Kinetics and mechanism of oxidation of chalcones by 1,3-dichloro-5,5-dimethyl hydantoin

Article abstract of DOI:10.14233/ajchem.2015.17582

Mechanistic study on oxidation of chalcone by 1,3-dichloro-5,5-dimethyl hydantoin (DCDMH or DCH) in aqueous acetic acid medium have been carried out. The reaction follows a fractional order (0.63) with respect to substrate and first order with respect to the oxidant. Increase in percentage of acetic acid increased the rate of the reaction. The added H⁺ did not affect the reaction rate. The oxidation reaction fails to initiate polymerization with acrylonitrile and free radical mechanism is ruled out. The thermodynamic and activation parameters for the oxidation have been determined and discussed. Based on the observations a probable mechanism and rate law were proposed.

Full text of DOI:10.14233/ajchem.2015.17582

Asian Journal of Chemistry; Vol. 27, No. 5 (2015), 1725-1728
A
SIAN
J
OURNAL OF HEMISTRY
C
http://dx.doi.org/10.14233/ajchem.2015.17582
Kinetics and Mechanism of Oxidation of Chalcones by 1,3-Dichloro-5,5-dimethyl Hydantoin
*
P. TAMIL SELVI and K. KARUNAKARAN
Department of Chemistry, Sona College of Technology, Salem-636 005, India
*
Corresponding author: E-mail: karunachemist@gmail.com
Received: 11 March 2014; Accepted: 26 May 2014;
Published online: 20 February 2015;
AJC-16879
Mechanistic study on oxidation of chalcone by 1,3-dichloro-5,5-dimethyl hydantoin (DCDMH or DCH) in aqueous acetic acid medium
have been carried out. The reaction follows a fractional order (0.63) with respect to substrate and first order with respect to the oxidant.
+
Increase in percentage of acetic acid increased the rate of the reaction. The added H did not affect the reaction rate. The oxidation reaction
fails to initiate polymerization with acrylonitrile and free radical mechanism is ruled out. The thermodynamic and activation parameters
for the oxidation have been determined and discussed. Based on the observations a probable mechanism and rate law were proposed.
Keywords: Chalcone, 1,3-Dichloro-5,5-Dimethylhydantoin, Kinetics, Oxidation.
1
4
aspects . N-Halo compounds have extensively been used as
INTRODUCTION
Chalcones (1,3-diaryl-2-propen-1-ones) are naturally
15-20
oxidizing agents for the catalyzed and uncatalyzed reactions
.
1
Since the N-halo compounds exhibit appreciable stability both
in acid and alkaline medium, it is used as an redox titrant in
analytical chemistry.
,3-Dichloro-5,5-dimethyl hydantoin is a N-chloro com-
pound and is usually represented as DCDMH or DCH. It has
found limited application and only few reports are available
of its uses in organic synthesis.
Literature survey reveals that no report was available on
the kinetics of oxidation of chalcones by 1,3-dichloro-5,5-
dimethyl hydantoin.
occuring compounds, largely distributed in plants, fruits and
vegetables. They belong to the flavanoid group of compounds.
2-5
Literature studies reveals that these compounds are of chemical
and pharmacological interest because they exhibit many
biological activities such as antimicrobial, antioxidant, anti-
malarial, antitumor, antidepressant, antiinflammatory, anti-
1
15-20
6
HIV and anticancer. Chalcones are α,β-unsaturated ketones
consisting of two aromatic rings (ringA and B) having diverse
array of substituents. Rings are interconnected by a highly
electrophilic three carbon α,β-unsaturated carbonyl system
that assumes linear or nearly planar structure. They contain
the ketoethylenic group (-CO-CH=CH-). Chalcones possess
conjugated double bonds and a completely delocalized
π-electron system on both benzene rings. They can be obtained
by several methods and Claisen-Schmidt's condensation (aldol
condensation) is the most used one.
EXPERIMENTAL
All the chemicals and solvents used were of analytical
grade (Merck, India). Chalcone and 1,3-dichloro-5,5-dimethyl
hydantoin (Sigma-Aldrich) were used as such.All the solutions
used in the study were made by using double distilled water.
All the reagents were freshly prepared just before the reactions
were carried out. All the reactions were carried out in a thermo-
stat and the temperature was maintained at ± 0.1 ꢀC.
Kinetic measurements: The kinetic studies were carried
out in an aqueous acetic medium. The reactions were perfor-
med by maintaining a large excess of [chalcone] over [DCDMH].
The reaction mixture was homogeneous throughout the course
of the reaction. The reaction rate was found by estimating the
amount of the unconsumed DCDMH iodometrically up to 80 %
of completion of the reaction. The rate constants (kobs) were
determined from the slopes of linear plots of log[titre] versus
Chalcones are oxidized by different oxidizing agents and
in all these reactions either the >C=C< or >C=O group of
the chalcone is attacked by the oxidant. The kinetics and
oxidation of chalcones by various oxidants like pyridinium
7
8
9
chloro-chromate , trichloro isocyanuric acid , chromic acid ,
10
11
quinal-dinium dichromate , chlorammine-T , N-chlorosucci-
1
2
13
nimide , quinaldinium fluorochromate have been extensively
carried out by many eminent workers. In recent years, the
kinetic studies on the oxidation and halogenation of organic
compounds by N-halo compounds are gaining importance due
to their applications in organic synthesis and also due to kinetic

1
726 Selvi et al.
Asian J. Chem.
time. Replicate runs showed that the rate constants for the
oxidation reactions were reproducible within + 3 %.
4
.44
.42
r
= 0.99403
slope = 0.63255
SD = 0.00808
4
Rate law:
+
4.40
−
d[DCDMH] K k [chalcone]+ K K k [chalcone][H ]
1
3
1
2
4
kobs
=
=
+
4.38
4.36
4.34
4.32
4.30
dt
K + K K [H ]
1
1
2
Stoichiometry: The stoichiometry of the reaction was found
out by doing several sets of experiments by varying [DCDMH].
The unconsumed DCDMH remained after completion of the
reaction showed 1:1 (chalcone : DCDMH) stoichiometry.
Product analysis: The concentration of DCDMH is kept
in excess over chalcone. The reaction was allowed to complete
by keeping in a thermostat at 40 ꢀC for 4 h. The solution was
shaken well with ether. The ethereal layer was washed with
water several times and kept on a water bath for ether evapo-
ration and cooled to get the product. Benzoic acid and phenyl
acetaldehyde were found to be the major products of oxidation
and were detected by TLC and compared with those of the
authentic samples.
4.28
4
.26
.24
4
0
.70
0.75
0.80
0.85
log [chalcone]
Fig. 1. Plot of log kobs versus log [chalcone] for oxidation of chalcone by
,3-dichloro-5,5-dimehyl hydantoin in acetic acid medium
0.90
0.95
1.00
1
Data analysis: Correlation studies were carried out using
Microcal origin (version 6) computer software. The goodness
of the fit was discussed using the correlation coefficient, r, in
the case of simple linear and R in the case of multiple linear
regression analysis.
28000
r
= 0.98688
slope = 1923.9
SD = 639.95
26000
24000
22000
20000
18000
16000
RESULTS AND DISCUSSION
Factors influencing the rate of oxidation of chalcone by
1
,3-dichloro-5,5-dimethyl hydantoin such as [chalcone],
+
[DCDMH], [H ] and dielectric constant have been studied.
Rate of the reaction and activation parameters were calculated.
+
Effect of [Chalcone]:At a constant [DCDMH], [H ] and
fixed percentage of acetic acid, kinetic runs were carried out
with various initial concentrations of chalcone, which yielded
rate constants whose values depended on [chalcone]. The
pseudo-first order rate constants (kobs) thus obtained were found
to increase with [chalcone] (Table-1). The plot (Fig. 1) of log
5
6
7
8
9
10
1/[chalcone]
Fig. 2. Plot of 1/kobs versus 1/[chalcone] for oxidation of chalcone by 1,3-
k
obs versus log[chalcone] is linear with a slope of 0.63 showing
that the reaction is fractional order in [chalcone]. The plot
Fig. 2) of 1/kobs versus 1/[chalcone] is linear with definite
dichloro-5,5-dimethyl hydantoin in acetic acid medium
(
Effect of [DCDMH]: The kinetics of oxidation of chalcone
intercept on the rate ordinate, which indicates the operation
of Michaelis-Menten mechanism.
has been studied at various initial concentrations of the oxidant
(DCDMH) (1-5 × 10 mol dm ) and at fixed concentrations
-3
-3
TABLE-1
PSEUDO-FIRST ORDER RATE CONSTANTS FOR THE OXIDATION OF
CHALCONE BY 1,3-DICHLORO-5,5 DIMETHYL HYDANTOIN AT 303 K
-3
3
-3
+
-3
5
-1
[
Chalcone] × 10 (mol dm )
[DCDMH] × 10 (mol dm )
[H ] × 10 (mol dm )
CH COOH % (V/V)
kobs × 10 (s )
3.77
3
1.0
1.2
1.4
1.6
1.8
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
5.0
5.0
5.0
5.0
5.0
1.0
2.0
3.0
4.0
5.0
5.0
5.0
5.0
5.0
5.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
2.4
4.0
5.6
7.2
8.8
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
4.33
4.63
5.15
5.71
9.87
6.14
5.49
3.97
3.77
3.38
3.77
4.53
5.20
5.67

Vol. 27, No. 5 (2015)
Kinetics and Mechanism of Oxidation of Chalcones by 1,3-Dichloro-5,5-dimethyl Hydantoin 1727
of other reactants. The plot of log [DCDMH] versus time was
found to be linear indicating first order dependence on the
reaction rate and from the slope of such plots pseudo-first order
rate constants were evaluated.
TABLE-3
EFFECT OF TEMPERATURE ON THE REACTION RATE
Temp. (K)
obs (s ) × 10
283
1.23
293
303
3.77
313
7.51 9.96
+
323
-1
5
k
[
2.33
+
+
-3
-3
-3
DCDMH] = 5 × 10 mol dm , [chalcone] = 0.1 mol dm , [H ] = 4 mol
Effect of [H ]: The required concentrations of [H ] solu-
-3
dm , Acetic acid:water = 40:10
tions were prepared from the stock solution of 4 M concen-
+
tration. The reaction rate constants measured with various [H ]
1
-3
3.25
3.20
(
2.4-8.8 × 10 mol dm ) were found to increase with increase
+
in [H ] ion concentration (Fig. 3). Order of the reaction with
r = 0.99509
+
respect to [H ] is 0.39.
3
.15
3.10
.05
.00
.95
.90
.85
.80
4.50
4.45
4.40
4.35
4.30
4.25
r
= 0.99575
slope = 0.39805
SD = 0.01975
3
3
2
2
2
2
0.0031
0.0032
0.0033
/T
0.0034
0.0035
0.0036
1
Fig. 4. Plot of ln kobs/T versus 1/T showing the effect of temperature on
reaction rate
-1
mol . It is clear from these values that both the parameters
0
.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
#
#
+
∆H and ∆S are important in controlling the rates of reactions
log [H ]
though the enthalpy factor appears to be more predominating.
+
+
Fig. 3. Plot of log kobs versus log [H ] showing the effect of H concentration
#
The value of ∆S is negative indicating the loss of entropy
on reaction rate
during the formation of activated complex.
Mechanism: Chalcone contains two functional groups,
viz. > C=C < and > C=O < and the whole molecule is in conju-
gation. In strongly acidic media, DCDMH may get protonated
producing electrophilic species and these species will get
hydrolyzed in the presence of acid to give the reactive species
Effect of dielectric constant: In order to determine the
effect of dielectric constant (polarity) of the medium on rate,
the oxidation of chalcone by 1,3-dichloro-5,5-dimethyl
hydantoin was studied in aqueous acetic acid mixtures of
various compositions (Table-2). The data clearly revealed that
the rate increases with increase in the percentage of acetic
+
+
such as HOCl and H
2
OCl . Since H OCl is the reactive species
2
8
it attacks either at >C=C< or at >C=O or as an alternate path
these can chlorinate phenyl rings.As no chlorination products
were detected, the nuclear chlorination of phenyl rings was
ruled out. The other possibility is the attack at >C=C< or >C=O.
The functional group >C=C< may not be attacked by HOCI
as >C=C< group is deactivated by two phenyl rings in
conjugation. The other alternative is the attack at >C=O. The
keto functional group is highly polarized and HOCl forms an
unstable intermediate complex with chalcone at this site. This
unstable complex decomposes in a rate determining step to
give the final products such as benzoic acid and phenyl acetal-
dehyde. In accordance with the above observations and stoichio-
metry of the reaction, the following reactions are involved to
constitute the most probable mechanism of the reaction
acid .
TABLE-2
EFFECT OF DIELECTRIC CONSTANT ON
THE REACTION RATE AT 303 K
a
5
-1
CH COOH:H O
D
kobs × 10 (s )
2.33
3
2
10:90
20:80
30:70
40:60
50:50
41.47
45.48
49.71
53.83
57.95
3.45
3.77
6.35
7.83
-3
-3
-3
+
[
DCDMH] = 5 × 10 mol dm , [chalcone] = 0.1 mol dm , [H ] = 4
mol dm , values are calculated from the values of pure solvent
-3 a
Test for free radical intermediate: Addition of monomers
like acrylonitrile to the reaction mixture under inert conditions
did not induce any polymerization indicating the absence of
(
Scheme-I).
Conclusion
In conclusion, 1,3-dichloro-5,5-dimethyl hydantoin
DCDMH) has been proven to be an excellent oxidant for the
8
free radicals in the reaction mixture .
Rate and activation parameters: The effect of tempe-
rature was studied in the range of 283-323 K and the results
were shown in Table-3. The Arrhenius plot of lnkobs/T versus
(
oxidation of chalcone. The kinetic and thermodynamic para-
meters for the oxidation of chalcone by DCDMH were deter-
mined and the reaction scheme was proposed. The thermo-
dynamic data obtained, supported the proposed mechanism.
1
/T was found to be linear (Fig. 4). The value of energy of
-1
-1
#
activation (E ) was found to be 9.74081 kJ mol K and ∆H =
7
a
-1 # -1 -1 #
.2204 kJ mol , ∆S = -196.60 J K mol , ∆G = 66.79 kJ

1
728 Selvi et al.
Asian J. Chem.
O
O
Cl
Cl
K
1
N
N
HOCl
+
H O
+
2
N
N
O
O
H
Cl
K2
H
OCl+
2
+
H+
HOCl
O
fast
HOCl
C
H
CH
C
+
OH
C
C
H
CH
O
Cl
Unstable intermediate complex
OH
C
+
CH
k3
+
C
H
CH
CH
O
O
Cl
-
+ Cl
HO
C
benzoic acid
+
CH
fast
H+
+
H
2
O
CH
CH
OH
+
CH
Keto enol tautomerism
CH2
CHO
Phenyl acetaldehyde
Scheme-I: Probable mechanism for the oxidation of chalcone by DCDMH
1
1
1
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