Kinetics and mechanism of Phosphotungustic Acid ... - IJGHC

IJGHC; December 2012 – February 2013; Vol.2, No.1, 91-99.
E-ISSN: 2278-3229
International Journal of Green and Herbal Chemistry
Research Article
Available online at www.ijghc.org
Green Chemistry
Kinetics and Mechanism of Phosphotungustic Acid
Catalyzed Oxidation of p-methyl Benzyl Alcohol by
N-Chlorosaccharin in Acetic Acid-Water Medium
H.D. Gupta 1 , S.K. Singh* 1 , Manoj Kumar Solanki 2 , O.P. Gupta 3 and
Santosh Kumar Singh 4
1 Goverment T.R.S. College, Rewa (M.P.) India.
2 Goverment Engineering, College, Rewa (M.P.) India.
3 Goverment Model science college, Rewa (M.P.) India.
4 Government College, Rampur Naikin, Sidhi (M.P.) India.
Received: 19 December 2012; Revised: 14 January 2013; Accepted: 21 January 2013
Abstract: Kinetic investigations in Keggin-type phosphotungstic acid, which is ecofriendly
and green catalyst, catalyzed oxidation of p-methyl benzyl alcohol by N-
chlorosaccharin, (NCSA) in aqueous acetic acid, have been studied. Oxidation kinetics
of p-methyl benzyl alcohol by N-chlorosaccharin in presence of Phosphotungstic acid
(PTA) shows a first order dependence on NCSA and fractional order on p-methyl
benzyl alcohol and PTA. The variation of [H + ] and [saccharin] (reaction product) have
insignificant effect on reaction rate. Activation parameters for the reaction have been
evaluated from Arrhenius plot by studying the reaction at different temperature. The
rate law has been derived based on obtained kinetics data and a plausible mechanism
has been proposed.
Keywords: Kinetics, N-chlorosaccharin, phosphotungstic acid, Keggin anion,
heteropoly acids.
INTRODUCTION
Environmental concern has forced the chemical industry to re-evaluate many of its processes to reduce or
eliminate the formation of waste produced in the synthesis of organic products. This need is specially
required in oxidation technology and can be addressed by the development of clean and safe oxidation
procedures. This is possible by establishment of green catalytic process by use of environmentally
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H.D. Gupta et al.
friendly oxidant or green catalysts. At present we want an ecofriendly and economic catalyst.
Phosphotungstic acid is a green and cheap catalyst. Phosphotungstic acid (PTA) or tungustophosphoric
acid (TPA) is a heteropoly acid with chemical formula H 3 PW 12 O 40 . In the recent years, studies of
oxidation of various organic compounds by heteropoly acids and Polyoxometalates especially those with
Keggin type structure under homogeneous and heterogeneous reaction conditions 1-9 have attracted
considerable attention of the researchers. Literature survey reveals that phosphotungstic acid (PTA) due to
its thermal stability and acidity make it efficient and eco-friendly catalyst in oxidation of organic
compounds, such as aromatic amines 10 , aromatic alcohols 11 , cyclic alcohols 12 , allyl alcohols 13 , oximes 14 ,
styrene 15 etc. The versatile nature of N-halo compounds is due to their ability to act as sources of
halonium ions, hypo halite species and nitrogen anions, which act as both bases and nucleophiles. They
have been widely used as oxidizing and halogenating reagents in organic compounds 16-19 .
N-chlorosaccharin (NCSA) is a potential oxidizing agent and has some definite advantages over other
N-halogeno oxidants, which has been extensively used in the estimation of organic substrates 20 - 24 . The
kinetics and mechanistic investigations of the oxidation of p-methyl benzyl alcohol by various oxidizing
agents have been studied earlier 25-31 . It seems that there are no reports about PTA catalyzed the kinetics of
oxidation of p-methyl benzyl alcohol by NCSA. The present work reports kinetics and mechanism of PTA
catalyzed oxidation of p-methyl benzyl alcohol by N-chlorosaccharin in 30 % acetic acid.
MATERIALS AND METHODS
The oxidant N-chlorosaccharin (Aldrich sample) used. Acetic acid (A.R. grade) purified by the literature
procedure. The standard solution of p-methyl benzyl alcohol (sigma chemicals) was prepared in acetic
acid. Double distilled water employed in all kinetic runs. To prevent photochemical effect, the freshly
prepared solution of N-chlorosaccharin was stored in an amber coloured bottle and its strength was
checked iodometrically 32 using 1 % solution of freshly prepared starch as an indicator.
Kinetic measurements: All kinetic measurements made under pseudo first order conditions, by keeping
large excess of p-methyl benzyl alcohol over oxidant N-chlorosaccharin. Mixture containing requisite
amount of solutions of p-methyl benzyl alcohol, and PTA in 30 % acetic acid equilibrated at 313 K. To
this mixture added a measured amount of pre-equilibrated (313 K) standard solution of N-chlorosaccharin.
To maintain the desired temperature (within ± 0.1 o C) the reaction mixture was kept in a thermo stated
water bath and the progress of the reaction was monitored iodometrically by withdrawing aliquots of the
reaction mixture at regular time of intervals.
Stoichiometry and Product Analysis: Stoichiometry of the reaction was ascertained by equilibrating the
reaction mixture containing an excess of N-chlorosaccharin over p-methyl benzyl alcohol and
phosphotungstic acid in 30 % acetic acid for 24 hrs. at 313 K. The un-reacted oxidant
(N-chlorosaccharin) was determined by iodometrically. The estimated amount of un-reacted
N-chlorosaccharin showed that one mole of p-methyl benzyl alcohol consumes one mole of
N-chlorosaccharin.
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The p-methyl benzaldehyde found as the end product of oxidation. These products were identified by
forming their 2,4-dinitrophenylhydrazone (p-methyl benzaldehyde 2,4-DNP, M.P. 239.2 0 C ), Which were
characterized by their melting point.
RESULTS AND DISCUSSION
Order on reactants: The kinetics of oxidation of p-methyl benzyl alcohol by NCSA in 30% acetic acid in
presence of phosphotungstic acid (H 3 PW 12 O 40 ) as a catalyst was carried at 313 K under pseudo first order
conditions. The Plot of log [NCSA] vs. time found to be linear indicating first order dependence of the
reaction rate and from the slopes of such plots pseudo first order rate evaluated. The rate constant (k’)
have been found to increase with increase in the concentration of p-methyl benzyl alcohol and plot of log
k' Vs log [p-methyl benzyl alcohol] was linear with slope less than unity, indicating a fractional order
dependence on rate of p-methyl benzyl alcohol (Table-1). The concentration of PTA was varied while the
concentration of p-methyl benzyl alcohol and, [NCSA] kept constant. Reaction is PTA catalyzed with in a
limit. The plot of k 1 vs. [PTA] is obtained linear with the passing through the origin in lower
concentrations but it bent towards x-axis at higher concentrations, indicating fractional order dependence
of rate on [PTA] (Fig. 3 , Table 1).
Effect of Hydrogen ion: The dependence of the reaction rate on hydrogen ion concentration has been
investigated at different initial concentrations of H 2 SO 4 and keeping the concentration of the other
reactants constant. There was no significant change in rate constant observed with variation of [H + ] ion.
Effect of Ionic Strength: The ionic strength of the reaction varied by the addition of NaClO 4 and the
influence of ionic strength on the reaction rate studied. It was found that the ionic strength of the reaction
medium has negligible effect on the reaction rate. This may presumably be due to the attack of an ion on a
neutral molecule in the rate-determining step.
Effect of Solvent Composition: The effect of changing solvent composition on the reaction rate was
studied by varying concentration of acetic acid from 20-60 %. The rate constants suggest that the rate of
reaction decreases with increasing acetic acid content of the solvent mixture. The plot of log k' Vs 1/D
was found to be linear with negative slope indicating the involvement of two dipoles or a negative ion–
dipole reaction.
Effect of Product and Free Radical Inhibitor: Variation of saccharin, one of the products of oxidation,
had negligible effect on the rate of reaction. The PTA catalysed oxidation reactions of p-methyl benzyl
alcohol with NCSA at different initial concentrations of acrylonitrile have been investigated. The reaction
neither induces polymerization nor retards the reaction rate, which may be attributed to the inertness
shown by free radicals.
Reactive Species and Mechanism: Earlier reports reveal that NCSA and N-chlorosuccinimide are stable
oxidizing and chlorinating agents and behave in a similar way. NCSA like other similar N-halo imides
may exist in various forms in acid medium viz. free NCSA, protonated NCSA, Cl + , HOCl, H 2 O + Cl. In
absence of mineral acid, the possibility of Cl + , NCSAH + , or H 2 O + Cl being the reactive oxidizing species is
ruled out. If HOCl is the reactive oxidizing species, then the rate of reaction should be an inverse function
of saccharin (Sac.) which is not observed in the present study. The rate constants suggest that the rate of
reaction decreases with increasing acetic acid content of the solvent mixture. This observation coupled
with slight enhancement in the reaction rate with ionic strength of the medium also supports the
participation of neutral molecule in the rate determining step. Therefore (free) NCSA is the probable
oxidizing species under the present experimental condition.
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The proton released during the reaction by PTA used for protonation of NCSA. Negligible effect of [H + ]
suggested that the protonated NCSA is not involved in reaction mechanism. Keggin structure of PTA
allows the molecule to hydrate and dehydrate without significant structural changes. In the present study
the catalyst [PW12O40] 3- Keggin anion gets converted into the oxidized form and acts as an outer sphere
reagent. Formation of an outer sphere complex by the replacement of one of the water molecules of
hydration is more probable transition state. Since probable transition, state is less solvated and in large it
will be more stabilized in the medium. Therefore irrespective of the nature of p-methyl benzyl alcohol the
transition state formed would be the same, making the rate of reaction independent of nature of p-methyl
benzyl alcohol. Oxidation of substrate by heteropoly anions occurred via overlap of the aromatic system
with the tungstate frame work 33 .
Based upon the experimental observations, the most probable mechanism is as following.
The rate of reaction may be expressed in terms of loss of [NCSA] as given below,
(4)
If [NCSA]T =Total concentration of [NCSA], then = [NCSA] + [X]
(5)
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But, Rate = k[x] according to equation (4), substituting value of [X] in it, where [BA] is substrate
Concentration, we have,
(6)
(7)
(8)
The order with respect to [NCSA] is one and fractional order with [PTA] and [S]. As [BA]>> [NCSA]
Equation (8) further reduced to,
(9)
And,
(10)
At a constant [PTA],
(11)
The ternary complex between oxidant, substrate and catalyst strongly evidence by the kinetic results. This
ternary complex has very short lifetime and decomposes in rds. Thus, mechanism is in good agreement
with the work reported by Binyahia 34 35, 36
and Gupta et al.
Effect of temperature: The rate of oxidation was determined at different temperatures and the Arrhenius
plots of log k vs. 1/T were all linear From this plot, the activation and thermodynamic parameter for
equilibrium step and rate determining step of the scheme was evaluated (Table 2). The observed DS #
values are large and negative. It may be interpreted that the fraction of collisions become more stringent
and decomposition of activation complex is a quite slow process. DH # indicates that the reactions are
enthalpy controlled. Further, the constancy in the calculated values of DG # for this oxidation reaction
indicates that the same type of the reaction mechanism could be operative for the reaction.
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CONCLUSION
Kinetic studies demonstrate that the ternary complex of Keggin-anion, substrate and oxidant decomposes
in a slow rate-determining step to give p-methyl benzaldehyde as the main product. The experimental
stoichiometry is in good agreement. First order to oxidant and fractional order to catalyst and substrate is
supported by derived rate law. In the present study, Keggin anion acts as an outer sphere electron transfer
reagent. The Keggin-type phosphotungstic acid catalyst is efficient homogeneous catalyst for oxidation of
p-methyl benzyl alcohol and it is participating in the reaction as Keggin anion.
Table- 1: Effect of variation of reactants on pseudo first-order rate constant k' at 313K
10 2 [Substrate]
(mol dm -3 )
10 3 [NCSA]
(mol dm -3 )
10 3 [H + ]
(mol dm -3 )
10 3 [PTA]
(mol dm -3 )
% HOAc-H 2 O k 1 x10 5
(s -1 )
1.00 2.50 - 1.25 30 11.43
1.25 2.50 - 1.25 30 14.05
2.00 2.50 - 1.25 30 15.82
2.50 2.50 - 1.25 30 16.12
4.00 2.50 - 1.25 30 16.44
1.25 1.00 - 1.25 30 14.33
1.25 2.00 - 1.25 30 14.49
1.25 2.50 - 1.25 30 14.05
1.25 4.00 - 1.25 30 14.13
1.25 5.00 - 1.25 30 14.27
1.25 2.50 1.00 1.25 30 13.87
1.25 2.50 1.25 1.25 30 13.59
1.25 2.50 2.00 1.25 30 13.18
1.25 2.50 2.50 1.25 30 13.00
1.25 2.50 4.00 1.25 30 13.87
1.25 2.50 - 1.00 30 11.43
1.25 2.50 - 1.25 30 14.05
1.25 2.50 - 2.00 30 16.81
1.25 2.50 - 2.50 30 18.52
1.25 2.50 - 4.00 30 20.12
1.25 2.50 - 1.25 20 14.14
1.25 2.50 - 1.25 30 14.05
1.25 2.50 - 1.25 40 13.90
1.25 2.50 - 1.25 50 13.81
1.25 2.50 - 1.25 60 13.75
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Table- 2: Thermodynamic parameters of p-methyl Benzyl alcohol-NCSA system
Substrate
Ea
(KJ mol -1 )
A
(s -1 )
∆H #
(KJ mol -1 )
∆G #
(KJ mol -1 )
∆S #
(JK mol -1 )
p-methyl Benzyl alcohol
(p-CH 3. BA)
42.03
±0. 66
2.36x10 7
±0.76
47.38
±0.97
-88.68
±0.58
-98.45
±0. 57
[NCSA]=2.50X10 -3 (mol.dm. -3 ), [p-methylBenzyl alcohol] =1.25X10 -2 (mol.dm. -3 ), [PTA] =1.25X 50X10 -3
(mol.dm.-3), HOAC-H 2 O=30% (V/V), Temp. =313K.
Fig.1: Dependence of rate on the initial concentration of NCSA (oxidant)
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[NCSA]=2.50X10 -3 (mol.dm. -3 ), [PTA] =1.25X 50X10 -3 (mol.dm. -3 ), HOAC-H 2 O=30% (V/V), Temp. =313K
. Fig.2: Dependence of rate on the concentration of Benzyl alcohol (Substrate)
[NCSA]=2.50X10 -3 (mol.dm. -3 ), [p-methyl Benzyl alcohol] =1.25X10 -2 (mol.dm. -3 ), HOAC-H 2 O=30% (V/V),
Temp. =313K.
Fig. 3 Dependence of rate on the concentration of phosphotungstic acid (PTA)
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Corresponding author: S.K. Singh;
Govt T.R.S. College, Rewa (M.P.) India
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