Studies on the ketone-catalyzed decomposition of caroate in aqueous alkaline medium

Article abstract of DOI:10.1002/kin.20093

The kinetics of ketone-catalyzed decomposition of caroate (peroxomonosulfate) was studied in aqueous alkaline medium at 25°C. The rate follows simple second-order kinetics, first order in each (Ketone) and [PMS]. The rate constant values are independent of hydroxide ion concentrations over the range from 0.05 M to 0.15 M. The experimental results suggest that the nucleophilic addition of SO₅^-2 ion at the carbonyl carbon leads to the formation of oxirane intermediate in the rate-determining step. Oxirane reacts rapidly with another SO₅^2- to give the parent ketone, oxygen, and SO₄^2-. The observed substituent effect on the activation energy E_A and Arrhenius (A) factor suggests that the electron-donating substituents stabilize the activated state and make the entropy of activation more negative. This can be explained by the fact that the mechanism of oxirane formation shifts from concentric to nonconcentric process as we go from acetone to 3-hexanone.

Full text of DOI:10.1002/kin.20093

Studies on the
Ketone-Catalyzed
Decomposition of Caroate
in Aqueous Alkaline
Medium
1
1
2
S. SELVARARANI, B. MEDONA, M. S. RAMACHANDRAN
1
Department of Chemistry, Fatima College, Madurai 625018, India
School of Chemistry, Madurai Kamaraj University, Madurai 625021, India
2
Received 1 December 2004; revised 15 February 2005
DOI 10.1002/kin.20093
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The kinetics of ketone-catalyzed decomposition of caroate (peroxomonosulfate)
◦
was studied in aqueous alkaline medium at 25 C. The rate follows simple second-order kinetics,
first order in each [Ketone] and [PMS]. The rate constant values are independent of hydroxide
ion concentrations over the range from 0.05 M to 0.15 M. The experimental results suggest
2
5
−
that the nucleophilic addition of SO ion at the carbonyl carbon leads to the formation of
2
5
−
oxirane intermediate in the rate-determining step. Oxirane reacts rapidly with another SO
2
−
to give the parent ketone, oxygen, and SO . The observed substituent effect on the activation
4
energy EA and Arrhenius (A) factor suggests that the electron-donating substituents stabilize
the activated state and make the entropy of activation more negative. This can be explained
by the fact that the mechanism of oxirane formation shifts from concentric to nonconcentric
process as we go from acetone to 3-hexanone. ꢀC 2005 Wiley Periodicals, Inc. Int J Chem Kinet
3
7: 483–488, 2005
INTRODUCTION
order equation (1).
−
−
Rate = k[HSO ][Ketone][OH ]
(1)
Montgomery [1] observed that small amounts of ke-
tones enhance the decomposition of Caro’s acid (per-
oxomonosulfate) in weak alkaline medium. The rate
law observed by this author follows a simple third-
5
Later detailed experimental study by Edwards et al. [2]
supports suggestions of Montgomery [1] that dioxi-
rane could be the possible intermediate compound
responsible for the catalytic effect of carbonyl com-
pound. Murray and Jeyaraman [3] isolated and charac-
terized dimethyl dioxirane from a buffered aqueous so-
lution of acetone and peroxomonosulfate. Oxiranes are
powerful oxygen transfer [4] and effective epoxidation
[5] reagents.
Correspondence to: M. S. Ramachandran; e-mail: msrchandran
@eth.net.
Contract grant sponsor: UGC, New Delhi, India.
ꢀ
c 2005 Wiley Periodicals, Inc.

484
SELVARARANI, MEDONA, AND RAMACHANDRAN
The mechanism proposed [2] for the ketone-
second dissociation constant of caro’s acid. These au-
thors [6] also showed that the Eq. (9) is valid in the
pH range 8–10, and the pseudo-first-order rate con-
stant reaches a maximum value at pH 10.5. The rate is
catalyzed decomposition of caro’s acid involves the
formation of dioxirane from the interaction of caro’s
acid anion with the carbonyl compound. The second
step involves the interaction between dianion of caro’s
approximately constant when pH>10.5.
−
The pKa of HSO is calculated [8] as 9.4 and there-
5
fore under the experimental conditions of the earlier
investigations, namely in the pH range 8–10, consid-
erable amount of [PMS] (caroate) would have existed
2
−
(2)
as the doubly ionized SO5 . However, the mechanisms
of the ketone-catalyzed decomposition of Caro’s acid
reported so far consider only the interaction of caro’s
−
2−
5
acidic anion HSO , and not the SO , with the
5
(
(
3)
5)
(
9)
carbonyl compound as the first step toward the forma-
tion of oxirane (reaction (2)). Since SO5 is a strong
2−
nucleophile, the formation of intermediate II (reaction
(3)) may also be possible from the simple addition re-
action with the carbonyl compound as in Eq. (9). If
reaction (2) is mainly responsible for the formation of
oxirane, then the ketone-catalyzed decomposition of
caroate may not be possible in strong alkaline medium
(
6)
2
−
acid and dioxirane. The detailed mechanism can be
summarized as in reactions (2)–(6), and the rate law
for the decomposition of PMS was shown to be as in
Eq. (7).
(pH >12) where PMS exists only as SO . Therefore,
5
to explore the possible mechanisms, the title reactions
are carried out in the presence of aqueous sodium hy-
droxide solution and the results are presented in this
paper.
−
−
−
d[PMS]/dt = K1 K2k1[HSO ][Ketone][OH ] (7)
5
EXPERIMENTAL
However, the plot of log(rate) vs. pH for cyclohexanone
catalyst showed a leveling off at ∼pH 9.5 and this was
explainedbyassumingapossibleratetermindependent
Peroxomonosulfate (Fluka Chemie Switzerland) in the
form of the triple salt 2KHSO5 · KHSO4 · K2SO4 was
used as received. Sodium hydroxide (Merck, India) and
other chemicals used were of highest purity available.
The ketones were from Aldrich or Fluka and were dis-
tilled from dry potassium carbonate prior to use.
The kinetics was carried out in the aqueous sodium
−
of [OH ] as in Eq. (8) in addition to Eq. (7).
−
−
d[PMS]/dt = k2[HSO ][Ketone]
(8)
5
Later Brauer and his coworkers [6,7] studied the forma-
tion of singlet oxygen by the ketone-catalyzed decom-
positionofcaro’sacid. Theseauthors, byexpressingthe
−
hydroxide solutions usually with [OH ] ≥ 0.05 M.
−
−
+
+
The rate of the reaction was determined by measur-
ing the unreacted peroxomonosulfate concentration by
iodometry at different time intervals. The rate follows
simple first-order equation (Fig. 1). The rate constants
were obtained from the linear regression of time vs.
log[PMS]t . Duplicate kinetic measurements showed
that the pseudo-first-order rate constants kobs are re-
producible within ± 5% error.
concentration of [HSO ] as [HSO ]t ·[H ] / ([H ]+
5
5
Ka2), showed that the rate equation for the mechanism
in Eqs. (2)–(6) can be expressed as in Eq. (9).
−
−
1/2 × d[PMS]/dt = k[HSO ]t [Ketone]
5
+
×
Kw/([H ] + Ka2)
(9)
−
In Eq. (9), [HSO ]t denotes the total concentration of
The evolution of oxygen gas was identified by
the color change with alkaline solution of sodium
5
caroate, Kw the ionic product of water, and Ka2 is the

STUDIES ON THE KETONE-CATALYZED DECOMPOSITION OF CAROATE
485
Figure 1 First-order plot.
◦
Figure 2 Plot of kobs vs. [Acetone] at 25 C.
dithionite activated with indigocarmine [9]. The
turnover number was determined by taking a known
Based on the experimental results, the following
mechanistic scheme may be proposed for the simple
alkyl ketone-catalyzed decomposition of PMS in aque-
ous sodium
−
excess of [PMS] over carbonyl compound at [OH ] =
0
.05 M. The unreacted PMS was estimated after 24 h.
A correction for the self-decomposition of PMS was
also made.
RESULTS AND DISCUSSION
(11)
Plots of log [PMS]t vs. time were linear even up to
two half-lives. The first-order linear plots were ob-
served (Fig. 1) when the concentrations of the carbonyl
compounds are less than the initial concentration of
PMS ([PMS]0) also. The first-order rate constants kobs
are independent of [PMS]0. This clearly shows that
peroxomonosulfate ion is involved in the rate-limiting
reaction.
(
12)
(13)
The effect of hydroxide ion on the rate was studied
by calculating the kobs values at different hydroxide ion
hydroxide solution.
The rate equation for the mechanistic scheme in re-
actions (11)–(13) can be expressed as in Eq. (14).
−
concentrations under the condition [OH ] ꢁ [PMS]0.
The values of kobs were found to be independent of hy-
droxide ion concentrations in the range from 0.05 M to
ꢀ
ꢁ
0
.15 M. Similarly the concentrations of sodium sulfate
2−
−
d[PMS]/dt = k1 K1 SO [CO]
(14)
(15)
5
and ionic strength have no effect on the rate.
The rate constant kobs values increase linearly with
increase in the concentrations of carbonyl compound
·
·
· kobs = k1 K1[CO]
Equation (15) explains all the experimental results. In
aqueous solution, peroxomonosulfate exists as a mix-
[
CO]. The plot of kobs vs. [CO] gives a straight line
almost passing through origin (Fig. 2).These experi-
mental results suggest that the rate of disappearance of
−
2−
5
ture of [HSO ] and SO due to the equilibrium (16).
5
[PMS] can be expressed by the Eq. (10).
(
16)
ꢀ
ꢁ
ꢂ
2−
5
−
d[PMS]/dt = k [CO] SO
(10)
Nolossofketone, forexampleacetone, wasobservedin
the decomposition reactions, and the turnover numbers
of greater than 2 (acetone > 10, ethyl methyl ketone
(17)
The dissociation constant Ka2 is calculated as
4 × 10− M at 25 C. The kinetics described in this
10
◦
>
4) indicate the carbonyl compounds act as a catalyst.

486
SELVARARANI, MEDONA, AND RAMACHANDRAN
report is studied with sodium hydroxide concentra-
tion of 0.05 M or greater. Therefore, under the ex-
by the nucleophilic addition as in reactions (16), (18),
and (19).
perimental conditions, ∼1% or even less of [PMS]
−
will exist as [HSO ] and we can approximate that
5
(18)
2
5
−
all the peroxomonosulfate exists as the dianion SO
.
We can also consider the reaction (17) as the source
−
for HSO . The equilibrium constant Kh value for the
5
2−
hydrolysis of SO5 in Eq. (17) can be calculated as
Kh = KW /Ka2 = 2.5 × 10− at 25 C. Very low Kh
5
◦
value also suggests that in strong alkaline medium, per-
2−
oxomonosulfate exists predominantly as SO5 and the
concentration of HSO5 can be neglected. Therefore,
(19)
−
−
−
under the experimental conditions ([OH ] ≥ 0.05 M)
The dissociation constant of HSO is very low
5
−
10
◦
the caroate exists exclusively as the doubly ionized an-
(4 × 10
M at 25 C) and the pK of intermediate
a
2
5
−
ion SO
.
I like compound R C-OH [18] is ∼17. Therefore, the
3
The addition to carbon-oxygen double bond [10]
is a well-documented reaction. Nucleophilic attack-
ing species always go to the carbon and the elec-
trophilic ones to the oxygen. A good nucleophile will
readily attack the carbonyl carbon, and a poor nucle-
ophile requires an acid catalyst to make the nucle-
ophilic addition reaction at a reasonable rate. In the
acid-catalyzed reactions, the mechanism involves the
formation of the intermediate II in reaction (3) through
the base-catalyzed ionization of hydroxyl type com-
pound (I) is less probable when compared with the
reaction (11). Moreover, according to the mechanism
−
in Eq. (11), as pH is increased more and more HSO5
2
5
−
will be converted to the powerful nucleophile SO
and when the pH >10.5 almost all the caroate ion will
2
5
−
exist as SO . Therefore, the rate should initially in-
+
electrophilic attack of H on the carbonyl oxygen fol-
crease and reach a limiting value with the increase in
pH. This conclusion is in accordance with the obser-
vation reported by earlier researchers [1,6]. Therefore,
we can come to the conclusion that the nucleophilic in-
lowed by the nucleophilic addition. Whatever may be
the sequence of attack, the rate-determining step is usu-
ally the nucelophilic addition. The self-decomposition
of peroxomonosulfate was studied in detail [8,11,12],
and the mechanism proposed involves the nucleophilic
2
5
−
−
teraction of the SO , and not HSO , with the carbonyl
5
group is the most probable first step in the mechanism
of the oxirane formation.
2
−
−
attack of SO on the peroxide oxygen of HSO . The
5
5
−
oxidation reactions with peroxomonosulfate (HSO )
The rate constant k K values are tabulated in Ta-
5
1
1
also proceed through the oxygen atom transfer from
the terminal peroxide following nucleophilic attack
at the peroxide moiety by the substrate [13–15]. A
large difference in the nucleophilcity of PMS species
ble I. Perusal of the results suggests that the electron-
donating substituents (relative to CH -group) at the
3
carbonyl carbon leads to a significant reduction in the
rate constant values. The inductive effect of the sub-
stituents will affect the electrophilicity of the carbonyl
carbon. Thus the electron-donating substituent should
retard the formation of tetrahedral intermediate I (in
Eq. (11)). The large decrease in the kinetic constants
with the size of the substituent suggests that the steric
interaction of the substituent has to be taken into ac-
count. This is also supported by the fact that the plot
of log (K k ) vs. ꢀσ* results a curve even within a
is manifested in the oxidation of biacetyl [16] and
−
aliphatic aldehydes [17] where HSO is approximately
5
six to eight orders of magnitude less reactive than
2
5
−
SO . These observations clearly show that the dou-
2
−
−
bly ionized SO is a better nucleophile than HSO ,
5
5
−
and in fact peroxide oxygen in HSO5 is an elec-
trophile. Therefore, the intermediate in Eq. (2) can be
formed by the electrophilic addition of H followed
+
1
1
◦
Table I Kinetic Parameters at 25 C
ꢂ
∗
2
−1
EA (kJ M )
Ketone
σ
10 × KIkI
log A
Acetone
Ethyl methyl ketone
Diethyl ketone
0.00
−0.10
−0.20
−0.215
–
10.0
8.3
3.8
1.9
5.7
59.6
53.2
34.0
30.2
43.3
9.5
8.3
4.5
3.6
6.3
3
-Hexanone
Levulinic acid anion
CH3 COCH2 CH2 COO )
−
(

STUDIES ON THE KETONE-CATALYZED DECOMPOSITION OF CAROATE
487
Figure 3 Mechanistic scheme.
small range of ꢀσ* values. However, with the limited
number of ketones, we could not make a meaningful
correlation of K1k1 with the inductive effect and steric
from a concerted to a nonconcerted one. This may also
be a reason for the observed curvature between log
(K1k1) vs. ꢀσ .
∗
interaction. A rough correlation between log (K1k1) vs.
The apparent log (A) can be correlated as log (A) =
∗
0
ꢀ
σ* shows that the reaction susceptibility constant ρ
log (A12) + (ꢁS /2.303R), where A12 is the Arrhe-
1
1
is ∼ +3(± 1). This large positive value indicates that
nius parameter for the reaction (12). The entropy for the
the ketone-catalyzed decomposition of SO2 may pro-
−
reaction ꢁS should be a negative value and may not
0
11
5
ceed through the mechanism as in reaction (11).
The activation energy and Arrhenius A factor were
calculated from the Arrhenius plot of log (K1k1) vs.
change with the structure of the ketones listed in Table
I. Therefore, the observed decrease in log (A) from ace-
tone to 3-hexanone means that electron-donating sub-
−
1
ꢃ=
T
, and the values are also tabulated in Table I.
stituents make the entropy of activation ꢁS more neg-
12
One interesting observation is that the better catalyst
acetone has higher activation energy than the poor
ative. The degree of nonconcerted process and hence
the contribution of charged activated complex increase
from acetone to 3-hexanone. The charged activated
complex may be more solvated, and the difference in
solvation may be responsible for the negative entropy
of activation. This may be the reason for the increase
3
-hexanone and the electron-donating alkyl groups
(relative to methyl) lower the EA value. Perusal of Eqs.
(
11) and (12) shows that the apparent activation energy
0
EA can be defined as EA = ꢁH + EA (12) where
1
1
0
ꢃ
ꢁ
H
denotes the enthalpy of the equilibrium (11) and
in −ꢁS .
1
1
12
EA (12) is the activation energy of the monomolecu-
lar reaction (12).The enthalpy for the nucleophilc sub-
stitution equilibrium (11) should be increased by the
electron-donating substituents (relative to methyl) at
the carbonyl carbon. This means that the activated state
of the reaction (12) stabilized to a larger extent by the
electron-donating alkyl groups so that the overall acti-
vationenergy EA isreduced. Themechanismofoxirane
formation in acetone (Eq. 12) may be a concerted pro-
cess. This mechanism may change to slightly a noncon-
certed process in 3-hexanone in which the peroxo O O
bond breaking precedes the oxirane O O bond forma-
tion (Fig. 3).The partial positive charge developed on
the oxygen atom in the transition state due to the non-
concerted process may be stabilized by inductive effect
of electron-donating substituents. The steric hindrance
of the substituents may also favor the nonconcerted
process. Therefore, as we replace the methyl group at
the carbonyl carbon by bulky electron-donating groups
such as ethyl, propyl etc., the mechanism may change
ACKNOWLEDGMENTS
SS and BM express their gratitude to UGC, New Delhi,
for financial assistance (FIP fellowship). The Principal
and Management of Fatima College are also acknowl-
edged for the encouragement and permission to avail
the FIP fellowship.
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