CHAPTER I V KINETICS AND MECHANISM OF OXIDATION OF HYDROQUINONE BY TETRABUTYLAMMONIUMTRIBROMIDE IN AQUEOUS ACETIC ACID Journal of Solution Chemistry (In Press)
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 twoelectron transfer process which has been carried out by various transition metal oxidants such as [Ni III (cyclam)] 3+[1], [Ni IV (oxime)] 2+[2], [Mn III -(EDTA)] [3], [Rh 2 (O 2 CCH 3 ) 4 (OH 2 ) 2 ] +[4], Iron(II) porphyrins [5], [Cu II (dmp) 2 ] 2+ [ 6], [Ru III (CN) 6 ] 3-[7], trans-[ru IV (tmc)(o)] 2+[8], [Ru IV (bpy) 2 (py)(o)] 2+[9], VO 2+[10], Cu 2+ (aq) [11] and [Fe 2 (- O)(phen) 4 (OH 2 ) 2 ] 4+[12]. The oxidation of hydroquinone usually proceeds 72
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-[ru IV (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 electrontransfer 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 Br 3 ion formed from TBATB dissociation. Further dissociation of Br 3 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 73
(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 pseudofirst-order conditions keeping large excess of hydroquinone. The solutions containing the reactants and all other constituents were thermally equilibrated at 25 0.1 o C 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 (NaHCO 3 ) 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 74
confirmed by melting point (0.864g, 80%, m.p.114 0 C, lit. m.p. = 115 o C [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 o C. 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/ k obs 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. 75
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 k obs 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 o C 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/k obs 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 H 2 Q and Br 3 respectively. The reaction was 76
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, k obs 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/ k obs 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 E o 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 77
Q + 2 H + + 2e H 2 Q ( E o = 0.69V) (1) H 2 Q. + + e H 2 Q ( E o = 1.10V) (2) Br 3 - Br 3 - + 2e 3Br - ( E o = 1.03V ) (3) + e Br 2 - + Br - ( E o = 1.92V) (4) of hydroquinone is thermodynamically more favorable than that of the oneelectron. 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 E o 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 Br 3 is two electron oxidant having a E o value of 1.03 V [21] with bromide ion as the product whereas the E o value of 1.92 V [22] - for Br 2 to 2Br - one electron change. The redox reactions of tribromide ion and protonated hydroquinone with the corresponding E o 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 G o 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 electrontransfer rather than the two electron-transfer process. Therefore, the oxidation of hydroquinone by TBATB involves one electron transfer process producing both H 2 Q +. and Br 2 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 10 9 mol -1 s -1 ) [8] to yield the product quinone. The mechanism in terms of - active TBATB, Br 3 and protonated hydroquinone, H 2 Q showing the formation of the complex and its slow decomposition to yield the products is shown in 78
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/k obs against 1/ [H 2 Q] 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, K c, and the rate constant k c for the slow step of the reaction were determined and are given in (Table 4.5). K Br - c 3 + H 2 Q Complex k c +. Complex H 2 Q -. + Br 2 -. fast Br 2 + H 2 Q H 2 Q +. + 2Br -. 2H 2 Q + fast H 2 Q + Q + 2H + + Br - Scheme 1 Structure of the complex in scheme 4.1. HO------Br--Br--Br - HO Rate k K [TBATB][H Q] c c (1 K [H Q]) c 2 2 (5) k k K [H Q] c c 2 obs (6) (1 K c [H2Q]) Further using the values of the k c 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 k obs against 1/D was non-linear, (where D is 79
dielectric constant of the medium) therefore the solvent effect was analysed according to the Grunwald Winstein [23] equation (7). logk logk o my (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 [(S N 1/S N 2) =0.5] pathways involved in the transition state. The S N 1/S N 2 value less than 0.5 indicates S N 2 pathway while the value more than 0.5 indicate S N 1 pathway. In the present study the plot of log k obs against Y [22] (Figure 4.7) was found to be linear with a slope (m) of 0.43 indicating the reaction follow the S N 2 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 S N 1 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 S N 2 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 S N 2 type mechanism was justified by the Grunwald Winstein plot for the effect of solvent with a slope less than 0.5. 80
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Table: 4.1 Sample run Oxidation of Hydroquinone by TBATB in 50% acetic acid-water medium at 25 o C. 10 2 [KBr] = 1.0 mol dm -3 Time (min) Absorbance at 394nm 10 3 [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 83
Table: 4.2 Effect of reactant concentration on the oxidation of Hydroquinone by TBATB in 50 % acetic acid- water medium at 25 0 c. 10 2 [KBr] = 1.0 mol dm -3 10 3 [TBATB] mol dm -3 10 2 [Hydroquinone] mol dm -3 10 3 k obs 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 84
Table: 4.3 Effect of acetic acid concentration (%v/v) on the oxidation of Hydroquinone by TBATB at 25 o C. 10 3 [TBATB] =1.0 mol dm -3, 10 2 [H2Q] = 1.0 mol dm -3 10 2 [KBr] = 1.0 mol dm -3 %(V/V) Acetic acid ph 10 3 k obs 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 85
Table: 4.4 Effect of temperature on the rate constants for the oxidation of Hydroquinone by TBATB in 50% acetic acid-water medium. 10 3 [TBATB] =1.0 mol dm -3, 10 2 [KBr] = 1.0 mol dm -3 10 2 [H 2 Q] mol dm -3 288K 293K 10 3 k obs 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 86
Table: 4.5 Effect of temperature on the rate constants, k c, and complex formation constant K c for the oxidation of Hydroquinone by TBATB. Temperature K Rate constant 10 3 k c s -1 Complex formation constant K c 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 87
Table: 4.6 Activation parameters for the oxidation of Hydroquinone by TBATB. Ea # H # - S # G kj mol -1 kj mol -1 JK -1 mol -1 kj mol -1 42.3 + 4 39.7 + 4 145.3 + 5 83.0 + 4 88
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 25 0 c. (Conditions as in Table 4.1) 0.6 0.5 log (a / a-x) 0.4 0.3 0.2 0.1 0 0 5 10 15 20 25 Time (min) 89
Figure: 4.2 GCMS of p- Benzoquinone. 90
Figure: 4.3 IR spectra of A) p-benzoquinone and B) Hydroquinone. Fig.3 IR Spectra of (A) p-benzoquinone and (B) Hydroquinone. 91
Figure: 4.4 Michaelis-Menten plot of 1/ k obs against 1/ [Hydroquinone] 10 3 [TBATB] = 1.0 mol. dm -3, 10 2 [KBr] = 1.0 mol dm -3 (Conditions as in Table 4.4). 3 10 3 (1 / k obs )s 2 1 288K 293K 298K 303K 313K 323K 0 0 25 50 75 100 (1/ [H 2 Q]) dm 3 mol -1 92
Figure: 4.5 Arrhenius plot for oxidation of Hydroquinone by TBATB. [Plot of logk against 1/T]. 2.8 2.6 - log k 2.4 2.2 2 3.2 3.25 3.3 3.35 3.4 3.45 3.5 10 3 (1/ T ) 93
Figure: 4.6 Eyring plot for oxidation of Hydroquinone by TBATB. [Plot of -log (k / T) against 1/T]. -log( k / T ) 5.2 5.1 5 4.9 4.8 4.7 4.6 3.2 3.25 3.3 3.35 3.4 3.45 3.5 10 3 ( 1/ T ) 94
Figure: 4.7 Plot of Y against log k obs for oxidation of Hydroquinone by TBATB. -2.9-3 0 0.5 1 1.5 2 2.5 log kobs -3.1-3.2-3.3-3.4-3.5 Y 95
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