kinetic and mechanistic study of oxidation of...
TRANSCRIPT
CHAPTER III
KINETIC AND MECHANISTIC STUDY OF
OXIDATION OF ACETOPHENONES BY
TETRABUTYLAMMONIUMTRIBROMIDE
Oxidation Communications (In Press)
Chapter-III
51
3.1 Introduction:
Bromination of organic substrates, particularly aromatics, has gained a
significant amount of attention in recent years owing to the commercial
importance of such compounds as potent antitumor, antibacterial, antifungal,
antineoplastic, antiviral and antioxidizing agents and also as industrial
intermediates for the manufacture of speciality chemicals, pharmaceuticals
and agrochemicals [1]. Unfortunately, the hazards associated with traditional
bromination are not trivial and cannot be ignored. Environmental problems
caused by the use of detrimental chemicals and solvents in classical
bromination methods and the anticipated legislations against their use are
some of the major concerns. Consequently, what is needed is a methodology
that would be environmentally friendly and clean and yet efficient, site
selective, operationally simple and cost-effective.
Bromination is an important transformation in organic chemistry. The
bromoderivatives of carbonyl compounds, especially -bromo ketones are
utilized in synthesis of variety of biologically important
molecules.Bromoorganics also constitute various industrial products such as
pesticides, herbicides and fire retardants [2].The general methods of
bomination of organic compounds such as use of either molecular bromine or
its complex with organic ammonium salts. The salts like tetrabutylammonium
tribromide containing active Br3- ion can also be prepared in situ by reaction of
bromide with peroxometal species and such methods have also been utilized
for bromination. The reduced hazardous effects of organic ammonium
bromides and environmentally friendly protocol to prepare bromoderivatives,
makes their use for bromination “a preferable one” than the methods with
molecular bromine. The hazardous nature of elemental bromine and
difficulties encountered in its handling has led to preparation [3] of new active
bromine reagents like tetraalkylammonium tribromides. These quaternary
ammonium tribromides are comparatively less hazardous, stable, solid and
environmentally benign reagents. These reagents can be synthesized [4] very
easily by oxidizing bromide to tribromide and then precipitating with
quaternary ammonium cation. The tetraalkylammonium polyhalides have
been used in various organic transformations like aryl thioureas to 2-amino
Chapter-III
52
benzothiozoles [5], carbonyl compounds to 1,3-oxathiolanes[6]synthesis of
aurones[7],tetrahydropyranylation or depyranylation[8], thioacetalisation and
trans-thioacetalisation[9] cleavage of dithioacatals[10], preparations of
thiosugars[11],transformation of tolyl sulfones to quinodimethanes[12] gem
diacylation[13] and cleavage of tert-butyldimethylsilyl ethers[14]. Apart from the
above uses of quarternary ammonium polyhalides these reagents have also
been used for oxidation of various organic and inorganic substrates [15, 16]. The
oxidations by TBATB were generally studied in 50% acetic acid, as the
reagent is stable in such a medium. The main reactive species of the reagent
in aqueous solutions is Br3- as a result of dissociation of TBATB. Further
dissociation of Br3- into bromide and molecular bromine also occur which can
be suppressed by adding excess of bromide ions in solution. The added
bromide ion is reported to be affecting the rate in almost all the reactions
except in case of phosphorous acids[16]. The general mechanism of the
reactions involves a complex formation between the substrate and tribromide
ion followed by its decomposition. The decomposition of complex formed may
proceed either by direct two electron transfer between the reactants or by
hydride ion transfer. Carbonyl compounds like acetones are used in organic
synthesis and oxidation of acetophenone by various oxidants leading to the
product
-bromoacetophenone has also been reported [3]. In continuation of
our work [17-19] on tetrabutylammonium tribromide (TBATB) oxidation of
inorganic and organic substrates, the present study on oxidation of
acetophenones by TBATB was undertaken.
3.2 Experimental:
3.2.1 Materials and method All the chemicals used were of reagent grade and doubly distilled water
was used throughout. The oxidant TBATB was synthesized by the reported
procedure [4] and the stock solution was prepared by dissolving known
quantity of TBATB in 50% V/V in acetic acid. The standardization of TBATB
was carried out both by iodometrically and spectrophotometrically.
Acetophenone (SD fine) was fractionally distilled. The solutions of
acetophenone were prepared by dissolving them in acetic acid–water
mixtures. The acetic acid (Thomas Baker) was distilled with usual method [20]
Chapter-III
53
and used. Potassium bromide (SD fine) was used throughout the study as
received.
3.2.2 Kinetic studies
The reaction mixture, in all the kinetic runs, contained a constant
quantity of potassium bromide (0.01 mol dm-3) in order to prevent the
dissociation of the tribromide ion. Kinetic runs were carried out under pseudo-
first-order conditions keeping large excess of acetophenone. The solutions
containing the reactants and all other constituents were thermally equilibrated
at 25
0.1oC separately, mixed and the reaction mixture was analyzed for
unreacted TBATB at 394 nm using Elico SL-177 Spectrophotometer. The
values of rate constants are reproducible within
5%. The example run is
given in (Table 3.1) and corresponding pseudo-first-order plot is shown in (Fig
3.1).
3.2.3 Product analysis and stoichiometry
The product analysis was carried out by using the acetophenone
(0.480g, 4 m mol) and TBATB (0.482g, 1.0 mmol).Reactants were taken in
acetic acid-water (1:1, V/V) and the reaction mixture was allowed to stand for
24 hours to ensure completion of the reaction. Then the reaction mixture was
extracted with ether and the acetic acid in the ether layer was neutralized by
using saturated sodium bicarbonate solution (NaHCO3) and washed with
distilled water. Then ether layer was separated and evaporated to obtain -
bromoacetophenone as the product. The product -bromoacetophenone was
confirmed by boiling point. (b p134 oC, lit.b.p =135oC). Similarly the product
analysis for the oxidation products of p-substituted acetophenones was
carried out. The products were the corresponding -bromoacetophenone.The
observed physical constants (M.P. / B.P.) of the corresponding products are
given in (Table 3.2).
To determine the stoichiometry, TBATB(0.482g,1.0mmol) and
acetophenone (0.060g,0.5mmol) were mixed in 1:1 (V/V) acetic acid-water,
this reaction mixture was allowed to stand for 24h and the unreacted TBATB
was determined spectrophotometrically at 394 nm. The stoichiometry of the
reaction was found to be 1:1.
Chapter-III
54
3.3 Results:
3.3.1 Effect of reactants
The oxidant, TBATB effect and reductant, acetophenone effect were
studied at 25Oc by keeping all other conditions constant. The [acetophenone]
and [oxidant] were varied from 1.0 x10-2 to 1.0 x 10-1 mol dm-3 and 5 x 10-4 to
5 x 10-3 mol dm-3 at constant [oxidant] 1.0 x10-3 mol dm-3 and
[Acetophenone] 2.0 x 10-2 mol dm-3 respectively. The values of rate
constants remain constant when the concentration of oxidant is varied
indicating first order dependence of the reaction on the oxidant concentration.
While the values of rate constants were found to be increased as
concentration of reductant increases. (Table 3.3).
3.3.2 Effect of solvent composition
The effect of solvent composition on the rate of the reaction was
carried out by varying the acetic acid content in the reaction mixture between
50 to 75% V/V. The pseudo- first-order rate constant kobs decreases (Table
3.4) as the acetic acid content increases.
3.3.3 Effect of added acrylonitrile
In order to understand the intervention of free radicals [21, 22], the
reaction was studied in presence of added acrylonitrile. There was also no
induced polymerization of the acrylonitrile, as there was no formation of the
precipitate and also it did not affect the rate of the reaction. (Table3.5).
3.3.4 Effect of temperature
The effect of temperature was studied at 15,20,25,30 and 40 oC and
the rate constants, kobs obtained at constant concentration of various para
substituted acetophenones and TBATB are shown in (Table3.6).The
corresponding activation parameters are given in (Table 3.7).
3.4 Discussion:
3.4.1 Mechanism and rate law
The reaction was carried out under pseudo-first-order conditions
keeping the large excess concentration of acetophenones in 50% acetic acid
solutions and also containing a constant quantity of 0.01mol dm3 potassium
bromide to suppress further dissociation of tribromide into bromine and
bromide. The pseudo-first-order plot was found to be linear for all the kinetic
Chapter-III
55
runs studied and rate constants, kobs, value remained constant when the
concentration of the oxidant was varied from 0.5 x10-3 to 5.0 x10-3 mol dm-3 at
constant concentration of acetophenone of 0.02 mol dm-3 indicating first
order dependence of the reaction on the oxidant concentration; while the
pseudo-first-order rate constant was found to be increased with increase in
the concentration of acetophenone between concentration range of 1.0 x 10-2
to 1.0 x 10-1 mol dm-3 at the constant concentration of TBATB 1.0 x 10-3 mol
dm-3. The order in reductant concentration was found to be unity (0.99) as
determined from the plot of log [acetophenone] against log kobs. (Fig.3.2).
In acidic solutions the acetophenone undergoes fast enolization as
represented by equation [23].
C
O
C
O
CH3
CH2
H
(1)
The interaction between the oxidant and the enol form of the
acetophenone occurs through electrophilic attack of Br3-, on the nucleophilic
- carbon atom of the carbonyl group. The intermediate thus formed undergoes
decomposition to give the corresponding -bromoacetophenone. The detailed
mechanism of the reaction can be represented as in Scheme3.1
Chapter-III
56
C
O
C
H
HH
Acetophenone
+ Br2 Br -k C
+
O-
CHH
Br +
+ 2Br - + H+
Complex
fast K
C
O
C
H
H
Br
Bromoacetophenone-Alpha -bromoacetophenone
Scheme3.1
The para substituted electron withdrawing groups (p-methoxy, p-
hydroxy, p-bromo, p-chloro and p-nitro) accelerate the reaction which is due
to the development of positive charge in the transition state as shown in
Scheme 3.1. The constancy in G# values for all the acetophenones
studied and the linearity of plot of logkobs (298) against logkobs (288) (Fig.3.3)
for all para substituted acetophenones indicate the similarity in the
mechanism. There was also no effect of added acrylonitrile, a free radical
scavenger, on reaction suggesting a mechanism involving complimentary
two electron transfer step. The increase in the acetic acid content was found
to decrease the rate of reaction. Since the reactant does not undergo
protonation, the effect of acetic acid is due to the decrease in solvent
polarity. The rate of the reaction decreases with increase in the acetic acid
content; this is due to the decrease in the water content in the reaction
mixture, which is essential for rate determining decomposition of the
intermediate. The rate law can be obtained as in equation (2) with the
expression for the observed pseudo-first-order rate constant by equation (3).
Rate = k [Acetophenone] [TBATB] (2)
Rate / [TBATB] = kobs = k [Acetophenone] (3)
The rate constants for six different p-substituted acetophenones are
listed in (Table3.6) and an activation parameters (Ea, H#, G# and S#) are
Chapter-III
57
determined from the Arrhenius plot of -log k versus (1/T) and Eyring plot of -
log (k / T) versus (1/T). These plots give good straight lines. (Fig.3.4 and 3.5).
From the slopes of above plots the values of activation parameters were
calculated and summarized in (Table 3.7). The reaction between reactant
molecules to form an ionic intermediate as shown in Scheme1 leads to
considerable decrease in entropy of activation. The constancy of G# value
indicates the operation of similar mechanism for all the acetophenones
studied. The fairly high positive value of H# indicates that, the transition state
is highly solvated, while the high negative value of entropy favours the
formation of a compact and a more ordered transition state.
3.5 Conclusion:
The reaction between acetophenone with tetrabutylammonium
tribromide was carried out in 50% aqueous acetic acid solution under
pseudo-first-order conditions keeping large excess of acetophenone over
that of oxidant. The electrophilic attack of Br3– ion on carbonyl carbon atom
of acetophenone is the rate-determining step. The rapid decomposition of
intermediate takes place to give -bromoacetophenone as the product.
Based on the observed results plausible mechanism is proposed and the
related rate law has been deduced.
Chapter-III
58
References:
[1] V. Kavala, S. Naik and B. K. Patel, J. Org. Chem., 70, 4267 (2005)
[2] M. Bora, G. Bose, M.K.Chaudhuri, S.S.Dhar, R. Gopinath, A. T. Khan
and B.K.Patel, Org. Lett., 2, 247 (2000) and references cited therein.
[3] G.Rothenberg, R.M.H. Beadnall, J.E. McGrady and J. H.Clark, J.
Chem. Soc, Perkin Trans, 2, 630 (2002).
[4] S. Kajigaeshi, T.Kakinami, T. Okamoto and S. Fujisaki, Bull.Chem.Soc.
of Japan, 60, 1159 (1987).
[5] A. D. Jordan, C. Luo and A. B. Reitz, J. Org. Chem., 68, 8693 (2003).
[6] E. Mondal, P. R. Sahu, G. Bose and A. T. Khan, Tetrahedron Lett., 43,
2843 (2002).
[7] G.Bose, E. Mondal, A. T. Khan and M. J. Bordoloi, Tetrahedron Lett.,
42, 8907 (2001).
[8] S. Naik, R. Gopinath and B. K. Patel, Tetrahedron Lett., 42, 7679
(2001).
[9] S. Naik, R. Gopinath, M. Goswami and B. K. Patel, Org. Biomol.
Chem., 2, 1670 (2004).
[10] E. Mondal, G. Bose and A. T.Khan, Syn.lett., 6,785 (2001).
[11] J. Wirsching and J. Voss, Eur. J. Org. Chem., 3, 691 (1999).
[12] B. D. Lenihan and H. Shechter, J. Org. Chem., 63, 2072 (1998).
[13] V. Kavala and B. K. Patel, Eur. J. Org. Chem., 2, 441 (2005).
[14] R. Gopinath and B. K. Patel, Org. Lett., 2, 4177 (2000).
[15] S. N. Zende, V. A. Kalantre and G. S. Gokavi, J. Sulfer Chem., 29,2,
171 (2008) and ref. cited therein.
[16] P. K. Sharma, Indian J. Chem., 41A, 1612 (2002).
[17] V. A. Kalantre and G. S. Gokavi, Indian J. Chem., 44A, 2048 (2005).
[18] V. A. Kalantre and G. S. Gokavi, Oxidation Commun., 29,385 (2006).
[19] V. A. Kalantre, S.P. Mardur and G.S. Gokavi, Transition Met. Chem.,
32,214 (2007).
[20] A.Weissberger,”Technique of Organic Chemistry”, Wiley Interscience,
New York, vol. VII, (1955).
[21] I. M. Kolthoff, E. J. Meehan and E. M. Carr, J. Am. Chem. Soc., 75,
1439 (1953).
Chapter-III
59
[22] R.T. Mahesh, M.B. Bellakki and S.T. Nandibewoor, Catal. Lett., 97, 91
(2004).
[23] M.P. Nath and K. K. Banerji, Aust. J. Chem., 29(a), 1939 (1976).
Chapter-III
60
Table: 3.1 Sample run
Oxidation of Acetophenone by TBATB in 50% acetic acid at 25oC
[KBr] = 0.01 mol dm-3,102[Acetophenone] = 2.0 mol dm-3
Time (min) Absorbance at 394nm 103 [TBATB]
mol dm-3
log (a / a-x )
0 0.107 1.0 0.000
2 0.099 0.93 0.033
4 0.090 0.84 0.075
5 0.086 0.80 0.094
6 0.080 0.75 0.124
8 0.074 0.69 0.163
10 0.066 0.62 0.209
12 0.060 0.56 0.251
14 0.055 0.51 0.288
16 0.050 0.47 0.330
18 0.047 0.44 0.357
20 0.044 0.41 0.385
Chapter-III
61
Table: 3.2
Products of oxidation of various substituted Acetophenones by TBATB and
their physical constants.
Substrate Product M.P. / B. P. oC
Acetophenone
-bromoactophenone 134
p-methoxy
acetophenone
p-methoxy -bromoactophenone
70
p-hydroxy
acetophenone
p-hydroxy
-bromoactophenone
130
p-bromo
acetophenone
p-bromo
-bromoactophenone
109
p-chloro
acetophenone
p-chloro
-bromoactophenone
250
p-fluoro acetophenone p- fluoro
-bromoactophenone 185
p-nitro acetophenone p-nitro
-bromoactophenone 183
Chapter-III
62
Table: 3.3 Effect of reactant concentrations on Acetophenone and TBATB reaction
in 50 % acetic acid at 25 0c
[KBr]= 0.01 mol dm-3
103[TBATB]
mol dm-3
102[Acetophenone]
mol dm-3
103 kobs s-1
0.5 2.0 0.74
1.0 2.0 0.72
2.0 2.0 0.71
3.0 2.0 0.72
4.0 2.0 0.72
5.0 2.0 0.72
1.0 1.0 0.38
1.0 2.0 0.72
1.0 4.0 1.04
1.0 6.0 2.14
1.0 8.0 3.07
1.0 10.0 4.10
Chapter-III
63
Table: 3.4
Effect of acetic acid content (% v/v) on the reaction
between Acetophenone and TBATB at 250C
103 [TBATB] = 1.0 mol dm– 3
10 2 [Acetophenone] = 2.0 mol dm-3 102 [KBr] = 1.0 mol dm-3
% Acetic acid (v/v) 103kobs s-1
50 0.72
55 0.62
60 0.48
65 0.44
70 0.40
75 0.32
Chapter-III
64
Table: 3.5
Effect of acrylonitrile (% v/v) on the reaction between Acetophenone and
Tetrabutylammonium tribromide (TBATB) in 50% acetic acid at 25oC.
103 [TBATB] = 1.0 mol dm–3
102 [Acetophenone] = 2.0 mol dm-3 102 [KBr] = 1.0 mol dm-3
% Acrylonitrile (v/v) 103kobs s-1
0 0.72
2 0.72
4 0.71
6 0.72
8 0.72
Chapter-III
65
Table: 3.6
Effect of temperature on oxidation of various substituted Acetophenones
by TBATB.
103[TBATB] = 1.0 mol dm-3, 102[Acetophenone] = 2.0 mol dm-3,
102[KBr] = 1.0 mol dm-3
Substrate 103kobs
s-1
288K 293K 298K 303K 313K
Acetophenone 0.38 0.55 0.72 1.10 2.4
p-methoxy Acetophenone
0.41 0.58 0.81 1.2 2.5
p-hydroxy Acetophenone 0.47 0.64 0.90 1.50 2.8
p-bromo Acetophenone 0.43 0.60 0.82 1.40 2.7
p-chloro Acetophenone 0.41 0.57 0.78 1.30 2.5
p-fluoro Acetophenone 0.40 0.60 0.78 1.20 2.3
p-nitro Acetophenone 0.44 0.54 0.84 1.14 2.5
Chapter-III
66
Table: 3.7
Activation parameters for oxidation of various substituted Acetophenones by TBATB
Substrate Ea kJ mol-1
H#
kJ mol-1 - S#
JK-1mol G#
kJ mol-1
Acetophenone 52.2 52.6 134.0 92.8
p-methoxy Acetophenone 57.4 47.8 116.0 92.4
p-hydroxy Acetophenone 53.6 51.0 127.8 89.0
p-bromo Acetophenone 51.1 49.8 138.6 92.0
p-chloro Acetophenone 47.8 50.4 141.3 91.0
p-fluoro Acetophenone 51.0 50.9 137.9 92.0
p-nitro Acetophenone 52.4 49.8 132.6 89.3
Chapter-III
67
Figure: 3.1 Plot of log (a / a-x) against time for oxidation of Acetophenone by TBATB in 50% (v/v) acetic acid at 250c (Conditions as in Table 3.1)
0
0.1
0.2
0.3
0.4
0 5 10 15 20 25
Time in min
log
( a /
a-x
)
Chapter-III
68
Figure: 3.2
Plot of log [Acetophenone] against log kobs.
(Conditions as in Table 3.2)
2
2.2
2.4
2.6
2.8
3
3.2
3.4
1 1.2 1.4 1.6 1.8 2 2.2
-log[acetophenone]
-log
kobs
Chapter-III
69
Figure: 3.3
Exner Plot for Acetophenones and TBATB reaction.
[Plot of log kobs (298K) against log kobs (288 K)].
(Conditions as in Table 3.5)
-3.16
-3.14
-3.12
-3.1
-3.08
-3.06
-3.04-3.44-3.42-3.4-3.38-3.36-3.34-3.32
logkobs(288K)
logk
obs (
298K
)
p-hydroxy
p-nitro
p-bromop-chloro
p-methoxy p-fluoro
Acetophenone
Chapter-III
70
Figure 3.4
Arrhenius plot for the oxidation of Acetophenone by TBATB.
[Plot of (–log k) against (1/ T)].
2.5
2.9
3.3
3.7
3.2 3.25 3.3 3.35 3.4 3.45 3.5
103(1/T )
- log
k
Chapter-III
71
Figure 3.5 Eyring plot for the oxidation of Acetophenone by TBATB.
[Plot of –log (k / T) against (1 / T)].
5
5.2
5.4
5.6
5.8
6
3.2 3.25 3.3 3.35 3.4 3.45 3.5
103 ( 1/ T )
- lo
g (
k /
T )
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