Kinetics and mechanism of the oxidation of vanillin by hexacyanoferrate(III) in aqueous alkaline medium

Article abstract of DOI:10.1007/s10953-006-8938-z

The title reaction was investigated in aqueous alkaline medium. A first-order dependence in hexacyanoferrate(III) concentration and a fractional order in both vanillin and alkali were obtained at the concentrations studied. 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. The reaction constants are calculated and used to regenerate the k _obs values, which are compared with the experimental values.

Full text of DOI:10.1007/s10953-006-8938-z

ꢀC
Journal of Solution Chemistry, Vol. 35, No. 1, January 2006 ( 2006)
DOI: 10.1007/s10953-006-8938-z
Kinetics and Mechanism of the Oxidation
of Vanillin by Hexacyanoferrate(III)
in Aqueous Alkaline Medium
Timy P. Jose,1 Sharanappa T. Nandibewoor,2
and Suresh M. Tuwar1,∗
Received June 6, 2005; accepted July 20, 2005
The title reaction wasinvestigated in aqueousalkaline medium. A first-order dependence
in hexacyanoferrate(III) concentration and a fractional order in both vanillin and alkali
were obtained at the concentrations studied. 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.
The reaction constants are calculated and used to regenerate the kobs values, which are
compared with the experimental values.
KEY WORDS: Vanillin; oxidation; reduction; kinetics; mechanism.
1. INTRODUCTION
Hexacyanoferrate(III) is a weak oxidant and has been widely used to oxidize
numerous organic and inorganic compounds in alkaline media. The authors(1,2)
have suggested that the alkaline hexacyanoferrate(III) ion simply acts as an
electron-abstracting reagent in redox reactions. However, Speakman and Water(3)
have suggested different paths for the oxidation of aldehydes, ketones, and ni-
troparaffines. Singh et al.,(4,5) while discussing the oxidations of formaldehy-
des, acetones, and ethyl methyl ketone, have suggested that the oxidation takes
place via an electron-transfer process resulting in the formation of a free-radical
intermediate.
1Department of Chemistry, Karnatak Science College, Dharwad 580001, India.
2Postgraduate Department of Studies in Chemistry, Karnatak University, Dharwad 580003, India;
e-mail: sureshtuwar@yahoo.co.in
51
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C
0095-9782/06/0100-0051/0 2006 Springer Science+Business Media, Inc.

52
Jose, Nandibewoor, and Tuwar
3-Methoxy-4-hydroxy benzaldehyde, commercially called p-vanillin and in
general Vanillin, occurs(6) in nature as a glucoside, which hydrolyzes to vanillin
and sugar. It is a very popular flavoring reagent in the food industry and is also
useful in the synthesis of drugs; 40% of vanillin is consumed in manufacturing
drugs such as Aldomet, L-Dopa,(7) and Trimethaprim. Vanillin is also used in
the preparation of perfume and as a catalyst to polymerize methyl methacrylate.(8)
Vanillin has both phenolic and aldehydic groups and is capable of undergoing three
different types of reactions: those of the aldehydic group, the phenolic hydroxyl,
and the aromatic nucleus. The aldehydic group undergoes certain typical aldehyde
condensation(9) reactions that allows various substitutions(10) for the aldehyde
group. The aldehyde group can also be partially or completely reduced.(11) How-
ever, as a p-hydroxybenzalehyde, Vanillin does not undergo some very common
aldehyde reactions, such as the Cannizzaro’s reaction and the bezoin condensation.
If the hydroxyl group in Vanillin is protected, it undergoes oxidation(12) to Vanillic
acid. As a phenol, Vanillin forms esters and ethers, and the nucleus is easily sub-
stituted by halogen and nitro groups. In comparison with most other aldehydes,
Vanillin is notable for its stability. Literature reveals that there are no reports on the
oxidation of Vanillin except in one case,(13) and in order to explore the mechanistic
aspects of hexacyanoferrate(III) oxidation in alkaline medium, we have chosen
Vanillin as a substrate. The present work deals with the title reaction to investigate
the redox chemistry of hexacyanoferrate(III) in this medium and arrive at a suitable
mechanism for the oxidation of Vanillin by alkaline hexacyanoferrate(III).
2. EXPERIMENTAL
2.1. Materials and Methods
All chemicals used were of reagent grade. Doubly distilled water (from
KMnO4 in an all-glass apparatus) was used throughout the work. Solutions of
vanillin (S. D. Fine) were prepared by dissolving an appropriate amount of recrys-
tallized(14) sample in distilled H2O. A solution of Fe(CN)36− was prepared by
dissolving K3Fe(CN)6 (BDH) in H2O and standardized iodometrically.(15a) The
product, Fe(CN)46− and the Vanillic acid solutions were prepared by dissolving
pure samples of K4Fe(CN)6 and Vanillic acid crystals (S. D. Fine) in distilled
water, respectively. NaOH and NaClO4 were employed to maintain the required
alkalinity and ionic strength, respectively.
2.2. Kinetic Measurements
All kinetic measurements were performed under pseudo first-order conditions
where [Vanillin] was always in excess over [Fe(CN)36−], at a constant ionic strength
of 0.80 mol·dm−3 in alkaline medium at a constant temperature of (30.0 0.1) ◦C,
respectively, unless otherwise stated. The reaction was initiated by mixing the

Kinetics and Mechanism of the Oxidation of Vanillin
53
thermostatted solutions of Fe(CN)36− and Vanillin, which also contained the re-
quired concentration of NaOH and NaClO4. The progress of the reaction was
followed by measuring the decrease in the absorbance of Fe(CN)36− in the reaction
solution in a 1 cm quartz cell of a thermostatted compartment of a Hitachi model
150-20 spectrophotometer at its absorbance maximum, 420 nm, as a function of
time where other constituents of the reaction mixture do not absorb significantly.
The applicability of Beer’s law for Fe(CN)36− at 420 nm under the reaction con-
dition had earlier been verified giving ε = (1060 20) dm3 mol−1 cm−1. Pseudo-
first-order rate constants, kobs, were obtained (Tables I and II) from the slopes of
plots of log10[Fe(CN)36−] versus time; the plots were linear up to 40% completion
of the reaction in alkaline medium and the kobs values were reproducible to within
5%, and deviation from linearity was observed above 40% of reaction due to the
retarding effect of one of the products, Fe(CN)46− (discussed elsewhere). The re-
sults are also interpreted in terms of the initial rate of the reaction to avoid the com-
plexities, which may arise due to interference by the products, such as Fe(CN)64−
.
Initial rates were obtained from the tangents at the initial stages of the concentration
versus time curves by the plane mirror method(16) and were reproducible to within
Table I. Effect of Variations of [Fe(CN)36−], [Vanillin], and [OH−] on the Oxidation
of Vanillin by Fe(CN)36− at 30 ◦C, I = 0.80 mol·dm−3
kobs × 103 (s−1
[OH−] (mol·dm3) Found Calculated
)
[Fe(CN)63−] × 104
[Vanillin] × 103
(mol·dm−3
)
(mol·dm−3
)
0.8
1.0
2.0
4.0
6.0
8.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
1.0
2.0
4.0
6.0
8.0
10
1.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
0.80
0.80
0.80
0.80
0.80
0.80
0.80
0.80
0.80
0.80
0.80
0.80
0.80
0.08
0.10
0.20
0.40
0.60
0.80
0.08
2.26
2.54
2.29
2.26
2.28
2.28
0.88
1.50
2.26
2.80
3.10
3.40
0.88
0.55
0.66
1.10
1.65
1.94
2.26
0.55
2.20
2.20
2.20
2.20
2.20
2.20
0.83
1.42
2.20
2.69
3.03
3.27
0.83
0.55
0.66
1.10
1.65
1.98
2.20
0.55

54
Jose, Nandibewoor, and Tuwar
Table II. Effect of Added Product, Fe(CN)46−, on
the Oxidation of Vanillin by Hexacyanoferrate(III)
in Aqueous Alkaline Medium at 30 ◦C
[Fe(CN)46−] × 104
(mol·dm−3
)
kobs × 103 (s−1
)
0.1
0.2
0.4
0.6
0.8
1.0
2.21
2.05
1.89
1.80
1.75
1.71
Note. [Fe(CN)63−] = 4.0 × 10−4 mol·dm−3, [Vanillin]
= 4.0 × 10−3 mol·dm−3, [OH−] = 0.80 mol·dm−3
,
and I = 0.80 mol·dm−3
.
5%. However, the kobs values were used to interpret the results throughout the
study.
In view of modest concentrations of alkali used in the reaction medium,
attention was also given to the effect of the surface of the reaction vessels on the
kinetics. Use of polythene/acrylic ware and quartz or polyacrylate cells gave the
same results as glass vessels and cells, indicating that the surfaces do not play an
important role in the reaction rate.
The effect of dissolved oxygen on the rate of reaction was checked by prepar-
ing the reaction mixture and following the reaction in an atmosphere of nitrogen.
No significant difference between the results obtained under nitrogen and in pres-
ence of air was observed. In the view of ubiquitous contamination of CO23−, its
effect was also studied on the rate of reaction. Added CO23− up to 1.0 × 10−3
mol·dm−3 showed no effect on reaction rate. However, fresh solutions were used
while performing the experiments.
2.3. Stoichiometry and Products Analysis
Reaction mixtures with different sets of concentrations of reactants where
[Fe(CN)36−] was in excess over [Vanillin]◦at a constant ionic strength and al-
kali were kept for about 6 h at (30 0.1) C in a closed vessel. The remaining
Fe(CN)36− was assayed spectrophotometrically by measuring the absorbance at
420 nm. The results indicated that two moles of Fe(CN)36− consumed 1 mol of
Vanillin as in Eq. (1).
(1)

Kinetics and Mechanism of the Oxidation of Vanillin
55
The stoichiometric ratio suggests that the main reaction products are Vanillic
acid and Fe(CN)46−. The Vanillic acid is identified by its spot test as in earlier
work.(13,17) It was also isolated by acidifying the reaction mixture followed by
ether extraction and it was confirmed by its melting at 206 ◦C (literature melting
point (208 2)◦C) and by its IR spectrum. The concentration of the other product,
Fe(CN)46−, was estimated(15b) by titrating against a Ce(IV) solution.
3. RESULTS AND DISSCUSSION
3.1. Reaction Order
The order with respect to [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.
3.2. Effect of [Hexacyanoferrate(III)]
The [Fe(CN)36−] was varied in the range, 8.0 × 10−5–8.0 × 10−4 mol·dm−3
at fixed [Vanillin], [OH−] and ionic strength. The non-variation of the pseudo-
first-order rate constant at various concentrations of Fe(CN)36− indicates the order
in [Fe(CN)36−] is unity (Table I). This was also confirmed from the linearity of
plots of log10 [Fe(CN)36−] versus time (r = 0.9886 and s ≤ 0.005) for up to 40%
completion of the reaction.
3.3. Effect of [Vanillin]
The substrate, Vanillin, was varied in the range of 1.0 × 10−3–1.0 ×
◦
10−2 mol·dm−3 at 30 C keeping all other reactant’s concentrations and con-
ditions constant (Table I). The kobs values increased with increasing concentration
of Vanillin indicating an apparent less than unit order dependence on [Vanillin]
(≈0.6).
3.4. Effect of [Alkali]
The effect of [OH−] on the rate of reaction was studie◦d at constant [Vanillin],
[Fe(CN)36−] and ionic strength at 0.80 mol·dm−3 at 30 C. The rate constants
increased with increasing [OH−] and the order was found to be less than unity
(≈0.6) (Table I).
3.5. Effect of Ionic Strength and Solvent Polarity
The effect of ionic strength was studied by varying the [NaClO4] in the
reaction medium. The ionic strength of the reaction medium was varied from
0.80–3.0 mol·dm−3 at constant [Fe(CN)36−], [Vanillin], and [OH−]. It was found

56
Jose, Nandibewoor, and Tuwar
Fig. 1. Effect of ionic strength and dielectric constant of the medium on
oxidation of Vanillin by hexacyanoferrate(III) at 30 ◦C.
that as ionic strength increases, the rate of the reaction increases (slope = 0.5)
(Fig. 1).
The relative permittivity (D) effect was studied by varying the t-butyl alcohol–
H2O content in the reaction mixture with all other conditions being constant.
Attempts to measure the relative permittivities were not successful. However, they
were computed from the values of the pure liquids as in earlier work.(18) There
was no reaction of the solvent with the oxidant under the experimental conditions
used. The rate constants, kobs, decreased with a decrease in dielectric constant of
the medium. The plot of log10 kobs versus 1/D was linear (Fig. 1) (r = 0.9829 and
s ≤ 0.005).
3.6. Effect of Initially Added Products
The effect of initially added products, Vanillic acid and Fe(CN)46−, on the rate
of reaction was also studied in the range of 1.0 × 10−4–1.0 × 10−3 mol·dm−3 at
3−
◦
30 C at constant [Fe(CN)6 ], [Vanillin], [OH−], and ionic strength. The added
Vanillic acid did not alter the rate of reaction. However, Fe(CN)46− was shown to
have a retarding effect on rate of reaction (Table II).
3.7. Effect of Temperature
The kinetics were also studied at 30, 35, 40, and 45 ◦C at [Fe(CN)63−] = 4.0 ×
10−4 mol·dm−3, [Vanillin] = 4.0 × 10−3 mol·dm−3, [OH−] = 0.80 mol·dm−3
I = 0.80 mol·dm−3 and the resulting kobs values were 2.27 × 10−3, 2.55 × 10−3
,
,

Kinetics and Mechanism of the Oxidation of Vanillin
57
Table III. Activation Parameters for the Oxidation of Vanillin by Hexacyanoferrate(III)
on Aqueous Alkaline Medium
Ea (kJ·mol−1
)
ꢀH# (kJ·mol−1
)
ꢀS# (J·K−1 mol−1
−247 10
)
ꢀG# (kJ·mol−1
)
17 0.2
14.7 0.2
91
4
2.80 × 10−3, and 3.15 × 10−3 s−1 at 30, 35, 40 and 45 ◦C, respectively. The acti-
vation parameters corresponding to these constants were evaluated from the plots
of log10 kobs versus 1/T (r = 0.998 and s ≤ 0.0049) and are shown in Table III.
3.8. Test for Free Radicals
Since the reaction is of a non-complementary type, the intervention of free
radicals during reaction is expected. This was tested by adding a monomer, acry-
lonitrile, during the course of reaction and on diluting with methanol, no copious
precipitate was obtained. It is contrary to expectation and discussed elsewhere.
The overall reaction is approximately second order, being first order in [ox-
idant], fractional order each in [Vanillin] and [OH−] and the retarding effect of
hexacyanoferrate(II) can be accommodated in the following Scheme 1.
The probable structure of the complex is
The oxidation of hexacyanoferrate(III) resembles that with Cu(II) in which
reaction occurs through intervention of a free radical with rapid oxidation.(19–21)
The redox potential of the Fe(CN)36−–Fe(CN)46− couple is greater than that for
Cu(II)–Cu(I). Hence, there might be a rapid oxidation of the free radical that could
mask the polymerization of acrylonitrile. As found experimentally, sometimes the
vinyl compounds themselves are oxidized under the experimental conditions used
and the test for the free radicals fails.(22)
The retardation of the rate of reaction due to the product, hexacyanoferrate(II),
is not shown in the scheme. The retardation is only of about 10–15%, (Table II).
This may be due to a secondary salt effect(23) or it readily forms a complex with
the substrate(24) compared to hexacyanoferrate(III). This complex may be less
reactive than the complex formed between hexacyanoferrate(III) and substrate as
shown in the Scheme 1.
In the present study, the oxidation proceeds via the formation of the anion
of Vanillin; first it forms a complex with the oxidant in a prior equilibrium state.

58
Jose, Nandibewoor, and Tuwar
Scheme 1.
This decomposes in a slow step to give a free radical and in further fast steps,
the anionic form of Vanillic acid is formed as shown in Scheme 1. The Scheme 1
leads the rate law (2).
ꢀ
ꢁ
ꢀ
ꢁ
kK1 K2 Fe(CN)36− [VAN]T [OH−]T
(1 + K1[OH−])(1 + K1 K2[VAN][OH−])
d Fe(CN)36−
T
Rate = −
=
(2)
dt

Kinetics and Mechanism of the Oxidation of Vanillin
59
Fig 2. Verification of the rate law for the oxidation of Vanillin by
hexacyanoferrate(III) at 30 ◦C.
The denominator on right-hand side of Eq. (2) should also contain a term
(1 + K2[Fe(CN)36−]) which is neglected as it almost tends to unity due to low con-
centrations of Fe(CN)36− used in the present study. The formation of the complex
is proven kinetically by the non-zero intercept graph of 1/kobs versus 1/[Vanillin]
(see Fig. 2). Equation (2) can also be written as:
kK1 K2[VAN]T [OH−]T
(1 + K1[OH−])(1 + K1 K2[VAN][OH−])
kobs
=
(3)
or
kK1 K2[VAN]T [OH−]T
kobs
=
(4)
(1 + K1[OH−] + K1 K2[VAN][OH−] + K12 K2[VAN][OH−])
The term K12K2[VAN][OH−] in Eq. (4) can be omitted due to the low concentra-
tions of vanillin and OH−. Thus Eq. (4) becomes,
kK1 K2[VAN]T [OH−]T
kobs
=
(5)
(1 + K1[OH−] + K1 K2[VAN][OH−])
Equation (5) is verified in the following form and the subscripts, T, are omitted.
1
1
1
1
=
+
+
(6)
kobs
kK1 K2[VAN][OH−] kK2[VAN]
k
The plots of 1/kobs versus 1/[OH−] (r = 0.999 and s ≤ 0.005) and 1/kobs
versus 1/[Vanillin] (r = 0.999 and s ≤ 0.0048) should be linear and were found

60
Jose, Nandibewoor, and Tuwar
to be so as shown in Fig. 2. From the slopes and intercept, the constants k, K1,
and K2 were calculated as: (4.8 0.2) × 10−3 s−1, (0.83 0.04) and (517 2)
dm3 mol−1, respectively. Using these values in Eq. (5), the rate constants for
various concentrations of Fe(CN)63−, Vanillin, and OH− were calculated and com-
pared with experimental values. There was a good agreement between these values
(Table I).
The effect of ionic strength on the rate of the reaction is also in the ex-
pected direction as similar charged species, Fe(CN)36−, and the anionic form of
Vanillin is involved in the reaction. Ion pairing between Fe(CN)36− and K+ is well
known.(18,25) Thus, ion pairing between Na+ and Fe(CN)36− appears to be reduced
due to the expected value of slope (i.e., 3 in Fig. 1). Similar ion paring is also
experienced for other reactions.(18,25)
The effect of solvent on the rate of reaction has been studied. An increase in
rate with an increase in dielectric constant of the medium supports the involvement
of similarly charged species (Scheme 1). The activated complex may be more
polar than the reactants, Fe(CN)36− and the Vanillin anion, which also may be
more solvated in water than in the low dielectric medium(26) as compared to
its reactants. The large value of ꢀS# also supports the reaction between highly
charged species(27) of Fe(CN)36− and the Vanillin anion. The activated complex
could be more ordered than the reactants. The small value of ‘k’ also indicates the
nature of activated complex.
4. CONCLUSIONS
The commercially more valuable Vanillin can be oxidized by hexacyanofer-
rate(III) in alkaline medium. This occurs through intervention of a free radical
with the retarding effect of product, Fe(CN)46−. The small value of ‘k’ in the rate-
determining step proves that the activated complex is more highly ordered than
the reactants. This is supported by the large negative value of ꢀS#.
ACKNOWLEDGMENTS
One of the authors, S. M. Tuwar, thanks the UGC, Regional Office Bangalore,
for providing financial assistance.
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Jose, Nandibewoor, and Tuwar
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