Kinetics and mechanistic study of oxidation of ethyl vanillin by alkaline hexacyanoferrate(III)

Article abstract of DOI:10.14233/ajchem.2015.18366

The kinetics of oxidation of ethyl vanillin by hexacyanoferrate(III) in aqueous alkaline medium was studied. The reactions are found to be fractional order with respect to substrate & hydroxide ion and first order with respect to oxidant. The rate-determi

Full text of DOI:10.14233/ajchem.2015.18366

Asian Journal of Chemistry; Vol. 27, No. 7 (2015), 2583-2586
A
SIAN
J
OURNAL OF HEMISTRY
C
http://dx.doi.org/10.14233/ajchem.2015.18366
Kinetics and Mechanistic Study of Oxidation of Ethyl Vanillin by Alkaline Hexacyanoferrate(III)
3
4
A. GRACE KALYANI1,2,*, R. JAMUNARANI and F.J. MARIA PUSHPARAJ
1Research & Development Centre, Bharathiar University, Coimbatore-641 046, India
2Department of Chemistry, Nehru Institute of Engineering & Technology, Coimbatore-641 105, India
3Department of Chemistry, Government Arts & Science College, Coimbatore-641 018, India
4Department of Chemistry, Government Arts & Science College, Ooty-643 002, India
*Corresponding author: E-mail: agracekalyani@gmail.com
Received: 23 June 2014;
Accepted: 6 September 2014;
Published online: 30 March 2015;
AJC-17084
The kinetics of oxidation of ethyl vanillin by hexacyanoferrate(III) in aqueous alkaline medium was studied. The reactions are found to
be fractional order with respect to substrate & hydroxide ion and first order with respect to oxidant. The rate-determining step is the outer-
sphere formation of Fe(CN)64– and free radicals, which is followed by the rapid oxidation of free radicals by Fe(CN)63– to give products.
The added product, hexacyanoferrate(II), had a retarding effect on the rate of reaction. Ionic strength and dielectric constant of the
reaction medium have little effect on the reaction rate. The effect of temperature on the rate of reaction has also been studied and activation
parameters have been evaluated. A mechanism based on the experimental results is proposed and the rate law is derived.
Keywords: Oxidation, Ethyl vanillin, Kinetics, Mechanism, Potassium hexacyanoferrate(III).
INTRODUCTION
EXPERIMENTAL
Potassium ferricyanide is an inorganic complex, used
effectively as an oxidant for several organic compounds. The
oxidation-reduction reactions of hexacyanoferrate(III) ion are
shown to be rapid whenever the process involves a simple
electron transfer and to be slow and of complex mechanism if
such a step cannot occur1.
All the chemicals used are of AR grade and of SD fine
chemicals Ltd. These were used without further purification.
A solution of hexacyanoferrate(III) was prepared by dissolving
K3[Fe(CN)6] in distilled water and standardized iodomet-
rically20a. NaOH and KCl were employed to maintain the
required alkalinity and ionic strength, respectively.
Kinetics and mechanistic study of oxidation of several
organic substrates by hexacyanoferrate(III) are well documen-
ted2-9. The oxidation of carbonyl compounds by this oxidant
Kinetic measurements: All kinetic measurements were
performed under pseudo first-order conditions where [ethyl
vanillin] was always in excess over hexacyanoferrate(III), at a
constant ionic strength in alkaline medium at a constant
temperature of (30 0.1) °C, respectively, unless otherwise
stated. The reaction was initiated by mixing the thermostatted
solutions of hexacyanoferrate(III) and ethyl vanillin, which
also contained the required concentration of NaOH and KCl.
The progress of the reaction was followed by observing the
disappearance of hexacyanoferrate(III), titrimetrically. Pseudo
first-order rate constants, kobs, were obtained (Tables 1 and 2)
from the slopes of plots of log10 [Fe(CN)63–] versus time; the
plots were linear and the kobs values were reproducible to within
5 %.
in alkaline medium has been studied by several workers10-16
.
The present study deals with the oxidation of ethyl vanillin by
alkaline hexacyanoferrate(III) to explore the redox chemistry
of hexacyanoferrate(III) in such media and to arrive at a suitable
mechanism for the oxidation of ethyl vanillin by alkaline
hexacyanoferrate(III) on the basis of kinetic results.
3-Ethoxy-4-hydroxybenzaldehyde, commercially known
as ethyl vanillin has flavoring power two to four times stronger
than vanillin17. It is also used as a chemical intermediate18 and
in perfumery19. The oxidation of ethyl vanillin by hexacyano-
ferrate(III) in aqueous alkaline medium is quite interesting
because one may expect the transfer of electron from hydroxyl
group which is an electron rich centre to oxidant. In contrary
aldehydic group becomes electron rich and thereby undergoes
oxidation because of immediate exclusion of proton from
hydroxyl group by alkali.
Stoichiometry and products analysis: Reaction mix-
tures with different sets of concentrations of reactants where
3–
[Fe(CN)6 ] was in excess over [ethyl vanillin] at a constant
ionic strength and alkali were kept for about 8 h at 30 °C in a
closed vessel. The remaining hexacyanoferrate(III) was estimated

2584 Kalyani et al.
Asian J. Chem.
dm–3 at fixed [ethyl vanillin], [OH–] and ionic strength. The
non-variation of the pseudo first-order rate constant at various
concentrations of hexacyanoferrate(III) indicates the order in
[hexacyanoferrate(III)] is unity (Table-1). This was also confir-
med from the linearity of plots of log10 [hexacyanoferrate(III)]
versus time (r = 0.998 and s ≤ 0.005) for up to 60 % completion
of the reaction.
Effect of [ethyl vanillin]: The substrate ethylvanillin,
was varied in the range of 4 × 10–2 to 7 ×10–2 mol dm–3 at
30 °C keeping all other reactant's concentrations and conditions
constant (Table-1). The kobs values were almost constant which
underlines the first order dependence on ethyl vanillin. A plot
of log kobs against log [ethyl vanillin] is linear with slope =
0.90.
Effect of [alkali]: The dependence of the reaction rate on
hydroxide ion has been studied in the range 0.35 to 0.95 mol dm-3.
Table-1 shows the values of the rate constants at different [OH–].
A plot of log kobs against log [OH–]actual is linear with slope =
0.63. An attempt was made to find out whether the reaction
would proceed in the absence of hydroxide ion or in the
presence of bases other than hydroxide ion. For this purpose,
a solution of the substrate in exactly equal molarity of hydro-
xide ion was prepared and reacted with hexacyanoferrate(III).
The reaction did not proceed at all. So the reaction may be
considered to be specific hydroxide catalyzed and not general
base catalyzed.
Effect of ionic strength and solvent polarity: The ionic
strength of the medium has been found to have considerable
effect on the rate of oxidation of ethyl vanillin by alkaline
hexacyanoferrate(III) Fig. 1. The change in the ionic strength
was effected by the addition of potassium chloride of known
strength. The values of rate constants at different ionic strengths
are calculated in Table-2.
TABLE-1
EFFECT OF VARIATIONS OF [Fe(CN)63-], [ETHYL
VANILLIN] AND [OH–] ON THE OXIDATION OF ETHYL
VANILLIN BY Fe(CN)63- AT 30 °C, I = 0.80 mol dm–3
[OH–]
(mol dm-3)
0.40
[Fe(CN)63-
]
[Ethyl vanillin]
kobs × 103 (s-1)
× 103 (mol dm-3) × 102 (mol dm-3)
2.0
3.0
4.0
5.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
5.0
5.0
5.0
5.0
4.0
5.0
6.0
7.0
5.0
5.0
5.0
5.0
1.46
1.46
1.46
1.42
0.82
0.93
1.16
1.50
1.40
1.84
2.10
2.70
0.40
0.40
0.40
0.40
0.40
0.40
0.40
0.35
0.55
0.65
0.75
TABLE-2
EFFECT OF VARYING IONIC STRENGTH ON REACTION RATE
[Substrate] = 5 × 10-2 mol dm–3; [OH–] = 0.4 mol dm–3;
[oxidant] = 3 × 10–3 mol dm–3
I
0.588
7.8
0.688
10.8
0.788
18.5
0.888
22.9
0.988
26.8
104 k (s-1)
titrimetrically. The results indicated that 2 moles of hexa-
cyanoferrate(III) consumed 1 mol of ethyl vanillin as in
eqn. 1.
CHO
2[Fe(CN) ]3-
4OH-
+
+
6
OC2H5
OH
COO-
1.5
[Ethyl vanillin] = 5 × 10–2
[OH–] = 0.4 M
M
2[Fe(CN)6]4-
3H2O
1.4
1.3
1.2
1.1
1.0
0.9
0.8
[Oxidant] = 3 × 10–3
M
+
+
OC2H5
Temperature = 30 °C
O-
The stoichiometric ratio suggests that the main reaction
products are 3-ethoxy 4-hydroxy benzoic acid and Fe(CN)64–.
The product 3-ethoxy 4-hydroxy benzoic acid is identified by
its spot test21. It was also isolated by acidifying the reaction
mixture followed by ether extraction and it was confirmed by
IR spectrum. The characteristic absorption bands at 2980, 1681
and 1282 cm-1 confirmed the presence of carboxylic acid group
and the same at 3691 cm-1 confirmed the hydroxyl group. The
4–
concentration of the reduction product, Fe(CN)6 , was esti-
0.75
0.80
0.85
0.90
0.95
1.00
mated by titrating against a Ce(IV) solution20.
I1/2
Fig. 1. Effect of ionic strength on oxidation of ethyl vanillin by hexa-
cyanoferrate(III)
RESULTS AND DISCUSSION
Reaction order: The order with respect to [ethyl vanillin]
and [alkali] were found by log10 kobs versus log10 (concentration)
plots; these orders were obtained by varying the concentration
of reductant and alkali in turn while keeping others constant.
Effect of [hexacyanoferrate(III)]: The [hexacyano-
ferrate(III)] was varied in the range, 2 × 10–3 to 5 × 10–3 mol
The addition of ethanol to the reaction mixture decreases
the rate of oxidation. It is observed that the value of the rate
constant decreases with decrease in the dielectric constant of
the medium. The plot of log10 kobs versus 1/D was linear with
r = 0.997 (Fig. 2).

Vol. 27, No. 7 (2015)
Kinetics and Mechanistic Study of Oxidation of Ethyl Vanillin by Alkaline Hexacyanoferrate(III) 2585
TABLE-4
[Ethyl vanillin] = 5 × 10–2
[OH–] = 0.4 M
M
EFFECT OF TEMPERATURE ON THE OXIDATION OF
ETHYLVANILLIN BY HEXACYANOFERRATE(III)
IN AQUEOUS ALKALINE MEDIUM
4.4
[Oxidant] = 3 × 10–3
Ionic strength = 0.42 M
Temperature = 30 °C
M
[Ethyl vanillin] = 5 × 10–2 mol dm–3; [OH–] = 0.4 mol dm–3;
4.0
[Hexacyanoferrate(III)] = 3 × 10–3 mol dm–3; I = 0.42 mol dm–3
Temperature (K)
kobs × 103 (s–1)
302
0.27
308
0.50
314
0.62
318
0.89
323
0.92
3.6
Entropy of activation at 302, 308, 314, 318 and 323 K are
-101.28, -100.45, -102.52, -103.08 and -105.42 kJ mol-1,
respectively. The free energy of activation at 302, 308, 314, 318
and 323 K are 76.15, 76.45, 77.65, 78.20 and 79.43 kJ mol-1,
respectively.
3.2
2.8
Test for free radicals: The interference of free radicals
was tested by adding few drops of methyl acrylate to the
mixture of solution of ethyl vanillin in NaOH and hexacyano-
ferrate(III). As there occurred turbidity, interference of free
radicals was confirmed.
Mechanism of reaction: The reaction being first order
in [oxidant], first order in [ethyl vanillin] and first order in
[OH–] and the retarding effect of hexacyanoferrate(II) can be
accommodated in the following Scheme-I. The oxidation was
initiated by the formation of the anion of ethyl vanillin. The
anion can transfer an electron to hexacyanoferrate(III), resul-
ting in the formation of a radical through slow step and is the
rate determining step. The second molecule of hexacyano-
ferrate(III) abstracts an electron from the radical and subse-
quently leads to the formation of products.
0.012
0.014
0.016
0.018
1/D × 102
Fig. 2. Effect of solvent polarity on oxidation of ethyl vanillin by hexa-
cyanoferrate(III) at 30 °C
Effect of initially added product: The effect of initially
added product hexacyanoferrate(III) on the rate of reaction
was also studied in the range of 1 × 10–3 to 4 × 10–3 mol dm–3
at 30 °C at constant [ethyl vanillin], [hexacyanoferrate(III)],
[OH-] and ionic strength. Hexacyanoferrate(II) was shown to
have a retarding effect on rate of reaction (Table-3).
TABLE-3
EFFECT OF ADDED PRODUCT, HEXACYANOFERRATE(II),
ON THE OXIDATION OF ETHYLVANILLIN BY
HEXACYANOFERRATE(III) IN AQUEOUS
ALKALINE MEDIUM AT 30 °C
The rate law is given as follows;
k1
[Ethylvanillin] = 5 × 10–2 mol dm–3; [OH–] = 0.4 mol dm–3;
EV–
–
H2O
[Hexacyanoferrate(III)] = 3 × 10–3 mol dm–3; I = 0.42 mol dm–3
+
+
EV OH
k–1
Slow k2
[Fe(CN)64-] × 103
kobs × 103 (s–1)
NIL
1.6
1
1.39
2
1.35
3
1.02
4
0.98
EV– + FC
FC'
Radical R +
k
Effect of temperature: The oxidation of ethyl vanillin
by alkaline hexacyanoferrate(III) was carried out in the tempe-
rature range 302-323 K and it was observed that the rate of
reaction increased with an increase in temperature (Table-4).
The activation parameters corresponding to the rate
constants were evaluated from the Arrhenius plots of log10 kobs
versus 1/T which was linear with r = 0.99 and the energy of
activation and enthalpy of activation are found to be 48.07
and 45.56 kJ mol-1, respectively.
k3
R + FC + OH–
Products
−dFC
dt
= k2[EV− ][FC]− k−2[R][FC']+ k3[R][FC]
Applying steady state approximation for radical R
k2[EV− ][FC]
[R] =
k3[FC]+ k−2[FC']
O-
.
O
O
-
O
O
O
C - H
C - H
C - H
C - H
C - H
C - H
k1
k
[Fe(CN)6]3-
[Fe(CN) ]4-
OH-
-e
H2O
+
OEt
;
+
OEt
;
;
+
OEt
6
+
Slow Step
OEt
OEt
OEt
O
O-
O-
O
O
OH
O
C
+
.
+
O
O
O
O
O
O
+
C - O
C - H
C - OH
+
C - H
C - H
C
+
H
+
OEt
OH
fast
[Fe(CN)6]3-
4-
;
-e
Fast Step
fast
[Fe(CN)
]
+
;
+
OEt
6
OEt
OEt
OEt
O-
OEt
OEt
O-
O-
O-
O
O
O
Scheme-I

2586 Kalyani et al.
Asian J. Chem.
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Substituting the terms in equations and simplifying we
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kk1k2[S][OH− ][FC]
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Rate =
(1+ k1[OH− ])(1+ k1k2[S][OH− ])(1+ k2[FC])
As the concentration of Fe(CN)64– used in the study is
very low, term (1 + k2[FC]) tends to unity. Then equation
becomes:
kk1k2[S][OH− ][FC]
Rate =
(1+ k1[OH− ])(1+ k1k2[S][OH− ])
Conclusion
It is hereby concluded that one of the most important
and widely used flavoring compounds ethyl vanillin can be
oxidized by hexacyanoferrate(III) in alkaline medium. This
happens through interference of free radical. The reaction may
be considered to be specific hydroxide catalyzed. Even though
it involves the retardation by one of the products the overall
mechanistic sequence described here is consistent with
product, mechanistic and kinetic studies.
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