Deoximation by cetyltrimethylammonium dichromate: A kinetic study

Article abstract of DOI:10.1002/kin.20576

The deoximation kinetics of some oximes was studied by using cetyltrimethylammonium dichromate (CTADC) in dichloromethane in the presence of acetic acid and a cationic surfactant. The rate of reaction is highly sensitive to the change in [CTADC], [oxime], [acid], [surfactant], polarity of the solvents, and reaction temperature. The reaction is found to be catalyzed by acid with an appreciable uncatalytic rate. The reaction is first order with respect to substrate. With increase in CTADC concentration, rate of the reaction increases with a fractional order dependency with respect to oxidant. Consistent to the observation, a mechanism has been proposed in which the substrate forms a complex with CTADC in the rate determining step followed by decomposition with a fast process to yield corresponding carbonyl compounds. The structure of the substituents has also a significant effect on the rate constant.

Full text of DOI:10.1002/kin.20576

Deoximation by
Cetyltrimethylammonium
Dichromate: A Kinetic Study
SANDHYAMAYEE SAHU,1 SABITA PATEL,2 BIJAY K MISHRA1
1Centre of Studies in Surface Science and Technology, School of Chemistry, Sambalpur University, Jyoti Vihar 768 019, India
2Department of Chemistry, NIT Rourkela, Rourkela 769 008, India
Received 9 November 2010; revised 26 March 2011, 20 April 2011; accepted 20 April 2011
DOI 10.1002/kin.20576
Published online 21 July 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: The deoximation kinetics of some oximes was studied by using cetyltrimethylam-
monium dichromate (CTADC) in dichloromethane in the presence of acetic acid and a cationic
surfactant. The rate of reaction is highly sensitive to the change in [CTADC], [oxime], [acid],
[surfactant], polarity of the solvents, and reaction temperature. The reaction is found to be
catalyzed by acid with an appreciable uncatalytic rate. The reaction is first order with respect to
substrate. With increase in CTADC concentration, rate of the reaction increases with a fractional
order dependency with respect to oxidant. Consistent to the observation, a mechanism has
been proposed in which the substrate forms a complex with CTADC in the rate determining step
followed by decomposition with a fast process to yield corresponding carbonyl compounds.
ꢀ
C
The structure of the substituents has also a significant effect on the rate constant.
Wiley Periodicals, Inc. Int J Chem Kinet 43: 482–488, 2011
2011
The insolubility of CTADC in water reduces con-
tamination of Cr (VI) in aqueous medium and thus can
be used as a green reagent. Furthermore, the reagent
is devoid of an acidic proton and thus, is relatively
milder than other Cr (VI) oxidants. In the absence of
acid, it exhibits some bizarre reactions with noncon-
ventional products. Aromatic amines are found to yield
corresponding diazo compounds [2] and arylaldoximes
yielded corresponding nitriles [3]. In an oxidation reac-
tion of cholesterol with CTADC, 7-dehydrocholesterol
was obtained instead of cholestenone [4]. This de-
hydrogenation was a rare event in Cr (VI) oxidation
and was explained through a remote functionalization
mechanism, where CTA+ provides a favorable envi-
ronment for proper orientation of the oxochromium
group making the removal of hydrogen becomes eas-
ier. Furthermore, the protonated dichromate oxidizes
the secondary hydroxy group of cholesterol to the cor-
responding ketone with the addition of acid.
INTRODUCTION
Our search for novel phase transferring oxidant has led
to the generation of cetyltrimethylammonium dichro-
mate (CTADC) as an emerging lipopathic agent. Un-
like other quaternary ammonium ions (tetrabutyl or
octylammonium ions), CTA+ has a relatively small
head group with more exposed charge and a well-
balanced hydrophobic group to carry the ion to
both water and organic medium. With bromide as
the counter ion, CTA+ can form micelle in aque-
ous medium and reversed micelle in chloroform.
Onium with its counter ion forms a tight ion pair
in organic solvents, whereas in aqueous medium it
dissociates [1].
Correspondence to: Bijay Mishra; e-mail: bijaym@hotmail
.com.
c
ꢀ
2011 Wiley Periodicals, Inc.

DEOXIMATION BY CETYLTRIMETHYLAMMONIUM DICHROMATE: A KINETIC STUDY
483
Table I Rate Constant of Oxidation of Cyclohexanone
Oximes by CTADC at 298 K in Dichloromethane
[CTADC] ×
[cyclohexanone
oxime] M
[Acetic
acid] M
kobs × 104
104M
in s−1
Scheme 1
0.47
0.94
1.89
2.83
3.78
4.7
1.89
1.89
1.89
1.89
1.89
1.89
1.89
1.89
0.05
0.05
0.05
0.05
0.05
0.05
0.005
0.01
0.02
0.1
3.24
3.24
3.24
3.24
3.24
3.24
3.24
3.24
3.24
3.24
3.24
0.81
1.62
6.48
21.30
15.08
10.17
8.41
6.87
6.18
1.11
1.96
3.61
19.69
42.35
4.41
6.29
16.35
In the present work, we report the kinetics of the re-
action of CTADC with the oxime derivatives of some
arylaldehydes and ketones in presence of acid in non-
aqueous medium. Since oximes have been considered
as the functional group equivalents of ketone or alde-
hyde in organic synthesis [5], the conversion of oximes
into their parent carbonyl compounds has received con-
siderable attention [6]. Based on the hydrolytic [7], re-
ductive [8], and oxidative [9–11] reactions a number of
methods have been developed for deoximation. Deox-
imation can be undertaken both in homogeneous and
heterogeneous conditions [12]. Oximes on treatment
with CTADC in presence of trace amount of acetic acid
in dichloromethane gave the corresponding aldehydes
whereas in absence of corresponding nitriles were ob-
tained (Scheme 1) [3].
0.2
0.05
0.05
0.05
Product Analysis
A solution of CTADC (300 mg, 0.38 mmol), p-
methoxybenzaldoxime (200 mg, 1.3 mmol), and acetic
acid (0.5 mL) in CH2Cl2 (25 mL) was refluxed for
2 h and evaporated. The residue was then chro-
matographed on silica gel by using chloroform as an
eluent to give p-methoxybenzaldehyde (yield, 90%)
b.p. 249◦C, m.p. of 2, 4-dinitrophenylhydrazone,
251◦C [18].
EXPERIMENTAL
Materials
Cetyltrimethylammonium dichromate (CTADC) was
prepared by the method reported earlier [13], and its
purity was checked by estimating Cr(VI) iodometri-
cally [14]. The organic solvents were purified by the
standard methods [15]. The melting point and boiling
point of the compounds were obtained by open cap-
illary method. For the determination of boiling points
of liquids Siwoloboff’s method was used [16]. Oximes
were prepared by the reaction of hydroxylamine hy-
drochloride with corresponding aldehyde and ketone
[17]. These are characterized from the melting point
and IR spectral data, recorded on a Perkin Elmer RX-1
FTIR spectrometer in KBr disk for solids and in nujol
for liquids [14].
Stoichiometry
The stoichiometry of the reaction was determined
by performing the experiment at 303 K, under the
condition of [CTADC] ≈ [cyclohexanone oxime] at
Kinetic Measurements
The oxidation kinetics of oximes by CTADC in the
presence of acetic acid was investigated by varying
[CTADC], [oximes], and [acid] and the kobs are pre-
sented in Table I. Oxidation of oximes was carried out
by monitoring the decrease in the optical density of
CTADC at a wavelength of 354 nm (Fig. 1). All the
experiments were repeated at least three times and the
rates of reactions were obtained with an error of 7%.
Figure 1 Repetitive scan of the spectra of CTADC with
cyclohexanone oxime in dioxane/acetic acid medium per
minute.
International Journal of Chemical Kinetics DOI 10.1002/kin

484
SAHU, PATEL, AND MISHRA
various oxime concentrations. The disappearance of
Cr(VI) was followed, until the absorbance values be-
come constant. The [CTADC] was estimated after 48 h.
A stoichiometry ratio, ꢀ [CTADC]/ꢀ [cyclohexanone
oxime] ≈0.33 was observed, which confirmed a 1:3
CTADC/cyclohexanone oxime relationship.
RESULT AND DISCUSSION
Oximes are prepared by the condensation of carbonyl
compounds with hydroxylamine and are mostly used
as the protecting group of the corresponding carbonyl
group. The deprotection can be achieved by hydrolytic
cleavage of the imine group in the presence of acid and
base. However, the exchange of nitrogen by oxygen
function in the imine bond may be considered as an
oxidation process which may not need a strong oxi-
dizing agent such as Cr(VI). In the present study, the
regeneration of the carbonyl group from the oxime by
CTADC in presence of acetic acid delineates the mild-
ness of the oxidant, which may be due to the CTA+
present as the counter ion of dichromate. The mildness
of the oxidant was established from the earlier reports
on the oxidation of oximes by CTADC yielding nitrile
derivatives through dehydration [3].
The reaction mixture consisting of CTADC, oxime,
and acetic acid in dichloromethane in almost all
cases turns to green after completion of the reaction.
During the oxidation process, Cr(VI) is reduced to
Cr(II), which in a very fast process disproportionate
with another Cr(VI) to Cr(III) and Cr(V). Cr(V) is
further reduced to Cr(III) on addition of two electrons.
The appearance of Cr(II) as reactive intermediates
in the reduction of Cr(VI) has been well established
[19]. The existence of Cr(III) in the product mixture
was also confirmed from the absorption maximum at
580 nm.
Figure 2 Plot of 104 kobs versus [Acid] in the oxidation of
cyclohexanone oxime at 298 K in dichloromethane.
solution of 1 M being 2.6. Thus acetic acid may be ac-
tivating the Cr(VI) by converting CTADC to HCr2O−7 ,
which may be the active oxidant with lipopathic char-
acteristics due to CTA+ counter ion.
With increasing oxime concentration the rate con-
stant increases linearly and the reaction is found to be
first order with respect to the substrate (Fig. 3). How-
ever, with increasing oxidant concentration the rate
constant experiences an asymptotic decrease (Fig. 4).
To attain pseudounimolecular condition, the sub-
strate concentration was taken 100 times more than the
oxidant concentration. The rate constant of the reaction
was determined by monitoring the change in concen-
tration of the oxidant. When the rate is calculated as
the product of rate constant and the oxidant concen-
tration, it is found that rate of reaction increases with
increase in concentration of the oxidant. In accordance
with the above results, a mechanism may be proposed
(Scheme 2) wherein CTADC initially equilibrates with
acetic acid to form HCr2O−7 which interacts with the
substrate to form a cyclic intermediate in the rate deter-
mining step. In a fast process, the intermediate breaks
The increase in amount of acetic acid increases the
rate linearly with an intercept indicating the catalytic
behavior of the acid (Fig. 2).
The uncatalytic rate constant is obtained from the
equation
kobs = a[H+] + b
(1)
where b is the uncatalytic rate constant. For a typical
example of cyclohexanone-oxime, the uncatalytic rate
constant is obtained to be 2.93 × 10−4 s−1 with R2 =
0.99. Bhandari et al. have also studied the oxidation ki-
netics of some oximes by pyridinium chlorochromate
(PCC) in DMSO without the addition of acid [20]. The
oxidation to carbonyl compound could be achieved
because PCC contains acidic hydrogen, the pH of the
Figure 3 Plot of 104 kobs versus [oxime] in the oxidation
of cyclohexanone oxime at 298 K in dichloromethane.
International Journal of Chemical Kinetics DOI 10.1002/kin

DEOXIMATION BY CETYLTRIMETHYLAMMONIUM DICHROMATE: A KINETIC STUDY
485
Table II Values of kobs of Oxidation of Cyclohexanone
Oxime by CTADC in Different Solvents at 298 K. [CTADC]
= 1.89 × 10−4M, [oxime] = 0.05M, [acetic acid] = 3.24M
Sl. No
Solvents
kobs × 104 in s−1
1
2
3
4
5
6
7
8
Toluene
Benzene
Chloroform
Dichloromethane
Benzonitrile
Dioxan
Dimethyl sulfoxide (DMSO)
Butanone
Tetrahydro furane(THF)
Ethyl acetate
Dimethyl formamide(DMF)
Acetonitrile
Dimethyl acetamide(DMA)
36.7
33.39
46.56
10.17
20.35
25.73
88
14.28
6.09
3.84
9
Figure 4 Plot of 104 kobs versus [CTADC] in the oxidation
of cyclohexanone oxime at 298 K in dichloromethane.
10
11
12
13
11.52
5.28
9.02
to afford the products. Accordingly, the rate of the re-
action can be expressed as per Eq. (2).
Rate = k[CTADC]0.5 [oxime]1 [H+]0.6
(2)
To investigate on the polarity of the transition state,
the oxidation reaction of a typical substrate (cyclo-
hexanone oxime) was conducted in different organic
solvents with varying polarity. Due to solubilization
constraint 13 solvents were used for the purpose
(Table II).
The kobs values were plotted against various solvent
parameters such as dielectric constant, π∗, ET (30),
dipole moment, anion solvating power (A), cation sol-
vating power (B) [21,22], and LogP (P being the parti-
tion coefficient of the probe between water and octanol
and hence referes to lipopathicity of the probe) [23].
The plots of kobs with dielectric constant and π∗ exhib-
ited scattered points indicating no definite relationship
exists between the transition state and the polarity of
the medium (Fig. 5). The plot of kobs versus LogP val-
ues led to the classification of the solvent into three
different classes (Fig. 6). The solvents such as toluene,
Figure 5 Plot of kobs versus π∗ (polarity parameter) in the
oxidation reaction of cyclohexanone oxime with CTADC at
298 K (number refers to the solvent of Table III).
Figure 6 Plot of kobs versus LogP polarity parameter in the
oxidation reaction of cyclohexanone oxime with CTADC at
298 K (number refers to the solvent of Table III).
Scheme 2
International Journal of Chemical Kinetics DOI 10.1002/kin

486
SAHU, PATEL, AND MISHRA
Table III Rate Constant of Oxidation of Oximes Derived from Various Carbonyl Compounds at Three Different
Temperatures in Dichloromethane. [CTADC] = 1.89 × 10−4M, [oxime] = 0.05M, [acetic acid] = 3.24M
kobs × 104 in s−1
Carbonyl Compound for
the Oxime Derivative
293 K
298 K
303 K
ꢀH= kJ mol−1
ꢀS= J mol−1 K−1
ꢀG= kJ mol−1
o-Chlorobenzaldehyde
p-Chlorobenzaldehyde
o-Methoxybenzaldehyde
p-Methoxybenzaldehyde
Benzaldehyde
Acetophenone
Benzoin
Cyclohexanone
Benzophenone
2.15
2.3
8.9
3.92
7.9
2.84
26.52
8.83
0.92
3.84
3.34
12.59
5.64
11.57
4.84
31.36
10.17
1.11
5.75
3.88
22.26
7.06
15.22
7.76
36.73
15.85
1.15
70.2
19.5
65.1
41.0
46.0
71.7
21.6
82.8
14.1
−75
−246
−82
92.5
92.8
89.5
91.5
89.7
91.9
87.3
90.1
95.5
−169
−147
−68
−220
−24
−273
benzene, chloroform, dichloromethane, and benzoni-
trile have high LogP values (1.44 to 2.73) and the kobs
values are found to increase linearly with increase in
LogP. As the LogP value relates to the lipopathicity of
the solvent, the increasing trend with LogP indicates
the transition state to be lipopathic, which is achieved
due to the presence of CTA+ group in the transition
state.
To find the other factors responsible for the change
in the reactivity due to solvation, all the parameters are
subjected to multiple regression analysis. Parameters
such as π∗, A, and B are found to show significant
correlation (R2 = 0.99) in a triparametric model for
benzene, toluene, chloroform, dichloromethane, and
benzonitrile (Eq. (3)). While π∗ and A have nega-
tive contribution toward the kobs value, with increas-
ing B value, the kobs increases. With solvents such as
butanone, tetrahydrofuran, ethyl acetate, DMSO, and
dioxan having negative LogP values, the kobs decreases
sharply with increasing LogP values. When the rate
constants in these solvents are regressed with, π∗, A,
and B as the variables, R2 is found to be 0.97 (Eq.
(4)). Solvents such as Acetonitrile, DMF, and DMA
decreases the kobs significantly. The polarity values of
these solvents, judged from the dielectric constant, are
high and thus the negative coefficient of π∗ also led to
the proposition of a nonpolar transition state.
analyzed. Among the substituted para-substituted al-
doxime including benzaldoxime, the trend in the kobs
is H > OMe > Cl (Table III).
This trend clearly supports the earlier proposition
that the reaction center is not controlled by the elec-
tronic factor. However, the ortho substitution favors the
oxidation process. Both the substituents in the present
study have lone pair of electrons, which may be respon-
sible for the close proximity of the reactant to the re-
action center through their interaction with the CTA+.
Steric factor also controls the reaction significantly.
When H attached to trigonal C in the benzaldoxime
was replaced by methyl group and phenyl group in ace-
tophenone and benzophenone oxime respectively, the
trend in the reactivity was found to be H > Me > Ph.
As electronic factor does not contribute significantly to
the reactivity, the drastic change in the reactivity may
be attributed to the steric factor. It gets support from
the high reactivity of cyclohexanone oxime, where the
steric contribution is less due to the two rigid methy-
lene groups adjacent to the trigonal C reaction center.
In the former cases, phenyl group attached to the C has
more flexibility to reduce the reactivity cone on C of
oxime.
Benzoin oxime has two reaction centers for oxida-
tion, a secondary hydroxyl group and an oxime. The
product analysis of the oxidation of benzoin oxime
revealed a selective oxidation by CTADC affording
benzoin only. As in the experimental condition the
concentration of CTADC is 100 times less than that
of the substrate, further oxidation of alcohol can be
ruled out. The exorbitant kobs for benzoin oxime can
be explained through neighboring group participation
by the hydroxyl group for the formation of cyclic in-
termediate (Scheme 3).
kobs = −360.73π∗ − 208.43A + 455.09B
+ 11.04, R2 = 0.99
(3)
(4)
kobs = −408.6π∗ + 460.16A + 428.05B
− 124.56, R2 = 0.97
With a change in the structure, the change in reactiv-
ity was found to be more prominent at high tempera-
ture and hence the kobs values at this temperature are
In the presence of a cationic surfactant such as
cetyltrimethylammonium bromide (CTAB), the kobs
was found to decrease instantaneously and then remain
International Journal of Chemical Kinetics DOI 10.1002/kin

DEOXIMATION BY CETYLTRIMETHYLAMMONIUM DICHROMATE: A KINETIC STUDY
487
Scheme 3
constant (Fig. 7). Earlier, a similar type of observation
was reported during the oxidation of alcohol, and the
result was attributed to the reversed micellization of
CTAB in organic solvents [24,25]. CTAB is known
to form reversed micelle with an inner hydrophilic
core. Due to compatibility of the hydrophobic chain
CTADC forms a mixed micelle with CTAB, where the
oxidant dichromate will be buried inside the core of
the reversed micelle thereby partitioning away from
the substrate to a large extent leading to decrease in
kobs. The transition point in the bilinearity of the plot
may be referred to the critical reversed micelle concen-
tration, above which the surfactant aggregates to form
reversed micelles. For cyclohexanone, acetophenone
and benzoin oximes, the critical reversed micellar con-
centrations are found to be around 1 × 10−3M. The
same values for the three different oximes indicate that
the oxime could not induce any change in reversed
micellization of the mixed surfactant systems, that is,
CTAB and CTADC. In the mixed micellar system, the
percent compositions of CTADC and CTAB are 1:5.
The thermodynamic parameters such as ꢀH=, ꢀS=,
and ꢀG= for the oxidation reactions with all the sub-
strates were determined and presented in Table III. A
high negative change in entropy of activation is an indi-
cation of a cyclic transition state. The enthalpy versus
entropy plot is found to be linear with R2 = 0.994. This
indicates an excellent linear compensation effect of en-
thalpy and entropy [26], which may be attributed to the
significant role played by the cyclic transition state.
The isokinetic temperature was found to be 295 K,
indicating similar mechanism for all the reactions at
that temperature [27]. Furthermore, the values of free
energies for activation of the reaction are found to be
more or less similar indicating an identical reaction
mechanism being followed in these reactions.
Thus, CTADC is found to be mild oxidizing
agent for regeneration of carbonyl group from its
oxime derivative through a mechanism as proposed
in Scheme 2. Furthermore, in these experimental con-
ditions, CTADC was found to be chemoselective, pre-
ferring oxidation of oxime over hydroxyl group.
BKM thanks the University Grants Commission, New Delhi,
for financial assistance through a research project. SP thanks
the Department of Science and Technology, New Delhi, for
a Fast Track project, and SS thanks the Council of Scientific
and Industrial Research, New Delhi, for a Senior Research
Fellowship.
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