Kinetics and mechanism of the oxidative regeneration of carbonyl compounds from oximes by pyridinium bromochromate

Article abstract of DOI:10.1002/kin.20185

The oxidative deoximination of several aldo- and keto-oximes by pyridinium bromochromate (PBC), in dimethylsulfoxide, exhibited a first-order dependence on both the reductant (oxime) and the oxidant (PBC). The oxidation of ketoximes is slower than that of aldoximes. The rates of oxidation of aldoximes correlated well in terms of the 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 subject 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 Taft's and Swain's multiparametric equations. A mechanism involving the formation of a cyclic intermediate, in the rate-determining step, has been proposed.

Full text of DOI:10.1002/kin.20185

Kinetics and Mechanism of
the Oxidative Regeneration
of Carbonyl Compounds
from Oximes by Pyridinium
Bromochromate
PRADEEP K. SHARMA
Department of Chemistry, J.N.V. University, Jodhpur 342 005, India
Received 7 November 2003; accepted 28 November 2005
DOI 10.1002/kin.20185
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The oxidative deoximination of several aldo- and keto-oximes by pyridinium bro-
mochromate (PBC), in dimethylsulfoxide, exhibited a first-order dependence on both the
reductant (oxime) and the oxidant (PBC). The oxidation of ketoximes is slower than that of
aldoximes. The rates of oxidation of aldoximes correlated well in terms of the 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 subject to steric hin-
drance by the alkyl groups. The reaction of acetaldoxime has been studied in 19 different organic
solvents. The solvent effect has been analyzed by Taft’s and Swain’s multiparametric equations.
A mechanism involving the formation of a cyclic intermediate, in the rate-determining step, has
been proposed. ꢀ 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 364–368, 2006
C
bromochromate. We have been interested in the kinet-
ics of oxidations by Cr(VI) species and have already
published a few reports on oxidation by pyridinium
[6,7] and quinolinium halochromates [8,9]. Therefore
in continuation of our earlier work, we report here the
kinetics of the oxidative deoximation of several aldo-
and keto-oximes by PBC in several organic solvents
but mainly in dimethylsulfoxide (DMSO). Mechanis-
tic aspects have also been discussed.
INTRODUCTION
Regeneration of carbonyl compounds from its deriva-
tives under mild conditions is an important process in
synthetic organic chemistry. Several oxidative meth-
ods are available for deoximation [1]. Pyridinium
bromochromate (PBC) has been used as a mild and
selective oxidant in synthetic organic chemistry [2]. A
few reports about the kinetic and mechanistic aspects of
oxidation by PBC are available in the literature [3–5].
There seems to be no report about the kinetics and
mechanism of oxidative deoximation by pyridinium
EXPERIMENTAL
Materials
Correspondence to: P. K. Sharma; e-mail: drpkvs27@yahoo.
com.
Oximes were prepared by the reported standard meth-
ods [10], and their melting points were checked with
Contract grant sponsor: DST, New Delhi, India.
c
ꢀ
2006 Wiley Periodicals, Inc.

KINETICS AND MECHANISM OF THE OXIDATIVE REGENERATION OF CARBONYL COMPOUNDS
365
the literature values. PBC was also prepared by the re-
ported method [2]. Solvents were purified by the usual
procedure [11].
RESULTS
Stoichiometry
Theanalysisofproductsindicatedthefollowingoverall
reaction:
Product Analysis
R2C N OH + 2CrO2BrO−PyH+
The oxidation of the oximes results in the regenera-
tion of corresponding carbonyl compounds, as con-
firmed by TLC (eluent: CCl4/Et2O). Isolation of the
product was attempted in the oxidation of oximes of
benzaldehyde and acetophenone. In a typical experi-
ment, oxime (0.2 mol) and PBC (0.02 mol) were dis-
solved in 50 mL of DMSO and allowed to stand for
ca. 10 h for the completion of the reaction. Silica gel
(5 g) was then added to the reaction mixture, and the
mixture was stirred for 15 min [12]. It was then fil-
tered, and the solid residue was washed with the sol-
vent (2 × 15 mL). The solvent was removed on a rotary
evaporator, and the residue was purified on a silica–gel
column(eluent:CCl4/Et2O). Evaporationofthesolvent
afforded the pure carbonyl compound. Yields of ben-
zaldehyde and acetophenone were 1.77 g (83%) and
2.12 g (88%), respectively. The presence of HNO2 in
completely reduced reaction mixtures was confirmed
by a positive starch–iodide test [13]. The oxidation
state of chromium in a completely reduced reaction
mixture, as determined by an iodometric method, is
3.95 0.05.
→ R2C O + HNO2 + 2CrOBrO−PyH+ (1)
PBC undergoes a two-electron change, which is in ac-
cord our earlier observation with structurally similar
PFC [6] and PBC [3].
Rate Laws
The reaction is first order with respect to PBC. The in-
dividual kinetic runs yielded linear (r2 > 0.995) plots
between log [PBC] and time. Further, the values of kobs
do not depend on the initial concentration of PBC. The
reaction exhibited a linear dependence on the concen-
tration of the oximes (Table I).
Effect of Temperature
The rate constants were determined at four different
temperatures, and the activation parameters were
calculated (Table II).
Effect of Solvents
The oxidation of acetaldoxime was studied in 19 dif-
ferent solvents. The choice of solvents was limited by
the solubility of the reactants and the reaction of PBC
with primary and secondary alcohols. There was no
Kinetics Measurements
The reactions were studied under pseudo-first-order
conditions by keeping a large excess (×10 or greater)
of the oxime over PBC. The solvent was DMSO, un-
less mentioned otherwise. The reactions were stud-
ied at constant temperature ( 0.1 K). The reactions
were followed, for up to 80% reaction, by moni-
toring the decrease in the concentration of PBC at
365 nm spectrophotometrically. The pseudo-first-order
rate constant, kobs, was evaluated from the linear least-
squares plots of log [PBC] versus time. Duplicate
kinetic runs showed the rate constants to be repro-
ducible to within 3%. The second-order rate constant,
k2, was calculated from the relation: k2 = kobs/[oxime].
Table I Rate Constants for the Oxidation of Acetald-
oxime by PBC at 308 K
×103 [PBC]
[Oxime]
(mol dm−3
×104 kobs
(mol dm−3
)
)
(s−1
)
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
2.0
3.0
4.0
5.0
0.1
0.2
0.4
0.6
0.8
1.0
1.5
0.2
0.2
0.2
0.2
0.2
27.8
54.2
110
165
218
272
414
The range of [oxime] is from 0.10 to 1.00 mol dm−3
.
The usual concentration of PBC is ranged from 0.001
to 0.005 mol dm−3. We have used coefficient of de-
termination (R2 or r2), standard deviation (sd), and
Exner’s parameter [14], ꢀ , as the measures of good-
ness of fit. Most of the rate data were obtained at 308 K
instead of 298 K, as the reaction was much slower at
298 K.
55.5
53.2
55.1
53.0
54.3

366
SHARMA
Table II Rate Constant and Activation Parameters for the Oxidation of Oximes (R1R2C N OH) by PBC
×104 k2 (dm3 mol−1 s−1
)
ꢀH∗
(kJ mol−1
ꢀS∗
(J mol−1 K−1
ꢀG∗
(kJ mol−1
)
Substituent
288
298
308
318 K
)
)
H
H
H
H
H
H
H
Me
Me
Et
H
Me
Et
Pr
PrI
ClCH2
Ph
Me
Et
Et
Ph
3440
131
101
53.6
37.4
242
216
3920
196
152
86.0
64.2
310
333
10.1
8.00
6.34
33.1
4460
272
227
124
98.6
393
513
5720
414
350
198
161
531
801
33.3
27.6
23.4
63.0
10.0 1.2
26.2 0.8
28.9 0.6
30.1 0.8
34.1 0.6
17.2 0.7
30.6 0.6
43.0 0.7
44.8 0.7
46.9 0.9
21.6 0.6
−220
−190
−183
−184
−173
−217
−171
−158
−155
−149
−221
3
3
2
3
2
2
2
2
2
3
2
75.3 1.0
82.8 0.6
83.4 0.5
84.8 0.7
85.6 0.5
81.6 0.6
81.4 0.4
90.1 0.6
90.6 0.6
91.2 0.8
87.1 0.5
5.49
4.25
3.29
18.0
14.6
11.8
45.0
Me
24.3
reaction with chosen solvents. The kinetics were simi-
lar in all the solvents. The values of k2 are recorded in
Table III.
complete range of solvent parameters was not avail-
able) were correlated in terms of the linear solvation
energy relationship (2) of Kamlet et al. [17].
log k2 = A0 + pπ∗ + bβ + aα
(2)
DISCUSSION
In this equation, ꢁ∗ represents the solvent polarity,
ꢂ the hydrogen bond acceptor basicities, and ꢃ is the
hydrogen bond donor acidity. A0 is the intercept term.
It may be mentioned here that of the 18 solvents, 12
have a value of 0 for ꢃ. The results of correlation anal-
yses in terms of (2), a biparametric equation involving
ꢁ∗ and ꢂ, and separately with ꢁ∗ and ꢂ, are given in
Eqs. (3)–(6)
The linear correlation (r2 = 0.9912, slope = 0.487
0.024) between the values of the activation enthalpies
and entropies of the reaction indicated the operation
of a significant compensation effect in this reaction
[15]. The reaction also exhibited an excellent isoki-
netic effect, as determined by Exner’s method [16].
An Exner’s plot between log k2 at 288 and 318 K is
linear (r2 = 0.9988, slope = 0.7838 0.01). The value
of isokinetic temperature is 513 27 K. A linear iso-
kinetic relationship is a necessary condition for the va-
lidity of linear free-energy relationships. It also implies
that all reactions so correlated follow a similar mecha-
nism [16].
log k2 = −3.61 + 1.75( 0.23)π∗
+ 0.24( 0.19)β + 0.15( 0.18)α
(3)
R2 = 0.8344, sd = 0.21, n = 18, ψ = 0.45
log k2 = −3.57 + 1.70( 0.22)π∗ + 0.29( 0.18)β
The rate constants, k2, of the oxidation of acetal-
doxime in 18 solvents (CS2 was not considered, as the
(4)
Table III Effect of Solvent on the Oxidation of Acetaldoxime by PBC at 308 K
×104 k2
×104 k2
Solvent
(dm3 mol−1 s−1
)
Solvent
(dm3 mol−1 s−1
)
Chloroform
1,2-Dichloroethane
Dichloromethane
DMSO
Acetone
Dimethylformamide
Butanone
Nitrobenzene
Benzene
Cyclohexane
83.2
91.2
79.4
272
74.1
120
51.3
87.1
22.4
1.78
Toluene
Acetophenone
Tetrahydronfuran
t-BuOH
Dioxane
1,2-Dimethoxyethane
Acetic acid
14.8
95.5
13.5
28.2
43.7
30.2
47.9
24.0
7.24
Ethyl acetate
Carbon disulfide

KINETICS AND MECHANISM OF THE OXIDATIVE REGENERATION OF CARBONYL COMPOUNDS
367
R2 = 0.8267, sd = 0.21, n = 18, ψ = 0.44
log k2 = −3.52 + 1.77( 0.22)π∗
of solvent effect indicated the formation of an activated
complex, which is more polar than the reactants. The
rate is affected by the solvent polarity.
(5)
(6)
r2 = 0.7970, sd = 0.22, n = 18, ψ = 0.46
log k2 = −2.58 + 0.59( 0.38)β
Correlation Analysis of Reactivity
r2 = 0.1317, sd = 0.46, n = 18, ψ = 0.96
We could not find any report about the mechanism of
the reaction between a C N bond and a halochro-
mate derivative. However, the reaction of alkenes
with chromium(VI) has been well studied [19]. Since
olefinic bonds are not usually subject to a nucle-
ophilic attack, it has been suggested that in the alkene-
chromate reaction, an organometallic derivative is
formed initially [19]. 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 II showed that the rate
of oxidation of ketoximes is much less as compared to
that of the aldoximes. The reason for the slower reac-
tion of ketoximes may be largely 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 also
supported by the correlation analysis of the reactivity
of the aliphatic aldoximes. The rate of oxidation of the
aliphatic oximes did not yield significant correlation
separately with Tafts’s ꢄ∗ and ES values (see Eqs. (12)
and (13)).
Kamlet’s [17] triparametric equation explains ca.
83% of the effect of solvent on the oxidation. However
by Exner’s criterion [14], the correlation is not even
satisfactory (cf. 3). The major contribution is of solvent
polarity. Italoneaccountedforca. 80%ofthedata. Both
ꢂ and ꢃ play relatively minor roles.
The data (including that of CS2) on the solvent effect
were also analyzed in terms of Swain’s equation [18]
of cation- and anion-solvating concept of the solvents
(7).
log k2 = aA + bB + C
(7)
Here A represents the anion-solvating power of the sol-
vent and B the cation-solvating power. C is the inter-
cept term. (A + B) is postulated to represent the solvent
polarity. Theratesindifferentsolventswereanalyzedin
terms of (7), separately with A and B and with (A + B).
log k2 = 1.47( 0.04)A + 1.68( 0.03)B − 4.10
(8)
R2 = 0.9964, sd = 0.03, n = 19, ψ = 0.06
log k2 = 1.21 0.34σ∗ − 2.44
r2 = 0.6475, sd = 0.52, n = 9, ψ = 0.63,
Temp. = 308 K
(12)
log k2 = 1.23( 0.55)A − 2.75
(9)
(10)
(11)
r2 = 0.4484, sd = 0.45, n = 19, ψ = 0.76
log k2 = 1.57( 0.26)B − 3.90
r2 = 0.6804, sd = 0.29, n = 19, ψ = 0.58
log k2 = 1.61 0.04(A + B) − 3.89
r2 = 0.9916, sd = 0.05, n = 19, ψ = 0.09
log k2 = 0.99 0.06ES − 2.74
r2 = 0.9753, sd = 0.14, n = 9, ψ = 0.17,
Temp. = 308 K
(13)
The rates of oxidation of acetaldoxime in different
solvents showed an excellent correlation in Swain’s
equation (cf. (8)) with both the anion- and cation-
solvating powers playing almost equal role. However,
individually A and B are able to account for only 45%
and 68% of the data only. The solvent polarity, repre-
sented by (A + B), also exhibited an excellent corre-
lation. In view of the fact that solvent polarity is able
to account for ca. 99% of the data, an attempt was
made to correlate the rate with the relative permittivity
of the solvent. However, a plot of log k2 against the
inverse of the relative permittivity is not very signifi-
cant (r2 = 0.4810, sd = 0.37, ꢀ = 0.74). The analysis
The rates were, therefore, correlated with Pavelich–
Taft’s [20] dual substituent-parameter equation (DSP)
(14).
log k2 = ρ∗σ∗ + δES + log k0
(14)
The rates exhibited excellent correlations in terms of
thePavelich–Taftequation(TableIV);thereactioncon-
stants are positive. The values of reaction constants
show that steric factor plays a major role as compared
to polar effects.

368
SHARMA
Table IV Reaction Constants for the Oxidation of
Aliphatic Oximes by PBCa
The author thanks Prof. K. K. Banerji for critical
suggestions.
Temp. (K)
ꢅ∗
ꢆ
R2
sd
ꢀ
288
298
308
318
0.46 0.01 0.94 0.01 0.9998 0.014 0.02
0.39 0.01 0.89 0.01 0.9999 0.006 0.02
0.33 0.01 0.83 0.01 0.9998 0.009 0.03
0.27 0.01 0.79 0.01 0.9997 0.013 0.03
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The low positive polar reaction constant points to an
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of the bond between chromate-oxygen and the carbon
is somewhat ahead of the formation of N O bond.
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the reactants. This indicated a nucleophilic attack by
a chromate-oxygen on the carbon. The positive steric
reaction constant points to a steric hindrance by the
substituents. Therefore, a mechanism (Scheme 1) is
proposed for the reaction. The mechanism is also sup-
ported by the values of activation parameters. The low
values of enthalpy of activation indicate that the bond-
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