Kinetics and mechanism of oxidation of hydroquinone by tetrabutylammonium tribromide ion in aqueous acetic acid

Article abstract of DOI:10.1007/s10953-010-9567-0

The oxidation of hydroquinone by environmentally benign tetrabutyl ammonium tribromide (TBATB) was carried out in 50% V/V aqueous acetic acid medium under pseudo-first-order conditions, keeping a large excess of hydroquinone over the oxidant. The main rea

Full text of DOI:10.1007/s10953-010-9567-0

J Solution Chem (2010) 39: 1178–1186
DOI 10.1007/s10953-010-9567-0
Kinetics and Mechanism of Oxidation of Hydroquinone
by Tetrabutylammonium Tribromide Ion in Aqueous
Acetic Acid
S.N. Zende · V. A. Kalantre · G.S. Gokavi
Received: 12 September 2009 / Accepted: 30 March 2010 / Published online: 14 August 2010
© Springer Science+Business Media, LLC 2010
Abstract The oxidation of hydroquinone by environmentally benign tetrabutyl ammonium
tribromide (TBATB) was carried out in 50% V/V aqueous acetic acid medium under pseudo-
first-order conditions, keeping a large excess of hydroquinone over the oxidant. The main
reactive species of oxidant and substrate were found to be the Br−3 ion and hydroquinone,
respectively. The reaction proceeds with prior complex formation between the reactants fol-
lowed by its slow decomposition to generate semiquinone and bromine radicals. The com-
plex formation was kinetically verified by its Michaelis–Menten plot. The solvent effect was
verified by using Grunwald–Winstein equation which is consistent with an SN2 mechanism.
The formation constants for the complex and rate constant for the slow decomposition step
were determined by studying the reaction at five different temperatures. The values of for-
mation constant of the complex and the rate constant for its decomposition were determined
at these temperatures. The activation parameters with respect to the slow step of the reaction
have also been determined.
Keywords Hydroquinone · Mechanism · Oxidation · Tetrabutylammonium tribromide
1 Introduction
The oxidative conversion of hydroquinone to p-benzoquinone is a two-electron process
which has been carried out by variety of oxidants, including: [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(μ-O)(phen)4(OH2)2]4+ [12].
S.N. Zende · G.S. Gokavi (
)
ꢀ
Kinetics and Catalysis Laboratory, Department of Chemistry, Shivaji University, Kolhapur 416004,
Maharashtra, India
e-mail: gsgokavi@hotmail.com
V.A. Kalantre
Balasaheb Desai College, Patan, Dist. Satara, Maharashtra, India

J Solution Chem (2010) 39: 1178–1186
1179
The oxidation of hydroquinone usually proceeds through an 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 disproportionation to give
the final product benzoquinone. Depending upon the reactive species of the hydroquinone
in aqueous acidic solutions, various possible 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 a proton transfer coupled electron transfer mechanism
is probable if the oxidant and its reduced state undergo protonation. In order to understand
the mechanism of oxidation of hydroquinone by an oxidant which is a strong electrolyte,
both in its oxidized or reduced form, the present study was undertaken. The oxidant in the
present study, tetrabutylammonium bromide, and its product bromide are both strong elec-
trolytes in aqueous acidic solutions thus providing an opportunity to study the nature of the
electron-transfer mechanism of the oxidation of hydroquinone. Tetrabutylammonium tribro-
mide is comparatively less hazardous, stable, solid and environmentally benign [13] reagent
than molecular bromine; it can be prepared by oxidizing bromide to tribromide and then pre-
cipitating with a quaternary ammonium cation [13]. Such tetraalkylammonium polyhalides
have been used in various organic transformations [14] and for oxidation of organic [15]
and inorganic [16] substrates. Oxidations by TBATB are 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 Br−3 formed from TBATB dissociation. Further dissociation of Br−3 into
bromide ion [17] and molecular bromine also occurs, which can be suppressed by adding an
excess of bromide ions to the solutions. Oxidations [17, 18] by TBATB generally follow a
mechanism involving prior complex formation with the substrate followed by its decompo-
sition. The decomposition of the 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 tetrabutylammonium tribromide (TBATB) for oxidation of inorganic and organic
substrates, the present work of oxidation of hydroquinone by TBATB was undertaken.
2 Experimental
2.1 Materials and Methods
All the chemicals used were of reagent grade and doubly distilled water was used through-
out. The oxidant TBATB was synthesized by the reported procedure [13] and stock solutions
was obtained by dissolving known quantity of TBATB in 50% acetic acid. The standard-
ization of TBATB was carried out both iodometrically and spectrophotometrically. Hydro-
quinone (SD fine) was used. The solution of hydroquinone was prepared by dissolving it
in distilled water. The acetic acid (Thomas Baker) and potassium bromide (SD fine) were
used as received. The reaction mixture, in all kinetic runs, contained a constant concentra-
tion of potassium bromide of 0.01 mol·dm−3 in order to prevent the dissociation [17] of the
tetrabutylammonium tribromide ion. Kinetic runs were carried out under pseudo-first-order
conditions keeping a large excess of hydroquinone. The solutions containing the reactants
and all other constituents were thermally equilibrated at 25 0.1 °C separately, mixed and
the reaction mixture was analyzed for unreacted TBATB at 394 nm using an Elico-SL-177
Spectrophotometer (ε = 107 2 dm3·mol−1·cm−1). The values of the rate constants were
reproducible to within 5%.

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J Solution Chem (2010) 39: 1178–1186
in 50% V/V acetic acid
Table 1 Effect of tetrabutylammonium tribromide (TBATB) concentration on k
at 25 °C. 10 [H Q] = 1.0 mol·dm , 10 [KBr] = 1.0 mol·dm
obs
2
−3
2
−3
2
3
−3
10 [TBATB] / mol·dm
0.5
1.1
1.0
1.1
2.0
1.1
3.0
1.1
4.0
1.0
5.0
1.1
3
−1
10 k /s
obs
2.2 Product Analysis
The product analysis was carried under kinetic conditions. In a typical experiment, the hy-
droquinone (1.101 g, 0.1 mol) and TBATB (0.482 g, 0.01 mol) were taken in 100 cm3 acetic
acid-water (1:1, V/V) and the reaction mixture was allowed to stand for 24 hours to en-
sure completion of the reaction. Then the 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 the ether layer was separated and evaporated to ob-
tain p-benzoquinone as the product. The product was purified, weighed and confirmed by
its melting point (0.864 g, 80%, m.p. 114 °C, lit. m.p. = 115 °C [19]). The FTIR spectral
analysis of the product and substrate were carried out by using a Perkin Elmer Spectrum
100 instrument. The product showed peaks at 1633, 1516 and 3212 cm−1 corresponding to
C = O, C = C and aromatic C–H stretches, while that of the hydroquinone shows peaks at
3400, 3212 and 1425 cm−1 corresponding to the O–H, aromatic C–H and C = C stretching
frequencies. Comparison of the spectra indicate that the peak of O–H stretching is absent
while the peak of C = O stretching appeared in the product as a result of conversion of hy-
droquinone to quinone. To determine the stoichiometry, TBATB (0.482 g, 1.0 mmol) and
hydroquinone (0.5505 g, 0.5 mmol) were mixed in 1:1 (V/V) acetic acid-water, this reac-
tion mixture was allowed to stand for 24 hours, and the unreacted TBATB was determined
spectrophotometrically at 394 nm. It was observed that the stoichiometry of the reaction
is 1:1.
3 Results and Discussion
3.1 Effect of Reactants
The effects of oxidant, TBATB and reductant, hydroquinone were studied at 25 °C.
The [hydroquinone] and [oxidant] concentrations were varied from 1.0 × 10−2 to 1.0 ×
10−1 mol·dm−3 and 5 × 10−4 to 5 × 10−3 mol·dm−3, respectively. The values of rate con-
stants remained constant (Table 1) as the concentration of oxidant is varied indicating first
order dependence on the oxidant concentrations, while the values of rate constants were
found to increase as the concentration of reductant increased. The Michaelis–Menten plot
of 1/kobs against 1/[hydroquinone] (Fig. 1) was found to be linear with the intercept indicat-
ing complex formation between the reactants prior to the rate determining step; 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.
3.2 Effect of Solvent Composition
The effect of solvent composition on rate of the reaction was studied by varying the acetic
acid content in the reaction mixture from 50 to 75% V/V. The pseudo-first order constant kobs
decreased (Table 2) as the acetic acid content increased.

J Solution Chem (2010) 39: 1178–1186
1181
Fig. 1 Michaelis–Menten plot of
1/k
against 1/[hydroquinone];
obs
3
−3
10 [TBATB] = 1.0 mol·dm
,
2
−3
10 [KBr] = 1.0 mol·dm
Table 2 Effect of acetic acid
3
−1
% V/V Acetic acid
pH
10 k /s
obs
content on the k
values at
obs
3
25 °C; 10 [TBATB] =
−3
2
2
50
55
60
65
70
75
1.38
1.29
1.21
1.14
1.06
0.98
1.1
1.0 mol·dm , 10 [H Q] =
2
−3
1.0 mol·dm , 10 [KBr] =
0.82
0.70
0.52
0.40
0.35
−3
1.0 mol·dm
Table 3 Effect of temperature on the complex formation constant, K , rate constants, k , and activation
c
c
parameters with respect to k
c
Temp/K
288
3.4
5.8
293
4.3
9.8
298
4.9
303
5.3
313
6.1
323
9.1
−3
K / mol·dm
c
3
−1
10 k / s
12.3
13.0
20.0
49.8
c
‡
−1
ꢀH / kJ·mol
39.7 0.4
‡
−1
−1
ꢀS / J·K ·mol
−145.3
5
‡
−1
ꢀG / kJ·mol
83.0 0.4
3.3 Effect of Temperature
The effect of [Hydroquinone] was studied at 15, 20, 25, 30, 40 and 50 °C and the formation
constant of the complex between the reactants and the rate constant for its decomposition
were determined from the linear 1/kobs against 1/[hydroquinone] plots (Fig. 1). The acti-
vation parameters with respect to the rate determining decomposition of the complex were
calculated and are given in Table 3.

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J Solution Chem (2010) 39: 1178–1186
Fig. 2 Structure of the complex
−
formed between H Q and Br
2
3
3.4 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 pHs of different acetic acid solutions (Table 2) were measured
and were found to vary from 1.38 to 0.98 for 50 and 75% V/V acetic acid solutions, re-
spectively. The reported pK value of 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] solutions and
further decompositions of the tribromide ion also occur [17] as shown in equilibria given by
Eqs. 1 and 2 respectively:
(C4H9)4N+Br3− ꢀ (C4H9)4N+ + Br−3
Br−3 ꢀ Br2 + Br−
(1)
(2)
The second equilibrium can be shifted to left hand side by using an excess of bromide ion in
the solutions. The active species of oxidant, TBATB, also does not undergo any protonation
and in the 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 hydro-
quinone, H2Q, and tribromide ion, Br−3 , respectively. The reaction was carried out under
pseudo-first order conditions keeping the concentration of hydroquinone in large excess in
50% acetic acid solutions and also containing a constant quantity of 0.01 mol·dm−3 potas-
sium bromide. The pseudo-first order plot was found to be linear for all kinetic runs and the
rate constant, kobs, values remained constant as the concentration of the oxidant was var-
ied from 0.5 × 10−3 to 5.0 × 10−3 mol·dm−3 at constant concentration of hydroquinone of
0.01 mol·dm−3, indicating an overall first order dependence of the reaction on the oxidant
concentration. However, the pseudo-first order rate constants were found to increase with
increases in the concentration of hydroquinone from 1.0 × 10−2 to 1.0 × 10−1 mol·dm−3
at constant TBATB concentration of 1.0 × 10−3 mol·dm−3. The Michaelis–Menten plot of
1/kobs against 1/[substrate] (Fig. 1) was found to be linear with intercept indicating that
the mechanism involves a prior complex formation between the oxidant and the substrate
followed by its rate determining decomposition. During the oxidation [17] of aliphatic al-
cohols by TBATB, prior complex formation between the non-bonded pairs of electrons of
the alcohol oxygen and the tribromide ion has been predicted. Similarly, hydroquinone also
contains an analogous phenolic OH group and the prior complex formation between the tri-
bromide ion with the substrate occurs with the non-bonded pairs of electron of the phenolic
oxygen (Fig. 2). The complex thus formed will decompose 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 existence of the 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 undergos disproportionation generating

J Solution Chem (2010) 39: 1178–1186
1183
the final product, quinone. The Eo values for the oxidation of various protonated forms of
hydroquinone and the semiquinone [8] radical are given Eqs. 3 and 4 respectively. From
Eqs. 3 and 4 it can be noticed that the two-electron oxidation
Q + 2H+ + 2e− −→ H2Q (Eo = 0.69 V)
H2Q+ + e −→ H2Q (Eo = 1.10 V)
Br−3 + 2e− −→ 3Br− (Eo = 1.03 V)
Br−3 + e− −→ Br·2− + Br− (Eo = 1.92 V)
(3)
(4)
(5)
(6)
of hydroquinone is thermodynamically more favorable than the one-electron oxidation. Even
though the oxidant is capable of oxidizing hydroquinone through a direct two-electron trans-
fer process, single electron transfer mechanisms have been proposed. Recently, the mech-
anism of oxidation of hydroquinone was studied using trans-dioxoruthenium(VI) [8] and
μ-oxo-bridged di-iron(III, III) [12] complexes. In the reaction utilizing the ruthenium(VI),
complex, it has been shown that the one electron-transfer mechanism is favored thermo-
dynamically even though the Eo value of the two-electron change from Ru(VI) to Ru(IV)
(0.96 V) is higher than that of the Ru(VI)/Ru(V) (0.56 V) redox couple. Similarly μ-oxo-
bridged-di-iron(III, III) follows a one-electron reduction producing the semiquinone radical.
In the present study also the oxidant Br3− is a two-electron oxidant having an Eo value of
1.03 V [20] with bromide ion as the product, whereas the Eo value is 1.92 V [21] for the
Br−2 to 2Br− one electron change. The redox reactions of tribromide ion and protonated hy-
droquinone with the corresponding Eo values are summarized in Eqs. 3 to 6. Combination
of two electron-transfer and one electron-transfer reactions of the oxidant and the reduc-
tant couples leads to ꢀGo values of 65.62 kJ·mol−1 and 79.13 kJ·mol−1 respectively. The
requirement of facile electron transfer is that ꢀGo < ꢀG∗ and both calculated values of
ꢀG◦ are less than the ꢀG∗ value obtained in the present study for the slow step of the reac-
tion. Therefore, in order to differentiate between the two possibilities, the second-order rate
o
constants have been calculated using k = 1013 × e−ꢀG /RT [9]. The calculated rate constant
for two-electron transfer at 298 K was 31.6 dm3·mol−1·s−1 and for a one-electron path it
was 0.134 dm3·mol−1·s−1. The experimentally obtained pseudo-first-order rate constant is
1.1×10−3 s−1 (Table 1) and for the concentration of hydroquinone of 1.0×10−2 mol·dm−3
the second order rate constant was found to be 0.1 dm3·mol−1·s−1, which is of the same
order as the calculated value considering the one-electron path. Thus the oxidation of hydro-
quinone follows the one-electron transfer mechanism rather than the two electron-transfer
mechanism which is also predicted in the oxidation of hydroquinone by both Ru(VI) and
μ-dioxo-bridged-di-iron(III,III) ions. The driving force for the one-electron transfer in ox-
idation of hydroquinone might be due to the fast disproportionation of the resulting semi-
quinone radical. The activation energy for semiquinone disproportionation is reported to be
15.5 kJ·mol−1 which is close to the diffusion controlled limit (12.5 kJ·mol−1) [22]. Thus
the combination of semiquinone radical as a result of a one-electron transfer step is facili-
tated over that of the direct two-electron step. Therefore, the oxidation of hydroquinone by
TBATB involves a 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 undergoes rapid disproportiona-
tion (k = 1.1 × 109 mol−1·dm3·s−1) [8] to yield the product quinone.
Scheme 1
K
c
Br−3 + H2Q ꢀ complex
(7)

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J Solution Chem (2010) 39: 1178–1186
k
c
complex −→ H2Q·+ + Br2·− + Br−
(8)
(9)
fast
Br·2− + H2Q −→ H2Q·+ + 2Br−
fast
2H2Q·+ −→ H2Q + Q + 2H+
(10)
The overall mechanism of the reaction can be summarized as in Scheme 1. According
to Scheme 1 the active species of the oxidant and the hydroquinone are respectively Br3−
and protonated hydroquinone. These two active species will form a complex as shown by
Eq. 7 of Scheme 1. The complex formed undergoes slow decomposition to generate the
semiquinone free radical and bromine free radical as a result of one-electron transfer be-
tween them as shown by Eq. 8. In step 9 the bromine free radical produced rapidly oxidizes
another hydroquinone, and in step 10 the fast disproportionation of the semiquinone occurs.
The rate of the reaction is given by Eq. 11:
Rate = kc[complex]
(11)
The concentration of the complex can be obtained from step (7) of Scheme 1 as in Eq. 12
where [H2Q]free is the concentration of free hydroquinone. The [H2Q]free can be expressed
as in Eq. 13 in terms of total concentration of hydroquinone.
[complex] = Kc[Br−3 ][H2Q]free
[H2Q]
(12)
(13)
[H2Q]free
=
1 + Kc[H2Q]
Substituting the value of [H2Q]Free in Eq. 12 and the resulting equation in Eq. 11
kcKc[Br−3 ][H2Q]
Rate =
(14)
(15)
1 + Kc[H2Q]
kcKc[H2Q]
kobs
=
1 + Kc[H2Q]
we get the rate law, Eq. 14.
The corresponding expression for the pseudo-first-order rate constant is given by Eq. 15.
Verification of Eq. 15 can be done by plotting 1/kobs against 1/[H2Q], which was found to
be linear. Such Michaelis–Menten plots were obtained at five different temperatures (Fig. 1)
and from the values of the intercept and slope of the linear plots the complex formation
constant, Kc, and the rate constant for the slow step of the reaction, kc, were determined
(Table 3). Further, using the values of the kc at different temperatures, the activation para-
meters for the slow step of the reaction were calculated and the values are given in Table 3.
The increase in the acetic acid content in the reaction medium changes both the pH as
well as the dielectric constant of the medium. The highest pH value of 1.38 is obtained in
solutions containing 50% V/V acetic acid. At this pH the hydroquinone will be present in the
protonated form as its pK value is low [8]. Therefore, the effect of acetic acid content on the
rate of the reaction will be only due to the change in the dielectric constant of the medium.
The increase in the acetic acid content was found to decrease the rate of reaction and the
plot of log10 kobs against 1/D, where D is dielectric constant of the medium, was non-linear.
Therefore the solvent effect was analyzed according to the Grunwald-Winstein [23] equation
(Eq. 16), where Y is the solvent parameter representing the ionizing power of the solvent

J Solution Chem (2010) 39: 1178–1186
1185
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,
and a value less than 0.5 indicates an SN2 pathway while a value greater than 0.5 indicates
an SN1 pathway. In the present study the plot of log10 kobs against Y [22] was found to be
linear (R = 0.998) with a slope of 0.43
log10 k = log10 ko + mY
(16)
which is less than 0.5 indicating an reaction follows an SN2 mechanism. Therefore, the
formation of the complex between hydroquinone and TBATB occurs through the interaction
between the non-bonded pairs of electrons of the phenolic OH and the tribromide ion as
shown in Fig. 2. The dissociation of the phenolic OH or the proton transfer before formation
of the complex would have resulted in a slope of more than 0.5 for the Grunwald-Winstein
plot and the mechanism would have followed an SN1 mechanism. Since, the dissociation
of hydroquinone to HQ+ is not possible under the reaction conditions and the slope of
Grunwald-Winstein plot also supports the formation of the complex as shown in Fig. 2.
The decrease in entropy of the reaction is also in support for the formation of a complex
analogous to that in the case of oxidation of aliphatic alcohols by TBATB [17].
4 Conclusions
The reaction between hydroquinone and tetrabutylammonium tribromide in 50% aqueous
acetic acid solution proceeds with formation of an SN2 type complex between the reactants
followed by its slow decomposition. The complex formation is supported by the Michaelis–
Menten plots. The active species of the oxidant and substrate were found to be tribromide
ion and hydroquinone respectively. The mechanism involving a single-electron transfer
is favored due to the facilitated disproportionation of the semiquinone radical generated
rather than that of two electron-transfers. The semiquinone radical disproportionates rapidly
to form the product quinone. The SN2 type mechanism was supported by the Grunwald-
Winstein plot with a slope less than 0.5 for the effect of solvent on the reaction.
Acknowledgements The authors S.N.Z. and V.A.K. gratefully acknowledge UGC, New Delhi for Sanc-
tioning Teacher fellowship under FIP and Minor Research Project (F-47-033/07- UGC-(XI plan), respec-
tively.
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