Oxidative regeneration of carbonyl compounds from oximes by pyridinium fluorochromate: A kinetic and mechanistic study

Article abstract of DOI:10.14233/ajchem.2013.13935

The oxidative deoximination of several aldoximes and ketoximes by pyridinium fluorochromate, in dimethyl sulphoxide, exhibited a first order dependence on pyridinium fluorochromate. A Michaelis-Menten type kinetics was observed with respect to oximes. The oxidation of ketoximes is slower than that of aldoximes. The rates of oxidation of aldoximes correlated well in terms of Pavelich-Taft dual substituentparameter equation. The low positive value of polar reaction constant indicated a nucleophilic attack by a chromate-oxygen on the carbon. The reaction is subjected to steric hindrance by the alkyl groups. The reaction of acetaldoxime has been studied in 19 different organic solvents. The solvent effect has been analyzed by multiparametric equations. A mechanism involving the formation of a cyclic intermediate, in the rate-determining step is suggested.

Full text of DOI:10.14233/ajchem.2013.13935

Asian Journal of Chemistry; Vol. 25, No. 5 (2013), 2792-2796
http://dx.doi.org/10.14233/ajchem.2013.13935
Oxidative Regeneration of Carbonyl Compounds from Oximes by
Pyridinium Fluorochromate: A Kinetic and Mechanistic Study
1
1
1
2
1,*
AJAY SHARMA , NEHA VYAS , A. CHOUDHARY , PTSRK PRASADRAO and VINITA SHARMA
1
2
Chemical Kinetics Laboratory, Department of Chemistry, Jai Narain Vyas University, Jodhpur-342 005, India
Department of Chemistry, PB Siddhartha College of Arts & Science, Siddhartha Nagar, Moghalraja Puram, Vijayawada-520 010, India
Corresponding author: E-mail: drvsharma29@gmail.com
Received: 19 April 2012;
*
(
Accepted: 23 November 2012)
AJC-12455
The oxidative deoximination of several aldoximes and ketoximes by pyridinium fluorochromate, in dimethyl sulphoxide, exhibited a first
order dependence on pyridinium fluorochromate. A Michaelis-Menten type kinetics was observed with respect to oximes. The oxidation
of ketoximes is slower than that of aldoximes. The rates of oxidation of aldoximes correlated well in terms of Pavelich-Taft dual substituent-
parameter equation. The low positive value of polar reaction constant indicated a nucleophilic attack by a chromate-oxygen on the carbon.
The reaction is subjected to steric hindrance by the alkyl groups. The reaction of acetaldoxime has been studied in 19 different organic
solvents. The solvent effect has been analyzed by multiparametric equations.A mechanism involving the formation of a cyclic intermediate,
in the rate-determining step is suggested.
Key Words: Carbonyl compounds, Halochromates, Kinetics, Mechanism, Oxidation, Oximes.
and acetophenone. In a typical experiment, the oxime (0.2
mol) and pyridinium fluorochromate (0.02 mol) were dissolved
in 50 mL of dimethyl sulphoxide and allowed to stand for ca.
INTRODUCTION
Regeneration of carbonyl compounds from its derivatives
under mild conditions is an important process in synthetic
organic chemistry. Several oxidative methods are available for
10 h for the completion of the reaction. Silica gel (5 g) was
then added to the reaction mixture and the mixture was stirred
13
for 15 min . It was then filtered and the solid residue was
1
deoximation . Halochromates have been used as mild and
2
-5
selective oxidizing reagents in synthetic organic chemistry .
washed with the solvent (2 % 15 mL). The solvent was
removed on a rotary evaporator and the residue was purified
6
Pyridinium fluorochromate is also one such reagent . We have
been interested in the kinetics of reactions of complexed Cr(VI)
species and have already published several reports on oxidation
4 2
on a silica-gel column (eluent: CCl /Et O). Evaporation of the
solvent afforded the pure carbonyl compound. Yields of
benzaldehyde and acetophenone were 1.83 g (86 %) and 2.11
7
,8
9,10
aspects of quinolinium and pyridinium halochromates.
Therefore in continuation of our earlier work, we report here
the kinetics of the oxidative deoximation of several aldo- and
keto-oximes by pyridinium fluorochromate in several organic
solvents but mainly in dimethyl sulphoxide. Mechanistic
aspects have also been discussed.
2
g (88 %) respectively. The presence of HNO in completely
reduced reaction mixtures was confirmed by a positive starch-
14
iodide test . The oxidation state of chromium in a completely
reduced reaction mixture, as determined by an iodometric
method, is 3.95 ± 0.15.
Kinetics measurements: The reactions were studied
under pseudo-first-order conditions by keeping a large excess
(× 10 or more) of the oxime over pyridinium fluorochromate.
The solvent was dimethyl sulphoxide, unless mentioned
otherwise. The reactions were studied at constant temperature
(± 0.1 K). The reactions were followed by monitoring the
decrease in the concentration of pyridinium fluorochromate
at 370 nm spectrophotometrically. The pseudo-first-order rate
constant, kobs, was evaluated from the linear least-squares plots
of log [PFC] versus time. Duplicate kinetic runs showed the
EXPERIMENTAL
11
Oximes were prepared by the reported standard methods
and their mp were checked with the literature values.
Pyridinium fluorochromate also was prepared by the reported
6
12
method . Solvents were purified by the usual procedure .
Product analysis: The oxidation of the oximes results in
the regeneration of corresponding carbonyl compounds, as
4 2
confirmed by TLC (eluent: CCl /Et O). Isolation of the product
was attempted in the oxidation of oximes of benzaldehyde

Vol. 25, No. 5 (2013)
Oxidative Regeneration of Carbonyl Compounds from Oximes by Pyridinium Fluorochromate 2793
rate constants to be reproducible to within ± 4 %. The second
order rate constants were evaluated from the relation k
reductant]. All reactions, other than those to study the effect
alcohols. There was no reaction with chosen solvents. The
kinetics were similar in all the solvents. The values of k
recorded in Table-4.
2
= kobs
/
2
are
[
+
of [H ], were performed in the absence of TsOH.
TABLE-1
RATE CONSTANTS FOR THE OXIDATION OF
ACETALDOXIME BY QFC AT 288 K
RESULTS AND DISCUSSION
Stoichiometry: The analysis of products indicated the
following overall reaction.
3
-3
-3
0 [PFC] (mol dm ) [Oxime] (mol dm )
4
−1
10 k (s )
1
obs
1
1
.0
.0
0.10
0.20
0.40
0.60
0.80
1.00
1.50
3.00
0.40
0.40
0.40
0.40
0.20
11.9
17.5
22.7
25.2
26.7
27.6
29.1
30.6
23.4
21.6
25.5
23.0
2 2 2 2
R C = N-OH + 2CrO ClOMH → R C = O + HNO + 2CrOClOMH(1)
Thus pyridinium fluorochromate undergoes a two electron
change. This is in accord with the earlier observations with
1.0
1.0
6
-8
other halochromates . It has already been shown that both
1
.0
15
16
1.0
pyridinium fluorochromate and pyridinium chlorochromate
act as two electron oxidants and are reduced to chromium(IV)
species.
1
1
2
4
.0
.0
.0
.0
Rate laws: The reactions were found to be first order with
respect to pyridinium fluorochromate (Fig. 1). In individual
2
kinetic runs, plots of log [PFC] versus time were linear (r >
6.0
8.0
*
1
.0
18.9
0
k
.995). Further, it was found that the observed rate constant,
obs, does not depend on the initial concentration of pyridinium
*
-3
Contained 0.001 mol dm acrylonitrile
fluorochromate. The order with respect to oxime was less than
one (Table-1). A plot of 1/kobs versus 1/[Oxime] was linear
with an intercept on the rate ordinate (Fig. 2). Thus Michaelis-
Menten type kinetics were observed with respect to oxime.
This leads to the postulation of following overall mechanism
0
0
0
.09
.08
.07
(
eqns. 2 and 3) and the rate law (4).
K
Oxime + PFC
0.06
0.05
[Complex]
Products
(2)
0
.04
k
2
[
Complex]
(3)
(4)
0.03
0.02
2
Rate = k K [Oxime] [PFC]/(1 + K [Oxime])
0
.01
0
2
1
0
.5
2
0
5
10
15
1
/oxime
Fig. 2. Oxidation of Acetaldoxime by PFC: A double reciprocal plot
.5
1
2
The linear correlation (r = 0.9478) between the values of
the activation enthalpies and entropies of the reaction indicated
the operation of a significant compensation effect in this
.5
0
1
7
reaction . The reaction exhibited an excellent isokinetic effect
(
temperature 508 ± 59 K) also, as determined by Exnerꢀs
18
2
method .An Exnerꢀs plot between log k at 288 K and at 318 K
0
200
400
600
800
1000
2
is linear (r = 0.9966, slope = 0.8048 ± 0.0236) (Fig. 3). The
value of isokinetic temperature is 567 ± 67 K. A linear
isokinetic relationship is a necessary condition for the validity
of linear free energy relationships. It also implies that all
Time (s)
Fig. 1. Oxidation of acetaldoxime by PFC: A typical Kinetic run
18
The dependence of kobs on the concentration of oxime
was studied at different temperatures and the values of K and
were evaluated from the double reciprocal plots. The
reactions so correlated follow a similar mechanism .
Solvent effect: The rate constants, k , of the oxidation of
2
k
2
acetaldoxime in 18 solvents (CS was not considered, as the
2
thermodynamic parameters for the complex formation and
activation parameters of the disproportionation of the comp-
complete range of solvent parameters was not available) were
correlated in terms of the linear solvation energy relationship
19
lexes, at 298 K, were calculated from the values of K and k
respectively at different temperatures (Tables 2 and 3).
2
(eqn. 2) of Kamlet et al. .
log k = A + pπ + bβ + aα
*
2
0
(2)
*
Effect of solvents: The oxidation of acetaldoxime was
studied in nineteen different solvents. The choice of solvents
was limited due to the solubility of the reactants and the reaction
of pyridinium fluorochromate with primary and secondary
In this equation, π represents the solvent polarity, β the
hydrogen bond acceptor basicities and α is the hydrogen bond
donor acidity. A is the intercept term. It may be mentioned
0
here that out of the 18 solvents, 12 have a value of zero for α.

2
794 Sharma et al.
Asian J. Chem.
TABLE-2
RATE CONSTANTS AND ACTIVATION PARAMETERS FOR THE OXIDATION OF PFC-OXIMES (R R C = N – OH ) COMPLEXES
1
2
4
0 k (dm mol s )
3
-1 -1
*
*
*
1
–∆H
–∆S
–∆G
2
Substituent
-1
-1
-1
-1
2
88 K
298 K
1090
51.8
41.8
21.8
16.2
63.8
99.0
5.10
3.96
3.24
9.30
308 K
1350
77.4
63.0
34.6
26.5
86.4
153
318 K
1590
117
(kJ mol )
14.2 ± 0.7
29.9 ± 0.3
31.2 ± 0.4
35.0 ± 0.4
35.8 ± 0.2
23.5 ± 0.7
29.8 ± 0.6
31.7 ± 0.7
30.2 ± 0.4
30.6 ± 0.4
33.3 ± 0.3
(J mol K )
216 ± 2
189 ± 1
187 ± 1
179 ± 1
179 ± 1
209 ± 2
184 ± 2
202 ± 2
209 ± 1
210 ± 1
192 ± 1
(kJ mol )
73.6 ± 0.6
86.1 ± 0.2
86.3 ± 0.3
88.2 ± 0.3
89.0 ± 0.2
85.6 ± 0.5
84.4 ± 0.5
91.8 ± 0.5
92.4 ± 0.3
92.9 ± 0.3
90.3 ± 0.2
H
H
Me
Et
822
H
H
32.4
25.5
12.8
9.50
44.5
65.7
3.24
2.58
2.07
5.75
97.2
55.8
43.2
126
H
Pr
I
H
Pr
H
ClCH2
Ph
H
234
Me
Me
Et
Me
Me
Et
7.83
6.12
4.95
15.3
12.6
9.36
7.65
23.4
Et
Ph
TABLE-3
FORMATION CONSTANTS AND THERMODYNAMIC PARAMETERS OF THE
1
OXIDATION OF PFC- OXIMES (R R C = N – OH) COMPLEXES
2
3
−1
*
*
*
K (dm mol )
298 K
∆H
–∆S
∆G
(kJ mol )
Substituent
-
1
-1
(J mol K )
-1
-1
288 K
308 K
4.73
4.26
3.92
4.23
3.51
4.62
4.86
4.30
4.10
4.77
4.52
318 K
3.96
3.42
3.15
3.45
2.79
3.78
4.05
3.50
3.33
3.95
3.70
(kJ mol )
15.1 ± 0.9
16.0 ± 0.8
16.7 ± 0.6
16.2 ± 0.7
18.1 ± 0.7
15.0 ± 0.6
14.3 ± 0.6
15.8 ± 0.7
16.4 ± 0.6
14.6 ± 0.6
14.9 ± 0.6
H
H
H
Me
Et
6.35
5.86
5.53
5.90
5.15
6.22
6.45
5.92
5.76
6.39
6.12
5.58
5.04
4.70
5.11
4.35
5.40
5.67
5.13
4.95
5.55
5.10
29±3
33±2
6.66 ± 0.5
6.46 ± 0.6
6.29 ± 0.7
6.48 ± 0.6
6.10 ± 0.3
6.64 ± 0.6
6.75 ± 0.3
6.49 ± 0.7
6.40 ± 0.3
6.71 ± 0.3
6.56 ± 0.3
H
35±2
33±2
H
Pr
I
Pr
ClCH2
Ph
H
41±2
29±2
H
H
26±2
32±2
Me
Me
Et
Me
Me
Et
34±2
27±2
Et
Ph
28±2
2
TABLE-4
EFFECT OF SOLVENTS ON THE OXIDATION OF
ACETALDOXIME BY PFC AT 298 K
r = 0.7918; sd = 0.22; n = 18; ψ = 0.47
log k = - 2.89 + 0.56 (±0.37) β
r = 0.1260; sd = 0.44; n = 18; ψ = 0.96
2
(6)
2
3
K (dm mol )
-1
5
Solvents
10 k
2
Here n is the number of data points and ψ is Exnerꢀs
20
statistical parameter .
Chloroform
5.44
45.7
42.7
35.5
117
1,2-Dichloroethane
5.45
Dichloromethane
DMSO
5.94
6
5
4
3
2
1
0
3.42
Acetone
4.76
33.9
63.1
21.9
39.8
11.7
0.85
7.94
38.9
14.8
20.9
17.4
9.77
4.47
19.1
12.1
DMF
5.85
Butanone
Nitrobenzene
Benzene
5.55
3.92
5.45
Cyclohexane
Toluene
5.63
4.60
Acetophenone
THF
5. 24
5.59
t-butylalcohol
4.76
1,4-Dioxane
5.45
1,2-Dimethoxyethane
4.33
CS2
5.51
Acetic acid
Ethyl acetate
4.59
0
2
4
6
5.13
2
+ log kobs at 318 K
The results of correlation analyses in terms of eqn. (2), a
*
biparametric equation involving π and β and separately with
Fig. 3. Exnerꢀs isokinetic relationship in the oxidation of oximes by PFC
*
π and β are given below [eqs. (3)-(6)].
19
Kamlet et al. triparametric equation explains ca. 86 %
*
= -3.93 + 1.77 (±0.21) π + 0.15 (±0.17) β + 0.34 (±0.16) α (3)
log k
2
of the effect of solvent on the oxidation. However, by Exnerꢀs
18
criterion the correlation is not even satisfactory (cf. eqn. 3).
2
R = 0.8615; sd = 0.19; n = 18; ψ = 0.41
*
= - 3.85 + 1.65 (±0.22) π + 0.27 (±0.17) β (4)
log k
2
The major contribution is of solvent polarity. It alone accounted
for ca. 79 % of the data. Both β and α play relatively minor
roles.
2
R = 0.8189; sd = 0.21; n = 18; ψ = 0.45
*
log k
2
= -3.80 + 1.72 (±0.22) π
(5)

Vol. 25, No. 5 (2013)
Oxidative Regeneration of Carbonyl Compounds from Oximes by Pyridinium Fluorochromate 2795
The data on the solvent effect were analyzed in terms of
The rates exhibited excellent correlations in terms of the
Pavelich-Taft equation (Table-4); the reaction constants (Table-
5) are being positive.
21
Swainꢀs equation of cation- and anion-solvating concept of
the solvents also [eqn. (7)].
log k
2
= aA + bB + C
(7)
HereA represents the anion-solvating power of the solvent
and B the cation-solvating power. C is the intercept term. (A +
B) is postulated to represent the solvent polarity. The rates in
different solvents were analyzed in terms of eqn. (7), separately
with A and B and with (A + B).
TABLE-5
REACTION CONSTANTS FOR THE OXIDATIVE
DEOXIMINATION OF ALIPHATIC ALDOXIMES BY PFC
a
*
ρ
2
R
Temp./K
δ
sd
ψ
2
2
3
3
88
98
08
18
0.36 ± 0.01
0.30 ± 0.02
0.25 ± 0.01
0.22 ± 0.01
0.99 ± 0.01
0.95 ± 0.02
0.90 ± 0.01
0.83 ± 0.02
0.9999 0.003
0.9998 0.002
0.9999 0.003
0.9989 0.002
0.01
0.02
0.01
0.04
log k
2
= 1.33 (±0.05) A + 1.64 (±0.04) B - 3.86
(8)
2
R = 0.9937; sd = 0.04; n = 19; ψ = 0.08
log k
r = 0.1937; sd = 0.44; n = 19; ψ = 0.92
log k = 1.54 (±0.24) B - 3.71
r = 0.7120; sd = 0.26; n = 19; ψ = 0.55
log k = 1.53 ± 0.05 (A + B) - 3.87
r = 0.9826; sd = 0.06; n = 19; ψ = 0.14
2
= 1.10 (±0.54) A - 3.02
(9)
a
Number of compounds is 6
2
2
(10)
(11)
2
Mechanism: The low positive polar reaction constant
points to an almost cyclic transition state in which the forma-
tion of the bond between chromate-oxygen and the carbon is
somewhat ahead of the formation of N-O bond. This supports
a nucleophilic attack by a chromate-oxygen on the carbon.
The positive steric reaction constant points to a steric hindrance
by the substituents. Therefore, the following mechanism
2
2
The rates of oxidation of acetaldehyde in different solvents
showed an excellent correlation in Swainꢀs equation [cf. eqn.
(
8)] with both the anion- and cation-solvating powers playing
almost equal role. However, individually A and B are able to
account for only 19 and 71 % of the data only. The solvent
polarity, represented by (A + B), also exhibited an excellent
correlation. In view of the fact that solvent polarity is able to
account for ca. 98 % of the data, an attempt was made to corre-
late the rate with the relative permittivity of the solvent.
(
Scheme-I) is proposed for the reaction. The mechanism is
supported by the values of activation parameters also. The low
values of enthalpy of activation indicate that the bond-cleavage
and bond-formation are almost synchronous. The large negative
entropies of activation support the formation of a rigid cyclic
activated complex from two acyclic molecules.
However, a plot of log k
permittivity is not very significant (r = 0.4604; sd = 0.36; ψ =
.75). The analysis of solvent effect indicated the formation
2
against the inverse of the relative
2
#
O
O
OPyH
F
0
K
C
O
N
O
OH
C
N
OH
+
Cr
of an activated complex which is more polar than the reactants.
The rate is affected by the solvent polarity.
Cr
Correlation analysis of reactivity: We could not find
any report about the mechanism of the reaction between a
C=N bond and a halochromate derivative. However, the
F
OPyH
22
reaction of alkenes with chromium(VI) has been well studied .
Since, olefinic bonds are not usually subject to a nucleophilic
attack, it has been suggested that in the alkene-chromate
C
O
+
CrOFOPyH
+
N
OH
22
CrO
2
FOPyH +
N
OH
CrOFOPyH
+ HNO2
reaction, an organometallic derivative is formed initially . The
organometallic derivative then changes to a chromium(IV)
diester in the rate-determining step. However, carbon-nitrogen
double bonds, being dipolar in nature, can be easily attacked
by a nucleophile. The data in Table-2 showed that the rate of
oxidation of ketoximes is much less as compared to that of
the aldoximes. The reason for the slower reaction of ketoximes
must be steric. As the central carbon changes from a trigonal
to a tetragonal state, the crowding around it increases. This
increase in the steric crowding will be more in the case of
ketoximes as compared to that in aldoximes. This observation
is supported by the correlation analysis of the reactivity of the
aldoximes also. The rate of oxidation of the aliphatic oximes
Scheme-I
The faster oxidation of benzaldoxime may be attributed
to the resonance stablization of the cyclic activated complex.
The oxidation of benzphenoxime is much slower. This may
well be due to steric hindrance by the bulky phenyl and methyl
groups. Hydroxynitrene (N-OH) has been recently reported
24
as a very reactive intermediate .
ACKNOWLEDGEMENTS
Thanks are due to University Grants Commission, New
Delhi for financial support in the form of Major Research
Project and to Professor KK Banerji for his critical suggestions
and helpful discussions.
*
did not yield significant correlation separately with Taftsꢀs σ
S
and E values [eqns. (12) and (13)]. The rates were, therefore,
23
correlated with Pavelich-Taftꢀs dual substituent-parameter
eqn. (14).
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*
= 0.68 ± 0.58 σ - 2.34
log k
2
(12)
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