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

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