Shift of reaction pathway by added chloride ions in the oxidation of aromatic ketones by dichloroisocyanuric acid-A kinetic study

Article abstract of DOI:10.1134/S0036024416030237

Role of added chloride ions on the shift of reaction pathway of oxidation of aromatic ketones (acetophenone, desoxybenzoin) by dichloroisocyanuric acid (DCICA) was studied in aqueous acetic acid-perchloric acid medium. Participation of enolic and protonated forms of ketones in the rate determining steps is manifested from zero and first orders with respect to the oxidant in absence and presence of added chloride ions, respectively. Positive and negative effects of acid and dielectric constant on the reaction rate were observed. The observations deduce plausible mechanisms involving (i) rate-determining formation of enol from the conjugate acid of the ketone (SH⁺) in the absence of added chloride ions and (ii) rapid formation of molecular chlorine species from HOCl (hydrolytic species of DCICA) in the presence of added chloride ions, which then interacts with SH⁺ in a rate-determining step prior to the rapid steps of product formation. The order of Arrhenius parameters substantiate the proposed plausible mechanisms based on order of reactants both in presence and absence of added chloride ions.

Full text of DOI:10.1134/S0036024416030237

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2016, Vol. 90, No. 3, pp. 552–559. © Pleiades Publishing, Ltd., 2016.
CHEMICAL KINETICS
AND CATALYSIS
Shift of Reaction Pathway by Added Chloride Ions in the Oxidation
1
of Aromatic Ketones by Dichloroisocyanuric Acid—A Kinetic Study
a
b
a
Y. Lakshman Kumar , R. Venkata Nadh , and P. S. Radhakrishnamurti
a
Department of Chemistry, TJPS (PG) College, Guntur-522006, India
GITAM University-Bengaluru Campus, Karnataka-561203, India
b
e-mail: doctornadh@yahoo.co.in
Received December 28, 2014
Abstract—Role of added chloride ions on the shift of reaction pathway of oxidation of aromatic ketones (ace-
tophenone, desoxybenzoin) by dichloroisocyanuric acid (DCICA) was studied in aqueous acetic acid—per-
chloric acid medium. Participation of enolic and protonated forms of ketones in the rate determining steps is
manifested from zero and first orders with respect to the oxidant in absence and presence of added chloride
ions, respectively. Positive and negative effects of acid and dielectric constant on the reaction rate were
observed. The observations deduce plausible mechanisms involving (i) rate-determining formation of enol
+
from the conjugate acid of the ketone (SH ) in the absence of added chloride ions and (ii) rapid formation of
molecular chlorine species from HOCl (hydrolytic species of DCICA) in the presence of added chloride ions,
+
which then interacts with SH in a rate-determining step prior to the rapid steps of product formation. The
order of Arrhenius parameters substantiate the proposed plausible mechanisms based on order of reactants
both in presence and absence of added chloride ions.
Keywords: dichloroisocyanuric acid, acetophenone, desoxybenzoin, kinetics, chloride ion, enolization, reac-
tion pathway.
DOI: 10.1134/S0036024416030237
INTRODUCTION
tinuation, the present study of kinetics of oxidation of
acetophenone and desoxybenzoin by DCICA in aque-
ous acetic acid—perchloric acid medium in absence
and presence of added chloride ions is intended (i) to
learn about the active species out of the two possible
forms (enol and keto) of aromatic ketones (ii) to
understand the nature of DCICA i.e., whether it halo-
genates and/or oxidizes these ketones (iii) to study the
effect of added chloride ions on the kinetics and
mechanism of oxidation of aromatic ketones (ace-
tophenone and desoxybenzoin) by the present N-halo-
genated reagent (iv) as the oxidation of ketones fre-
quently provides a useful synthetic route to
α-hydroxyketones, α-diketones, carboxylic acids, etc.
and aromatic ketones are convenient substrates in that
the reaction site is localized by the flanking phenyl
rings leading to minimization of secondary reactions.
The nature of end products can be understood easily
in this case.
Recent literature survey shows that kinetics of oxida-
tion of aromatic ketones by different inorganic oxidants
like potassium permanganate [1], thallium (III) per-
chlorate [2], iodate [3], permanganate ion [4], quino-
linium dichromate [5], as well as in the presence of
amino acids [6] and cetyltrimethylammonium bromide
[
7, 8] is well studied. The earlier works reported that end
products are carboxylic acids [9, 10] formed via enol
form of ketones [11, 12] and diketones [13] via the pro-
tonated form of ketone which undergoes the oxidation
[
14]. However, halogenation of ketones was reported
with halogen/N-halogenated oxidants [15, 16].
Though, the effect of chloride ions on the kinetics
of oxidations involving inorganic oxidants was
explained on the basis of a chloride ion bridge acti-
vated transition state [17], very few such studies were
carried out on the reactions involving N-halogenated
oxidizing agents [18, 19]. Recently the authors have
reported a pronounced effect of the added chloride ion
on the oxidation of aliphatic ketones [20] and cyclic
ketones [21] with N-halogenated oxidizing agent in
which a change in reaction path was observed com-
EXPERIMENTAL
AR grade acetophenone and desoxybenzoin were
pared to that of absence of added chloride ion. In con- redistilled and recrystallized, respectively, before use.
Dichloroisocyanuric acid was from Fluka. All other
chemicals used were of ExcelaR grade from Glaxo.
1
The article is published in the original.
552

SHIFT OF REACTION PATHWAY
553
Table 1. Oxidation of aromatic ketones by DCICA in Table 2. Oxidation of aromatic ketones by DCICA in pres-
absence of added chloride ions ([DCICA] = 0.0005 M, ence of added chloride ions ([DCICA] = 0.0005 M, [Sub-
+
+
[
Substrate] = 0.005 M, [H ] = 0.1 M, temperature = 35°C, strate] = 0.005 M, [H ] = 0.1 M, temperature = 35°C, sol-
–
solvent composition (AcOH : H O) = 70 : 30 vol/vol
vent composition (AcOH : H O) = 70 : 30 vol/vol, [Cl ] =
2
2
0.00125M)
k × 10 , mol L s–1
8
–1
0
–
5
–1
Varied conditions
k × 10 , s
1
acetophenone desoxybenzoin
Varied conditions
acetophenone desoxybenzoin
[
DCICA]
0.00025
.0005
.001
0.00125
2.39
2.36
2.40
–
1.31
–
1.31
1.34
1.36
0.34
0.57
0.85
1.34
–
[
DCICA]
0.00025
.0005
34.1
35.2
34.9
–
27.5
26.6
25.0
7.03
12.7
15.2
26.6
–
0
0
0
[
Substrate]
0.001
0
0
0
0
0
0
.0025
.00375
.005
.0125
.025
[Substrate]
0.00125
0.0025
0.00375
20.3
–
2.36
4.62
0
0
0
0
.005
35.2
88.2
126.0
245.0
38.4
33.4
35.2
34.2
31.3
35.2
40.0
51.9
63.6
35.2
44.1
52.9
96.3
11.5
23.9
1.40
–
–
.0125
.025
.375
–
+
[
H ]
0.025
0.95
1.02
1.34
1.40
–
–
–
–
1.34
1.58
1.93
1.34
3.58
6.87
–
.0375
–
0
0
0
0
.05
.1
.15
.3
2.09
2.36
4.52
6.86
1.16
1.58
1.68
2.36
2.73
4.35
2.36
4.06
6.61
+
[H ]
0.025
26.1
25.0
26.6
42.1
89.4
26.6
31.0
38.4
46.1
26.6
34.3
69.3
101.0
0
0
0
0
.05
.1
.15
.3
Solvent
composition
AcOH : H O
(
15 : 85
35 : 65
55 : 45
70 : 30
80 : 20
90 : 10
35
[Cl–]
0.00125
2
vol/vol)
0
0
0
.0025
.005
.01
35
Temperature
(
Temperature
°C)
°C)
(
40
40
45
50
45
11.2
50
The reaction was followed by estimating the concen-
tration of unreacted DCICA by iodometry at pH 1–2.
RESULTS AND DISCUSSION
Reaction Orders of the Oxidant and Substrate
−
+
C O N H(Cl) + 4I + 2H
3
3
3
2
In the oxidation of aromatic ketones (acetophe-
none and desoxybenzoin) by DCICA in absence of
added chloride ions, the reactions have been found to
(1)
−
→
C O N (H) + 2I + 2Cl .
3 3 3 3 2
Compared to the rates of present reaction, self be zero order in oxidant (DCICA) concentration as
decomposition rates of DCICA were negligibly evidenced by the linear plots of x versus time (Table 1).
smaller under the conditions employed. The rate con- Such a zero order dependence on oxidant was
stants remained practically unaltered in air or in a de- observed in the oxidation of acetophenone in acidic
aerated atmosphere. Only a representative set of the medium using Tl (III) perchlorate in perchloric acid
average values of kinetic data in aqueous acetic acid media [2], N-bromosuccinimide [22] in which eno-
medium was presented here. Zero or first order rate lized form of ketone is involved in the rate determining
2
constants were evaluated from the linear plots (r > step whereas, it was first order in the oxidation of ace-
0
.993) of [reacted DCICA] or log [unreacted DCICA] tophenone by potassium permanganate [1], the reac-
against time. Rate constants and other determined tion is first order with respect to [oxidant], as ketone
values were reproducible to ±2ꢀ. was the active form of ketone. Hence, based on the
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 90 No. 3 2016

5
54
LAKSHMAN KUMAR et al.
zero order nature of oxidant in the present case, it is positive effect of acetic acid composition on the rate of
evident that enolized ketone form involves in the rate reaction can be attributed to an improvement of enol
determining step. However, in the presence of added concentration due to catalytic nature of acetate
chloride ions, the oxidation of acetophenone and ion/acetic acid on the process of enolization. Retarda-
desoxybenzoin were first order with respect to tion in reaction rate was observed by carrying out sol-
[
DCICA], as seen from the constancy of k values vent isotope studies in the medium of heavy water
1
+
(
Table 2) obtained with different initial oxidation con- which can be explained on the fact that D O is a
3
centrations and the plots of log (a–x) versus time are stronger acid than hydronium ion and hence this
linear confirming unit dependence with respect to observation supports the proposed mechanism.
[
DCICA]. Both in the absence and presence of added
chloride ions, the reaction order on substrates (ace-
tophenone and desoxybenzoin) concentration is unit
Effect of Temperature
(
Tables 1, 2).
To investigate the effect of temperature and to eval-
uate the various Arrhenius activation parameters the
oxidation of acetophenone and desoxybenzoin by
DCICA has been carried out at different temperatures
Effect of Variation of Acid Concentration
In the absence of added chloride ions, as the rate of in absence and presence of added chloride ions
+
reaction increased with an increase in [H ] (Table 2), (Tables 1, 2) and thermodynamic parameters were cal-
it is evident that the reaction (enolization) is catalyzed culated from plots of logk vs. 1/T and tabulated
0
in the presence of acid, and enol form is active species. (Table 3). In the absence of added chloride ions, the
In the absence of added chloride ions, the reactions
have positive fractional order with respect to acid con-
centration. In the presence of added chloride ions, the
oxidation of acetophenone is acid independent and in
the case of desoxybenzoin, initially it is acid concen-
tration independent and later first order.
≠
fairly high positive value of ΔH indicates that, the
transition state is highly solvated. The high negative
value of entropy in the oxidation of acetophenone
favors the formation of a compact and a more ordered
transition state compared to the oxidation of desoxy-
≠
benzoin. Almost constant values of ΔG for both aryl
ketones may be indicative of the operation of the same
mechanism of oxidation, namely a fast nucleophilic
attack of oxidant on enol forms of ketones to form the
products.
Effect of Variation of Solvent Composition
Rates of the reactions increased with an increase in
the percentage of acetic acid in the aqueous—acetic
acid medium (Table 1) and the observation can be
substantiated with an increase in the rate of enoliza-
tion of acetophenone with the increase in acetic acid
Absence of Induced Polymerization
[
23] and similarly in the case with desoxybenzoin.
To understand if the involvement of free radicals is
However, a decrease in rate of the oxidation of ace- present or not, the reactions were studied in presence
tophenone by potassium permanganate with increase of added acrylonitrile [25]. Non-appearance of pre-
in the acetic acid content was studied in 50ꢀ aqueous cipitate formation (due to lack of induced polymeriza-
acetic acid-medium [1] and those results were tion of the acrylonitrile) gives evidence to absence of
explained on the basis of a decrease in the water con- induced polymerization. No effect of acrylonitrile on
tent in the reaction mixture, which is essential for rate rate of the reaction and the negative polymerization
determining decomposition of the intermediate. But, test rules out the possibility of in situ generation of free
it may be the case when the composition of acetic acid radicals during the course of the reaction.
is more than 95ꢀ in aqueous—acetic acid mixture. In
the present case, plots of logk vs. 1/D gave straight
0
Product Analysis and Stoichiometry
lines with positive slopes indicating that the reaction is
between dipolar molecule and a positive ion. In cor-
roboration with the work of Swain [24], the observed ketone (10 mmol) and DCICA (1 mmol) was carried
To conduct the product analysis, reaction between
Table 3. Arrhenius parameters for oxidation of acetophenone and desoxybenzoin
Addition of
chloride ions
≠
≠
≠
–1
≠
Substrate
logPZ
ΔE , kJ/mol
ΔH , kJ/mol −ΔS , J K /mol
ΔG , kJ/mol
Acetophenone
Desoxybenzoin
Acetophenone
Desoxybenzoin
–
–
+
+
85.32
133.23
52.75
82.76
130.67
50.19
122.68
–28.64
149.14
70.27
6.84
14.74
5.46
120.54
121.85
96.13
77.77
75.21
9.58
96.85
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SHIFT OF REACTION PATHWAY
555
out in a mixture of acetic acid-water (70 : 30 vol/vol). Table 4. Relative rates of oxidation of ketones by DCICA
+
(
[DCICA] = 0.0005 M, [Substrate] = 0.005 M, [H ] = 0.1 M,
The reaction mixture was allowed to stand for 48 h to
make sure of completion of reaction. From the reac-
tion mixtures, the organic layer was extracted with
diethyl ether. The acetic acid in the ether layer was
neutralized with saturated sodium bicarbonate solution.
The extract was washed with distilled water, dried over
anhydrous sodium sulphate and evaporated. Obtained
product was recrystallised from hot water and washed
with chloroform.
temperature = 35°C, solvent composition (AcOH : H O) =
2
7
0 : 30 vol/vol)
8
–1
Substrate
σ
k × 10 , mol s
0
Acetone
0.1
4.80
4.60
2.36
1.34
2
-Butanone
0.115
0.13
0.145
Acetophenone
Desoxybenzoin
Formation of benzoic acid and formic acid in the
oxidation of acetophenone and, benzoic acid in the
oxidation of desoxybenzoin was observed from spot
tests [26, 27]. Formation of benzoic acid is evident
from thin layer chromatographic analysis in which
product is eluted on a silica gel G TLC plate (еthanol :
ammonia—9 : 1). Air dried plate was sprayed with per-
oxide—ferric chloride reagent (mixture of equal vol-
umes of freshly prepared 2ꢀ ferric chloride and 0.5ꢀ
hydrogen peroxide solution) where benzoic acid shows
benzil, benzaldehyde, and benzoic acid [32]. Forma-
tion of benzoin as the final product in the oxidation of
desoxybenzoin can be denied on the basis earlier
observation that the oxidation of benzoin occurs at a
rate 500 times as great as that of desoxybenzoin [32].
In addition, in the recent publication of [5], it was
reported that in the oxidation arylalkyl ketones by
QDC, cleavage of the carbon–carbon bond yields car-
boxylic acids.
Reaction mixtures of varying proportions of
ketones and DCICA were equilibrated for 48 h at room
temperature in perchloric acid medium. The unre-
acted DCICA was determined by iodometry and the
stoichiometry was found to be 3 : 2 (DCICA : ketone).
The stoichiometry also confirms the above said prod-
ucts formation.
a mauve colored spot (R = 0.5) [28]. The presence of
f
an absorption band corresponding to characteristic of
–1
O–H stretching (nearer to 2950 cm ) in addition to
the peak of C=O stretching in IR spectrum confirms
the formation of benzoic and formic acids.
The literature survey shows that in the of oxidation
of acetophenone, ester was the product by peroxomo-
nosulphate [29], benzoic acid was the product of
potassium permanganate oxidation [1], benzoic acids
and formic acid were the products using Tl (III) [2],
phenylglyoxal using N-bromosuccinimide [22], ben-
zoic acid and formaldehyde using manganese (III)
Lower Rate of Desoxybenzoin
Both in the absence and presence of added chloride
[
30] and quinolinium dichromate [5]. Oxidation of ions, the lower order of reactivity of desoxybenzoin
desoxybenzoin with 1 M HNO at gives benzoin compared to acetophenone (Tables 1, 2) is in accor-
3
(
(
46ꢀ), benzil (11ꢀ), and the degradation products dance with the structural changes. Addition of one
41ꢀ) (due to C–C bond fission products, i.e., ben- more phenyl group to alkyl side of acetophenone
zoic and p-nitrobenzoic acids and benzaldehyde) [31]. enhances electron density at the carbon atom adjacent
In the chromic acid oxidation of desoxybenzoin in to the carbonyl group. The ease of deprotonation from
9
1ꢀ acetic acid, benzoin was shown to be the interme- this carbon atom in desoxybenzoin would decrease as
diate in the reaction, and the source of the products, compared that in acetophenone.
1
0 + logk
0
2.8
2.7
2.6
2.5
2.4
2.3
2.2
2.1
Acetone
Butanone
y = −13.015x + 4.0553
R = 0.9142
2
Acetophenone
Desoxybezoin
0.09
0.10
0.11
0.12
0.13
0.14
0.15
σ
Hammet’s plot for oxidation of ketones by DCICA in absence of added chloride ions.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 90 No. 3 2016

5
56
LAKSHMAN KUMAR et al.
+
Table 5. Effect of acid containing chloride ([DCICA] =
.0005 M, [Substrate] = 0.005 M, solvent composition
AcOH : H O) = 70 : 30 vol/vol, temperature = 35°C)
H OCl in the present case is further substantiated
2
0
(
from the earlier reports [20, 21, 33] in the case of tri-
chloroisocyanuric acid. In both oxidation and electro-
philic substitution reactions, hypohalous acidium ion
was reported to be the active species [34].
2
k × 10 mol L s–1
8
–1
Conc. of
acid, M
0
Acid
acetophenone desoxybenzoin
HClO4
HCl
0.1
0.1
2.36
21.23
1.34
22.04
Molecular Chlorine Is the Active Oxidizing Species
in Absence of Chloride Ions
The added chloride ions play a vital role in the
kinetics of oxidation of aromatic ketones by DCICA as
The Hammet’s Plot
To understand the nature of substrates in the oxi- they have a positive effect on the rate of the reaction.
dation by DCICA, Hammet’s plot was drawn from the Formation of molecular chlorine (Cl ) was observed
2
rate constants obtained at similar conditions of reac- with different N-halogenated oxidants [20, 21, 35, 36].
+
tants (Table 4 and figure). The Hammet’s plot is linear
and a reaction constant of –13.01 is obtained. This
value indicates that electron-donating groups retard
the process which is in agreement with the suggested
mechanism.
Apart from H ions, chloride ions also exhibited cata-
lytic effect in the present case and molecular chlorine
was reported to be the efficient active oxidizing species
in acidic medium [37] and similar reports can be
observed in the literature in the oxidation of organic
compounds [38, 39]. Hence, in the present case,
molecular chlorine is formed from DCICA in pres-
ence of added chloride ions and it is the active oxidant.
The set of following reactions clearly explain the for-
Effect of Variation of Chloride Concentration
-
The reaction rates increased with an increase in Cl
concentration in the oxidation of both the substrates mation of molecular chlorine in the presence of added
(
Table 2) indicating that the reaction proceeded chloride ions.
–
through a Cl mediated route. Such an increase in the
rate of oxidation of chalcones by trichloroisocyanuric
acid [33], aliphatic and cyclic ketones [20, 21] was
observed with an increase in concentration of chloride
DCICA + H O ꢀ MCICA + HOCl,
(2)
2
MCICA + H O ꢀ Cyanuric acid + HOCl, (3)
2
‒
ions. In the present case, plots of logk vs. log[Cl ]
1
gave straight lines with slopes of 0.292 and 0.268
respectively in the oxidation of acetophenone and
desoxybenzoin, respectively, indicating positive frac-
tional order with respect to chloride. The catalytic
nature of chloride ions is further evidenced from the
K1
+
+
HOCl + H ꢀ H OCl ,
(4)
(5)
2
K3
+
–
H OCl + Cl ꢀ Cl + H O.
2
2
2
kinetic studies of replacing HClO with HCl (which
4
supplies free chloride ions) where an increase in rate of
reaction was observed which further augments the
proposed involvement of chloride ions in the genera-
tion of active oxidizing agent—chlorine (Table 5).
Mechanism and Rate Law in Absence
of Added Chloride Ions
In the oxidation of acetophenone and desoxyben-
zoin in acetic acid—perchloric acid medium in
absence of added chloride ions, the rate controlling
step is enolization of arylketones in view of (i) zero
order dependence on oxidant (ii) increase in rate con-
stant with increase in concentration of hydrogen ion as
enolization is acid catalyzed reaction. This is in cor-
roboration with reports in literature [20, 21, 40]. Par-
ticipation of a dipole (enol form of ketone) and an ion
+
H OCl Is the Active Oxidizing Species in Absence
2
of Chloride Ions
The pH of the medium dictates the type of oxidiz-
ing species and different reactive species of
dichloroisocyanuric acid in acidic medium may be
either unprotonated (HOCl/DCICA) or protonated
+
+
forms (H OCl /DCICAH ) [20, 21]. The possibility
2
of participation of either unprotonated/protonated
(hypohalous ion) in a rate determining step can be
+
oxidant molecules (DCICA/DCICAH ) can be elim- ascertained from negative dielectric effect of solvent.
inated as there is no effect of added cyanuric acid. Out This is further supported by the effect of ionic strength
of the remaining two species, involvement of HOCl on the rate of reaction as there is no effect of ionic
can be ruled out from positive effect of acid concentra-
tion on rate of reaction and hence, in acidic medium
strength, the reaction might be between an ion and
molecule. In acidic solutions the acetophenone
+
the remaining only one option is H OCl which forms undergoes fast enolization as represented by equation
2
in protonation equilibrium. Active participation of [9] as given below.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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SHIFT OF REACTION PATHWAY
557
enolization involve in the rate determining step. Such
a rate controlling step of keto-enolization was pro-
posed in the oxidation of acetophenones using Tl (III)
perchlorate in perchloric acid media [2].
O
OH
C
C CH3
CH2
Based on the reaction conditions, ketones can react
The kinetics of oxidation of acetophenone/desoxy-
directly or via its enol form. In general, ketone is the benzoin by dichloroisocyanuric acid (DCICA), the
active species when (i) the rate of oxidation is faster reaction stoichiometry is [ketone] : [DCICA] : 2 : 3
compared to the enolization rate, (ii) oxidant transfers i.e., [ketone] : [HOCl] : 1 : 3 (because each DCICA
one electron in the reactions, and (iii) mechanism releases two HOCl molecules). The kinetic and ther-
involves a free radical. Direct oxidation of ketone form modynamic parameters strongly suggest the mecha-
was observed in the oxidation of ketones by various nism of a fast protonation of the ketone (S) to give
3
+
+
oxidants like Ce(IV), Mn(IV) [41], Co [42].
(SH ), followed by slow and rate-determining conver-
+
Active participation of enol form can be observed if sion of SH to enol (EN) by keto-enol tautomerism.
+
(
i) rate of oxidation equals enolization (ii) oxidant The enol (EN) is quickly attacked by two H OCl in
2
transfers two electron in the reactions, and (iii) zero
order dependence on [oxidant]. Participation of enol
form was observed in the oxidation of ketones by oxi-
concomitant fashion to give diketones, which react in
+
a fast a step with another H OCl to decompose into
2
the products. In the oxidation of acetophenone the
−
4
−
3
dants like Tl(III), MnO , and Hg(II) [43], BrO [44] products were benzoic and formic acids, whereas,
and manganic pyrophosphate [45]. Based on the benzoic acid was the only product from oxidation of
experimental observation in the present investigation, desoxybenzoin.
−
O
OH
H
H
K
2
2
R C C C H5 H+
R C C C H ,
6
6
5
OH
OH
H
k0
2
R C C C H5
R C C C H ,
6
6
5
slow
H
OH
O O
2
H2OCl+
fast
+
R C C C H5
R C C C H + 2HCl +2H O + 2H ,
6 5 2
6
H
O O
2
H2OCl+
H2O
−
+
6
R C C C H5
C H COOH + RCOOH + Cl + H ,
6 5
where R=H/C H .
6
5
+
+
Rate Law in Absence of Added Chloride Ions
DCICA + H O ꢀ MCICA + HOCl,
[S] = [S] + [SH ] = [S] + K [S] [H ]
T 2
+
(2)
2
= [S]{1 + K [H ]},
2
MCICA + H O ꢀ Cyanuric acid + HOCl, (3)
2
[S]T
[
S] =
,
+
1
+
+ K [H ]
2
K1
+
+
HOCl + H ꢀ H OCl ,
(6)
(7)
(8)
2
+
Rate = k [SH ] = k K [S] [H ],
0
0
2
K2
+
+
+
S + H ꢀ SH ,
k K [S] [H ]
0
2
T
rate =
.
+
1
+ K [H ]
2
+
k0
+
SH → EN + H ,
The given above rate law explains the observed
reaction orders i.e., zero order in [oxidant], first order
+
fast1
EN + H OCl ⎯ ⎯→ Products,
(9) in [substrate], and fractional order in [acid].
2
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 90 No. 3 2016

5
58
LAKSHMAN KUMAR et al.
Mechanism and Rate Law in Presence
of Added Chloride Ions
As the reactions are first order in concentrations of
lar chlorine (Cl ) were the active oxidizing species in
2
the oxidation of ketones. The oxidation of aromatic
ketones by DCICA is acid catalyzed in absence of
added chloride ions. Hence, the protonated ketone
both oxidant and substrate, probably, the formation of
complex between them is not viable or the so formed
complex can be assumed as fragile in nature. The pos-
sible steps in the oxidation of aromatic ketones by
undergoes enolization in a rate determining step and
+
subsequently, the enol interacts with H OCl in a
2
faster step to give benzoic acid and formic acid in oxi-
dation of acetophenone, where as benzoic acid was the
product in oxidation of desoxybenzoin. In the pres-
ence of chloride ions, same dicarboxylic acids are pro-
duced by decomposition of an intermediate which is
formed in a slow step due to reaction between proton-
+
DCICA in the presence of added chloride ions and H
are summarized below.
(
a) In acid medium, the protonated forms oxidant
+
+
(
H OCl ) and ketone (SH ) exist.
2
(
b) Molecular chlorine is formed due to reaction ated ketone and molecular chlorine.
+
between H OCl and added chloride ions.
2
(
c) In a rate determining step, complex formation
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(
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