Kinetics and mechanism of oxidation of aliphatic secondary alcohols by benzyltrimethylammonium chlorobromate

Article abstract of DOI:10.14233/ajchem.2014.16081

Oxidation of several secondary alcohols by benzyltrimethylammonium chlorobromate (BTMACB) in aqueous acetic acid leads to the formation of corresponding ketones. The reaction is first order with respect to BTMACB and the alcohols. The reaction failed to induce the polymerization of acrylonitrile. There is no effect of tetrabutylammonium chloride on the reaction rate. The proposed reactive oxidizing species is chlorobromate ion. The oxidation of benzhydrol-α-d (PhCDOHPh) exhibited a substantial primary kinetic isotope effect (k_H/k_D = 5.61 at 298 K). The effect of solvent composition indicated that the rate increases with an increase in the polarity of the solvent. The reaction is susceptible to both the polar and steric effects of the substituents. A mechanism involving transfer of a hydride ion in the ratedetermining step has been proposed.

Full text of DOI:10.14233/ajchem.2014.16081

Asian Journal of Chemistry; Vol. 26, No. 9 (2014), 2702-2706
ASIAN JOURNAL OF CHEMISTRY
http://dx.doi.org/10.14233/ajchem.2014.16081
Kinetics and Mechanism of Oxidation of Aliphatic Secondary
Alcohols by Benzyltrimethylammonium Chlorobromate
PRADEEP K. SHARMA
Department of Chemistry, J.N.V. University, Jodhpur-342 005, India
Corresponding author: E-mail: drpkvs27@yahoo.com
Received: 22 July 2013; Accepted: 27 January 2014;
Published online: 28 April 2014;
AJC-15101
Oxidation of several secondary alcohols by benzyltrimethylammonium chlorobromate (BTMACB) in aqueous acetic acid leads to the
formation of corresponding ketones. The reaction is first order with respect to BTMACB and the alcohols. The reaction failed to induce
the polymerization of acrylonitrile. There is no effect of tetrabutylammonium chloride on the reaction rate. The proposed reactive oxidizing
species is chlorobromate ion. The oxidation of benzhydrol-α-d (PhCDOHPh) exhibited a substantial primary kinetic isotope effect (kH/kD
= 5.61 at 298 K). The effect of solvent composition indicated that the rate increases with an increase in the polarity of the solvent. The
reaction is susceptible to both the polar and steric effects of the substituents. A mechanism involving transfer of a hydride ion in the rate-
determining step has been proposed.
Keywords: Alcohols, Correlation analysis, Chlorobromate, Kinetics, Mechanism, Oxidation.
Benzhydrol was recrystallized from ethanol. Benzyltrimethyl-
ammonium chlorobromate was prepared by reported method1
and its purity was checked by iodometric method. Benzhydrol-
α-d (PhCDOHPh) was also prepared by reported method12.
The isotopic purity of PhCDOPh, as ascertained by its NMR
spectra, was 93 5 %. Acetic acid was refluxed with chromic
oxide and acetic anhydride for 6 h and then it was fractionally
distilled. All other reagents were commercial products and
were purified by the usual methods13.
INTRODUCTION
Benzyltrimethylammonium chlorobromate [BTMACB or
(PhCH2NMe3)+(Br2Cl)–] is one of the quaternary ammonium
polyhalides which has been used as an effective halogenating
and oxidizing agent in synthetic organic chemistry1-3. Polymeric
benzyltrimethylammonium dichloroiodate and dibromoiodate
have been used for the addition of halogens to olefins4. The
polyhalides are more suitable than molecular halogens because
of their solid nature, ease of handling, stability, selectivity and
excellent product yield. We have been interested in the kinetic
and mechanistic aspects of oxidation by polyhalogen com-
pounds. A few reports (including BTMACB) regarding the
kinetic and mechanistic studies of the reactions of polyhalides
are available in the literature5-8. There seems to be no report
on the oxidation of aliphatic secondary alcohols by BTMACB.
Therefore, in continuation of our earlier work on the oxidation
studies by polyhalides9-11, We report here the kinetics of the
oxidation of eight aliphatic secondary alcohols, 1-phenyl-
ethanol and benzhydrol by BTMACB in aqueous acetic
acid solution. Attempts have been made to correlate rate and
structure in this reaction. A suitable mechanism has also been
proposed.
Product analysis: The product analysis was carried out
under kinetic conditions i.e. with an excess of alcohol over
BTMACB. In a typical experiment, 2-propanol (0.05 mol),
KBr (0.2 mol) and BTMACB (0.01 mol) were made up to 100
mL in 1:1 (v/v) aqueous acetic acid and kept in the dark for
ca. 10 h to ensure completion of the reaction. The solution
was then treated with an excess (200 mL) of a saturated solution
of 2,4-dinitrophenylhydrazine in 2 mol dm-3 HCl and kept
overnight in a refrigerator.
The precipitated 2,4-dinitrophenyl-hydrazone (DNP) was
filtered off, dried, weighed, recrystallized from ethanol and
weighed again. The yields of DNP before and after
recrystallization were 2.21 g (93 %) and 1.97 g (81 %), respec-
tively, based on the consumption of BTMACB. The DNP was
found to be identical (m.p. and mixed m.p.) with the DNP of
acetone. In similar experiments, with other alcohols, the yields
of the DNP were in the range of 78-89 % after recrystallization.
EXPERIMENTAL
All the alcohols were commercial products (Fluka) and
were dried over magnesium sulphate and than fractionated.

Vol. 26, No. 9 (2014)
Kinetics and Mechanism of Oxidation of Aliphatic Secondary Alcohols 2703
Kinetic measurements: The reactions were studied under
pseudo-first-order conditions by keeping an excess (× 15 or
more) of the substrate over BTMACB. The solvent was 1:1(v/
v) acetic acid-water, unless mentioned otherwise. The reactions
were studied at constant temperature ( 0.1 K) and were follo-
wed by monitoring the decrease in the concentration of BTMACB
at 394 nm for up to 80 % reaction. Pseudo-first-order rate
constants, kobs, were evaluated from linear plots (r > 0.995) of
log [BTMACB] against time. Duplicate kinetic runs showed
that the rate constants are reproducible to within 3 %. Simple
and multivariate regression analyses were carried out by the
least-squares method.
2.5
2.0
1.5
1.0
0.5
0
RESULTS AND DISCUSSION
The rate constants and other experimental data were
obtained for all the alcohols. Since the results are similar, only
representative data are reproduced here.
The oxidation of alcohols by BTMACB results in the
formation of corresponding ketones. The overall reaction may
be represented as equation (1).
0
500
1000
Time (sec)
R2CHOH + Br2Cl– → R2CO + 2H+ + 2Br– + Cl– (1)
Rate laws: The reactions are of first order with respect to
BTMACB. Further, the pseudo-first order rate constants, kobs are
independent of the initial concentration of BTMACB. The rate
constant increases linearly with an increase in the concentration
of the alcohol (Table-1). Fig. 1 depicts a typical kinetic run.
Fig. 1. Oxidation of 2-propanol by BTMACB: A typical kinetic run
Induced polymerization of acrylonitrile:The oxidation
of 2-propanol, in an atmosphere of nitrogen, failed to induce
the polymerization of acrylonitrile. Further, the addition of
acrylonitrile had no effect on the rate of oxidation (Table-1).
Kinetic isotope effect: To ascertain importance of the
cleavage of the α-C-H bond in the rate-determining step, the
oxidation of benzhydrol-α-d (PhCDOHPh) was studied. Results
showed the presence of a substantial primary kinetic isotope
effect (Table-2).
TABLE-1
RATE CONSTANTS FOR THE OXIDATION OF
2−PROPANOL BY BTMACB AT 298 K
104 kobs
(s-1)
103[BTMACB]
(mol dm-3)
[Alcohol]
(mol dm-3)
103 k2
(dm3 mol-1 s-1)
1.0
0.10
0.20
0.40
0.60
0.80
1.00
1.50
0.20
0.20
0.20
0.20
0.40
3.58
7.10
14.4
21.3
28.2
35.1
53.4
7.15
6.75
7.29
6.93
15.3*
3.58
3.55
3.60
3.55
3.53
3.51
3.56
3.75
3.38
3.65
3.47
3.83
Effect of tetrabutylammonium chloride: Addition of
tetrabutylammonium chloride (TBACl) had no effect on the
rates of oxidation (Table-3).
1.0
1.0
1.0
Effect of solvent composition: The rate constant of oxida-
tion was determined in solvents containing different amounts
of acetic acid and water. It was observed that the rate constant
increased with an increase in the amount of water in the solvent
mixture (Table-4).
1.0
1.0
1.0
2.0
4.0
6.0
Effect of temperature: The rate constant of oxidation of
ten secondary alcohols was determined at different tempera-
tures and the activation parameters were calculated at 298 K
8.0
1.0
*contained 0.005 mol dm-3 acrylonitrile
TABLE-2
RATE CONSTANTS AND ACTIVATION PARAMETERS FOR THE OXIDATION OF SECONDARY ALCOHOLS BY BTMACB
104 k2 ( dm3 mol–1 s–1)
∆H*
−∆S*
∆G*
Alcohol
(kJ mol–1)
(J mol–1 K–1)
(kJ mol–1)
288 K
16.2
26.1
47.7
77.4
126
298 K
35.1
57.6
92.7
144
308 K
318 K
153
225
333
486
621
360
4.68
29.7
783
837
161
5.20
Me
Et
75.6
117
180
279
387
207
2.03
13.5
405
432
79.2
5.45
54.6
0.3
109
114
127
131
149
128
100
106
118
118
112
1
1
1
2
1
2
1
1
2
3
3
87.0 0.3
85.8 0.3
84.6 0.3
83.5 0.4
82.4 0.3
84.3 0.5
96.4 0.3
91.4 0.2
82.6 0.6
82.4 0.6
86.7 0.6
–
52.1 0.3
46.9 0.4
44.5 0.6
38.1 0.3
42.6 0.2
66.6 0.4
60.1 0.3
47.5 0.7
47.4 0.7
50.5 0.8
–
n-Pr
i-Pr
i-Bu
225
n-Bu
53.1
0.31
2.52
110
108
ClCH2
MeOCH2
Ph
0.80
5.94
207
Benzhydrol
117
225
19.8
5.91
40.1
5.61
Benzhydrol-α-D
kH/kD
–

2704 Sharma
Asian J. Chem.
TABLE-3
EFFECT OF BENZYLTRIMETHYLAMMONIUM CHLORIDE OR POTASSIUM
BROMIDE ON THE RATE OF OXIDATION OF 2-PROPANOL BY BTMACB
[BTMACB] = 0.001 mol dm-3
[Alcohol] = 1 mol (dm-3)
Temperature = 298 K
103[BTMACl] or [KBr] (mol dm-3)
0.00
0.5
1.0
2.0
3.0
4.0
104 kobs (s-1)
BTMACl
KBr
35.1
35.1
32.4
36.1
33.3
34.0
36.0
33.1
34.2
35.2
35.0
34.5
TABLE-4
EFFECT OF SOLVENT COMPOSITION IN THE OXIDATION OF 2-PROPANOL BY BTMACB
[BTMACB] = 0.001 mol dm-3
Temperature = 298 K
[2-Propanol] = 1.00 mol dm−
3
AcOH %
104 k2 ( s-1 )
25
40
50
60
72
180
68.4
35.1
17.1
6.30
(Table-2). A plot of log k2 at different temperature is linearly
related to the inverse of the absolute temperature (Fig. 2). The
Arrhenius equation is, therefore, valid.
4.5
4.0
3.5
3.5
3.0
3.0
2.5
2.0
1.5
2.5
Me
n-Pr
2.0
1.5
1.0
0.5
0
MeOCH2
Ph
1.0
0.5
0
0
1
2
3
4
3.47
3.35
3.29
3.14
log 318
1/(T) × 103
Fig. 3. Exner's isokinetic relationship in the oxidation of alcohols by
BTMACB
Fig. 2. Oxidation of alcohols by BTMACB: Effect of temperatures
acetic acid-water mixture. Therefore, BTMACB can be consi-
dered as an ionic compound, which exists under our reaction
conditions as benzyltrimethylammonium and chlorobromate
ions [eqn.(2)]. No effect of added benzyltrimethylammonium
ion also indicates that the equilibrium (2) lies far towards the
right.
There is a fair correlation between the activation enthalpies
and entropies of the oxidation of eight alcohols (r2 = 0.9281),
indicating the operation of a compensation effect14. A corre-
lation between the calculated values of enthalpies and entropies
is often vitiated by the experimental errors associated with
them. Exner15 has suggested an alternative method of testing
the validity of the isokinetic relationship. An Exner's plot
between log k2 at 288 and 318 K was linear (r2 = 0.9966; slope
0.8272 0.0198) (Fig. 3). The value of isokinetic temperature
evaluated from this plot is 650 94 K. The linear isokinetic
correlation suggests that all the alcohols are oxidized by the
same mechanism and the changes in the rates are governed by
the changes in both the enthalpy and entropy of the activation.
Reactive oxidizing species: Some conductivity measu-
rements have been carried out to determine the nature of
BTMACB in aqueous acetic acid solution16. It was observed
that acetic acid has very low conductivity.Addition of BTMACB
increases the conductivity of acetic acid. We measured the
conductivity of BTMACB in solvents containing different
proportions 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 limiting value in about 60 %
PhCH2Me3NBr2Cl
The following equilibria may also exist in the solution.
PhCH2Me3N+ + Br2Cl- (2)
Br2Cl–
Br2 + H2O
Br2 + Cl–
HOBr + HBr
(3)
(4)
The probable oxidizing species in a solution of BTMACB
are, therefore, chlorobromate ion, molecular bromine or hypo-
bromous acid. The equilibria (3) and (4) are likely to be suppre-
ssed by the addition of BTMACl or potassium bromide. The
absence of any added BTMACl and bromide ion on the reaction
rate rules out role of Br2 and HOBr in the oxidation process.
The oxidation of benzyl alcohol by bromine, in 1:1(v/v) acetic
acid-water, is retarded by the addition of bromide ions17
whereas the oxidation by BTMACB is unaffected by bromide
ions. This indicates that in the present reaction bromine is not
the active oxidizing species. Similar results were obtained in
the oxidation of aliphatic aldehydes by BTMACB16. Thus in

Vol. 26, No. 9 (2014)
Kinetics and Mechanism of Oxidation of Aliphatic Secondary Alcohols 2705
TABLE-5
TEMPERATURE DEPENDENCE OF THE REACTION CONSTANTS
ρ*
δ
Ψ
Temperature (K)
R2
sd
288
298
308
318
0.9999
0.9998
0.9999
0.9989
0.009
0.010
0.008
0.007
0.01
0.02
0.01
0.04
−1.80 0.01
−1.71 0.01
−1.62 0.02
−1.54 0.01
−0.73 0.01
−0.64 0.01
−0.55 0.02
−0.46 0.01
the present reaction also the reactive oxidizing species is the
chlorobromate ion.
The values of the substituent constants were obtained from
the compilation by Wiberg19. The correlations are excellent;
reaction constants being negative (Table-5). There is no
significant collinearity (r2 = 0.2136) between σ* and Es of the
nine substituents.
Solvent composition effect: The increase in the rate
constant with an increase in the polarity of the medium suggests
that the transition state is more polar than the reactants. The
plot of log k2 against the inverse of the dielectric constant is
nonlinear. The solvent effect was analysed using Grunwald-
Winstein equation18.
The negative polar reaction constant indicates an electron-
deficient carbon centre in the transition state of the rate-deter-
mining step. The negative steric reaction constant, points to a
steric acceleration of the reaction. This may be explained on
the basis of the high ground state energy of the sterically
crowded alcohols. Since the crowding is relieved in the product,
acetone as well as in the transition state leading to it, the transi-
tion state energy of the crowded and uncrowded alcohols do
not differ much and steric acceleration, therefore, results. The
faster oxidation of 1-phenylethanol and benzhydrol may well
be due to the stabilization of the electron-deficient carbon centre
in he transition state by phenyl groups through resonance.
Mechanism: There is no kinetic evidence for the forma-
tion of an intermediate complex in the present reaction. However,
its formation in small amounts cannot be ruled out. It is possible
that in the present reaction also an intermediate complex is
formed in a rapid pre-equilibrium but the equilibrium constant
is very small. The presence of a substantial primary kinetic
isotope effect (kH/kD = 5.61 at 298 K) confirms that the α-C-H
bond is cleaved in the rate-determining step. In view of any
effect of radical scavenger, acrylonitrile, on the reaction rate,
it is unlikely that a one-electron oxidation giving rise to free
radical is operative in this reaction. However, with the present
log k2 = log k0 + mY
(5)
The plot of log k2 versus Y is linear (r2 = 0.9998) with m
= 0.78 0.01. The value of m suggests a transition state, which
is more polar than the reactants. Thus considerable charge
separation takes place in the transition state of the reaction.
Correlation analysis of reactivity: The rate constants,
k2, of oxidation of alcohols failed to yield any significant corre-
lation separately with Taft's19 σ* and Es. The correlation with
σ* accounts for 94 % of the data but correlation is not accep-
table by Exner's criterian20.
log k2 = - 1.83 ( 0.16) ∑σ* - 2.23
(6)
r2 = 0.9540; sd = 0.19; n = 8; ψ = 0.23; T = 298 K
log k2 = - 1.38 ( 1.02) ∑Es - 2.90
(7)
r2 = 0.2343; sd = 0.78; n = 8; ψ = 0.94; T = 298 K
Here n is the number of data points and ψ is the Exner's
statistical parameter20. The rate constants were, therefore, corre-
lated in terms of Pavelich-Taft's21 dual substituent-parameter
eqn (8).
log k = ρ* ∑σ* + δ ∑Es + log ko
(8)
-
R2
C
H
O
H
Br2Cl-
R2
C
H
OH
+
Br
Br
Cl
Slow
-
R2
C
H
O
H
C+
OH + HBr + BrCl-
R2
Br
Br
Cl
Fast
R2
C
O
+
H+
Scheme-I

2706 Sharma
Asian J. Chem.
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the complex. Heasley and co-workers22 have postulated the
formation of an intermediate π-complex in the reaction of
alkenes with pyridinium hydrobromide perbromide. The
formation of similar complexes has also been postulated in
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from the alcohol to the oxidant, in the rate-determining step, is
suggested.
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ACKNOWLEDGEMENTS
Thanks are due to UGC, New Delhi for financial support
in the form of BSR-One Time Grant No. F. 4 - 10/2010 (BSR)
dated 07.03.2012.
24. H. Gangwani, P.K. Sharma and K.K. Banerji, J. Chem. Res. (S), 180
(1999).
25. D. Mathur, P.K. Sharma and K.K. Banerji, J. Chem. Soc., Perkin Trans.
II, 205 (1993).
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