Kinetics and mechanism of the oxidation of aliphatic aldehydes by tetrabutylammonium tribromide

Article abstract of DOI:10.1002/kin.1033

The oxidation of six aliphatic aldehydes by tetrabutylammonium tribromide (TBATB) in aqueous acetic acid resulted in the formation of corresponding carboxylic acids. The reaction is first order with respect to TBATB and the aldehydes. The oxidation of deuteriated acetaldehyde (MeCDO) showed the presence of substantial kinetic isotope effect (k_H/k_D = 5.92 at 298 K). Addition of tetrabutylammonium chloride has no effect on the reaction rate. Tribromide ion has been proposed as the reactive oxidizing species. The rate constants correlate well with Taft's σ* values, the reaction constant being negative. A mechanism involving a hydride-ion transfer from the aldehyde hydrate to the oxidant in the rate-determining step has been suggested.

Full text of DOI:10.1002/kin.1033

Kinetics and Mechanism
of the Oxidation
of Aliphatic Aldehydes by
Tetrabutylammonium
Tribromide
MANJU BAGHMAR, PRADEEP K. SHARMA
Department of Chemistry, J.N.V. University, Jodhpur 342 005, India
Received 24 July 2000; accepted 8 February 2001
ABSTRACT: The oxidation of six aliphatic aldehydes by tetrabutylammonium tribromide
(TBATB) in aqueous acetic acid resulted in the formation of corresponding carboxylic acids.
The reaction is first order with respect to TBATB and the aldehydes. The oxidation of deuter-
iated acetaldehyde (MeCDO) showed the presence of substantial kinetic isotope effect
(
k /k ϭ 5.92 at 298 K). Addition of tetrabutylammonium chloride has no effect on the reaction
H D
rate. Tribromide ion has been proposed as the reactive oxidizing species. The rate constants
correlate well with Taft’s ␴* values, the reaction constant being negative. A mechanism in-
volving a hydride-ion transfer from the aldehyde hydrate to the oxidant in the rate-determining
step has been suggested. ᭧ 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 390–395, 2001
INTRODUCTION
oratory [9–11]. There seems to be no report on the
oxidation of aliphatic aldehydes by tetrabutylammo-
nium tribromide (TBATB). We are interested in the
kinetic and mechanistic studies of the newer oxidizing
agents and report here the kinetics of the oxidation of
six aliphatic aldehydes by TBATB in aqueous acetic
acid solution. Attempts have been made to correlate
rate and structure in this reaction. A suitable mecha-
nism has also been proposed.
Tetraalkylammonium polyhalides are widely used as
halogenating reagents in synthetic organic chemistry
[
(
1–3]. Recently, tetrabutylammonium tribromide
TBATB) has been used for the bromination of some
selected organic substrates [4]. There are, however,
only a few reports regarding their use as oxidizing and
brominating agents in synthetic chemistry [5–8].
These compounds are more suitable than molecular
halogens because of their solid nature, ease of han-
dling, stability, selectivity, and excellent product
yields. We have been interested in the kinetic and
mechanistic studies of the reactions of polyhalides,
and many reports have already emanated from our lab-
EXPERIMENTAL
Materials
All the aldehydes were commercial products and were
purified by the method described earlier [12]. TBATB
was prepared by the reported method [1] and its purity
Correspondence to: Pradeep K. Sharma
2001 John Wiley & Sons, Inc.
᭧

OXIDATION OF ALIPHATIC ALDEHYDES BY TETRABUTYLAMMONIUM TRIBROMIDE
391
checked by an iodometric method. Deuteriated acet-
RESULTS
aldehyde (MeCDO) was obtained from Sigma Chem-
icals. Acetic acid was refluxed with chromic oxide and
acetic anhydride for 6 h and then fractionated [13].
The rate and other experimental data were for all the
aldehydes. Since the results are similar, only repre-
sentative data are reproduced here.
The oxidation of aldehydes resulted in the forma-
tion of corresponding carboxylic acids. Product anal-
ysis indicated the following overall reaction as eq. (1).
Product Analysis
Since the expected product of the oxidation of ali-
phatic aldehydes is he carboxylic acid, this set of ex-
periments was carried in aqueous solutions. In a typ-
ical experiment, acetaldehyde (4.4 g, 0.1 mol) and
TBATB (4.82g, 0.01 mol) were made up to 100 ml
in water and kept in the dark for ca. 10 h to ensure
completion of the reaction. The reaction mixture was
ϩ
Ϫ
RCHO ϩ (C H )N Br ϩ H O !:
4
9
3
2
ϩ
Ϫ
RCOOH ϩ (C H )N Br ϩ 2HBr (1)
4
9
Rate Laws
neutralized with solid NaHCO . The solution was
The reactions were found to be first order with respect
to TBATB. The individual kinetic runs were strictly
first order to TBATB. Further, the pseudo-first-order
rate constant does not depend on the initial concentra-
tion of TBATB. The reaction rate increases linearly
with an increase in the concentration of aldehyde (Ta-
ble I).
3
evaporated to dryness under reduced pressure. The res-
idue was acidified and extracted with ether (3 ϫ 50
ml). The ether extract was dried with anhydrous mag-
nesium sulfate and treated with thionyl chloride (10
ml). The solvent was allowed to evaporate. Dry meth-
anol (7 ml) was added and the HCl formed was re-
moved in a current of dry air. The residue was dis-
solved in ether (200 ml), and the ester content was
determined colorimetrically as iron (III) hydroxymate
by the method of Hall and Schaefer [14]. The yield of
acetic acid was 88%, based on the consumption of
TBATB.
Induced Polymerization of Acrylonitrile
The oxidation of aldehydes by TBATB in an atmo-
sphere of nitrogen failed to induce polymerization of
acrylonitrile. Further, an addition of acrylonitrile had
no effect on the rate (Table I).
Kinetic Measurements
The reactions were carried out under pseudo-first-or-
der conditions by maintaining a large excess of the
aldehyde (ϫ15 or greater) over TBATB. The solvent
was 1:1 (v/v) acetic acid–water. To suppress the dis-
sociation of tribromide ion, the reactions were carried
out in the presence of an excess (0.2mol dm ) of
KBr. The reactions were followed at constant temper-
ature (Ϯ0.1 K). The reactions were followed by mon-
itoring the decrease in [TBATB] spectrophotometri-
cally for at least three half-lives at 394 nm. The
Effect of Tetrabutylammonium Chloride
Addition of tetrabutylammonium chloride (TBACl)
had no effect on the rates of oxidation (Table II).
Ϫ3
Table I Rate Constants for the Oxidation of
Acetaldehyde by TBATB at 298 K
[RCHO]/mol dm 3 10 [TBATB]/mol dm
Ϫ
3
Ϫ
3
10 kobs/s
3
Ϫ1
0
0
0
0
0.8
1.0
0.4
0
0
0
0
.1
.21.00
.4
1.00
3.10
1.00
1.00
1.00
1.00
2.00
4.00
6.00
8.00
3.41*
1.56
pseudo-first-order rate constants, k , were evaluated
obs
from linear plots (r Ͼ 0.995) of log[TBATB] against
time. Duplicate kinetic runs showed that the rate con-
stants were reproducible to within Ϯ3%. The average
error in the values of the rate constant is 3%. The ex-
6.14
9.25
12.4
15.4
6.40
6.74
6.61
6.18
.6
perimental second-order rate constant, k , was calcu-
ex
.4
.4
.4
.21.00
lated from the relation k ϭ k /[aldehyde].
ex
obs
Preliminary experiments showed that the reaction
is not sensitive to the changes in the ionic strength.
Hence, no attempt was made to keep the ionic strength
constant.
Ϫ
3
* Contained 0.005 mol dm acrylonitrile.

3
92
BAGHMAR AND SHARMA
Table II Effect of Tetrabutylammonium Chloride on the Rate of Oxidation of Acetaldehyde by TBATB
TBATB] ϭ 0.001 mol dm 3
Ϫ
[RCHO] ϭ 1.0 mol dm 3
Ϫ
Temp. ϭ 298 K
[
3
Ϫ3
1
1
0 [TBACl]/mol dm
0.20.4 0.6
15.4 15.6
0.8
15.0
1.0
15.9
3
Ϫ1
0 kobs/s
15.1
ϩ
Ϫ
Kinetic Isotope Effect
(C H ) NBr GH PhCH Me N ϩ Br
(2)
4
9 4
3
2
3
3
To ascertain importance of the cleavage of the alde-
hydic C9H bond in the rate-determining step, the
oxidation of deuteriated acetaldehyde was studied. Re-
sults showed (Table III) the presence of a primary ki-
netic isotope effect (k /k ϭ 5.92at 29 8 K).
Tribromide ion is known to dissociate to bromine and
bromide ion and the value of the dissociation constant
has been reported [15]. However, in the presence of
the large excess of bromide ion, the reaction (3) will
be suppressed. Thus, in the present reaction the reac-
tive oxidizing species is the tribromide ion.
H
D
Effect of Solvent Composition
Ϫ
Ϫ
Br3 GH Br ϩ Br
(3)
2
The rate of oxidation of aldehydes was determined in
solvents containing different proportions of acetic
acid and water. The results showed that the rate in-
creases with an amount of acetic acid in the solution
The decrease in the value of k with an increase in
ex
the amount of the acetic acid indicates that an increase
in the solvent polarity facilitates the reaction. This can
be explained if one assumes that in the rate-deter-
mining step, the transition state is more polar than
(Table IV).
the reactant. The correlation of log k against the in-
ex
DISCUSSION
verse of the dielectric constant is not significant
2
(
r ϭ 0.9428). The effect of solvent composition
We have carried out some conductivity measurements
to determine the nature of TBATB in aqueous acetic
acid solution. It was observed that acetic acid has very
low conductivity. Addition of TBATB increases the
conductivity of acetic acid. We measured the conduc-
tivity of TBATB in solvents containing different pro-
portions of acetic acid (100–30%) and water also. We
found that the conductivity increases sharply as the
water content is initially increased but reaches a lim-
iting value in about 60% acetic acid–water mixture.
Therefore, TBATB can be considered as an ionic com-
pound that exists under our reaction conditions as tet-
rabutylammonium and tribromide ions as in eq. (2).
No effect of added tetrabutylammonium ion also in-
dicates that the equilibrium (2) lies far towards the
right.
was analyzed using the Grunwald–Winstein equa-
tion [16].
log k ϭ log k ϩ mY
(4)
2
0
A plot of log kex against Y was linear (r ϭ 0.9996)
with m ϭ 0.78 Ϯ 0.01. The value of m is quite close
to that for S 1 reaction.
N
Aliphatic aldehydes are known to be hydrated to
varying extent in aqueous solution, and the equilib-
rium constant of the following reaction has been re-
ported [17]:
K
d
RCH(OH) G
R
H RCHO ϩ H O
(5)
2
2
Table III Kinetic Isotope Effect in the Oxidation of Acetaldehyde by TBATB
TBATB] ϭ 0.001 mol dm 3
Ϫ
[MeCHO] ϭ 1.00 mol dm 3
Ϫ
[
03 kex dm mol
3
Ϫ
1
s
Ϫ1
1
Aldehyde
298 K
308 K
318 K
328 K
MeCHO
MeCDO
k /k
15.4
2.60
5.925.74
29.7
5.17
60.5
10.7
112
20.3
5.65
5.52
H
D

OXIDATION OF ALIPHATIC ALDEHYDES BY TETRABUTYLAMMONIUM TRIBROMIDE
393
Table IV Effect of Solvent Composition in the Oxidation of Acetaldehyde by TBATB
TBATB] ϭ 0.001 mol dm 3
Ϫ
[MeCHO] ϭ 1.0 mol dm 3
Ϫ
Temp. ϭ 298 K
[
%
AcOH
25
77.6
40
30.0
50
15.4
60
7.69
72
2.75
3
3
Ϫ
1
Ϫ1
1
0 k /dm mol
s
ex
Table V Rate Constants for the Oxidation of Hydrate and Free Aldehyde Forms at 298 K
3
3
Ϫ
1
Ϫ1
1
0 (Rate Constant)/dm mol
s
Aldehyde
Kd
kex
kA
kHy
Ϫ
HCHO
MeCHO
EtCHO
PrCHO
5.5 ϫ 10 4
0.67
1.40
2.10
3.41
15.4
16.0
13.6
17.3
6200
38.5
27.6
20.0
24.7
9440
3.41
25.7
38.2
42.4
57.6
0.34
i
Pr CHO
2.30
Ϫ
5
CCl CHO
3.6 ϫ 10
0.34
3
Table VI Rate Constant and Activation Parameters of the Oxidation of Aldehyde Hydrates, RCH(OH) , by TBATB
2
3
3
Ϫ
1
Ϫ1
1
0 k /dm mol
s
⌬H*
⌬S*
⌬G*
Hy
kJ mol 1
Ϫ
J mol 1 K 1
Ϫ
Ϫ
kJ mol 1
Ϫ
R
298 K
308 K
318 K
328 K
H
3.41
25.7
.3 38.282180
42.4
8.53
56.0
21.2
126
46.6
261
66.3 Ϯ 0.5
58.5 Ϯ 0.6
Ϯ 0.6
56.4 Ϯ 0.9
54.8 Ϯ 0.9
77.4 Ϯ 0.4
Ϫ66 Ϯ 285.9
Ϫ76 Ϯ 281.1
Ϫ77 Ϯ 280.1
Ϫ80 Ϯ 3
Ϫ82 Ϯ 3
Ϫ48 Ϯ 1
Ϯ 0.4
Ϯ 0.6
Ϯ 0.7
Me
Et
Pr
Pr
CCl3
371
57.3
87.2
118
0.98
193
248
2.76
395
511
7.12
79.9 Ϯ 0.4
79.2 Ϯ 0.6
91.5 Ϯ 1.0
i
57.6
0.34
Table VII Temperature Dependence of the Reaction Constants
Temp. (K)
298
308
318
328
␳
r2
*
Ϫ1.79 Ϯ 0.01
0.9998
Ϫ1.67 Ϯ 0.01
0.9999
Ϫ1.58 Ϯ 0.01
0.9989
Ϫ1.48 Ϯ 0.01
0.9998
sd
0.008
0.003
0.004
0.009

3
94
BAGHMAR AND SHARMA
From the values of kex and K , two sets of data were
21. The correlation was tested and found genuine by
d
computed. The values of kHy were obtained by assum-
ing that only the hydrated form participates in the ox-
idation process and the rate law has the form (6). The
data are recorded in Table V.
Exner’s criterion [21]. A plot between log k at 298
and 328 K was linear (r ϭ 0.9996; slope 0.8363 Ϯ
2
2
0.01). The value of isokinetic temperature evaluated
from this plot is 676 Ϯ 24 K, which is in excellent
agreement with the value obtained earlier. The linear
isokinetic relationship is a necessary condition for the
validity of linear free-energy relationships. It also im-
plies that all the compounds so correlated react by the
same mechanism [21].
Rate ϭ k [TBATB][RCH(OH) ]
(6)
Hy
2
Similarly, the values of k were calculated assuming
A
participation of only the free aldehyde form, according
to the rate law (7).
Rate ϭ k [TBATB][RCHO]
(7)
Mechanism
A
The presence of a substantial kinetic isotope effect
confirms the cleavage of the C9H bond in the rate-
determining step. A hydrogen abstraction mechanism
leading to the formation of free radicals may be dis-
counted in view of the failure to induce polymerization
of acrylonitrile. The negative polar reaction constant
points to an electron-deficient carbon center in the
transition state. The large negative reaction constant
and a substantial deuterium kinetic isotope effect sug-
gest a considerable carbocation character in the tran-
sition state. Therefore, the following mechanism
The rates of oxidation of the aldehyde hydrate, kHy,
correlates very well with Taft’s [19] eq. (8).
log k ϭ ␳* ␴* ϩ log k0
(8)
Here, ␴* is the substituent constant and ␳* is the re-
action constant; log k and k refer to the rate constants
0
of the substituted and unsubstituted compound, re-
spectively. The reaction constant ␳* has a negative
value.
On the other hand, no such correlation exists with
the rates of oxidation of free aldehyde form. In partic-
ular, formaldehyde and trichloroacetaldehyde(chloral)
react much faster as compared to other aldehydes. No
satisfactory correlation was obtained even after ne-
glecting the data of formaldehyde and chloral.
If one assumes that the aldehyde reacts via the hy-
drate form as eq. (6), the rate of oxidation of formal-
dehyde and chloral compares favorably with the reac-
tivities of the other aldehydes. If the oxidation of the
aldehyde form is assumed, then formaldehyde and
chloral are ca. 250 and 350 times more reactive than
the other aldehydes. This makes a direct hydrogen
transfer from the free aldehyde to the oxidant highly
unlikely. The existence of a good structure–reactivity
correlation in the oxidation of aldehyde hydrate further
supports the involvement of aldehyde hydrates in the
oxidation process.
(Scheme I) involving transfer of a hydride ion from
the aldehyde hydrate to the oxidant, in the rate-deter-
mining step, is suggested:
slow
+
Ϫ
Ϫ
RCH(OH) ϩ Br
;9: RC(OH) ϩ HBr ϩ 2Br
2
3 2
+
slow
ϩ
RC(OH) ;
9
: RCOOH ϩ H
2
Scheme I
The observed negative entropy of activation also
supports it. As the charge separation takes place, the
charged ends become highly solvated. This results in
an immobilizationof a large number of solvent mole-
cules, reflected in the loss of entropy [22].
The rate constants, k , were determined at different
ex
temperatures, and the values of kHy were calculated.
Thanks are due to the Council of Scientific and Industrial
Research (India) for financial assistance and to Professor K.
K. Banerji for his valuable help and suggestions.
The equilibrium constants, K , at different tempera-
d
tures were calculated from the data compiled by Bell
[18]. The activation parameters were also calculated
(
Table VI). The rate constants, k , at different tem-
hy
peratures were correlated in terms of Taft’s [18] eq.
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OXIDATION OF ALIPHATIC ALDEHYDES BY TETRABUTYLAMMONIUM TRIBROMIDE
395
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