Oxidation of substituted 2-furaldehydes by quinolinium dichromate: A kinetic study

Article abstract of DOI:10.1002/poc.698

The kinetics of oxidation of substituted 2-furaldehydes by quinolinium dichromate in sulfuric acid, using 50% acetic acid as the solvent, was studied. The rate of the reaction was first order in each of the substrate, oxidant and acid. The kinetic data are discussed with reference to the aldehyde hydration equilibria. The kinetic results support a mechanistic pathway proceeding via a rate-determining oxidative decomposition of the chromate ester of the aldehyde hydrate. Copyright 2003 John Wiley & Sons, Ltd.

Full text of DOI:10.1002/poc.698

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2004; 17: 83–87
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.698
Oxidation of substituted 2-furaldehydes by quinolinium
dichromate: a kinetic study
Girija S. Chaubey, Bansiewdor Kharsyntiew and Mahendra K. Mahanti*
Department of Chemistry, North-Eastern Hill University, Shillong 793022, India
Received 25 September 2002; revised 13 March 2003; accepted 9 July 2003
ABSTRACT: The kinetics of oxidation of substituted 2-furaldehydes by quinolinium dichromate in sulfuric acid,
using 50% acetic acid as the solvent, was studied. The rate of the reaction was first order in each of the substrate,
oxidant and acid. The kinetic data are discussed with reference to the aldehyde hydration equilibria. The kinetic
results support a mechanistic pathway proceeding via a rate-determining oxidative decomposition of the chromate
ester of the aldehyde hydrate. Copyright # 2003 John Wiley & Sons, Ltd.
KEYWORDS: kinetics; oxidation; 2-furaldehydes; quinolinium dichromate
INTRODUCTION
strates by quinolinium dichromate in general,7 and
aldehydes in particular.8
The oxidation of heterocyclic aldehydes by Mn(VII)
highlighted the kinetic aspects and nature of the product
formed.1 Cerium(IV) ion in an acetic acid (25%, v/v)
medium was used to oxidize 2-furaldehyde to the acid;
the reaction involved a free-radical mechanism.2 Thalliu-
m(III) was used for the oxidation of 2-furaldehyde in
perchloric acid solution, and it was observed that the rate
of the reaction did not depend on the concentration of Hþ
ions.3 The kinetics of the oxidation of heterocyclic
aldehydes by bromic acid in H2SO4–HOAc medium
showed that the reaction was first- order in both oxidant
and substrate concentrations, but showed a second-order
dependence on the concentration of Hþ ions.4 Kinetic
studies on the oxidation of 2-furaldehyde by chromic
acid5 and quinolinium chlorochromate6 in acetic acid–
water media showed a first-order dependence on each of
the concentrations of substrate, oxidant and acid.
EXPERIMENTAL
Materials. All the substrates (Aldrich) were purified by
distillation under reduced pressure. The oxidant, QDC,
was prepared by the reported method,9 and its purity was
checked by spectral analysis. The infrared spectrum
(KBr) exhibited bands at 930, 875, 765 and 730 cmꢀ1
,
characteristic of the dichromate ion. Acetic acid (SD, AR
grade) was distilled before use. Sulfuric acid (Merck) was
used after a check of its physical constants. 2-Furalde-
hyde-1-d was prepared by the reported method.10 IR
spectra were recorded on an FT-IR spectrophotometer
(DA-8, Bomem).
Kinetic measurements. The reactions were performed
under pseudo-first-order conditions, maintaining a large
excess of the substrate with respect to the oxidant. The
reactions were performed at constant temperature (0.1 K)
and were followed by monitoring the UV-visible absorp-
tion band at 440 nm using a DU-650 spectrophotometer
(Beckman), as described earlier.7 All the reactions were
performed under nitrogen. The rate constants were eval-
uated from the linear (r > 0.996) plots of log[QDC]
against time. The values of the rate constants reported
are the means of two or more runs and were reproducible
to within ꢁ 3%. The reactions were carried out in aqu-
eous medium, and water–acetic acid mixtures were used
for studying the effect of dielectric constant on the rates
of the reactions. The reaction mixtures remained homo-
geneous in the solvent systems used.
In the oxidation of heterocyclic aldehydes, there is a
possibility of the reaction taking place either at the
heteroatom or at the aldehydic function. The aims of
the present investigation were (a) to highlight the effect of
the heteroatom on the rate of the reaction and (b) to
determine the site of attack of the oxidant. For this
purpose, we carried out a kinetic study of the oxidation
of substituted 2-furaldehydes by a chromium(VI) reagent,
quinolinium dichromate [QDC, (C9H7NþH)2Cr2O72ꢀ], in
acidic medium [50% (v/v) acetic acid–water] under a
nitrogen atmosphere. This study forms part of our con-
tinuing efforts concerning the oxidation of organic sub-
*Correspondence to: M. K. Mahanti, Department of Chemistry, North-
Eastern Hill University, Shillong 793022, India.
E-mail: mkmahanti@yahoo.com
Contract/grant sponsor: University Grants Commission.
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 83–87

84
G. S. CHAUBEY, B. KHARSYNTIEW AND M. K. MAHANTI
Product analysis. Doubly distilled water (30 ml) was
cooled in ice and concentrated H2SO4 (7.9 g, 0.08 mol)
was added slowly with constant cooling. When the acid
solution had cooled to room temperature, QDC (9.52 g,
0.02 mol) was added and the mixture was warmed to
313 K for complete dissolution of the QDC. To this
mixture, 0.015 mol of substrate (1.45 g of 2-furaldehyde,
1.66 g of 5-methyl-2-furaldehyde and 1.63 g of 5-bromo-
2-furaldehyde), taken in 25 ml of 50% acetic acid–water,
was added. The reaction mixture was stirred at 313 K for
48 h under nitrogen. The organic layer was extracted
three times with diethyl ether (25 ml each time) and the
combined organic extracts were washed with water and
dried over anhydrous Na2SO4. The oxidized products (the
corresponding 2-furancarboxylic acids) were obtained
after complete removal of the ether (melting-points
were in agreement with literature values; yields 85–
90%). Each reaction product was subjected to IR (KBr)
0.66–0.69 were obtained, which conformed to the follow-
ing overall equation:
3C5H4O2 þ 2CrVI þ 3H2O ! 3C5H4O3 þ 2CrIII þ 6Hþ
ð2-furaldehydeÞ
ð1Þ
Kinetic results
Using pseudo-first-order conditions, individual kinetic
runs were observed to be first order in QDC. The
pseudo-first-order rate constants (k) were independent
of the initial concentration of the oxidant. 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
[Hþ]. The order with respect to the concentration of acid,
at constant [aldehyde] and [QDC], was found to be unity.
Table 1 shows the kinetic results. In the range of acid
concentrations used, the protonation of the aldehydes was
less significant,11 although it cannot be ruled out. It was
not possible to decide whether protonation of the alde-
hydes or protonation of the dichromate resulted in the
observed acid catalysis, since these two processes could
not be distinguished on the basis of the data obtained.
Since the acid concentrations used were in the range 0.5–
1.5 mol, the dichromate ion was suggested to be the
predominant species in these oxidation reactions. Earlier
reports have established the involvement of protonated
Cr(VI) species in chromic acid oxidation reactions.12,13
analysis, and characterized as follows: 2-furancarboxylic
—
acid, ꢀ ¼ 3000, 2860 (br, s, OH), 2583, 1690 (s, C O),
—
1470, 1305, 1245, 1020, 930, 760 cmꢀ1; 5-Methyl-2-
—
furancarboxylic acid, ꢀ ¼ 2860 (br, s, OH), 1642 (s, C
—
O), 1525, 1375, 1160, 1030, 940, 760 cmꢀ1; 5-bromo-2-
furancarboxylic acid, ꢀ ¼ 3130, 2560 (br, s, OH), 1705 (s,
—
—
C
O), 1590, 1280, 1160, 940, 760 cmꢀ1; 5-nitro-2-
furancarboxylic acid, ꢀ ¼ 3120, 2580 (br, ꢀs,1 OH), 1690
—
(s, C O), 1570, 1250, 1170, 930, 760 cm
—
.
RESULTS AND DISCUSSION
The oxidation of 2-furaldehydes by QDC resulted in the
formation of the corresponding acids. Under the present
experimental conditions, there was no further oxidation
of the acids.
Effect of solvent
The oxidation of the substrates was studied in solutions
containing varying proportions of water and acetic acid.
The dielectric constants ("r) of water–acetic acid mixtures
were calculated from the dielectric constants of the pure
solvents (at 313 K: water ¼ 73.28, acetic acid ¼ 6.29).14 It
Stoichiometry
The stoichiometry of the reaction was determined.7
Stoichiometric ratios, Á[QDC]/Á[substrate], in the range
Table 1. Rate data for the oxidation of substituted 2-furaldehydes at 313 K
104 k (sꢀ1
)
[Substrate] (102 M)
[QDC]
(103 M)
[H2SO4]
(M)
2-Furaldehyde
5-Bromo-2-
furaldehyde
5-Methyl-2-
furaldehyde
5-Nitro-2-
furaldehyde
1.0
2.5
5.0
7.5
10
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.75
1.0
1.25
1.50
1.25
3.12
6.21
9.32
12.5
1.22
1.25
1.24
1.27
1.88
2.50
3.20
3.80
1.1
2.6
5.6
8.3
11.0
1.12
1.09
1.1
1.15
1.7
1.5
3.8
7.8
12.1
15.6
1.52
1.5
1.45
1.43
2.3
1.03
2.54
5.12
7.71
10.2
1.02
1.06
1.01
1.03
1.5
1.0
0.75
0.50
0.25
0.10
1.0
1.0
1.0
1.0
2.15
2.7
3.4
3.1
3.7
4.6
2.05
2.6
3.1
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 83–87

OXIDATION OF SUBSTITUTED 2-FURALDEHYDES
85
Table 2. Solvent effect for oxidation of substituted 2-furaldehydesa by QDC at 313 K
104 k (sꢀ1
)
Dielectric
constant, "r
2-Furaldehyde
5-Bromo-2-
furaldehyde
5-Methyl-2-
furaldehyde
5-Nitro-2-
furaldehyde
H2O:AcOH (%, v/v)
50:50
45:55
40:60
35:65
30:70
39.79
36.44
33.09
29.74
26.39
1.25
1.82
2.51
3.80
6.30
1.1
1.3
1.7
2.4
3.4
1.5
2.2
3.1
4.5
7.5
1.03
1.24
1.5
2.2
3.2
a
[Substrate] ¼ 1.0 ꢃ 10ꢀ2 M; [QDC] ¼ 1.0 ꢃ 10ꢀ3 M; [H2SO4] ¼ 0.5 M.
Table 3. Temperature and activation parametersa for the oxidation of substituted 2-furaldehydesb by QDC
104 k (sꢀ1
)
Temperature (ꢁ0.1 K)
2-Furaldehyde
5-Bromo-2-furaldehyde
5-Methyl-2-furaldehyde
5-Nitro-2-furaldehyde
303
308
313
318
323
0.62
0.94
1.25
1.78
0.55
0.83
1.1
1.6
2.2
0.75
1.1
1.5
2.1
0.51
0.75
1.03
1.5
2.0
55
2.54
2.8
¼
ÁH (kJ molꢀ1
)
51
ꢀ152
53
ꢀ151
49
ꢀ157
¼
ÁS (J molꢀ1 Kꢀ1
)
ꢀ148
a
¼
¼
Error limits: ÁH ꢁ 2 kJ molꢀ1; ÁS ꢁ 5 J molꢀ1 Kꢀ1
.
b
[Substrate] ¼ 1.0 ꢃ 10ꢀ2 M; [QDC] ¼ 1.0 ꢃ 10ꢀ3 M; [H2SO4] ¼ 0.5 M.
was observed that there was an increase in the rate of the
reaction with decrease in the dielectric constant of the
medium (Table 2). The magnitude of this effect suggested
that, for the equilibrium 2HCrO4ꢀ Ð Cr2O72ꢀ þ H2O, a
decrease in the dielectric constant of the medium (in-
crease in the acetic acid content) favored the dichromate
form over the chromate form. Although the range of
dielectric constants used for these reactions was not large,
plots of log k versus 1/"r were found to be linear, with
positive slopes, which suggested that the reactions were
of the ion–dipole type.15
aldehydes by QDC. The aldehyde hydrate dissociation
constants (Kd) 10 for the reaction
Kd
Ð
RCHðOHÞ2
RCHO þ H2O
ð2Þ
are also given. From k and Kd, two sets of rate constants
for the oxidation of the aldehyde in only one of the forms
present in solution were calculated. The values of kHy
were obtained by assuming that only the hydrate form
appears in the rate law:
v ¼ kHy½QDCꢂ ½RCHðOHÞ2ꢂ
ð3Þ
Effect of temperature
Similarly, the values of kA were calculated using the
concentration of free aldehydes according to the rate law
The oxidation of the substrates was studied over the
temperature range 303–323 K; the rate data and activa-
tion parameters are given in Table 3. The negative values
v ¼ kA½QDCꢂ ½RCHOꢂ
ð4Þ
¼
of ÁS provided support for the formation of a rigid
activated complex which was strongly solvated.
Table 4. QDC oxidation of substituted 2-furaldehydes at
313 K
Kd 104k
(sꢀ1
kHy
kA
Hydrated form of the substrate
Aldehyde
)
(Mꢀ2 sꢀ1
)
(Mꢀ2 sꢀ1
)
It has been shown that aldehydes are extensively hydrated
in aqueous solutions and many oxidation reactions pro-
ceed via the hydrate form.11,16–21 Table 4 lists the
experimental rate constants (k) for the oxidation of the
2-Furaldehyde
1.29 1.25 16.5 ꢁ 1.28 12.8 ꢁ 1.28
5-Bromo-2-furaldehyde 1.16 1.1 12.7 ꢁ 1.2 10.9 ꢁ 0.27
5-Methyl-2-furaldehyde 1.23 1.5 18.4 ꢁ 1.15 15.5 ꢁ 0.63
5-Nitro-2-furaldehyde 1.09 1.03 11.2 ꢁ 1.0 9.77 ꢁ 0.31
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 83–87

86
G. S. CHAUBEY, B. KHARSYNTIEW AND M. K. MAHANTI
Table 5. Kinetic isotope effect at 313 Ka
The values of kHy and kA are given in Table 4. Using
the ꢁþ values derived from consideration of the electro-
philic substitution for the hetero systems,22 a plot of
log kHy against ꢁþ was linear (r ¼ 0.999), with a slope of
ꢂ ¼ þ0.509. On the other hand, the correlation of ꢁþ with
kA gave ꢂ ¼ þ0.423 (r ¼ 0.979). This might suggest the
involvement of the aldehyde hydrate in the oxidation
process. Hence a mechanism involving a direct hydro-
gen-transfer reaction between a free aldehyde and QDC
was very unlikely. Earlier work on the oxidation of
benzaldehyde by chromic acid had shown a pathway
involving the intermediate formation of the chromic
acid ester of hydrated benzaldehyde.23 This similarity
provided additional support for the mechanistic pathway
suggested, in the present investigation, that the rate-
determining step involved the oxidative decomposition
of the chromate ester of the aldehyde hydrate.
Substrate
104 kH (sꢀ1
)
104 kD (sꢀ1
)
kH/kD
2-Furaldehyde
2-Furaldehyde-1-d
1.25
—
—
0.215
—
5.8
a
[Substrate] ¼ 0.01 M; [QDC] ¼ 0.001 M; [H2SO4] ¼ 0.5 M.
This rate expression shows that the reaction exhibits a
first-order dependence with respect to the concentrations
of each of the substrate, oxidant and acid. Hence
ꢀ2:303dlog½QDCꢂ=dt ¼ k ¼ K1K2k3½Aꢂ ½QDCꢂ ½Hþꢂ
ð7Þ
This rate law explains all of the experimentally observed
results.
Rate law
Structure and reactivity
From the kinetic results, the rate law was derived as
follows:
For the oxidation of the substrates, the order of reactivity
is 5-methyl-2-furaldehyde > 2-furaldehyde > 5-bromo-
2-furaldehyde > 5-nitro-2-furaldehyde (Table 1), which
is in conformity with the inductive effects of the
substituents. The inference is that the electron-
releasing substituent (methyl) increases the oxidation
rate by increasing the equilibrium concentration of
the chromate ester of the aldehyde hydrate, and the
electron-withdrawing substituent (nitro) decreases the
oxidation rate.
ꢀd½QDCꢂ=dt ¼ k3½Eꢂ ¼ k3½Hyꢂ ½PQꢂ
ð5Þ
where [PQ] ¼ K1 [QDC] [Hþ] and [Hy] ¼ K2[A] [H2O].
Substituting the values of [PQ] and [Hy] in Eqn (5)
(taking the activity of water to be unity), we obtain
ꢀd½QDCꢂ=dt ¼ K1K2k3½Aꢂ ½QDCꢂ ½Hþꢂ
ð6Þ
Scheme 1
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 83–87

OXIDATION OF SUBSTITUTED 2-FURALDEHYDES
87
Mechanism
heterocyclic aldehydes, which could prove to be a regio-
selective route for the synthesis of carboxylic acids.
The mechanistic pathway involves the formation of the
ester of the aldehyde hydrate (step 1), followed by the
slow oxidative decomposition of this ester (step 2). A
cyclic structure for the reaction intermediate explains all
of the features of the oxidation reaction. The large
Acknowledgment
Financial support from the University Grants Commis-
sion, New Delhi, under the Special Assistance Program,
is gratefully acknowledged.
¼
negative entropies of activation (ÁS ) are consistent
with the formation of a cyclic intermediate in a bimole-
cular reaction. Electron flow in a cyclic transition state
has been considered,24 and has been rationalized as
follows: if the chromium is coordinated through the OH
group (of the aldehyde hydrate), then the electron flow is
through the carbon–oxygen–chromium bonds, allowing
the formation of the chromate ester and enhancing its
ease of conversion to the product.
The slow step of the reaction involves the participation
of the aldehyde hydrate, protonated QDC and two elec-
trons in a cyclic system; being a Hu¨ckel-type system
(4n þ 2), this is an allowed process.25 Removal of the
hydrogen (on the carbon) is part of this step, as evidenced
from the kinetic isotope effect for the oxidation of 2-
furaldehyde-1-d (Table 5), indicating cleavage of the
carbon–hydrogen bond in the rate-determining step of
the reaction.
The sequence of reactions for the oxidation of hetero-
cyclic aldehydes by QDC has been shown (Scheme 1). In
acidic medium, the oxidant QDC is converted to the
protonated dimetallic chromium(VI) species (PQ).
The substrate (A) is converted to the hydrated form
(Hy). The reaction of the hydrated form (Hy) with the
protonated QDC (PQ) results in the formation of
the monochromate ester (E) and a Cr(VI) monomer.
The monochromate ester (E) undergoes decomposition
in the rate-determining step to give the product (the
corresponding acid), along with the Cr(IV) species. The
conversion of Cr(IV) to Cr(III) proceeds by a dispropor-
tionation reaction.26,27
The data collected demonstrate that the application of
QDC to the oxidation of heterocyclic aldehydes leads to
the formation of carboxylic acids, substantiating the
mechanism of the oxidation reaction wherein there is
an attack of the oxidant on the aldehydic function,
leaving the heteroatom site intact. While highlighting
the importance of QDC as an oxidant, this study
emphasizes the efficiency of the reactions of QDC with
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