another substrate molecule through sulfur. Such a possibility decreases the
scope for an effective electron transfer from sulfur to ruthenium, and most likely an equilibrium
in electron distribution may exist in the S–Ru–S (C2) moiety, thus decreasing the rate of the
reaction.
• In a similar way, there is the possibility of attachment of HCF(III) to ruthenium(III) in C1 via
cyanide bridge to form an intermediate (I). In this intermediate, ruthenium(III) facilitates the
electron transfer from substrate to HCF(III), thus catalyzing the oxidation of methionine by
HCF(III).
These arguments are in support of the following intimate mechanism proposed for this
reaction.

270 B.D. Rao et al.
Intimate mechanism
2+
3+
OH2
OH2
Ru
H2O
H2O
H2O
H2O
OH
OH2
OH2
K1
+
OH-
Ru
+ H2O
OH2
OH2
OH2
….(1)
+
2+
OH
Ru
2
OH
2
CH
S
OH
3
(
H O
2
OH
H O
2
K
2
CH )
CH COO
+
2
Ru
2
(
S
CH )
2
CH COO
H O
2
2
NH
2
O
H O
2
H
2
H O
2
O
NH
2
H
2
CH
3
( C )
1
….(2)
OH
Ru
2
OH
CH
S
H O
3
2
K
3
(
CH
)
+
CH COO
C
2
1
2
(
S
CH )
2
CH COO
NH
OOC
(
S
HC
)
2
H C
2
NH
2
2
H O
2
CH
CH
2
3
3
….(3)
_
OH 2
Ru
Fe(CN)
OH 2
HO
- S
6
k
+
HFC(III)
C
1
slow
_
OOC - HC - (H C)2
NH 2
2
OH 2
CH3
( I )
….(4)
….(5)
_
+
I
CH3 - S - (CH2)2 - CH (NH2) - COO +[Ru(H2O)5OH]2+
+
HCF(II)
fast
O
CH - S - (CH ) - CH (NH ) - COO
_
_
+
H O
2
fast
CH - S - (CH ) - CH (NH ) - COO
+
HCF(III)
+
HCF(II)
3
2 2
2
3
2 2
2
….(6)
O
O
_
_
CH - S - (CH ) - CH (NH ) - COO + HCF(III)
CH - S - (CH ) - CH (NH ) - COO + HCF(II)
3 2 2 2
+
3
2 2
2
fast
….(7)

Journal of Sulfur Chemistry 271
O
O
_
_
_
H O
2
fast
CH - S - (CH ) - CH (NH ) - COO
+
+
HCF(III)
HCF(III)
CH - S - (CH ) - CH (NH ) - COO
+ HCF(II)
3
2 2
2
3
2 2
2
+
O
O
….(8)
O
.
CH - S - (CH ) - CH - NH + CO
2
+ HCF(II)
CH - S - (CH ) - CH (NH ) - COO
3
2 2
2
3
2 2
2
fast
O
O
….(9)
O
O
.
__
__
CH –S - (CH ) –CH - NH
+
3HCF(III)
CH - S - (CH ) - C __ N
+
3 HCF(II)
3
2 2
2
3
2 2
fast
O
O
….(10)
4. Experimental
Kinetic measurements were performed on a Milton Roy Spectronic-1201 UV–visible spectropho-
tometer at the HCF(III) absorption maximum 420 nm, where the other constituents of the reaction
mixture did not have significant absorbance.
4.1. Materials and methods
The standard solutions of dl-methionine in 0.1 mol/dm3 NaOH were prepared afresh by
dissolving the sample in a requisite volume of sodium hydroxide and its strength was deter-
mined iodometrically (13). The other chemicals used were HCF(III), NaOH, ruthenium(III) and
NaClO4. All chemicals used were of AR grade. Double-distilled water was used throughout the
investigation.
HCF(III) (E.Merck) was prepared by dissolving the requisite amount of potassium HCF(III)
in double-distilled water. The solution was standardized by measuring the absorbance using a
Milton Roy UV–visible 1201 spectrophotometer at 420 nm (ε = 1060 50 dm3/mol/cm). Ruthe-
nium(III) solution was prepared by dissolving ruthenium chloride in 2.0 mol/dm3 sulfuric acid
(Johnson Matthey, London) and its strength was determined by the titration method suggested by
Beamish andValnoon (14). Sodium hydroxide (E.Merck) solution was prepared and standardized
by potassium hydrogen phthalate.
4.2. Instruments used
A Milton Roy Spectronic-1201 UV–visible spectrophotometer, Shimadzu IR-Affinity-1 spec-
trophotometer, Agilent MSDS Mass spectrometer and Siskin Julabo V Constant Temperature
liquid circulatory bath were used.
4.3. Kinetic measurements
All kinetic measurements were performed under pseudo-first-order conditions where [methionine]
is in excess over [HCF(III)] at a constant ionic strength of 0.3 mol/dm3 in alkaline medium in the
presence of ruthenium(III) at a constant temperature of 30 0.1◦C. The progress of the reaction
was followed by measuring the absorbance of HCF(III) at 420 nm. As no other species except

272 B.D. Rao et al.
HCF(III) had any significant absorption at this wavelength under the conditions employed, the
absorbance of the solution was taken as a measure of the residual concentration of [HCF(III)] at
time ‘t’. Plots of log(absorbance) versus time were found to be linear up to 80% completion of
the reaction, indicating that the reaction is first order with respect to [HCF(III)]. The pseudo-first-
order rate constants calculated from these plots were denoted by kꢀ and the rate constants were
found to be reproducible within 5%.
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