Delayed autocatalytic behavior of Mn(II) ions at a critical ratio: The effect of structural isomerism on permanganic oxidation of L-norleucine

Article abstract of DOI:10.1002/kin.20131

The kinetics of the permanganic oxidation process of L-norleucine, L-leucine, L-isoleucine, and L-tert-leucine in strong acid medium has been investigated using a spectrophotometric technique. Conclusive evidences have proven autocatalytic activity of Mn(II) for these reactions in strong acid medium analogous to weak acid medium, but in the former, ratio of Mn(II) to amino acid concentration must reach a certain amount for autocatalytic phenomenon to emerge, which we call "critical ratio." This critical ratio depends on the nature of the amino acid employed. Thus considering "delayed autocatalytic behavior" of Mn(II) ions, rate equations satisfying observations for both catalytic and noncatalytic routes have been presented. Kinetic data in a noncatalytic pathway have been fitted to a biparametric equation including inductive, steric, and hyperconjugation correction effects, and it is determined that by shifting the side branch on a carbon chain toward an α-carbon atom (adjacent to amino acid's functional group) and also adding branches to the α-carbon atom, the reaction rate in the noncatalytic pathway decreases. Inductive and steric hindrance factors in amino acid's carbon chain are effective on processes' rate both in catalytic and noncatalytic pathways.

Full text of DOI:10.1002/kin.20131

Delayed Autocatalytic
Behavior of Mn(II) Ions at
a Critical Ratio: The Effect
of Structural Isomerism
on Permanganic Oxidation
of L-Norleucine
HOMAYOON BAHRAMI, MANSOUR ZAHEDI
Department of Chemistry, Faculty of Sciences, Shahid Beheshti University, Evin, Tehran 19839, Iran
Received 3 February 2005; accepted 22 June 2005
DOI 10.1002/kin.20131
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The kinetics of the permanganic oxidation process of L-norleucine, L-leucine, L-iso-
leucine, and L-tert-leucine in strong acid medium has been investigated using a spectropho-
tometric technique. Conclusive evidences have proven autocatalytic activity of Mn(II) for these
reactions in strong acid medium analogous to weak acid medium, but in the former, ratio of
Mn(II) to amino acid concentration must reach a certain amount for autocatalytic phenomenon
to emerge, which we call “critical ratio.” This critical ratio depends on the nature of the amino
acid employed. Thus considering “delayed autocatalytic behavior”of Mn(II) ions, rate equations
satisfying observations for both catalytic and noncatalytic routes have been presented. Kinetic
data in a noncatalytic pathway have been fitted to a biparametric equation including inductive,
steric, and hyperconjugation correction effects, and it is determined that by shifting the side
branch on a carbon chain toward an α-carbon atom (adjacent to amino acid’s functional group)
and also adding branches to the α-carbon atom, the reaction rate in the noncatalytic pathway
decreases. Inductive and steric hindrance factors in amino acid’s carbon chain are effective on
processes’ rate both in catalytic and noncatalytic pathways. ꢀ 2005 Wiley Periodicals, Inc. Int
C
J Chem Kinet 38: 1–11, 2006
ceived considerable attention since the 1970s [1–16].
In most of these studies, the reaction has been con-
sidered as a process that follows a perfect second-
order kinetics at constant [H+] and without any au-
tocatalytic effect present. There are two reports of a
“double stage” process in which every stage is shown to
have kinetics identical to a pseudo-first-order reaction
[10,14].
INTRODUCTION
Kinetic and mechanistic study of amino acid oxida-
tion in strong acid medium by permanganate has re-
Correspondence to: Mansour Zahedi; e-mail: m-zahedi@cc.
sbu.ac.ir.
c
ꢀ
2005 Wiley Periodicals, Inc.

2
BAHRAMI AND ZAHEDI
In our previous studies [15,16], we have reported
corresponding aldehyde was isolated as its 2,4-dinitro-
phenylhydrazone derivatives. The melting points of the
abovementioned 2,4-dinitrophenylhydrazone deriva-
tives were compared with the reference compounds and
confirmed [20]. Reaction mixtures containing various
ratios of amino acid to MnO−4 were mixed in the pres-
ence of 3.16 mol dm−3 H2SO4, then equilibrated for
24 h at room temperature. An estimate of the unre-
acted MnO−4 showed that 1 mol of MnO−4 consumed
2.5 mol of the amino acid [12]. In view of these results
and taking into account that the reduction final product
of permanganate in acid medium and in large excess
of reducing species is the Mn2+ ion [1,3,11,21], the
following equation was obtained:
the presence of an autocatalytic effect in which Mn2+
is an autocatalytic agent. In the present investigation,
wehaveexaminedoxidationofL-norleucine, L-leucine,
L-iso-leucine, and L-tert-leucine in strong acid medium
by permanganate. The aforementioned amino acids are
all structural isomers, which differ just with respect to
their branching of carbon chain. Thereby, employing
such isomers enable us to investigate the effect of struc-
tural isomerism regarding the carbon chain branching
on permanganic oxidation of amino acids. Conclusive
evidences show that these reactions are autocatalized,
and Mn2+ is the autocatalytic agent. However, it should
be noted that for the delayed autocatalytic phenomenon
to appear, a certain ratio of [Mn2+] to amino acid con-
centration requires to be reached. This concentration
ratio, which we call “critical ratio,” depends on the
nature of the amino acid used. As such, erroneous re-
sults may be yielded by not following the process long
enough well after 50% completion. Also it has been
found that the autocatalytic activity starts with a cer-
tain delay after the onset of the reaction. Thus, it is not
possible to figure out whether such an activity does ac-
tually exist if one just suffices to a common sigmoid
profile of absorbance versus time curves. Rather curves
of rate–time data would clearly indicate the autocat-
alytic activity at such instances.
2Mno−4 + 5R-CH(N+H3)COOH + 6H+
−→ 5R-CHO + 5NH+4 + 5CO2 + 2Mn2+ + 3H2O
(1)
where R is H2C CH2 CH2 CH3 for L-norleucine,
H2C CH(CH3) CH3 for L-leucine,
HC(CH3)
CH2 CH3 for L-iso-leucine, and C(CH3)3 for L-
tert-leucine.
Rate Equation
Figure 1 illustrates the permanganate absorption vs.
wavelength curves for L-leucine oxidation, obtained
at 4 min intervals as the reaction proceeded. From
previous studies [22–30], it is known that Mn(IV)
ions absorb in the 400–650 nm region, so the ab-
sorption profiles suggest that MnO2 is not a reaction
product. Furthermore, since no rise and fall in ab-
sorption is observed at 418 nm, it is concluded that
Mn(IV) ions do not intervene as possible oxidizing
agents unless they are short lived. Also the same results
have been obtained for L-norleucine, L-iso-leucine, and
L-tert-leucine, while all amino acids have been treated
with 3.2–7 M H2SO4.
In Fig. 2 reaction rate-time curves for L-norleucine,
L-leucine, L-iso-leucine, and L-tert-leucine have been
shown. The reaction rate vs. time curves for L-norleu-
cine, L-iso-leucine, and L-tert-leucine show that the re-
action rate initially falls and then grows after 35%,
45%, and 50% reaction completion, respectively. Such
percentages are judged by the differences between the
absorbance for the reaction’s onset vs. start of the au-
tocatalysis process. Therefore, a certain species must
have acted as a catalyst. For the case of L-leucine, how-
ever, there is no increase in reaction rate while the re-
action progresses to completion.
EXPERIMENTAL
All the reagents were obtained from Merck or Sigma.
The solutions were prepared with tridistilled water
deionized and boiled. The permanganate solutions
were prepared and tested by the Vogel method [17].
The kinetic progress of the reaction was followed by
measuring the absorbance of the permanganate ions at
525nmwithathermostated(ShimadzuTB-85, 0.1K)
Shimadzu (1605 model) UV–VIS spectrophotometer
within the temperature range of 25–45◦C. For the per-
manganate molar absorption coefficient at 525 nm and
in a strong acid solution, a value of 2.26 × 103 dm3
mol−1 cm−1 was obtained. In all the kinetic measure-
ments, amino acid was used in an excess amount rela-
tive to permanganate.
RESULTS AND DISCUSSION
Stoichiometry and Product Analysis
For all amino acids studied, the following reaction
products were detected: ammonium ions [18], carbon
dioxide [19], and the corresponding aldehyde. The
Figure 3 illustrates rate–time curves for L-norleu-
cine in the absence and presence of considerable

DELAYED AUTOCATALYTIC BEHAVIOR OF Mn(II) IONS AT A CRITICAL RATIO
3
Figure 1 Spectral changes in the oxidation of leucine by permanganate ion. [KMnO4] = 4 × 10−4 mol dm−3, [L-leucine] =
0.1 mol dm−3, [H2SO4] = 3.16 mol dm−3, T = 323 K. Scanning time intervals = 4 min. [Color figure can be viewed in the
online issue, which is available at www.interscience.wiley.com.]
initial concentration of Mn(II). Considering Fig. 3,
it is obvious that the reaction rate increase portion
of the reaction rate vs. time curve is annihilated by
increasing Mn2+, while at the same time a sharp de-
crease in reaction half-life is observed. Same trends for
L-iso-leucine and L-tert-leucine similar to L-norleucine
have been observed, so it could be concluded that Mn2+
is the autocatalytic agent. But in contrast to our previ-
ous studies [15,16], for Mn2+ to show “delayed au-
tocatalytic characteristic,” it should acquire a certain
concentration, which we call “critical concentration.”
This critical concentration depends on the nature of
amino acid used. As has been claimed elsewhere [31],
a sigmoidal profile for absorbance vs. time plot can
be taken as an evidence for an autocatalytic process.
However, such profiles could not be observed in our
certain experiments, which have proven to follow an
autocatalytic behavior. So, common sigmoidal profiles
cannot be seen and judged as an autocatalytic activity
in cases where such behavior starts in later stages of the
process. As such, it would be therefore impossible to
fit the kinetic data from the initiation of reaction to its
70% completion with just a single rate equation. Thus
for delayed autocatalytic reactions mentioned above,
such an autocatalytic effect could not be detected. This
phenomenon has led to the false conclusion of the
Figure 2 Reaction rate vs. time plot. [amino acid] = 0.1 mol dm−3, [KMnO4] = 4 × 10−4 mol dm−3, [H2SO4] =
3.16 mol dm−3, T = 313 K.

4
BAHRAMI AND ZAHEDI
Figure 3 Effect of the added initial concentration of ions on the reaction rate–time plots. [L-Norleucine] = 0.1 mol dm−3
[KMnO4] = 4 × 10−4 mol dm−3, [H2SO4] = 3.16 mol dm−3, T = 313 K.
,
existence of a “double stage” process in some of the
reports [10,14].
According to the above discussion, the following
rate equation is proposed for L-norleucine, L-leucine,
L-iso-leucine, and L-tert-leucine until 35%, 75%, 45%,
and 50% reaction completion, respectively:
k1ꢁ values have been obtained by fitting the rate data
to a first-order rate equation for L-leucine. Initial an-
ticipation of k1ꢁ for L-norleucine, L-iso-leucine, and L-
tert-leucine has been made by fitting the initial portion
of the rate data to a first-order rate equation. This is
because there is no catalytic effect in the early stages
of the corresponding reactions.
Moreover, our kinetic run in the presence of man-
ganese sulfate(II) with a concentration more than that
of the permanganate ion satisfies the following rate
equation:
−d[MnO−4 ]/dt = k1ꢁ[MnO−4 ]
(2)
And with more progress of reaction, the following
equation for L-norleucine, L-iso-leucine, and L-tert-
leucine is yielded:
ln[a(b + x)/b(a − x)] = k2ꢁ (a + b)t
(6)
−d[MnO−4 ]/dt = k1ꢁ[MnO−4 ] + k2ꢁ[MnO−4 ][Mn2+
]
where b represents the initial concentration of man-
ganese sulfate(II).
(3)
The calculated rate constants under this condi-
tion have provided a reasonable initial guess for k2ꢁ .
Equation (5) illustrates the fact that both k1ꢁ and k2ꢁ ap-
pear not only as intercept and slope but also on the
left-hand side of that expression. Thus having previ-
ously determined preliminary values of k1ꢁ and k2ꢁ , this
equation must be used for the simultaneous determi-
nation of the kinetic data of the reaction rate increase
region by taking advantage of iterative methods [22–
25,31–36]. Figure 4 demonstrates the typical results of
the fitting process of rate data for L-norleucine at two
temperatures and under the same conditions. Almost
perfect fits to the data in Fig. 4 (R2 values of 0.9995)
not only corroborate the validity of the applied kinetic
method, but also confirm the autocatalytic effect of the
Mn2+ species and point to such a delayed activity at
where k1ꢁ and k2ꢁ are pseudo-order rate constants for the
noncatalytic and catalytic processes, respectively. The
amino acid concentration, which is always kept in large
excess, and the constant sulfuric acid concentration in
eachexperimentareincludedinthesepseudo-orderrate
constants.
The integrated form of rate equations gives
ln[a/(a − x)] = k1ꢁ t
(4)
ln[(k1ꢁ /k2ꢁ + x)/(a − x)] = (k1ꢁ + k2ꢁ a)t − ln(k2ꢁ a/k1ꢁ )
(5)
where a represents the initial concentration of perman-
ganate, and x is the amount of permanganate ion con-
sumed in time t.
certain critical concentrations of Mn2+
.

DELAYED AUTOCATALYTIC BEHAVIOR OF Mn(II) IONS AT A CRITICAL RATIO
5
Figure 4 Integrated rate-law plots for the oxidation of L-norleucine by potassium permanganate at various temperatures.
[L-Norleucine] = 0.1 mol dm−3, [KMnO4] = 4 ×10−4 mol dm−3, [H2SO4] = 3.16 mol dm−3
.
Such a sawtooth effect has been observed in
aliphatic amines reaction with nitrous acid [37] and has
been related to the presence of NH2 group, and for-
mation of a complex between nitrous acid and amine is
suggested as the cause. A similar behavior for straight
aliphatic amine’s photolysis in hexane has been re-
ported [38]. In the kinetic investigation of permanganic
oxidation of some carboxylic acids in 3.2 M sulfuric
acid medium, no explanation for a relation between rate
constants and carboxylic acid structure is found [39];
however, the sawtooth effect in rate constant depen-
dence on carbon chain is reported. Also in the kinetic
study of oxidation of some linear chain amino acids in
weak acidic medium, this effect is observed, which has
The Effect of Structural Isomerism on
Noncatalytic Pathway Using Taft Analysis
This effect has been analyzed by comparing val-
ues obtained for k1ꢁ under equal conditions for vari-
ous amino acids. Figure 5 illustrates variation of k1ꢁ
with side chain approaching toward carbon atom ad-
jacent to COOH group in different concentrations
of amino acids. L-Norleucine, L-leucine, L-iso-leucine,
and L-tert-leucine are distinguished by the differences
in carbon chain branching and are designated by R1,
R2, R3, and R4, respectively. Similar plots for tempera-
ture variations are shown in Fig. 6. As it is evident from
both Figs. 5 and 6, sawtooth plots are encountered.
Figure5 Effectoftheisomerismofcarbonchainontheratepseudoconstantsatseveralaminoacidconcentrations. L-Norleucine,
L-leucine, L-iso-leucine, and L-tert-leucine are distinguished by the differences in carbon chain branching and are designated by
R1, R2, R3 and R4, respectively. [KMnO4] = 4 × 10−4 mol dm−3, [H2SO4] = 3.16 mol dm−3, T = 313 K.

6
BAHRAMI AND ZAHEDI
Figure 6 Effect of the isomerism of carbon chain on the rate pseudoconstants at different temperatures. L-Norleucine, L-leucine,
L-iso-leucine, and L-tert-leucine are distinguished by the differences in carbon chain branching and are designated by R1, R2,
R3 and R4, respectively. [Amino acid] = 0.1 mol dm−3, [KMnO4] = 4 × 10−4 mol dm−3, [H2SO4] = 3.16 mol dm−3
.
been related to zwitterionic structure or side chain in
amino acid [40].
and steric hindrance corrected by the hyperconjugation
effects, respectively.
The fact that in all of the abovementioned stud-
ies NH2 and COOH groups were present guides
us to the probability of their being responsible for
the sawtooth effect. While investigating the effect of
the amino acid carbon chain length on their perman-
ganic oxidation [31], a convincing explanation was not
presented. Arrizabalaga et al. [31] related the appear-
ance of sawtooth effect to complication of the reaction
mechanism. They also suggested that variations exper-
imented in the rate pseudo-constants, when the length
of the amino acid carbon chain is modified, were not
determined by only one elemental step of the process.
The authors believed that formation and dissociation of
permanganate–amino acid and amino acid–Mn2+ com-
plexes are among those steps, which are involved in the
experimental rate constants.
An effort to fit the kinetic data of all four amino
acids to the above equations was fruitless, namely a
linear relation while all four acids included could not
be achieved. It was then found that by eliminating the
data for L-norleucine, the aforementioned flaw could
be surmounted. Plots of log k1ꢁ vs. σ∗, Es, and Esc at
313 K have been illustrated in Fig. 7. Not a satisfac-
tory linear relationship (R2 = 0.9818) is found in plot
of log k1ꢁ against σ∗. A more reasonable linear behav-
ior (R2 = 0.9977) is evident for log k1ꢁ vs. Es and Esc,
where the slopes are δ = 3.46 and δc = 1.76, respec-
tively. Positive δ values are an indication of the fact that
the reaction rate is decreased when steric hindrance is
increased. Loweringofδ valuewhilethehyperconjuga-
tion correction is considered is indicative of the effect
of losing ꢀ-hydrogens by accumulating more branches
on an ꢀ-carbon. Such a lowering has the further effect
of decreasing the reaction rate. Finally it was consid-
ered that taking into account the above single parameter
fits to the kinetic data, it would be more effective if the
experimental results could fit a biparametric equation
in agreement with the following expression [42]. This
fact was justified provided that L-norleucine data are
eliminated, and therefore no sawtooth shape for plots
in Figs. 5 and 6 is encountered:
Inordertofindapossiblerelationbetweentheexper-
imental behavior observed in this work and the struc-
tural characteristic of the amino acids, the possible in-
ductive, steric, and hyperconjugation effects are dis-
cussed immediately. Thus the following relations have
been considered:
•
log(k/k0) = ρσ*, where σ* is Taft’s polar effect
constant [41];
•
log(k/k0) = δEs, where Es is Taft’s steric hin-
log(k/k0) = ρσ∗ + δc Esc
(7)
drance constant [41];
log(k/k0) = δc Esc, where Esc is Hancock’s steric
•
A perfect fit to experimental rate constants with
the biparametric equation was obtained in which
Hancock’s steric hindrance constant plays an essen-
tial role (Eq. (7)). This fact is illustrated in Fig. 8. ρ
and δc values for various temperatures are summa-
rized in Table I. Taking a close look at the negative
hindrance constant (Taft’s steric hindrance con-
stant corrected by the hyperconjugation effect of
the bonds σ(C H) in ꢀ position) [42].
ρ, δ, and δc in the above expressions are the efficacy
parameters associated with inductive, steric hindrance,

DELAYED AUTOCATALYTIC BEHAVIOR OF Mn(II) IONS AT A CRITICAL RATIO
7
Figure 7 The plots of log kꢁ vs. σ*, Es, and Esc. [Amino acid] = 0.1 mol dm−3, [KMnO4] = 4 × 10−4 mol dm−3, [H2SO4] =
3.16 mol dm−3, T = 313 K.1
results of ρ in Table I, it is obvious that carbon
chain’s electron-donating property causes an increase
in reaction rate. Also positive δc values indicate that in-
creasing steric hindrance toward the ꢀ-carbon causes a
decrease in the reaction rate in a noncatalytic pathway.
Considering the mechanism mentioned in our previous
study[16]forthesereactions, radicalreleaseisaneffec-
tive step in the determination of reaction rate. Since by
increasing electron-donating character of carbon chain,
radical production is eased, then the reaction rate is
increased as expected. As it is evident from Fig. 7,
change in δ values suggests that the hyperconjuga-
tion effect influences the reaction rate. This can be
explained by an increase in number of ꢀ-hydrogens
from L-tert-leucine to L-leucine, the mentioned radical
is stabilized thereby increase in the reaction rate results.
Also initially, the amino acid and permanganate form a
complex, which is hindered by the steric hindrance in-
crease in carbon chain that is accompanied by decreas-
ing of the reaction rate. As such the results presented
by the above Taft’s analysis fully corroborates the
mechanism presented in our previous studies [15,16].
Ignoring L-norleucine data from Figs. 5 and 6 as sug-
gested above indicated that the reaction rate decreases
by shifting from L-leucine to L-tert-leucine. Although
the electron-donating capability in L-tert-leucine
is more than that of L-leucine, but due to the
larger steric hindrance of the former, steric hin-
drance governs the reaction rate and is of more
importance than the induction factor. This latter argu-
ment can be more accurately justified by taking a closer
look at R2 values presented in Fig. 7.
In a simultaneousresearch (manuscriptunder prepa-
ration), we have also studied effect of carbon chain
Figure 8 General Taft equation for k1ꢁ . [Amino acid] = 0.1 mol dm−3, [KMnO4] = 4 × 10−4 mol dm−3, [H2SO4] =
3.16 mol dm−3, T = 313 K.

8
BAHRAMI AND ZAHEDI
Table I Steric and Inductive Parameters Accompanied
with for Hyperconjugation Correction of α-Hydrogen
Atoms for Noncatalytic Process at Different
observation is indicative of the fact that for the auto-
catalytic effect to show up, a certain ratio of [Mn2+
]
to amino acid concentration should be provided by the
reaction mixture. It can also be pointed out that in con-
centrations below 0.1 M of L-leucine there might be a
significant probability of autocatalytic oxidation.
The explanation offered above leads us to the use of
“critical ratio,” being the concentration ratio of Mn2+
to amino acid. Such a critical ratio, whose magnitude
depends on the nature of amino acid used, must be met
before any autocatalytic activity comes to existence.
Also by increasing sulfuric acid to more than 4.3 M, the
autocatalytic effect of the said ion diminishes; Fig. 10
is an illustrative example. Thus, sulfuric acid con-
centration is also important in the Mn2+ autocatalytic
performance.
Based on the mechanism proposed in the literature
[16,32–36] for these reactions, as a crucial stage
Mn2+ and amino acid form a complex. By considering
the above discussion, it is obvious that formation of
the complex depends on the “critical ratio” and the
sulfuric acid concentration. So in the concentration
range studied, the complex for L-leucine is not stable
and that for L-iso-leucine is destroyed by the increasing
amino acid concentration. k2ꢁ values for L-norleucine
and L-tert-leucine at various temperatures are sum-
marized in Table II, and those at various amino acid
concentrations are mentioned in Table III. Looking
at Table II, it becomes clear that the steric hindrance
in the carbon chain decreases the reaction rate. Such
an observation means that by elimination of steric
hindrance, the complex formation between Mn2+
and amino acid is advanced and the reaction rate is
Temperatures. [Amino Acid] = 0.1 mol dm−3, [KMnO4] =
4 × 10−4 mol dm−3, [H2SO4] = 3.16 mol dm−3
Temperature
ρ
δc
318
313
308
303
298
−4.57
−6.54
2.23
2.69
2.87
4.31
5.36
−6.56
−15.23
−21.49
length in some straight chain amino acid reactions. In
that study, it is concluded that inductive effect is in-
fluential in the noncatalytic pathway while steric hin-
drance is of no importance when carbon chain branch-
ing is avoided. Thus elimination of L-norleucine from
the above analyses seems logical because its oxidation
in the noncatalytic pathway is solely controlled by the
inductive effect in contrast to L-leucine, L-iso-leucine,
and L-tert-leucine.
The Effect of Structural Isomerism
on Catalytic Pathway
As mentioned previously, in these processes Mn2+ con-
centration should reach a certain amount in order to
allow the catalytic process to emerge. As it is evi-
dent from Fig. 9, while L-iso-leucine’s concentration
increases from 0.1 M to 0.5 M, the autocatalytic ef-
fect gradually decreases and ultimately vanishes. This
Figure 9 The reaction rate–time plots for oxidation of L-iso-leucine at various concentration of amino acid. [KMnO4] = 4 ×
10−4 mol dm−3, [H2SO4] = 3.16 mol dm−3, T = 313 K.

DELAYED AUTOCATALYTIC BEHAVIOR OF Mn(II) IONS AT A CRITICAL RATIO
9
Figure 10 Effect of the added sulfuric acid on the reaction rate–time plots. [L-Norleucine] = 0.1 mol dm−3, [KMnO4] = 4 ×
10−4 mol dm−3, [H2SO4] = 3.16 mol dm−3, T = 298 K.
increased from L-tert-leucine to L-norleucine. Thus,
it seems that formation of the complex depends on
both electron-donating and steric hindrance factors in
the amino acid carbon chain and therefore they should
be considered as mutual factors. So that in spite of more
steric hindrance in carbon chain for L-tert-leucine
relative to L-leucine, more electron-donating ability
in tert-butyl group relative to iso-butyl causes that
L-tert-leucine be able to form a complex with Mn2+
in contrast to L-leucine. It should be noted that the
above explanation is in accord with a mechanism
presented elsewhere [16]. Thus, as an effective step in
the determination of the reaction rate for the catalytic
pathway, a radical is released which itself is eased by
increase in electron donating in the amino acid carbon
chain and thus speeding up the oxidation process.
Therefore in the catalytic pathway, inductive and
steric hindrance effects intervene in the reaction rate in
such a way that oxidation process speeds up by increase
in electron donating and decrease in steric hindrance
effects in the amino acid carbon chain.
CONCLUSIONS
The kinetics of the permanganic oxidation pro-
cess of L-norleucine, L-leucine, L-iso-leucine, and
L-tert-leucine in strong acid medium was investigated
using a spectrophotometric technique. By investigating
rate–time curves, it is concluded that after passing a de-
termined portion of the reaction time for L-norleucine,
L-iso-leucine, and L-tert-leucine, a parallel reaction is
encountered, which in the case of L-leucine is not ob-
served. By increasing the concentration of Mn(II) and
observing increase in reaction rate as well as fitting the
kinetic data into appropriate kinetic equations, it is de-
termined that Mn(II) ion is the autocatalytic agent. For
the autocatalytic activity to initiate its action, a certain
concentration of the said species is required, which we
call “critical concentration.” Also the ratio of Mn2+ to
the amino acid concentration is influential in the auto-
catalytic effect to start its course of action, this ratio is
called “critical ratio.” Moreover, the autocatalytic phe-
nomenon in such reactions vanishes in concentrations
Table II Effect of Temperature on k2ꢁ ; [Amino Acid] =
Table III Variation of k2ꢁ vs. the Concentration of
0.1 mol dm−3, [KMnO4] = 4 × 10−4 mol dm−3
,
Amino Acid; [KMnO4] = 4 × 10−4 mol dm−3
,
[H2SO4] = 3.16 mol dm−3
[H2SO4] = 3.16 mol dm−3, T = 313 K
k2ꢁ (×102 dm3 mol−1 s−1
)
k2ꢁ (× 102 dm3 mol−1 s−1
)
[Amino Acid]
(mol dm−3
Temperature
L-Norleucine
L-Tert-leucine
)
L-Norleucine
L-Tert-leucine
298
303
308
313
318
17.32
26.45
39.62
62.46
93.34
15.98
21.32
30.10
40.48
53.95
0.1
62.46
125.73
185.60
267.93
332.79
40.48
85.20
134.22
153.18
209.80
0.2
0.3
0.4
0.5

10
BAHRAMI AND ZAHEDI
of sulfuric acid that are greater than 4.3 M. This “crit-
ical concentration” and “critical ratio” in addition to
the nature of amino acid used depend on the amount of
sulfuric acid present in the matrix.
In most of researches carried out on this subject, the
autocatalytic effect has neither been investigated nor
reported. This may be due to the “critical ratio” phe-
nomenon, required for the initiation of the autocatalytic
effect.
6. Ameta, C. S.; Pande, P. N.; Gupta, H. L.; Chowdhry,
H. C. Acta Chim Acad Sci Hung 1982, 110, 7.
7. Bharadwaj, L. M.; Nigam, P. C. Indian J Chem 1981,
20A, 793.
8. Rao, V. S.; Sethuram, B.; Rao, T. N. Int J Chem Kinet
1979, 11, 165.
9. Rao, V. S.; Sethuram, B.; Rao, T. N. Oxid Commun 1986,
9, 11.
10. Mudaliar, U. D.; Chourey, V. R.; Verma, R. S.; Shastry,
V. R. Indian J Chem Soc 1983, 60, 561.
Inductive and steric hindrance factors in amino
acid’s carbon chain are effective on processes’ rate both
in catalytic and noncatalytic pathways. The reaction
rate decreases in the noncatalytic pathway by shifting
from L-leucine to L-iso-leucine and to L-tert-leucine
due to the steric hindrance increase in amino acid’s
carbon chain. Such an increase diminishes probability
of complex formation between amino acid and perman-
ganate, while at the same time lowering of the number
of ꢀ-hydrogens in amino acids results in a decrease in
stability of the radical produced. Although the electron-
donating capability in L-tert-leucine is more than that
of L-leucine, but due to the larger steric hindrance of
the former, steric hindrance governs the reaction rate
and is of more importance than the induction factor.
The steric hindrance in carbon chain decreases the re-
action rate in the catalytic pathway. Thus, by elimina-
tion of steric hindrance, the proposed complex, which
is supposedly formed between Mn2+ and amino acid,
is promoted and the reaction rate is increased from
L-tert-leucine to L-norleucine. In spite of more steric
hindrance in carbon chain for L-tert-leucine relative to
L-leucine, the delayed autocatalytic activity has been
observed for the former and not for the latter. This
fact suggests that more electron-donating ability in tert-
butyl group relative to iso-butyl causes L-tert-leucine
to be able to form a complex with Mn2+ in contrast
to L-leucine. This is indicative that in contrast to the
noncatalytic pathway, the inductive effect is more im-
portant than the steric hindrance factor in the catalytic
pathway.
11. Girgis, H. M.; Hassan, R. M.; El-Shahawy, A. S. Bull
Fac Sci Univ 1987, 16(1), 41.
12. Hassan, R. M.; Mousa, M. A.; Wahdan, M. H. J Chem
Soc, Dalton Trans 3 1988, 605.
13. Iloukani, H.; Bahrami, H. Int J Chem Kinet 1999, 31,
95.
14. Sahu, B. R.; Chourey, V. R.; Pandey, S.; Shastry, L. V.;
Shastry, V. R. Indian J Chem Soc 2000, 76, 131.
15. Zahedi, M.; Bahrami, H. Kinet Catal 2004, 45(3), 351.
16. Bahrami, H.; Zahedi, M. Can J Chem 2004, 82, 430.
17. Vogel, A. I. Quimica Analitica Cuantitative; Kapelusz:
Buenos Aires, 1960; Vol. 1, p. 382.
18. Felig, F. Spot Tests in Inorganic Analysis; Elsevier:
Amsterdam, 1972; p. 334.
19. Vogel, A. I. Quimica Analitica Cualitative; Kapelusz:
Buenos Aires, 1953; Vol. 1, p. 250.
20. Roberts, R. M.; Gilbert, J. C.; Rodwald, L. B.; Wingrove,
A. S. Modern Experimental Organic Chemistry, 2nd ed.;
Saunders: Philadelphia, PA, 1985; p. 700.
21. Banerji, K. K.; Nath, P. Bull Chem Soc Japan 1969, 42,
2038.
22. Andre´s Ordax, F. J.; Arrizabalaga, A.; Martinez de Ilar-
duya, J. I. An Quim 1984, 80, 531.
23. Andre´s Ordax, F. J.; Arrizabalaga, A.; Martinez Perez
de mendiola, R. Stud Chem 1986, 11, 303.
24. Andre´s Ordax, F. J.; Arrizabalaga, A.; Ortega, K. An
Quim 1989, 85, 218.
25. Perez Benito, J. F.; Mata Perez, F.; Brillas, E. Can J Chem
1987, 65(10), 2329.
26. Brillas, E.; Garrido, J. A.; Perez Benito, J. F. Collect
Czech Chem Commun 1988, 53, 479.
27. Garrido, J. A.; Perez Benito, J. F.; Rodrigouez, R. M.;
De Andre´s, J.; Brillas, E. J Chem Res 1987, 11, 380.
28. De Andre´s, J.; Brillas, E.; Garrido, J. A.; Perez Benito,
J. F. J Chem Soc, Perkin Trans 2 1988, 107.
29. Rodrigouez, R. M.; De Andre´s, J.; Brillas, E.; Garrido,
J. A.; Perez Benito, J. F. New J Chim 1988, 2(2–3), 143.
30. De Andre´s, J.; Brillas, E.; Garrido, J. A.; Perez Benito,
J. F. Gazz Ital 1988, 118, 203.
BIBLIOGRAPHY
1. Verma, R. S.; Reddy, M. J.; Shastry, R. J Chem Soc,
Perkin Trans 2 1976, 469.
2. Frost, A. A.; Pearson, R. G. Kinetics and Mechanisms;
Wiley: New York, 1972; p. 152.
31. Arrizabalaga, A.; Andre´s Ordax, F. J.; Ferna´ndez
Ara´nguiz, M. Y.; Peche, R. Int J Chem Kinet 1997, 29,
181.
3. Ameta, C. S.; Pande, P. N.; Gupta, H. L.; Chowdhry,
H. C. Acta Phys Chem 1980, 26, 89.
32. Andre´s Ordax, F. J.; Arrizabalaga, A.; Casado, J.; Peche,
R. React Kinet Catal Lett 1991, 44, 293.
4. Ameta, C. S.; Pande, P. N.; Gupta, H. L.; Chowdhry,
H. C. Z Phys Chem (Leipzig) 1980, 261, 1222.
5. Ameta, C. S.; Pande, P. N.; Gupta, H. L.; Chowdhry,
H. C. Z Phys Chem (Leipzig) 1980, 261, 802.
33. Andre´s Ordax, F. J.; Arrizabalaga, A.; Peche, R.; Quin-
tana, M. A. An Quim 1992, 87, 828.
34. Andre´s Ordax, F. J.; Arrizabalaga, A.; Peche, R.;
Quintana, M. A. An Quim 1992, 88, 440.

DELAYED AUTOCATALYTIC BEHAVIOR OF Mn(II) IONS AT A CRITICAL RATIO
11
35. Insausti, M. J.; Mata-pe`rez, F.; Alvarez-macho, M. P. Int
J Chem Kinet 1995, 27, 507.
39. Rao, V. S.; Sethuram, B.; Rao, T. N. Orient J Chem 1987,
3(1), 83.
36. Arrizabalaga, A.; Andre´s Ordax, F. J.; Ferna´ndez
Ara´nguiz, M. Y.; Peche. R. Int J Chem Kinet 1996, 28,
799.
37. Senent, S.; Casado, J. An Quim 1967, 63, 495.
38. Pouyet, P. Bull Soc Chim Fr 1964, 2582.
40. Pokrovskaya, O. G. Izv Sib Otd Akad Nauk SSSR, Ser
Khim Nauk 1959, 8, 50.
41. Hammett, L. P. J Am Chem Soc 1937, 59, 96.
42. Takayama, C.; Fujita, T.; Nakajima, M. J Org Chem
1979, 44(16), 2871.