CHAPTER IV KINETICS AND MECHANISM OF OXIDATION OF...
Transcript of CHAPTER IV KINETICS AND MECHANISM OF OXIDATION OF...
CHAPTER I V
KINETICS AND MECHANISM OF
OXIDATION OF HYDROQUINONE BY
TETRABUTYLAMMONIUMTRIBROMIDE
IN AQUEOUS ACETIC ACID
Journal of Solution Chemistry (In Press)
Chapter-IV
72
4.1 Introduction:
Hydroquinone is a white crystalline solid, which can found both in free
and conjugated forms in bacteria, plants and some animals. It is extensively
used as a photographic developer or stabilizer for certain materials that
polymerize in the presence of free radicals, and as a chemical intermediate
for the production of antioxidants, antiozonants, agrochemicals and polymers.
Hydroquinone is also used in cosmetics and medical preparations. Due to its
physicochemical properties, hydroquinone will be distributed mainly to the
water compartment when released into the environment. It degrades both as
a result of photochemical and biological processes; consequently, it does not
persist in the environment. Hydroquinone has been measured in mainstream
smoke from non-filter cigarettes in amounts varying from110 to 300 µg per
cigarette, and also in sidestream smoke. Hydroquinone has been found in
plant-derived food products (e.g., wheat germ), in brewed coffee, and in teas
prepared from the leaves of some berries where the concentration sometimes
exceeds 1%. Photo hobbyists can be exposed to hydroquinone dermally or by
inhalation. Dermal exposure may also result from the use of cosmetic and
medical products containing hydroquinone, such as skin lighteners.
Absorption via the skin is slower but may be more rapid with carriers such as
alcohols. It is metabolized to p-benzoquinone and other oxidized products,
and is detoxified by conjugation to monoglucuronide, monosulfate and
mercapturic derivatives. The excretion of hydroquinone and its metabolites is
rapid and occurs primarily via the urine.
Hydroquinone and its derivatives react with different biological
components such as macromolecules and low molecular weight molecules,
and they have effects on cellular metabolism. Direct hydroxylation of phenol
by cytochrome P-450-enriched extracts of Streptomyces griseus to form
hydroquinone has been reported, when phenol was used as a substrate.
The oxidative conversion of hydroquinone to p-benzoquinone is a two-
electron transfer process which has been carried out by various transition
metal oxidants such as [NiIII(cyclam)]3+[1], [NiIV(oxime)]2+[2], [MnIII-(EDTA)][3],
[Rh2(O2CCH3)4(OH2)2]+[4], Iron(II) porphyrins[5], [CuII(dmp)2]
2+[6], [RuIII(CN)6]3-[7],
trans-[RuIV(tmc)(O)]2+[8], [RuIV(bpy)2(py)(O)]2+[9], VO2+[10], Cu2+(aq)[11] and [Fe2(-
O)(phen)4(OH2)2]4+[12]. The oxidation of hydroquinone usually proceeds
Chapter-IV
73
through initial rate determining one electron-transfer step, irrespective of
whether the oxidant is a one or two-electron oxidant, generating the
semiquinone radical. The second fast step of the reaction involves either
further oxidation of the semiquinone radical by the oxidant or the
disproportionation to give final product benzoquinone. Depending upon the
reactive species of the hydroquinone in aqueous acidic solutions, various
possibilities of mechanisms involving proton and electron transfer for the
oxidation of hydroquinone by trans-[RuIV(tmc)(O)]2+ have been verified[8] . The
mechanism involving slow concerted proton and electron transfer has been
found to be thermodynamically favorable. Such proton transfer coupled
electron transfer mechanism is probable if the oxidant and its reduced state
undergo protonation. In order to understand the mechanism and
thermodyanamic probability of oxidation of hydroquinone by an oxidant which
is a strong electrolyte both in its oxidized or reduced form, the present study
was carried out. The oxidant in the present study, tetrabutylammonium
tribromide and its product bromide ion are strong electrolytes in aqueous
acidic solutions, thus providing an opportunity to know the nature of electron-
transfer mechanism of oxidation of hydroquinone. Tetrabutylammonium
tribromide is comparatively stable, solid and environmentally benign reagent
which can be prepared by oxidizing bromide to tribromide and then
precipitating with quaternary ammonium cation[13] and is less hazardous than
molecular bromine. Such tetraalkylammonium polyhalides have been used in
various organic transformations [14] and for oxidation of organic [15] and
inorganic [16] substrates.
The oxidations by TBATB were generally studied in 50% acetic acid due
to its stability in such a medium. The main reactive species of the reagent in
aqueous solutions is Br3- ion formed from TBATB dissociation. Further
dissociation of Br3 – ion into bromide ion [17] and molecular bromine also
occurs which can be suppressed by adding excess of bromide ions (KBr) in
the solution. The oxidations [17, 18] by TBATB generally follow a mechanism
involving prior complex formation with the substrate and its subsequent
decomposition. The decomposition of complex formed may proceed either by
one-electron or by direct two electron transfer between the reactants. In
continuation of our work [15, 16] in the use of tetrabutylammoniumtribromide
Chapter-IV
74
(TBATB) for oxidation of inorganic and organic substrates the present work of
oxidation of hydroquinone by TBATB was carried out.
4.2 Experimental:
4.2.1 Materials and methods
All the chemicals used were of reagent grade and doubly distilled water
was used throughout. The oxidant TBATB was synthesized by the reported
procedure [13] and stock solution was prepared by dissolving known quantity of
TBATB in 50% acetic acid. The standardization of TBATB was carried out
both iodometrically and spectrophotometrically. Hydroquinone (SD fine) was
used and the solution of hydroquinone was prepared by dissolving it in
distilled water. The acetic acid (Thomas Baker) was distilled with usual
method [19] and used. Potassium bromide (SD fine) was used throughout the
study as received.
4.2.2 Kinetic studies
The reaction mixture, in all the kinetic runs, contained a constant
quantity potassium bromide of 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 hydroquinone. 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 could be reproducible within 5%. The data of
example run is given in (Table 4.1) and corresponding pseudo-first-order plot
is shown in (Fig. 4.1).
4.2.3 Product analysis and stoichiometry
The product analysis was carried under kinetic conditions. In a typical
experiment, the hydroquinone (0.5505g, 5mmol) and TBATB (0.482g, 1 mmol)
were taken in acetic acid-water (1:1, V/V) and the reaction mixture was
allowed to stand for 24 hours to ensure the completion of the reaction. Then
reaction mixture was extracted with ether and the acetic acid in the ether layer
was neutralized using saturated sodium bicarbonate (NaHCO3) and washed
with distilled water. Then ether layer was separated and evaporated to obtain
p-benzoquinone as the product. The product was purified, weighed and
Chapter-IV
75
confirmed by melting point (0.864g, 80%, m.p.1140C, lit. m.p. = 115oC [20]).
The GCMS analysis of the product p- benzoquinone was carried out by using
GCMS –QP2010, Shimadzu instrument (Fig.4.2). The mass spectrum shows
a molecular ion peak at m / z = 108 amu, confirming p-benzoquinone as the
product of the reaction. The FTIR spectral analysis of the product and
substrate were carried out by using Perkin Elmer Spectrum 100 instrument
(Fig.4.3). The product shows peaks at 1633, 1516 and 3212 cm-1
corresponding to C=O, C=C and aromatic C–H stretching while hydroquinone
shows peaks at 3400, 3212 and 1425 cm-1 corresponding to the O-H,
aromatic C-H and C=C stretching frequencies respectively. Comparison of
both the spectra indicates that the peak of O-H stretching is absent while the
peak of C=O stretching is appeared in the product as a result of conversion of
hydroquinone to p-benzoquinone.
To determine the stoichiometry, TBATB (0.482g,1.0mmol) and
hydroquinone(0.05505g,0.5mmol) were mixed in 1:1 (V/V) acetic acid-water,
this reaction mixture was allowed to stand for 24hours and the unreacted
TBATB was determined spectrophotometrically at 394 nm. It was observed
that the stoichiometry of the reaction is 1:1.
4.3 Results:
4.3.1 Effect of reactants
The effects of oxidant, TBATB and reductant, hydroquinone were
studied at 25 oC. The [hydroquinone] and [oxidant] were varied from 1.0 x 10-2
to 1.0 x 10-1 mol dm-3 and 5 x 10-4 to 5 x 10-3 mol dm-3 respectively. The
values of rate constants remained constant (Table 4.2) as the concentration of
oxidant is varied indicating first order dependence of the reaction on the
oxidant concentrations. While the values of rate constants were found to be
increased as concentration of reductant increases. The Michaelis-Menten plot
of 1/ kobs against 1/ [Hydroquinone]( Fig. 4.4) was found to be linear with
intercept indicating the prior complex formation between the reactants
therefore, the effect of [Hydroquinone] was studied at different temperatures
to evaluate the formation constant of the complex and the rate constant for its
decomposition.
Chapter-IV
76
4.3.2 Effect of solvent composition
The effect of solvent composition on rate of the reaction was carried
out by varying acetic acid content in the reaction mixture between 50 to 75%
V/V.The pseudo-first-order rate constant kobs decreases (Table4. 3) as the
acetic acid content increases.
4.3.3 Effect of added acrylonitrile
In order to understand the intervention of free radicals, the acrylonitrile
was added to the reaction mixture, the gel formation was observed due to
induced polymerization of the acrylonitrile and this gel formation indicates
presence of free radical in the reaction.
4.3.4 Effect of temperature
The effect of [Hydroquinone] was studied at 15, 20, 25, 30, 40 and 50 oC and the values of rate constants are given in (Table 4.4). The formation
constant of the complex between the reactants and the rate constant for its
decomposition were determined from the 1/kobs against 1/ [Hydroquinone]
plots (Fig.4.4). The activation parameters with respect to the rate determining
decomposition of the complex were calculated and are given in (Table4.6).
4.4 Discussion:
4.4.1 Mechanism and rate law
The reaction medium used in the present study is 50% v/v acetic acid
in order to stabilize the oxidant in solution. The acetic acid content was further
varied from 50 to 75% v/v in the reaction mixture. The pH of different acetic
acid solutions (Table 4.3) were measured and it was found to vary from 1.38
to 0.98 for 50 and 75% v/v of the acetic acid solutions respectively. The
reported pK value of the hydroquinone [8] is 9.85. Therefore, it exists in the
protonated form in the pH range of the present study. The oxidant, TBATB,
dissociates into a tetrabutylammonium ion and the tribromide ion in aqueous
acetic acid [16] solution and further decomposition of the tribromide ion also
occurs [17]. In order to suppress the dissociation of tribromide ion into bromine,
excess of bromide ion was used in the reaction mixture. The active species of
oxidant, TBATB does not undergo any protonation in presence of excess
bromide ion used in the present study, it exists as tribromide ion only. Thus
the reactive species of the oxidant and the substrate are protonated
hydroquinone, tribromide ion H2Q and Br3- respectively. The reaction was
Chapter-IV
77
carried out under pseudo-first-order conditions keeping the concentration of
hydroquinone large excess in 50% acetic acid solutions and also containing a
constant quantity of 0.01 mol dm-3 potassium bromide.
The pseudo-first-order plot was found to be linear for all kinetic runs
studied and rate constants, kobs value remained constant as the concentration
of the oxidant was varied from 0.5 x 10-3 to 5.0 x 10-3 mol dm-3 at the
constant concentration of hydroquinone 0.02 mol dm-3 indicating first order
dependence of the reaction on the oxidant concentration, while pseudo-first
order rate constants were found to be increased with increase in the
concentration of hydroquinone from 1.0 x10-2 to 1.0 x10-1 mol dm-3 at constant
concentration of TBATB 1.0 x10-3 mol dm-3. The Michaelis-Menten plot of 1/
kobs against 1/ [Substrate] (Fig.4.4) was found to be linear with intercept
indicating that the mechanism involve a prior complex formation between the
oxidant and the substrate followed by its rate determining decomposition.
During the oxidation [17] of aliphatic alcohols by TBATB a prior complex
formation between the non-bonded pair of electrons of the alcoholic oxygen
and the tribromide ion has been reported. Similarly, the hydroquinone also
contains analogous phenolic -OH group and the prior complex formation
between the tribromide ion with the substrate occurs with the non-bonded pair
of electrons of the phenolic oxygen. The complex thus formed decomposes in
the rate determining step. The oxidation of hydroquinone to quinone is a two
electron transfer process and in most of the reactions of hydroquinone the
interference of semiquinone radical has been predicted irrespective of
whether the oxidant is a two-electron or one-electron transfer reagent. The
semiquinone radical thus produced preferentially undergoes
disproportionation generating the final product quinone. The Eo values for the
oxidation of various protonated forms of hydroquinone 8] and the semiquinone [8] radical are given equations (1) and (2) respectively. From the equations (1)
and (4) it can be noticed that the two-electron oxidation
Chapter-IV
78
Q + 2 H + + 2e H2Q ( Eo = 0.69V) (1)
H2Q.+ + e H2Q ( Eo = 1.10V) (2)
Br3- + 2e 3Br - ( Eo = 1.03V ) (3)
+ e Br2- + Br - ( Eo = 1.92V) (4)
Br3-
of hydroquinone is thermodynamically more favorable than that of the one-
electron. Even though, the oxidant is capable of oxidizing the hydroquinone
through direct two electron transfer process, the single electron transfer
mechanisms have been proposed. Recently, the mechanism of oxidation of
hydroquinone was studied by trans-dioxoruthenium (VI) [8] and -oxo-bridged
diiron (III, III) [12] complexes. In the reaction utilizing the ruthenium(VI),
complex it has been shown that one electron-transfer mechanism if favored
thermodynamically even though the Eo value of two-electron change from
Ru(VI) to Ru(IV)(0.96V) is higher than that of the Ru(VI)/Ru(V)( 0.56 V) redox
couple. Similarly the -oxo-bridged-diiron (III, III) follow one-electron reduction
producing the semiquinone radical.
In the present study also the oxidant used Br3- is two electron oxidant
having a Eo value of 1.03 V [21] with bromide ion as the product whereas the
Eo value of 1.92 V[22] for Br2-
to 2Br - one electron change. The redox
reactions of tribromide ion and protonated hydroquinone with the
corresponding Eo values are summarized in equations (1) to (4) respectively.
Combination of two electron-transfer and one electron-transfer reactions of
the oxidant and the reductant couples leads to the Go values as 65.62 kJmol-
1 and 79.13 kJmol-1 respectively. The experimentally obtained value of G# for
the slow step of the reaction is 83.04 kJmol-1 which favors the one electron-
transfer rather than the two electron-transfer process. Therefore, the oxidation
of hydroquinone by TBATB involves one electron transfer process producing
both H2Q+
and Br2. radicals. The bromine radical reacts with another
hydroquinone molecule in a fast step to generate the protonated semiquinone
radical. The semiquinone radical undergo rapid disproportionation (k = 1.1 x
109 mol-1 s-1) [8] to yield the product quinone. The mechanism in terms of
active TBATB, Br3- and protonated hydroquinone, H2Q showing the formation
of the complex and its slow decomposition to yield the products is shown in
Chapter-IV
79
Scheme 4.1. The rate law according to Scheme 4.1 is given in equation (5)
and the corresponding expression for the pseudo-first-order rate constant is
given by equation (6). The verification of equation (6) can be done by plotting
1/kobs against 1/ [H2Q] which was found to be linear with an intercept.
Michaelis-Menten plots were obtained at five different temperatures (Fig.4.4)
and from the values of intercept and the slope of the plots the complex
formation constant, Kc, and the rate constant kc for the slow step of the
reaction were determined and are given in (Table 4.5).
Br3- + H2Q Complex
Kc
Complex H2Q
.
+ .+ Br2
- .+ Br -kc
Br2-.
+ H2QH2Q
+ .+ 2Br
-fast
2H2Q+ H2Q + Q + 2H +fast
Scheme 1
Structure of the complex in scheme 4.1.
HO
HO------Br--Br--Br-
])QH[K1(
]QH][TBATB[KkRate
2c
2cc (5)
])QH[K1(
]QH[Kkk
2c
2ccobs (6)
Further using the values of the kc at different temperatures the activation
parameters for the slow step of the reaction were calculated (Figure4.5 and
figure4.6) and the values are given in (Table 4.6).
The increase in the acetic acid content was found to decrease the rate
of reaction and the plot of log kobs against 1/D was non-linear, (where D is
Chapter-IV
80
dielectric constant of the medium) therefore the solvent effect was analysed
according to the Grunwald Winstein [23] equation (7).
mYklogklog o
(7)
where Y is the solvent parameter representing the ionizing power of the
solvent and m is the measure of the extent of ion-pair formation in the
transition state. The value of m is also taken as the ratio of [(SN1/SN2) =0.5]
pathways involved in the transition state. The SN1/SN2 value less than 0.5
indicates SN2 pathway while the value more than 0.5 indicate SN1 pathway. In
the present study the plot of log kobs against Y [22] (Figure 4.7) was found to be
linear with a slope (m) of 0.43 indicating the reaction follow the SN2
mechanism.
Therefore, the formation of the complex between the hydroquinone and
TBATB occurs through the interaction between the tribromide ion and the
non-bonded pairs of electrons of the phenolic –OH. The dissociation of the
phenolic -OH or the proton transfer before formation of the complex would
have resulted in the slope value of more than 0.5 of the Grunwald Winstein
plot and the mechanism would have followed SN1 mechanism. Since, the
value of slope is observed to be m=0.43, dissociation of hydroquinone to HQ+
is not possible under the reaction conditions which supports the formation of
the complex. The decrease in entropy of the reaction is also in support of the
formation of complex analogous to that in case of oxidation of aliphatic
alcohols by TBATB [17].
4.5 Conclusions:
The reaction between hydroquinone with tetrabutylammonium
tribromide in 50% aqueous acetic acid solution proceeds with formation of SN2
type complex between the reactants followed by its slow decomposition. The
complex formation is also supported by the Michaelis-Menten plot. The active
species of the oxidant and substrate were found to be tribromide ion and
hydroquinone respectively. The mechanism involving single electron transfer
is thermodynamically favored than that of two electron transfer. The
semiquinone radical disproportionate rapidly to form the product quinone. The
SN2 type mechanism was justified by the Grunwald Winstein plot for the effect
of solvent with a slope less than 0.5.
Chapter-IV
81
References:
[1] J. C. Brodovitch, A. McAuley and T. Oswald, Inorg. Chem., 21, 3442
(1982).
[2] D. H. Macartney and A. McAuley, J. Chem. Soc. Dalton Trans., 103
(1984).
[3] G. Giraudi and E. Mentasti, Transition Met. Chem., 6, 230 (1981).
[4] J. W. Herbert and D. H. Macartney, J. Chem. Soc. Dalton Trans., 1931
(1986).
[5] C. E. Castro, G. M. Hathway and R. Havlin, J. Am. Chem. Soc., 99,
8032 (1977).
[6] J. D. Clemmer, G. K. Hogaboom and R. A. Holwerda, Inorg. chem., 18,
2567 (1979).
[7] J.M.A.Hoddenbagh and D. H. Macartney, J. Chem. Soc. Dalton Trans.,
615 (1990).
[8] W. W.Y.Lam, M.F.W. Lee, and T.C. Lau, Inorg. Chem., 45, 315 (2006).
[9] R. A. Binstead, M. E. McGuire, A. Dovletoglou, W. K. Seok, L. E.
Roecker, and T. J. Meyer, J. Am. Chem. Soc., 114, 173 (1992).
[10] K. Kustin, S.T. Liu, C. Nicolini and D. L. Toppen, J. Am. Chem.
Soc.,96, 7410 (1974).
[11] P. Kamau and R. B. Jordan, Inorg. Chem., 41, 3076 (2002).
[12] J. Bhattacharyya and S. Mukhopadhyay, Transition Met. Chem., 31,
256 (2006).
[13] S. Kajigaeshi, T. Kakinami, T. Okamoto and S. Fujisaki, Bull. Chem.
Soc. of Japan, 60, 1159 (1987).
[14] E. Mondal, G. Bose and A.T. Khan, Synlett., 6, 785 (2001).
[15] S.N.Zende, V.A. Kalantre and G.S. Gokavi, Journal of Sulfur Chemistry,
29, 2, 171 (2008).
[16] V.A. Kalantre, S.P. Mardur and G. S. Gokavi, Transition Met.Chem.,
32,214 (2007).
[17] M. Baghmar and P.K. Sharma, Proc. Indian Acad. Sci., (Chem.Sci.),
113, 139 (2001).
[18] J. Gosain and P. K. Sharma, Proc. Indian Acad. Sci., (Chem. Sci.),
115, 135 (2003).
Chapter-IV
82
[19] A.Weissberger,”Technique of Organic Chemistry”, Wiley Interscience,
New York, vol. VII, (1955).
[20] A.I.Vogel, “Practical Organic Chemistry”, p-788, 4th Ed, ELBS and
Longman, Londan (1978).
[21] Lurie and Ju, “Handbook of analytical chemistry”, p-301, Mir Publishers,
Moscow (1975).
[22] T. Beitz, W. Bechmann and R. Mitzner, J. Phys. Chem. A, 102, 6766
(1998).
[23] E.Grunwald and S. Winstein, J. Am. Chem. Soc., 70, 846 (1948).
Chapter-IV
83
Table: 4.1 Sample run
Oxidation of Hydroquinone by TBATB in 50% acetic acid-water medium at
25oC.
102 [KBr] = 1.0 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 0.095 0.88 0.052
4 0.082 0.77 0.115
5 0.077 0.72 0.142
6 0.072 0.67 0.172
8 0.064 0.60 0.223
10 0.056 0.52 0.281
12 0.050 0.47 0.330
14 0.043 0.40 0.427
16 0.038 0.36 0.449
18 0.034 0.32 0.497
20 0.030 0.28 0.552
Chapter-IV
84
Table: 4.2
Effect of reactant concentration on the oxidation of Hydroquinone by TBATB
in 50 % acetic acid- water medium at 25 0c.
102 [KBr] = 1.0 mol dm-3
103[TBATB]
mol dm-3
102[Hydroquinone]
mol dm-3
103 kobs s-1
0.5 2.0 1.1
1.0 2.0 1.1
2.0 2.0 1.1
3.0 2.0 1.1
4.0 2.0 1.0
5.0 2.0 1.1
1.0 1.0 0.64
1.0 2.0 1.1
1.0 4.0 1.44
1.0 6.0 1.68
1.0 8.0 1.85
1.0 10.0 2.10
Chapter-IV
85
Table: 4.3
Effect of acetic acid concentration (%v/v) on the oxidation of
Hydroquinone by TBATB at 25 oC.
103 [TBATB] =1.0 mol dm-3, 102[H2Q] = 1.0 mol dm-3
102 [KBr] = 1.0 mol dm-3
%(V/V) Acetic acid pH 103kobs s-1
50 1.38 1.1
55 1.29 0.82
60 1.21 0.70
65 1.14 0.52
70 1.06 0.40
75 0.98 0.35
Chapter-IV
86
Table: 4.4
Effect of temperature on the rate constants for the oxidation of Hydroquinone
by TBATB in 50% acetic acid-water medium.
103 [TBATB] =1.0 mol dm-3, 102 [KBr] = 1.0 mol dm-3
102[H2Q]
mol dm-3 288K 293K
103kobs
298K 303K 313k 323K
1.0 0.35 0.40 0.58 0.67 0.80 1.00
2.0 0.67 0.77 1.10 1.25 1.54 2.12
3.0 0.86 1.18 1.44 1.67 2.22 3.22
4.0 1.28 1.60 2.11 2.28 2.64 3.90
6.0 1.68 2.05 2.87 3.14 4.17 5.00
8.0 1.92 2.28 3.84 4.25 5.00 6.25
10.0 2.22 2.50 4.25 5.00 6.07 8.34
Chapter-IV
87
Table: 4.5
Effect of temperature on the rate constants, kc, and complex formation
constant Kc for the oxidation of Hydroquinone by TBATB.
Temperature K Rate constant
103 kc s-1
Complex formation
constant Kc
288K 5.8 3.44
293K 9.8 4.31
298K 12.3 4.97
303K 13.0 5.32
313K 20.0 6.14
323k 49.8 9.08
Chapter-IV
88
Table: 4.6
Activation parameters for the oxidation of Hydroquinone by TBATB.
Ea
kJ mol-1
H#
kJ mol-1
- S#
JK-1 mol-1
G#
kJ mol-1
42.3 + 4 39.7 + 4 145.3 + 5 83.0 + 4
Chapter-IV
89
Figure: 4.1 Plot of log (a / a-x) against time for oxidation of Hydroquinone by TBATB in 50% (v/v) in acetic acid at 250c .
(Conditions as in Table 4.1)
0
0.1
0.2
0.3
0.4
0.5
0.6
0 5 10 15 20 25
Time (min)
log (a /
a-x)
Chapter-IV
90
Figure: 4.2
GCMS of p- Benzoquinone.
Chapter-IV
91
Figure: 4.3 IR spectra of A) p-Benzoquinone and B) Hydroquinone.
Fig.3 IR Spectra of (A) p-Benzoquinone and (B) Hydroquinone.
Chapter-IV
92
Figure: 4.4
Michaelis-Menten plot of 1/ kobs against 1/ [Hydroquinone]
103[TBATB] = 1.0 mol. dm-3, 102 [KBr] = 1.0 mol dm-3
(Conditions as in Table 4.4).
0
1
2
3
0 25 50 75 100
(1/ [H2Q]) dm3 mol-1
10
3 (1 /
k obs
)s
288K
293K
298K
303K
313K
323K
Chapter-IV
93
Figure: 4.5
Arrhenius plot for oxidation of Hydroquinone by TBATB.
[Plot of –logk against 1/T].
2
2.2
2.4
2.6
2.8
3.2 3.25 3.3 3.35 3.4 3.45 3.5
103 (1/ T )
- lo
g k
Chapter-IV
94
Figure: 4.6
Eyring plot for oxidation of Hydroquinone by TBATB.
[Plot of -log (k / T) against 1/T].
4.6
4.7
4.8
4.9
5
5.1
5.2
3.2 3.25 3.3 3.35 3.4 3.45 3.5
103 ( 1/ T )
-log(
k /
T )
Chapter-IV
95
Figure: 4.7
Plot of Y against log kobs for oxidation of Hydroquinone by TBATB.
-3.5
-3.4
-3.3
-3.2
-3.1
-3
-2.90 0.5 1 1.5 2 2.5
Y
log
k obs
This document was created with Win2PDF available at http://www.win2pdf.com.The unregistered version of Win2PDF is for evaluation or non-commercial use only.This page will not be added after purchasing Win2PDF.