Kinetics and mechanism of oxidation of indole by HSO5 -

Article abstract of DOI:10.1002/kin.20215

Mechanistic studies on the oxidation of indole [IND] by HSO ₅^- in aqueous CH₃CN medium (80:20 v/v) have been carried out, and the reaction is characterized by the rate law -d[HSO ₅^-]/dt = k[IND][HSO₅^-]HSO5 ^- and SO₅^2- are probably the respective electrophiles in acidic and basic mediums. Nucleophilic attack of the ethylenic bond on the persulfate oxygen is envisaged to explain the reactivity. The reaction fails to initiate polymerization, and a radical mechanism is ruled out. Thermodynamic parameters very much suggest a bimolecular process. No significant catalytic activity is observed for the reaction system in the presence of Ag⁺, Cu²⁺, and heteroaromatic N-bases.

Full text of DOI:10.1002/kin.20215

Kinetics and Mechanism
of Oxidation of Indole
by HSO−5
SUBBIAH MEENAKSHISUNDARAM, N. SARATHI
Department of Chemistry, Annamalai University, Annamalainagar 608 002, India
Received 3 April 2006; revised 14 August 2006, 3 September 2006; accepted 3 September 2006
DOI 10.1002/kin.20215
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Mechanistic studies on the oxidation of indole [IND] by HSO−5 in aqueous CH3CN
medium (80:20 v/v) have been carried out, and the reaction is characterized by the rate law
−d[HSO−5 ]/dt = k[IND][HSO−5 ]HSO5− and SO25− are probably the respective electrophiles in
acidic and basic mediums. Nucleophilic attack of the ethylenic bond on the persulfate oxygen
is envisaged to explain the reactivity. The reaction fails to initiate polymerization, and a radical
mechanism is ruled out. Thermodynamic parameters very much suggest a bimolecular process.
No significant catalytic activity is observed for the reaction system in the presence of Ag+, Cu2+
,
and heteroaromatic N-bases. ꢀ 2006 Wiley Periodicals, Inc. Int J Chem Kinet 39: 46–51, 2007
C
carboxylic acids, alkenes, arenes, phenols, amines, and
sulfides [11,12]. It hydroxylates alkanes and aromatic
compounds [13]. The peroxo anion oxidation of in-
dole has been carried out because its distinct reactivity
is appropriate for an independent and precise kinetic
study on the steps of the mechanism [14].
INTRODUCTION
Indole, an electron-rich heteroaromatic nitrogen het-
erocycle, is found predominantly in various naturally
occurring compounds, such as various plant alkaloids
and fungal metabolites [1,2], usually as metabolites
of tryptophan. The metabolic oxidation investigation
of indole to indoxyl [3] and the oxidation studies by
sodium performate in the presence of acetone and
methyl paraoxane [4] have been reported. Indoles are
oxidized into oxindoles with peracetic [5] and persul-
furic acids [6]. Lawson and Witkop have shown that
N-bromosuccinimide can be used to convert indoles to
oxindoles [7,8]. Rangappa et al. [9] have examined the
kinetics of oxidation of indole by chloramine-T in the
presence of Os(VIII) in alkaline medium.
EXPERIMENTAL
All the chemicals used were reagent grade prod-
ucts. Indole (Aldrich, Bangalore, India) and oxone
(2KHSO5·KHSO4·K2SO4) (Aldrich) were used as
such. Heavy water (99.4%) was supplied by Bhabha
Atomic Research Centre, India.
The kinetic studies were carried out in an aqueous
acetonitrile medium. The reactions were performed by
maintaining a large excess of [substrate] over [oxidant].
The reaction mixture was homogeneous throughout
the course of the reaction. The reaction’s progress was
monitored for at least two half-lives by iodometric esti-
mation of unchanged oxidant at regular time intervals.
The first-order rate constants were evaluated from the
The synthetic utility of potassium peroxomonosul-
fate as an oxidant has been studied in detail [10]. It
is a powerful oxidant and oxidizes alcohols, ketones,
Correspondence to: Subbiah Meenakshisundaram; e-mail:
meenas1957@gmail.com.
c
ꢀ
2006 Wiley Periodicals, Inc.

KINETICS AND MECHANISM OF OXIDATION OF INDOLE BY HSO5−
slopes of linear plots of log titer versus time. The least- equation,
47
square method was adopted. All the rate constants are
the average of two or more determinations.
−d[HSO−5 ]
dt
= k[IND][HSO−5 ]
Stoichiometry and Product Analysis
The rate of conversion is independent of ionic
strength. The insensitivity of rates to added acryloni-
trile as shown in Table II rules out a mechanism in-
volving homolytic fission of the peroxide bond. The
reactivity of indole increases as the amount of water
in the solvent is increased (Table III). This probably
indicates that the transition state is more polar than the
reactants in the initial state [15–17]. Linear plot of log
kobs versus D−1 (r = 0.966; s = 0.18) with a negative
slope very much suggests that there is an involvement
of negative ion in the rate-limiting step.
D2O has a small insignificant retarding effect on the
rate of conversion as shown in Table III. A facile oxi-
dation process in the presence of high [OH−] and [H+]
is observed. Ag+, Cu2+, and heteroaromatic N-bases,
viz., 1,10-phenanthroline (phen), 2,2ꢁ-bipyridyl (bipy),
and picolinic acid (PA), have no significant catalytic ef-
fect on the reactivity. The temperature dependence of
the oxidation of indole is given in Table IV. The activa-
tion parameters were evaluated using an Eyring’s plot
of ln kobs/T versus 1/T (r = 0.999; s = 0.03).
The amount of oxindole formed corresponds to the
amount of HSO−5 consumed, which gives a stoichiom-
etry of 1:1 of indole and HSO−5 .
The reaction mixture, after 48 h, was extracted with
chloroform, dried over anhydrous Na2SO4, and then
evaporated. The residual brown solid obtained was sub-
jected to column chromatography using silica gel and
eluted with benzene and then evaporated. The melting
point of the compound (116–118◦C) (lit. 118–120◦C)
(yield: 55–60%) confirms oxindole as the main product
of the reaction. The UV–Vis absorption spectra of the
oxidation product of indole by HSO−5 were recorded
as shown in Fig. 1.
RESULTS AND DISCUSSION
MECHANISM
The first-order dependence on [HSO−5 ] was evidenced
by the linear semilogarithmic plots of titer versus time
(Fig. 2). The plot of kobs as a function of [IND] at con-
stant temperature is linear with a zero intercept (Fig. 3)
(r = 0.981; s = 0.72). The kinetic data are summarized
in Table I. The reaction can be characterized by the rate
Peroxomonosulfate exists [18] as HSO−5 in solution.
Although many peroxy anions are effective nucle-
ophiles, HSO−5 is a very weak nucleophile [19,20].
In spite of the fact that free radicals can arise from
the facile homolysis of the oxygen–oxygen bond [21],
Figure 1 The absorption spectra of (a) indole and (b) indole and HSO−5 (after 24 h).
International Journal of Chemical Kinetics DOI 10.1002/kin

48
MEENAKSHISUNDARAM AND SARATHI
Figure 2 Plots of log titer versus time: (a) 103 [Ox] = 3.03 mol dm−3, (b) 103 [Ox] = 2.02 mol dm−3, and (c) 103 [Ox] = 1.01
mol dm−3
.
an ionic mechanism is favored in certain reactions in-
volving oxidations by peroxides. In the present investi-
gation, no observed polymerization in the presence of
acrylonitrile rules out a free radical process. Generally,
the enhancement of the electrophilic activity of perox-
ide will minimize the importance of undesirable free
radical pathways, resulting in a mixture of products
[22].
and Maruthamuthu [23] examining the oxidation of
dimethyl sulfoxide with peroxomon−osulfate found that
the oxidant exists as an −O O SO3 at pH >9. HSO−5
exists [24] principally as SO25− around pH 10.
HSO− + H O
SO25− + H3O+
(1)
ꢀ
ꢁ
2
5
In the presence of high concentration of OH−
ions, the dianion is the reactive species. Pandurenga
The catalytic activity of perchloric acid is signif-
icant, and this conversion exhibits fractional order
Figure 3 Effect of substrate concentration on the reaction rate.
International Journal of Chemical Kinetics DOI 10.1002/kin

KINETICS AND MECHANISM OF OXIDATION OF INDOLE BY HSO5−
49
Table I Effect of Reactants’ Concentration on the
Reaction Rate in the Oxidation of Indole by HSO−5 at
313 K
Table III Effect of Dielectric Constant and D2O on the
Reaction Rate at 313 K
CH3CN:H2O:D2O
Da
104 k (s−1
)
obs
103 [Ox]
(mol dm−3
102 [IND]
(mol dm−3
103 [NaClO4]
104 k
(s−1
)
obs
50:50:0
70:30:0
80:20:0
80:17.5:2.5
80:12.5:7.5
80:10:10
58.85
50.31
46.04
46.04
46.04
46.04
32.9
8.1
3.6
3.0
2.9
2.7
)
)
(mol dm−3
)
1.0
2.0
3.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
4.0
5.0
6.0
2.0
2.0
2.0
2.0
–
–
–
–
–
–
–
–
3.9
3.6
3.0
3.6
8.3
9.1
10.6
3.6
3.3
3.6
3.4
[Ox] = 2.0 × 10−3 mol dm−3; [IND] = 2.0 × 10−2 mol dm−3
.
a Values are calculated from the values of pure solvent.
1.1
2.2
4.4
The pKa of the equilibrium is 9.4, and under the weakly
basic experimental conditions, the oxidant exists pre-
dominantly as HSO−5 [24].
CH3CN: H2O = 80:20 (v/v).
In the current investigations, kinetic data can be
rationalized by a biomolecular process that involves
the nucleophilic attack of the ethylenic bond on the
peroxidic oxygen.
dependence on acidity. The rate equation can be written
as
kobs = a + b[H+]
(2)
K
ꢀ
ꢁ
IND + HSO−5
C1
(4)
(5)
where a and b are rate constants for the uncatalyzed
and acid-catalyzed reactions, respectively. The plot of
kobs versus pH is found to be a U-shaped curve as re-
ported earlier [25]. This shows that different species
are involved at different pH. It is interesting to note
that the reaction rate was unaffected by [H+] in the ox-
idation of indole-3-acetic acid by peroxomonosulfate
[26] and peroxodisulfate [27].
C1 −→ Oxindole + HSO−
k
slow
4
HSO−5
SO25− + H+
(3)
ꢀ
ꢁ
The absorption spectra (Fig. 1) show spectral
changes supporting the complex formation. This type
of intermediate (C1) in the present study closely resem-
bles the intermediate proposed in the oxidation of sul-
fides [19], sulfoxides [10], and alkenes [28] by HSO−5 .
Anyway, we are unable to isolate and identify without
ambiguity the intermediate for this reaction.
Carmona et al. [29] were successful in synthesizing
and isolating the intermediate (I) in the oxidation reac-
tions of 2,3-dialkylindole derivatives by peroxomono-
sulfate anions.
Table II Effect of NaOH, H+ and Acrylonitrile
Concentration on the Reaction Rate at 313 K
102 [NaOH]
102 [H+]
(mol dm−3
103 [Acry]
104 k
obs
(mol dm−3
)
)
(mol dm−3
)
(s−1
)
–
–
–
–
2.2
4.4
6.5
8.7
–
–
–
–
–
–
–
–
–
3.6
15.1
28.9
4.8
5.9
6.8
0.9
1.8
–
–
–
–
–
7.6
3.6
–
–
–
–
–
–
1.1
1.8
2.4
2.9
2.9
3.4
[Ox] = 2.0 × 10−3 mol dm−3; [IND] = 2.0 × 10−2 mol dm−3
CH3CN:H2O = 80:20 (v/v).
;
International Journal of Chemical Kinetics DOI 10.1002/kin

50
MEENAKSHISUNDARAM AND SARATHI
Table IV Rate Constants and Activation Parameters for the Oxidation of Indole by HSO5−
104 k (s −1
)
obs
293 K
0.8
303 K
1.7
313 K
3.6
323 K
6.9
ꢀH# (kJ mol−1
)
–ꢀS# (J K−1 mol−1
)
ꢀG#(313K) (kJ mol−1
)
r
s
55
135
98
0.99
0.03
[Ox] = 2.0 × 10−3 mol dm−3; [IND] = 2.0 × 10−2 mol dm−3; CH3CN:H2O = 80:20 (v/v).
Table V Activation Parameters for Some HSO−5 Reactions
Reaction
ꢀH# (kJ mol−1
)
−ꢀS# (J K−1 mol−1
)
Reference
Indole oxidation
55
56
50
40
–
135
128
99
175
104
180
Present study
[32]
Ru(III) inhibition on the oxidation of anilines
Diphenyl sulfoxide oxidation
Aniline oxidation in acetonitrile medium
Nitrite oxidation
[10]
[17]
[33]
[14]
2,3-Dimethylindole oxidation
32
Similar type of intermediate is not entirely ruled out in
the present investigations.
observed with Ag(I) in this peroxomonosulfate oxida-
tion reaction.
The activation parameters for some reactions that
proceed by oxygen transfer mechanism are listed in
Table V. It appears that the oxygen transfer reactions
are associated with large, negative entropies of activa-
tion, and significant enthalpies of activation.
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