Kinetic and mechanistic study of the oxidative deamination and decarboxylation of L-valine by alkaline permanganate

Article abstract of DOI:10.1007/s007060070075

The kinetics of the oxidation of L-valine, (L-Val) by permanganate in aqueous alkaline medium at a constant ionic strength of 0.50 mol · dm^-3 was studied spectrophotometrically. The reaction is of first order in [permanganate ion] and of fractional order in both [L-Val] and [alkali]. Addition of products has no significant effect on the reaction rate. However, increasing ionic strength and decreasing dielectric constant of the medium increase the rate. The oxidation process in alkaline medium has been shown to proceed via two paths, one involving the interaction of L-valine with permanganate ion in a slow step to yield the products, and the other path the interaction of alkali with permanganate ion to give manganate. Some reaction constants involved in the mechanism were determined; calculated and observed rate constants agree excellently. The activation parameters were computed with respect to the slow step of the mechanism.

Full text of DOI:10.1007/s007060070075

Monatshefte fuÈr Chemie 131, 739±748 (2000)
Kinetic and Mechanistic Study of the Oxidative
Deamination and Decarboxylation of L-Valine
by Alkaline Permanganate
Abdulazizkhan L. Harihar, Mohammadsaleem R. Kembhavi,
Ã
and Sharanappa T. Nandibewoor
Post-Graduate Department of Studies in Chemistry, Karnatak University, Dharwad-580003, India
Summary. The kinetics of the oxidation of L-valine, (L-Val) by permanganate in aqueous alkaline
3
medium at a constant ionic strength of 0.50 mol Á dm was studied spectrophotometrically. The
reaction is of ®rst order in [permanganate ion] and of fractional order in both [L-Val] and [alkali].
Addition of products has no signi®cant effect on the reaction rate. However, increasing ionic strength
and decreasing dielectric constant of the medium increase the rate. The oxidation process in alkaline
medium has been shown to proceed via two paths, one involving the interaction of L-valine with
permanganate ion in a slow step to yield the products, and the other path the interaction of alkali with
permanganate ion to give manganate. Some reaction constants involved in the mechanism were
determined; calculated and observed rate constants agree excellently. The activation parameters were
computed with respect to the slow step of the mechanism.
Keywords. Kinetics; Oxidation; L-Valine; Activation parameters.
Introduction
Oxidation with permanganate has found extensive application in organic syntheses
[1±7], especially after the advent of phase transfer catalysis [3, 4, 6] which permits
the use of solvents like dichloromethane and benzene. The manganese chemistry
involved in these multistep redox reactions is an important source of information as
the manganese intermediates are relatively easy to identify when they have
suf®ciently long life times. In strongly alkaline medium, the stable reduction
product of permanganate is the manganate ion, MnO24 [8]. No mechanistic
information is available allowing to distinguish between a direct one electron
reduction to manganese (VI) (Scheme 1) or the prior formation of hypomanganate
in a two electron step followed by a fast reaction (Scheme 2).
Ã
Corresponding author

740
A. L. Harihar et al.
k10
Á
MnꢀVII† ‡ S ! MnꢀVI† ‡ S
k20
MnꢀVII† ‡ SÁ ! MnꢀVI† ‡ Products
ꢀS ˆ substrate; k20 ꢁ k10 †
Scheme 1
k30
MnꢀVII† ‡ S ! MnꢀV† ‡ Products
k40
MnꢀVII† ‡ MnꢀV† ! 2MnꢀVI†
ꢀS ˆ substrate; k40 ꢁ k30 †
Scheme 2
Amino acids have been oxidised by a variety of oxidants [9] leading to different
products [10]; the dependence of the behaviour of amino acids towards the oxidant
is of importance because of their biological signi®cance.
A variety of organic [11] and inorganic [8] substrates are oxidized by
permanganate in aqueous alkaline medium. However, there are only a few reports
on the oxidation of amino acids by aqueous alkaline permanganate [12]. Although
the oxidation of rac-serine by alkaline permanganate has been studied recently
[12], the behaviour of L-valine towards the same reagent is of interest to verify
nature of the active species, the actual site of oxidation, changes in mechanism, etc.
Results and Discussion
Reaction order
The reaction orders were determined from the slopes of logkobs vs. log c plots by
varying the concentration of oxidant, reductant, and alkali in turn while keeping the
others constant. The potassium permanganate concentration was varied in the
5
3
range of 5.0 Â 10 to 5.0 Â 10 4 mol Á dm , and the linearity of plots of
log[MnO4 ] vs. time indicates an order of 1 in [MnO4 ]. This was con®rmed by the
fact that variation of [MnO4 ] did not in¯uence the pseudo-®rst-order rate constants
(Table 1). [L-Val] was varied in the range of 5.0 Â 10 4 to 5.0 Â 10 3 mol Á dm 3 at
27ꢂC; the order with respect to [L-Val] was found to be less than unity (Table 1).
Effect of initially added products
Initially added reaction products, such as manganate, aldehyde, and ammonia, did
not show any signi®cant effect on the rate of the reaction.
Effect of alkali
The effect of alkali on the reaction was studied at constant concentrations of L-
3
valine and potassium permanganate, I ˆ 0.50 mol Á dm , and 27ꢂC. The rate
constants increased with increasing [OH ] (Table 1).

Oxidation of L-Valine by Alkaline MnO4
741
Table 1. Effects of [L-valine], [MnO4 ], and [OH ] on the oxidation of L-valine by permanganate in
3
aqueous alkaline medium at 27ꢂC (I ˆ 0.5 mol Á dm
)
‰L-ValŠ  103
‰MnO4 Š  104
‰OH Š
kobs  103
3
3
3
1
s
mol Á dm
mol Á dm
mol Á dm
Expt.
Calcd.a
0.5
1.0
2.0
3.0
5.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
0.5
1.0
2.0
3.0
5.0
2.0
2.0
2.0
2.0
2.0
0.3
0.49
0.76
1.15
1.70
2.60
1.15
1.15
1.15
1.15
1.15
0.32
0.56
0.82
1.15
1.54
0.50
0.70
1.10
1.51
2.32
1.10
1.10
1.10
1.10
1.10
0.32
0.55
0.85
1.10
1.45
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.05
0.10
0.20
0.30
0.50
a
1
Calculations of rate constants are performed on the basis of Eq. (2) using 5.14 dm3 Á mol
,
0.66 dm3 Á mol Á s 1, and 1.0 Â 10 3 dm3 Á mol Á s for K, k1, and k2, respectively
1
1
1
Effect in ionic strength and solvent polarity
The effect of ionic strength was studied by varying the sodium perchlorate
3
concentration from 0.3 to 2.0 mol Á dm at constant concentrations of permanga-
nate, L-valine, and alkali. It was found that the rate of reaction increased with
increasing concentration of NaClO4; the plot of logkobs vs. I1=2 is linear with a
positive slope (Fig. 1).
The relative permittivity effect was evaluated by variation of the t-butanol
content while keeping all other conditions constant. Attempts to measure the
relative permittivities failed; however, they were computed from the values of pure
liquids as in Ref. [13]. There was no reaction of the solvent with the oxidant. The
rate constants kobs increase with decreasing dielectric constant of the medium, and
the plot of logkobs vs. 1/D is linear (Fig. 1).
Test for free radicals
The intervention of free radicals was examined as follows. The reaction mixture, to
which a known quantity of acrylonitrile scavenger had been added initially, was
kept in an inert atmosphere for one hour. Upon diluting the reaction mixture with
methanol, a precipitate resulted, suggesting the participation of free radicals in the
reaction.
The permanganate ion is a powerful oxidant in aqueous alkaline medium. As it
exhibits a multitude of oxidation states, the stoichiometric results and the pH of

742
A. L. Harihar et al.
Fig. 1. Plot of logkobs vs. I1=2 and logkobs vs. 1/D
reaction media play a signi®cant role. Diode array rapid scan spectrophotometer
(DARSS) studies have shown that at pH > 12 the product of the reaction of Mn(VII)
is Mn(VI), and no further reduction was observed [8]. However, upon prolonged
standing Mn(VI) is slowly reduced to Mn(IV).

Oxidation of L-Valine by Alkaline MnO4
743
Scheme 3
The reaction between permanganate and L-valine in alkaline medium has a
stoichiometry of 2:1 with fractional order on alkali and L-valine concentrations and
®rst order on permanganate concentration. The observed kinetic and other results
may be explained by a mechanism proceeding via two pathways, one of which
depends on substrate concentration, whereas the other does not (Scheme 3).
The observed fractional order in alkali concentration may be due to the
existence of L-valine in its anionic form in alkaline medium [14]. This anionic
form will react with MnO4 under decarboxylation in a slow step, giving a radical
of L-valine which reacts further with another molecule of permanganate in a fast
step to afford the products. Since permangante is a one-electron oxidant in alkaline
medium, the reaction between substrate and oxidant would give rise to a radical
intermediate as indicated by a free radical scavenging experiment. This type of
radical intervention in the oxidation of amino acids has also been observed earlier
[12].
The combined rate law is given by Eqs. (1) and (2):
d‰MnO4 Š K Á k1 Á ‰MnO4 Š Á ‰L ValŠ Á ‰OH Š
ˆ
‡ k2 Á ‰MnO4 Š Á ‰OH Š ꢀ1†
dt
1 ‡ K Á ‰OH Š
K Á k1 Á ‰L ValŠ Á ‰OH Š
kobs
ˆ
‡ k2 Á ‰OH Š ꢀ2†
1 ‡ K Á ‰OH Š
According to Eq. (2), a plot of kobs vs. [L-valine] gave a better correlation than a
reciprocal plot (Fig. 2). From intercept of the linear plot, the value of k2 was
1
1
obtained as 1.0 Â 10 3 dm3 Á mol Á s .
Rearrangement of Eq. (2) leads to
1
1
1
ˆ
‡
ꢀ3†
kobs k2 Á ‰OH Š K Á k1 Á ‰L ValŠ Á ‰OH Š k1 Á ‰L ValŠ

744
A. L. Harihar et al.
Fig. 2. Plot of kobs vs. [L-valine] and 1/kobs vs. 1/[L-valine] (conditions as in Table 1)
According to Eq. (3), a plot of 1/kobs k2[OH ] vs. 1/[OH ] is expected to be linear
1
1
(Fig. 3). Intercept and slope of this plot lead to k1 ˆ 0:66 Æ 0:03 dm3 Á mol Á s
and K ˆ 5:14 Æ 0:25 dm3 Á mol at 27ꢂC. These values were used to calculate the
rate constants under different experimental conditions. There is a reasonable
agreement between the calculated and observed rate constants (Table 1); the value of
k2 thus obtained agrees with earlier investigations [15].
1
Effect of temperature
The rate constants (k1 and k2) of the slow steps of Scheme 3 were obtained from the
intercepts of plots of kobs vs. [L-valine] and 1=kobs k2‰OH Š vs. 1/[OH ] at different
temperatures and used to calculate the activation parameters. The values of k1 are
1
1
3
3
0.66, 1.0, 1.34, and 1.99 dm3 Á mol Á s , those of k2 1.0 Â 10 , 1.66 Â 10 , 2.33
 10 , and 3.20  10 3 dm3 Á mol Á s at 27, 32, 37, and 42ꢂC, respectively. Two
sets of activation parameters (for Path I and Path II of Scheme 3) corresponding to
these constants were evaluated from the plots of log(k1 or k2) vs. 1/T (Table 2).
The effect of increasing ionic strength on the rate of reaction explains
qualitatively the reaction between two negatively charged ions (Scheme 3). However,
increasing the content of t-butanol in the reaction medium leads to an increase of the
3
1
1

Oxidation of L-Valine by Alkaline MnO4
745
1
1
Fig. 3. Plot of
vs.
(conditions as in Table 1)
kobs k2‰OH Š
‰OHŠ
Table 2. Activation parameters for the oxidation of L-valine by alkaline permanganate with respect to
the slow step of the reaction (Scheme 3)
Activation parameters
Path I
Path II
1
Ea/kJ Á mol
42 Æ 4
51 Æ 5
logA
ÁH #/kJ Á mol
9.6 Æ 3.0
40 Æ 3
7.6 Æ 2.0
48.6 Æ 4.0
140 Æ 7
1
1
1
ÁS #/J Á K Á mol
116 Æ 6
75 Æ 4
ÁG #/kJ Á mol
90 Æ 4
1
reaction rate, which is contrary to the expected slower reaction between ions of the
same polarity in media of lower relative permittivity. Perhaps the effect is opposed
substantially by an increased formation of active reaction species in low-permittivity
media, thus leading to the observed net increase in the rate [16]. The values of ÁS #,
within the range for radical reactions, have been ascribed to the nature of electron
pairing and unpairing processes and to the loss of degrees of freedom formerly
available to the reactands upon the formation of a rigid transition state [17].
It is interesting to note that the oxidant species [MnO4 ] requires pH > 12; at
lower values, the system gets disturbed, and the reaction proceeds further to give a

746
A. L. Harihar et al.
reduced product of the oxidant, Mn(IV), which slowly, develops yellow turbidity.
Thus, the role of pH is crucial in this reaction. It is also noteworthy that under the
conditions studied the reaction occurs in two successive one-electron reductions
(Scheme 3) rather than as a two-electron reduction in a single step (Scheme 2).
Experimental
Materials
Series of stock solutions of L-valine (Sisco-Chem. Ltd.) and KMnO4 (BDH) were prepared by
dissolving the appropriate amounts of samples in doubly distilled water. The stock solution of
KMnO4 was standardized against oxalic acid [18]. K2MnO4 solution was prepared as described in
Ref. [19] as follows: A solution of KMnO4 was heated to boiling (above 120ꢂC) in an aqueous 8.0 M
KOH solution until a green colour developed. The solid K2MnO4 formed upon cooling was
recrystallized from the same solvent. The stock solution of K2MnO4 was prepared in aqueous
potassium hydroxide. The solution was standardized by measuring its absorbance using a Hitachi
1
150 20 spectrophotometer with a 1 cm cell at 608 nm (" ˆ 1530 Æ 20 dm3 Á mol Á cm 1). All other
reagents were of analytical grade purity, and their solutions were prepared by dissolving the required
amounts of sample in doubly distilled conductivity water. NaOH and NaClO4 were used to provide
the required alkalinity and to maintain the correct ionic strength, respectively.
Kinetic measurements
All kinetic measurements were performed under pseudo-®rst-order conditions with [L-valine]:
[MnO4 ] ꢃ 10:1 at a constant ionic strength of 0.5mol Á dm 3. The reaction was initiated by mixing
previously thermostatted solutions of MnO4 and L-valine which also contained the required quantities
of NaOH and NaClO4 to maintain the required alkalinity and ionic strength. The temperature was kept
at 27Æ 0.1ꢂC. The progress of the reaction was followed by monitoring the decrease in the absorbance
of MnO4 in a 1 cm quartz cell of a Hitachi 150 20 spectrophotometer at 526 nm as a function of time.
Preliminarily it was checked that there is negligible interference from other reagents at this
wavelength. The application of Beer's law for MnO4 at 526 nm had earlier been veri®ed, giving
1
1
" ˆ 2083 Æ 50 dm3 Á mol Á cm (Ref. [8]: " ˆ 2200). The ®rst-order plots were linear up to 85%
completion of the reaction in almost all cases, and kobs were reproducible within Æ 5%. In the course
of the measurements the colour of the solution changed from violet to blue and further to green. The
spectrum of the green solution was identical to that of MnO24 . It was empirically found that the blue
colour originated from a mixture of violet (MnO4 ) and green (MnO24 ) rather than from the formation
of hypomanganate. The formation of Mn(VI) was also evidenced by the decrease of the absorbance of
Mn(VII) at 526 nm and the increase of that of Mn(VI) at 608 nm during the course of reaction (Fig. 4).
The effect of dissolved oxygen on the rate of reaction was checked by preparing the reaction
mixture and following the reaction in an atmosphere of nitrogen. No signi®cant difference between
the results was observed. Added carbonate had no effect on the reaction rate.
Stoichiometry
The reaction mixtures containing an excess of permanganate over L-valine (see above) were mixed
in the presence of 0.30 M NaOH adjusted to a constant ionic strength of 0.50 mol Á dm 3. After
completion of the reaction, solid KI was added, followed by acidi®cation by 10% H2SO4. The
remaining MnO4 was then titrated against standard sodium thiosulfate [20]. The results indicated
that two moles of MnO4 consumed one mole of L-valine (Eq. (4)). The main oxidation products

Oxidation of L-Valine by Alkaline MnO4
747
Fig. 4. Spectral changes during the oxidation of L-valine by alkaline permanganate; [MnO4 ] ˆ 2 Â
10 4, [L-valine] ˆ 2  10 3, [OH ] ˆ 0.30, I ˆ 0.5 mol Á dm 3, scanning time interval 2.0 min, 27ꢂC
were identi®ed as isobutylaldehyde (spot test [21]), ammonia (Nessler's reagent [22]), and MnO42
(UV/Vis spectrum). CO2 was qualitatively detected by bubbling N2 gas through the acidi®ed reaction
mixture and passing the liberated gas through a tube containing lime water. The nature of the
aldehyde was con®rmed by recording its IR spectrum and by preparing its 2,4-DNP derivative. The
1
1
IR spectrum showed a carbonyl stretching vibration at 1729 cm and a band at 2928 cm for
aldehydic C-H stretching, thus con®rming the presence of isobutylaldehyde. The aldehyde did not
undergo further oxidation under the applied experimental conditions.
ꢀCH3† CH CH
ꢀCH †
CH CHO ‡ CO2 ‡ NH3 ꢀ4†
COOH ‡ 2MnO4 ‡ 2OH
!
3
2
2
‡2MnO22 ‡ H2O
j
NH2

748
A. L. Harihar et al.: Oxidation of L-Valine by Alkaline MnO4
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Received December 30, 1999. Accepted (revised) March 6, 2000