Kinetics of Oxidation of Benzaldehydes by Quinolinium Dichromate

Article abstract of DOI:10.1515/znb-2003-1207

Quinolinium dichromate (QDC) in sulfuric acid oxidizes benzaldehydes to the corresponding acids in a 50% (v/v) acetic acid-water medium. The reaction is first order each in [QDC], [substrate] and [H⁺]. The reaction rates have been determined at different temperatures and the activation parameters calculated. The rate decreases with an increase in the water content of the medium. The effects of substituents have been studied. A suitable mechanism is proposed.

Full text of DOI:10.1515/znb-2003-1207

Kinetics of Oxidation of Benzaldehydes by Quinolinium Dichromate
Hesham A. A. Medien
Department of Chemistry, Faculty of Science, Ain Shams University, Cairo, Egypt
Reprint requests to Prof. Hesham A. A. Medien. Fax: 202-4831836.
E-mail: hmadian59@yahoo.com
Z. Naturforsch. 58b, 1201 – 1205 (2003); received June 23, 2003
Quinolinium dichromate (QDC) in sulfuric acid oxidizes benzaldehydes to the corresponding acids
in a 50% (v/v) acetic acid-water medium. The reaction is first order each in [QDC], [substrate] and
[H+]. The reaction rates have been determined at different temperatures and the activation parameters
calculated. The rate decreases with an increase in the water content of the medium. The effects of
substituents have been studied. A suitable mechanism is proposed.
Key words: Kinetic, Oxidation, Aromatic Aldehydes, Quinolinium Dichromate
Introduction
Reactions were carried out under pseudo-first order con-
ditions by maintaining a large excess of the aldehyde over
[QDC]. The solvent was a 1:1 (v/v) acetic acid-water mix-
ture. The reactions were carried out at constant temperatures
( 0.1 K) and were followed up to 80% completion. The
amount of unreacted QDC was estimated iodometrically. The
pseudo-first order rate constants (k1 were computed from
A number of reports on the mechanism of oxidation
of several substrates by quinolinium dichromate are
available. Quinolinium dichromate (QDC) is shown to
oxidize primary and secondary alcohols to the corre-
sponding aldehydes [1, 2], cyclic alcohols to the cor-
responding cyclic ketones [3], bicyclic alcohols [4] linear least squares plot of [QDC] vs. time. Duplicate ki-
netic runs showed that the rate constants were reproducible
to within 3%. The second order rate constant was obtained
from the relation, k2 = k1/[substrate].
and benzyl alcohol [5]. The α-hydroxyacids [6] and
α-ketoacids [7] are found to be oxidized with QDC
and their reactions are studied kinetically. Quinolin-
ium dichromate (QDC) oxidizes cinnamic and cro-
tonic acids smoothly in N,N-dimethylformamide in
the presence of an acid [8, 9] to give aldehydes. α,β-
Unsaturated aldehydes [10], aliphatic dialdehydes [11]
and heterocyclic aldehydes [12] are oxidized by QDC.
Other substrates such as phenols [13], amino acids
[14], methoxytoluenes [15], thallium(I) [16], styrenes
[17] and diols [18] are also oxidized by QDC.
The present work reports the kinetics of oxidation of
substituted benzaldehydes by QDC and evaluates the
reaction constants. Mechanistic aspects are also dis-
cussed.
Product analysis
Benzaldehyde (0.2 mol) and QDC (0.4 mol) were mixed
together with sulfuric acid (0.1 mol) in 50% (v/v) aqueous
acetic acid (total volume 100 ml). The reaction mixture was
set aside for about 24 h to ensure completion of the reaction.
It was then evaporated and extracted with ether. The layer
was separated and dried. The residue was confirmed as ben-
zoic acid by the m.p. 121 ◦C, TLC and spectral analysis. The
percentage yield was calculated to be 97%.
Results and Discussion
The oxidation of aromatic aldehydes by QDC re-
sulted in the formation of the corresponding acids. Un-
der the present experimental conditions, there was no
further oxidation of the acids.
Experimental Section
Materials and methods
All benzaldehydes used were of A.R. grade (E
Merck), the liquid benzaldehydes were used after dis-
tillation. The oxidant quinolinium dichromate (QDC,
[(C9H7N+H)2Cr2O2−]) (Aldrich) was used. Acetic acid
(A.R. grade) was7 distilled before use. Sulfuric acid
(E. Merck) was used after a check of its physical constants.
The stoichiometry of the reaction was determined
[19]. Stiochiometric ratios, in the range of 0.65 to 0.68
were obtained, which conformed to the following over-
all equation:
3ArCHO+2QDC+H+ → 3ArCOOH+2Cr3+
c
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H. A. A. Medien · Kinetics of Oxidation of Benzaldehydes by Quinolinium Dichromate
Table 1. Rate data for the oxidation of benzaldehyde at 303 K
in 50% (v/v) acetic acid-water.
species. Earlier reports have established the involve-
ment of protonated Cr(VI) species in chromic acid ox-
idation reactions [21, 22].
The oxidation of substrates was studied in solutions
containing varying proportions of water and acetic
acid. It was observed that there was an increase in the
rate of the reaction with a decrease in the dielectric
constant [23] of the medium (Table 2).
102[BA]
mol·dm−3
1.0
2.5
5.0
7.5
10.0
1.0
103[QDC]
mol·dm−3
1.0
1.0
1.0
1.0
1.0
0.75
0.5
0.25
0.1
[H2SO4]
mol·dm−3
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.75
1.0
1.25
1.5
105k1
s−1
3.96
9.90
19.80
29.59
39.02
3.94
3.90
3.95
3.90
5.84
7.92
9.90
11.88
Mechanism and rate law
1.0
1.0
1.0
1.0
1.0
1.0
1.0
The kinetic results showed that the rate of oxida-
tion of benzaldehydes was dependent on the first pow-
ers of the concentrations of each (substrate, oxidant
and acid). The acid catalysis of the reaction must be
related to the structure of the oxidant (QDC), which
was converted to a protonated species at the concentra-
tions of the mineral acid used. Quinolinium dichromate
(QDC) is an anionic condensed form of chromic acid.
Aqueous solutions of chromic acid contain ions such
as CrO24−, HCrO4− and Cr2O27−, besides other proto-
nated species such as H2Cr2O7, HCr2O−7 and H2CrO4
[11, 24]. The ionization constant for the HCrO−4 ion
was 3.0×10−7 mol/l; hence, in dilute aqueous acid, the
concentration of CrO24− ions was negligible. This has
been amply substantiated by Michel et al. [25], who
examined the Raman spectra of chromate, dichromate
and chlorochromate species, and found that the proto-
nated form of chromate HCrO−4 did not exist in aque-
ous solutions of Cr(VI) compounds. The ionization
constant for the HCr2O−7 ion was 0.85 mol/l; hence, in
solutions where pH = 1, the ionization may be consid-
ered essentially complete. Consequently, of all the ions
involving hexavalent chromium, the only ones present
in large concentrations in solutions of dilute mineral
acid would be HCrO−4 and Cr2O27−. These ions are in
equilibrium with each other according to the equation
1.0
1.0
1.0
1.0
Table 2. Solvent effect for the oxidation of benzaldehyde by
QDC at 303 K.
H2O:AcOH Dielectric 104 k H2O:AcOH Dielectric 104 k
(%, v/v)
70:30
60:40
constants D s−1
(%, v/v)
40:60
30:70
constants D s−1
72.0
63.3
56.0
2.98
3.41
3.96
45.5
38.5
5.32
6.86
50:50
[BA] = 1×10−2 mol·dm−3; [QDC] = 1×10−3 mol·dm−3; [H2SO4]
= 0.5 mol·dm−3
.
Using pseudo-first-order conditions, individual ki-
netic runs were observed to be first order in QDC. The
pseudo-first-order rate constants (k) were independent
on the initial concentration of the oxidant (Table 1).
The order of the reaction with respect to the substrate
concentration was obtained by varying the aldehyde
concentration and observing the effect on the rate at
constant [QDC] and [H+]. The results are given in Ta-
ble 1. The order with respect to the concentration of
acid was obtained at constant aldehyde and QDC con-
centrations. The data given in Table 1 indicate a first
order dependence on the concentration of the acid. Un-
der the acid concentrations used in the present inves-
tigation, the protonation of the aldehydes would be
less significant. The possibility of the aldehydes get-
ting protonated can be ruled out on the basis that alde-
hydes are extensively hydrated in an aqueous medium,
and are present as equilibrium mixtures of the carbonyl
and hydrated forms. The formation constants are thus
not dependent on the acidity or alkalinity [20].
2HCrO−4 ꢀ Cr2O27− +H2O,
with a value of Kd = 35.5. According to this equilib-
rium, an increase of the hydrochromate concentration
should be significant with dilution. When the Raman
lines were examined under dilution, it was established
that at pH = 11, the Cr(VI) ion was only present in the
form of the CrO24− ion, whereas at pH = 1.2, it existed
only as the Cr2O27− ion [25]. Hence, at concentrations
Hence, it would be justified to propose that in of acid larger than 0.05 M, the dichromate ion (and its
the range of acid concentrations used the oxidant protonated forms) would be the predominant species.
QDC was converted to the protonated chromium(VI) In aqueous solutions of K2Cr2O7, spectral studies have
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H. A. A. Medien · Kinetics of Oxidation of Benzaldehydes by Quinolinium Dichromate
1203
shown that Cr2O27− was the predominant species [26]. ing equation was obtained:
In the present investigation, since the concentrations
– d[QDC]/dt = K1K2k3 [A] [QDC] [H+].
of acid used were in the range of 0.5 to 1.5 M, the
dichromate ion (or its protonated form) would be the
predominant species. Moreover, the protonated Cr(VI)
species would be a more reactive electrophile capable
of increasing its rate of coordination to the substrate.
The effect of a change in the solvent composition
(water-acetic acid, %, v/v) on the rate of oxidation for
the substrates was studied. It was observed that an in-
crease in the water content of the medium showed a
decrease in the rate of oxidation (Table 2). The mag-
nitude of this effect could be analyzed by suggesting
that, for the equilibrium 2 HCrO−4 ꢀ Cr2O72− +H2O,
a decrease in the dielectric constant of the medium
(increase in the acetic acid content) would favor the
dichromate form over the chromate form. If ion pairs
were to be formed in this medium, it would be expected
that they have a higher ion-pair association constant
for the dichromate ion, which would again favor the
dichromate ion. Plots of logk versus 1/D were found
to be linear, with positive slopes, which indicated that
the reactions were of the ion-dipole type [27].
From this rate expression, it is clear that the reaction
exhibited a first-order dependence with respect to the
concentrations of each (substrate, oxidant, and acid).
This rate law explains all of the experimentally ob-
served results.
It has been shown that aldehydes are extensively hy-
drated in aqueous solutions and many oxidation reac-
tions proceed via the hydrate form [20, 28– 33].
Scheme 1.
The sequence of reactions for the oxidation of ben-
zaldehydes by QDC, in an acid medium is shown in
Scheme 1. In an acid medium, the oxidant QDC is
converted to the protonated bimetallic chromium(VI)
species (PQ), in the acid range used for the present
investigation, the protonated QDC would have the
Cr(VI) existing mainly as Cr2O27−. The substrate (A)
was converted to the hydrated form (HY). The reaction
of the hydrated form (HY) of the substrate with the
protonated QDC (PQ) resulted in the formation of the
monochromate ester [11] which under decomposition
in the rate determining step give the product.
Effect of substituents
Effect of substituents on the rate of oxidation has
been studied with a number of para- and meta- substi-
tuted benzaldehydes. In general, the rate of reaction is
accelerated by electron-withdrawing substituents and
Table 3. Temperature effect and thermodynamics for the ox-
idation of substituted benzaldehydes by QDC.
Subst.
103 k2 (dm3·mol−1·s−1
)
∆H#
(kJ·mol−1
35.0
∆S#
(J·K−1·mol−1
175.60
Based on the mechanism shown in Scheme 1, the
rate law has been derived as follows:
303 K 313 K
3.96 6.51
1.35 2.51
0.60 1.23
5.09 7.85
23.30 32.70
17.32 24.30
2.81 4.87
323 K
9.92
4.31
)
)
H
p-CH3
p-OCH3
p-Cl
p-NO2
p-CN
m-CH3
44.91
52.56
37.46
23.95
25.85
39.20
34.75
35.58
23.99
32.92
151.80
133.32
165.30
198.90
193.50
164.58
175.10
168.34
-d[QDC]/dt = k3[E] = k3[HY][PQ],
2.31
where
11.99
44.00
34.70
7.80
11.60
17.20
28.70
13.60
[PQ] = K1 [QDC][H+],
and
m-OCH3 4.66 7.63
m-Br
m-NO2
p-Br
7.50 11.50
15.00 21.20
5.71 8.82
[HY] = K2 [A][H2O].
200.80
179.40
Substituting the values of [PQ] and [HY], the follow-
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1204
H. A. A. Medien · Kinetics of Oxidation of Benzaldehydes by Quinolinium Dichromate
Table 4. Correlation of rates of
oxidation of para- and meta-
substituted benzaldehydes by
QDC with substituent parame-
ters at 303 K.
Substituents
constants
ρI
ρR
R
SD
λ
f
para-substituted
0.987
σI, σo
1.09
1.05
0.97
1.75
1.05
1.11
0.30
0.26
0.28
1.61
1.00
1.14
0.09
0.03
0.10
σI, σR
+
0.999
0.981
σI, σR
−
R
σI, σ(AB)
1.12
1.50
0.978
meta-substituted
0.29
1.34
0.06
R
o
σI, σR
0.530
0.563
0.501
0.463
0.281
0.405
0.996
0.994
0.858
0.38
0.25
0.21
0.873
0.499
0.808
0.04
0.07
0.03
σI, σ
+
σI, σR
−
R
σI, σ(AB)
0.555
0.450
0.960
0.25
0.811
0.06
R
Table 5. Temperature depen-
dence of the reaction constants.
Temp (K)
ρI
ρR
R
SD
λ
f
para-substituted
0.9998
303
313
323
1.05
0.998
0.975
1.050
0.997
0.970
0.27
0.25
0.2
1.00
0.999
0.995
0.03
0.04
0.05
0.9997
0.9995
meta-substituted
0.9993
303
313
323
0.530
0.545
0.521
0.463
0.351
0.383
0.38
0.38
0.39
0.873
0.644
0.735
0.04
0.04
0.04
0.9983
0.9853
retarded by electron releasing ones (Table 3). A plot of
log (k2/k2o) vs σ gives a positive slope with ρ values of
1.269 (r = 0.93). Since the reaction rates failed to show
a significant correlation with any single-parameter sub-
stituents constants, the rates have been analyzed in
terms of the dual substituents parameter equation of
Taft [34],
those for other scales. This agrees with the observation
of Ehrenson et al. [36] that the correlation of meta-
substituted compounds is generally best with σRo scale
and meta-substituted compounds are less discriminat-
ing.
logk = logko +σIρI +σRρR.
The reaction constants and the statistical data at dif-
ferent temperature are given in Table 5. The value
of λ (ca. 0.983) shows that the oxidation of para-
The rates of oxidation of para- and meta-substituted
benzaldehydes were separately correlated with σI and
one of four σR values, viz. σRo, σR(AB), σR+ and σR−. The substituted benzaldehydes is more susceptible to the
results are summarized in Table 4, which are in good resonance effect than to the field effect. The selectivity
agreement with the results obtained by A. Jameel [35]. of the reaction is decreased at higher temperature, but
The results show that the rates of para-substituted this decrease is of similar order for both the field and
benzaldehydes show excellent correlation with σI and resonance effects resulting in the almost constant value
σR+ values. The correlation with the other three σR val- of λ at different temperature.
ues are relative poor. The coefficient of multiple cor-
relation (R), the standard deviation (SD), and f as the
measure of the goodness of fit. f has been defined by
Dayal et al. [34] as f = SD/RMS, where RMS is the
root mean square of the data points (here logk/ko) The
comparison of f values shows that f is smaller for σR+
scale than those for other scales. Therefore, it is ap-
parent that the rates of the oxidation para-substituted
benzaldehydes by QDC correlate best with σI and σR+.
The rates of meta-substituted benzaldehydes show
excellent correlation with σI and σRo. The comparison
In the oxidation of the meta-substituted compounds,
the value of λ (ca. 0.751) indicates greater importance
of the field effect. There is a small increase in the mag-
nitude of the reaction constants with an increase in the
temperature but this increase is of similar order for
both the field and resonance effects, reflected in the al-
most constant value of λ at different temperatures.
Effect of temperature
The rate constants for the quinolinium dichromate
of f values shows that f is smaller for σRo scale than oxidation of aromatic aldehydes at different tempera-
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H. A. A. Medien · Kinetics of Oxidation of Benzaldehydes by Quinolinium Dichromate
1205
tures are listed in Table 3. The electron-withdrawing where β is the isokinetic temperature and ∆Ho the en-
groups enhance and electron-releasing groups retard thalpy of activation when ∆S# = 0; usually ∆Ho has no
the oxidation rate. The activation parameters have been physical significance. The isokinetic temperature ob-
computed from a plot of logk vs. 1/T in the tempera- tained from the slope is 429.85 K (r = 0.9916). This
ture range 303 – 323 K (Table 3). The activation en- linear correlation indicates that a single mechanism is
thalpies and entropies are linearly related by the equa- operating throughout the series [38]. The negative val-
tion [37]
ues of ∆S# provided support for the formation of a rigid
activated complex.
∆H# = ∆Ho +β∆S#,
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