Kinetics of oxidation of α,β-unsaturated aldehydes by quinolinium dichromate

Article abstract of DOI:10.1139/v03-022

A series of α,β-unsaturated aldehydes (crotonaldehyde, cinnamaldehyde, acrylaldehyde, and methacrylaldehyde) were oxidized by quinolinium dichromate in sulfuric acid to the corresponding acids in 50% (v/v) acetic acid water medium. The kinetic data have been 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.

Full text of DOI:10.1139/v03-022

204
Kinetics of oxidation of ␣,␤-unsaturated aldehydes
by quinolinium dichromate
Girija S. Chaubey, Simi Das, and Mahendra K. Mahanti
Abstract: A series of α,β-unsaturated aldehydes (crotonaldehyde, cinnamaldehyde, acrylaldehyde, and methacrylalde-
hyde) were oxidized by quinolinium dichromate in sulfuric acid to the corresponding acids in 50% (v/v) acetic acid –
water medium. The kinetic data have been 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.
Key words: kinetics, oxidation, unsaturated aldehydes, quinolinium dichromate.
Résumé : Le dichromate de quinolinium dans l’acide sulfurique utilisé en solution d’acide acétique aqueux à 50%
(v/v) permet d’oxyder une série d’aldéhydes α,β-insaturés (crotonaldéhyde; cinnamaldéhyde; acrylaldéhyde; méthacry-
laldéhyde) en acides correspondants. On discute des données cinétiques en fonction de l’équilibre d’hydratation de
l’aldéhyde. Les données cinétiques sont en accord avec un mécanisme réactionnel impliquant une décomposition oxy-
dante de l’ester chromique de l’hydrate d’aldéhyde qui serait cinétiquement limitante.
Mots clés : cinétique, oxydation, aldéhydes insaturés, dichromate de quinolinium.
[Traduit par la Rédaction] Chaubey et al. 208
Introduction
the dichromate ion. Acetic acid (SD, AR grade) was distilled
before use. Sulfuric acid (E. Merck) was used after a check
of its physical constants. The deuterated compounds were
prepared by the reported method (8). The IR spectra were
recorded on a Bomem DA-8 FT-IR spectrophotometer.
The method used for the kinetic determinations has been
described previously (5). All the reactions were performed
under nitrogen. The rate constants have been evaluated from
the linear (r > 0.995) plots of log[QDC] against time, and
the values reported are the mean of two or more runs
(reproducibility 3%). The solvent was 50% aqueous acetic
acid, and the dielectric constant was varied using acetic acid –
water mixtures. The reaction mixtures remained homoge-
neous in the solvent systems used.
In the oxidation of α,β-unsaturated aldehydes, it has been
suggested that the reaction could proceed by way of (a) eno-
lization being the rate-determining step (1, 2); (b) hydration
of the double bond to form a β-hydroxyaldehyde (3); or
(c) oxidation of the aldehyde hydrate (4).
With a view to establishing the mechanistic pathway for
this oxidation process, we have carried out a kinetic study of
the oxidation of α,β-unsaturated aldehydes (crotonaldehyde,
cinnamaldehyde, acrylaldehyde, and methacrylaldehyde) by
quinolinium dichromate (QDC, (C9H7N+H)2Cr2O72–) in 50%
acetic acid – water medium under a nitrogen atmosphere.
This study forms part of our continuing efforts, concerning
the quinolinium dichromate oxidation of organic substrates
in general (5) and aldehydes in particular (6).
Product analysis
Thirty mL of water was taken and cooled in ice. Concen-
trated H2SO4 (7.9 g, 0.08 M) was added slowly with con-
stant cooling. When the acid solution had cooled to room
temperature, quinolinium dichromate (QDC 9.52 g, 0.02 M)
was added and the mixture was warmed to 313 K for com-
plete dissolution of the QDC. To this mixture, 0.01 M of
substrate (0.71 g of methacrylaldehyde, 0.57 g of acrylalde-
hyde, 0.71 g of crotonaldehyde, and 1.33 g of cinnamal-
dehyde) taken up in 25 mL of a 50% acetic acid – water
solution was added. The reaction mixture was stirred at
313 K for 48 h under nitrogen. The organic layer was ex-
tracted thrice with ether (25 mL each time) and the com-
bined organic extracts were washed with water and dried
over anhydrous Na2SO4. The oxidized products (methacrylic
acid from methacrylaldehyde, acrylic acid from
acrylaldehyde, crotonic acid from crotonaldehyde, and
cinnamic acid from cinnamaldehyde) were obtained after the
complete removal of ether (boiling and melting points were
Experimental
Materials and methods
Acrylaldehyde and methacrylaldehyde (Aldrich) and
crotonaldehyde and cinnamaldehyde (Merck-Schuchardt)
were used without further purification. The oxidant,
quinolinium dichromate (QDC, (C9H7N+H)2Cr2O72–) was
prepared by the reported method (7), and its purity was
checked by spectral analysis. The IR spectrum (KBr) exhib-
ited bands at 930, 875, 765, and 730 cm–1, characteristic of
Received 22 July 2002. Published on the NRC Research
Press Web site at http://canjchem.nrc.ca on 12 March 2003.
G.S. Chaubey, S. Das, and M.K. Mahanti.1 Department of
Chemistry, North-Eastern Hill University, Shillong 793 022,
India.
1Corresponding author (e-mail: mkmahanti@yahoo.com).
Can. J. Chem. 81: 204–208 (2003)
doi: 10.1139/V03-022
© 2003 NRC Canada

Chaubey et al.
205
Table 1. Rate data for the oxidation of α,β-unsaturated aldehydes in 50% acetic acid at 313 K.
103 k (s–1)
102 [Substrate]
(M)
102 [QDC]
(M)
[H2SO4]
(M)
Methacrylaldehyde
Acrylaldehyde
Crotonaldehyde
Cinnamaldehyde
3.1
0.31
0.30
0.31
0.30
0.32
0.12
0.06
0.04
0.03
0.31
0.32
0.30
0.31
0.51
0.52
0.51
0.51
0.53
0.50
0.52
0.50
0.51
0.75
1.02
1.25
1.50
1.3
4.6
7.6
11
1.2
4.0
7.1
9.9
12
1.2
1.3
1.2
1.2
1.8
2.4
3.1
3.7
0.51
1.5
2.9
4.1
4.9
0.52
0.51
0.54
0.53
0.81
1.1
0.080
0.26
0.41
0.62
0.78
0.081
0.080
0.082
0.083
0.12
10.0
17.0
25.0
30.0
3.0
3.2
3.1
3.0
3.1
14
1.3
1.3
1.3
1.3
2.0
2.7
3.4
4.0
3.1
3.0
3.2
0.16
0.21
0.25
1.4
1.6
in agreement with literature values; yields ≈ 85–90%). Each
mined. The dielectric constants (D) of water – acetic acid
mixtures have been calculated (at 313 K: water = 73.28, ace-
tic acid = 6.29) (9). The data in Table 3 shows that a de-
crease in D of the medium results in an increase in the rate
of the reaction (Table 3). The magnitude of this effect could
be analyzed by suggesting that, for the equilibrium
reaction product was characterized by IR analysis.
Results and discussion
The oxidation of α,β-unsaturated aldehydes (crotonalde-
hyde, cinnamaldehyde, acrylaldehyde, and methacrylalde-
hyde) by QDC results in the formation of the corresponding
acids. Under the present experimental conditions, there is no
further oxidation of the acids.
The stoichiometry of the reaction was determined (5).
Stoichiometric ratios, ∆[QDC]/∆[substrate], in the range
0.66 to 0.69 conform to the following overall equation (rep-
resentative):
2–
2HCrO4 ꢀ Cr2O7 + H2O, a decrease in D of the medium
(increase in the acetic acid content) favors the dichromate
form over the chromate form. If ion pairs are formed in this
medium, they would have a higher ion-pair association con-
stant for the dichromate ion, and this would again favor the
dichromate ion. Although the range of D used for these reac-
tions is not large, plots of log k versus 1/D are linear, with
positive slopes, suggesting an ion–dipole type of interaction
(10).
[1]
3C3H4O + 2Cr(VI) + 3H2O → 3C3H4O2
+ 2Cr(III) + 6H+ (acrylaldehyde)
There is no induced polymerization of acrylonitrile (11),
which indicates that a one-electron oxidation is quite un-
likely. Control experiments, performed in the absence of the
substrate, did not show any appreciable change in the con-
centration of QDC.
Variations in the ionic strength of the medium, using so-
dium perchlorate (µ = 0.01–0.25 M), did not influence the
rates of these reactions.
Aldehydes are extensively hydrated in aqueous solutions
and many oxidation reactions have been reported to proceed
via the hydrated form (12–17). Table 4 records the experi-
mental rate constants (k) for the oxidation of the aldehydes
by QDC. The aldehyde hydrate dissociation constants (Kd)
pertaining to the reaction:
Using pseudo-first-order conditions, individual kinetic
runs are first order in QDC. The pseudo-first-order rate con-
stants (k) are independent of the initial concentration of the
oxidant (Table 1). The order of the reaction with respect to
substrate concentration has been obtained by varying the al-
dehyde concentration and observing the effect on the rate at
constant [QDC] and [H+]. The order with respect to the con-
centration of acid, at constant [aldehyde] and [QDC], is
unity. The kinetic results are shown in Table 1. In the range
of acid concentrations used, the protonation of the aldehydes
would be less significant, though it cannot be ruled out. At
present, it may not be possible to decide whether protonation
of the aldehyde or protonation of the dichromate results in
the observed acid catalysis, since these two processes cannot
be distinguished on the basis of the data obtained. Since the
acid concentrations used were in the range 0.5 to 1.5 M, it
would be justified to suggest that the dichromate ion would
be the predominant species in these oxidation reactions.
The oxidation of the substrates by QDC has been studied
over the temperature range 303–323 K. The rate data and ac-
tivation parameters are shown in Table 2. The negative val-
ues of ∆S* provide support for a polar bimolecular reaction.
The effect of a change in the solvent composition (water –
acetic acid, %, v/v) on the rate of oxidation has been deter-
[2]
RCH(OH)2
RCHO + H2O
are also given (12). From k and Kd, two sets of rate constants
for the oxidation of the aldehyde in only one of the forms
present in solution are computed. The values of kHy (Hy =
hydrated form of the substrate) are obtained by assuming
that only the hydrate form appears in the rate law:
[3]
v = kHy [QDC] [RCH(OH)2].
Similarly, the values of kA (A = substrate) are calculated
using the concentration of free aldehyde according to the
rate law:
© 2003 NRC Canada

206
Can. J. Chem. Vol. 81, 2003
Table 2. Temperature and activation parameters for the oxidation of α,β-unsaturated aldehydesa in 50% acetic acid by QDC.
103k (s–1)
T (K)
Methacrylaldehyde
Acrylaldehyde
Crotonaldehyde
Cinnamaldehyde
0.71
1.0
1.3
2.0
2.7
0.62
0.91
1.2
1.9
2.5
0.25
0.39
0.51
0.77
1.10
0.031
0.062
0.081
0.130
0.16
303.0
308.1
313.0
318.0
323.1
∆H* (kJ mol–1)
52 1.9
–135
54 2.6
–129
57 2.2
–127
63 2.4
∆S* (J mol–1 K–1)
6
7
6
–123
8
a[Substrate] = 0.03 M; [QDC] = 0.003 M; [H2SO4] = 0.5 M.
Table 3. Solvent effect for oxidation of α,β-unsaturated aldehydesa by QDC at 313 K.
103 k (s–1)
H2O:AcOH
(%, v/v)
Dielectric
constants D
Methacrylaldehyde
Acrylaldehyde
Crotonaldehyde
Cinnamaldehyde
60:40
55:45
50:50
45:55
40:60
46.48
43.14
39.79
36.44
33.09
0.79
1.1
1.3
1.7
2.1
0.64
0.93
1.2
1.5
1.9
0.36
0.43
0.51
0.59
0.67
0.053
0.065
0.080
0.10
0.13
a[Substrate] = 0.03 M; [QDC] = 0.003 M; [H2SO4] = 0.5 M.
which would undergo decomposition in the rate-determining
step (21). The ester of the aldehyde hydrate would be in
equilibrium with the free aldehyde and the aldehyde hydrate
and could be formed either by a carbonyl addition reaction
to the free aldehyde or by the esterification of the hydrate
(22). It may be added that the esterification reaction has
more utility, since it helps one to understand and predict al-
dehyde oxidation reactions and their relationship to the
closely related oxidation of alcohols.
Table 4. QDC oxidation of α,β-unsaturated aldehydes at 313 K.
103 k
kHy
(M–1 s–1)
kA
(M–1 s–1)
a
Kd
Aldehydes
(s–1)
Methacrylaldehyde 2.3 1.3
34 0.23
23 0.24
5.9 0.25
0.51 0.22
15 0.30
13 0.23
5.4 0.27
0.85 0.24
Acrylaldehyde
Crotonaldehyde
Cinnamaldehyde
1.7 1.2
1.1 0.51
0.6 0.080
aReference 12.
The rate data shows that both acrylaldehyde and meth-
acrylaldehyde are oxidized at approximately equal rates,
with methacrylaldehyde reacting marginally faster than
acrylaldehyde (Table 1), which suggests that the α-CH3
group has little influence on the rates. It is justified to as-
sume that the oxidation process does not involve an eno-
lization step. The enolization of acrylaldehyde would have
yielded hydroxyallene, which would be improbable. Further,
the oxidation of methacrylaldehyde is a rapid reaction even
though there is no enolizable hydrogen atom. The argument
that unsaturated aldehydes could undergo a reaction involv-
ing the hydration of the double bond to form a β-
hydroxyaldehyde can be ruled out, since no such intermediate
could be isolated from the reaction. The order of reactivity
is: methacrylaldehyde > acrylaldehyde > crotonaldehyde >
cinnamaldehyde (Table 1). The presence of the methyl group
(in methacrylaldehyde) accelerates the reaction by increas-
ing the electron availability at the oxygen of the aldehydic
carbonyl group. In crotonaldehyde, the methyl group at the
β-position is far removed from the site of reaction, suggest-
ing that β-substitution influences the rate of the reaction to a
much lesser extent. Hence, the reactivity of crotonaldehyde
is much less than that of methacrylaldehyde. The presence
of the phenyl group in cinnamaldehyde exerts a deactivating
influence on the rate of the reaction and hence its reactivity
is the lowest in the series.
[4]
v = kA [QDC] [RCHO].
The values of kHy and kA have been shown in Table 4.
Using the σ values as reported by Taft and co-workers (18),
a plot of log kHy against σ is found to be linear, with a slope
of ρ = –0.92 (r = 0.993). On the other hand, the correlation
of σ with kA gave a value of ρ = –0.56 (r = 0.997). This
might suggest the involvement of the aldehyde hydrate in the
oxidation process. Hence, a mechanism involving a direct
hydrogen transfer reaction between a free aldehyde and
QDC is very unlikely. In the chromic acid oxidation of
benzaldehyde, it has been shown that the reaction proceeds
via the chromic acid ester of hydrated benzaldehyde as the
intermediate (19). This similarity provides additional support
for the mechanistic pathway suggested in the present investi-
gation: that the rate-determining step involves the oxidative
decomposition of the chromate ester of an aldehyde hydrate.
Since aldehyde hydrates very closely resemble alcohols
both in structure and in many aspects of oxidation, a similar-
ity in the nature of the two oxidation reactions is expected.
In the oxidation of alcohols by chromic acid, the rate-
determining step has been shown to be the decomposition of
the protonated acid chromate ester (20). By analogy, the oxi-
dation of aldehydes would proceed via the formation of a
similar intermediate (an ester of the aldehyde hydrate),
© 2003 NRC Canada

Chaubey et al.
207
Scheme 1.
Table 5. Kinetic isotope effect at 313 K.a
The mechanistic pathway involves the formation of the es-
ter of the aldehyde hydrate (step 1), followed by the slow
oxidative decomposition of this ester (step 2). A cyclic struc-
ture for the reaction intermediate would explain all the fea-
tures of the oxidation reaction. The manner of electron
transfer must be established. Electron flow in a cyclic transi-
tion state has been considered (23) and can be rationalized
by assuming that if the chromium is coordinated through the
-OH group (of the aldehyde hydrate), then the process of
electron transfer could take place through the carbon—
oxygen—chromium bonds, enabling the formation of the
chromate ester and enhancing the ease of conversion to the
product.
The slow step of the reaction involves the participation of
the aldehyde hydrate, protonated QDC, and two electrons in
a cyclic system. Removal of the hydrogen (on the carbon) is
part of this step, as seen from the kinetic isotope effect for
the oxidation of the respective aldehyde-d1 compounds
(Table 5), which indicates a cleavage of the carbon—hydro-
gen bond in the rate-determining step of the reaction.
Substrate
103 kH (s–1)
103 kD (s–1)
kH/kD
Crotonaldehyde-d1
Cinnamaldehyde-d1
0.51
0.080
0.084
0.013
6.25
6.15
a[Substrate] = 0.03 M; [QDC] = 0.003 M; [H2SO4] = 0.5 M.
The last step would be rate-determining, in accord with the
observation of the deuterium kinetic isotope effect. Further,
the oxidation rate for the aldehydes is not increased as rap-
idly by a decrease in the water concentration in acetic acid –
water mixtures (Table 3), which suggests that a molecule of
water is involved in a kinetically important stage in the oxi-
dation of the aldehydes.
The sequence of reactions for the oxidation of α,β-unsatu-
rated aldehydes by QDC is shown in Scheme 1. In acidic
medium, the oxidant QDC is converted to the protonated
dimetallic chromium(VI) species (PQ) (in the acid range
used for the present investigation, the Cr(VI) in the
protonated QDC would exist mainly as Cr2O72–). The sub-
strate (A) is converted to the hydrated form (Hy). The reac-
tion 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) under-
goes decomposition in the rate-determining step to give the
product (the corresponding acid), along with the Cr(IV) spe-
cies. The conversion of Cr(IV) to Cr(III) is a dispropor-
This step allows one to envisage a reaction via an
electrocyclic mechanism involving six electrons; being a
Hückel-type system (4n+2), this is an allowed process (24).
The kinetic isotope effect could be interpreted to indicate
that the reaction proceeds via the formation of the chromate
ester of the aldehyde hydrate as an intermediate, and that
this is converted to the product in the rate-determining step.
© 2003 NRC Canada

208
Can. J. Chem. Vol. 81, 2003
4. J. Rocek. The chemistry of carbonyl group. Edited by S. Patai.
Vol. 1. Interscience, London. 1966. Chap 10.
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(25), and this reaction proceeds rapidly. The Cr(V)–Cr(III)
couple has a potential of 1.75 V, which enables the rapid
conversion of Cr(V) to Cr(III) after the reaction of Cr(V)
with the substrate (25, 26).
If the mechanism shown in Scheme 1 is correct, then the
attack of the protonated QDC (PQ) on the aldehyde hydrate
(Hy) is crucial and would be favored by the formation of the
cyclic chromate ester (E). The rate law has been derived as
follows:
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[5]
–d[QDC]/dt = k3[E] = k3[Hy][PQ],
where [PQ] = K1[QDC][H+] and [Hy] = K2[A][H2O].
Hence, –d[QDC]/dt = K1K2 k3[A][QDC][H+], which
shows a first-order dependence on each of the concentrations
(substrate, oxidant, and acid). Hence, –2.303d(log[QDC])/dt =
k = K1K2 k3[A][QDC][H+]. This rate law explains all the ex-
perimentally observed results.
The data collected demonstrates that the QDC oxidation
of α,β-unsaturated aldehydes results in the formation of
carboxylic acids, substantiating the mechanism wherein
there is an attack of the oxidant on the aldehyde hydrate.
There is no cleavage of the carbon—carbon bond, thus rul-
ing out the possibility of any enolization. This study empha-
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aldehydes, suggesting a regioselective route for the synthesis
of carboxylic acids.
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Acknowledgment
Financial support from the University Grants Commis-
sion, New Delhi, under the Special Assistance Program, is
gratefully acknowledged.
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© 2003 NRC Canada