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Electrochemical oxidation of acetaminophen in aqueous solutions: Kinetic evaluation of hydrolysis, hydroxylation and dimerization processes
Electrochimica Acta 54 (2009) 7407–7415
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Electrochemical oxidation of acetaminophen in aqueous solutions: Kinetic evaluation of hydrolysis, hydroxylation and dimerization processes D. Nematollahi a,∗ , H. Shayani-Jam a , M. Alimoradi b , S. Niroomand b a b
Faculty of Chemistry, Bu-Ali-Sina University, Mahdiyeh St., Hamadan, Zip Code 65174, Iran Department of Chemistry, Faculty of Science, Arak Branch, Islamic Azad University, Arak, Iran
a r t i c l e
i n f o
Article history: Received 23 June 2009 Received in revised form 20 July 2009 Accepted 26 July 2009 Available online 4 August 2009 Keywords: Acetaminophen Electrochemical oxidation Cyclic voltammetry Observed homogeneous rate constants Dimerization reaction
a b s t r a c t Electrochemical oxidation of acetaminophen (paracetamol) has been studied in various pHs using cyclic voltammetry and controlled-potential coulometry. The results indicate that electrochemically generated N-acetyl-p-benzoquinone-imine (NAPQI) participates in different type reactions based on solution’s pH. It is hydrolyzed in strong acidic media and hydroxylated in strong alkaline media and also, it is dimerized in intermediate pHs. Furthermore, in this work, a simple method for electrochemical synthesis of acetaminophen’s dimer is described. In addition, in various pHs, based on related mechanism, the observed homogeneous rate constants (kobs ) of hydrolysis, hydroxylation and dimerization reactions were estimated by comparing the experimental cyclic voltammetric responses with the digital-simulated results. The most amounts of kobs are calculated in pHs less than 2 and more than 9. In this study, the least observed homogeneous rate constant (kobs ) belongs to pHs 5.0 and 9.0. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Electrochemical methods are more and more widely used for the study of electroactive compounds in pharmaceutical forms and physiological fluids due to their simple, rapid, and economical properties [1]. As an electroactive substance, acetaminophen (paracetamol) has also attracted much interest. Acetaminophen (paracetamol) (N-acetyl-p-aminophenol) is a popular, antipyretic and non-steroidal anti-inflammatory drug [2]. It is the preferred alternative to aspirin, particularly for patients who cannot tolerate aspirin [3], and its use is one of the most common causes of poisoning worldwide [4]. At the recommended dosage, there are no side effects. However, overdoses cause liver and kidney damage. It is suspected that a metabolite of acetaminophen (paracetamol) is the actual hepatotoxic agent [5]. It is reported, in therapeutic doses, 60–90% of the drug is metabolized by conjugation to form acetaminophen glucuronide and sulphate; 5–10% is oxidized by mixed-function oxidase enzymes such as cytochrome P-450 to form highly reactive N-acetyl-p-benzoquinone-imine (NAPQI), which is immediately conjugated with glutathione and subsequently excreted as cysteine and mercapturate conjugates. Only 1–4% of a therapeutic dose of acetaminophen (paracetamol) is excreted unchanged in the urine [6–11]. Because electrochemical oxidation very often parallels the cytochrome P-450 catalyzed oxi-
∗ Corresponding author. Tel.: +98 811 8282807; fax: +98 811 8257407. E-mail addresses: [email protected], [email protected] (D. Nematollahi). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.07.077
dation in liver microsomes, it was interesting to study the anodic oxidation of acetaminophen (1) in various pHs. This encourages us to study electrochemical oxidation of acetaminophen (1) in different pHs and conditions. In addition, the observed homogeneous rate constants (kobs ) of hydrolyses, hydroxylation and dimerization reactions of NAPQI derived from acetaminophen (1) have been estimated by digital simulation of cyclic voltammograms. 2. Experimental 2.1. Apparatus Cyclic voltammetry and controlled-potential coulometry were performed using an Autolab model PGSTAT 20 potentiostat/galvanostat. The cathodic peak current referred to the base line obtained as the extension of the anodic current detected immediately after the inversion potential in the reverse scan. The homogeneous rate constants were estimated by analyzing the cyclic voltammetric responses using the DigiElchSB simulation software [12]. An excellent fit between the experimental and simulated data was obtained over this range of experimental conditions for the following kinetic parameter values. The cell used was a simple and undivided cell. The working electrode used in the voltammetry experiment was a glassy carbon disk (1.8 mm diameter) and a platinum wire was used as the counter electrode. The working electrode used in controlled-potential coulometry was an assembly of four carbon rods (6 mm diameter and 4 cm length) and
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large platinum gauze constituted the counter electrode. The working electrode potentials were measured versus SCE (all electrodes were obtained from AZAR Electrodes). 2.2. Electro-organic synthesis of dimer 11 In a typical procedure, 80 ml of phosphate buffer in water (0.2 M, pH = 7.0) was pre-electrolyzed at 0.45 V versus SCE, in an undivided cell; then, 2 mmol of acetaminophen (1) was added to the cell. The electrolysis was terminated when the decay of the current became more than 95%. The process was interrupted during the electrolysis and the carbon anode was washed in acetone in order to reactivate it. The precipitated solid was collected by filtration and recrystallized from mixture of methanol + acetone. After recrystallization (yield 72%), product was characterized by: IR, 1 H NMR, 13 C NMR and MS.
Fig. 2. Cyclic voltammograms of 1 mM acetaminophen (1) at glassy carbon electrode, in buffer solution with various pHs. pH from a to e are: 1.2, 5.0, 7.0, 9.0 and 13.0. Scan rate: 50 mV s−1 . t = 25 ± 1 ◦ C.
IR(KBr) = 3290, 2823, 1760, 1704, 1664, 1607, 1553, 1501, 1421, 1369, 1314, 1228, 1021, 879, 813 cm−1 . 1 H NMR (300 MHz, DMSOd6 ): ı = 1.99 (s, 6H, acetyl protons), 6.79 (d, 2H, J2,3 = 9.3, H-2 proton in two ring), 7.34 (d, 4H, H-3 and H-5 protons in two ring), 9.03 (s, 2H, two OH proton), 9.76 (s, 2H, two NH proton). 13 C NMR (75 MHz, DMSO-d6 ) ı = 24.2 (acetyl carbon), 116.0 (C-2 carbon), 120.0 (C-5 carbon), 123.1 (C-3 carbon), 126.0 (C-6 carbon), 131.4 (C-4 carbon), 150.6 (C-1 carbon), 168.0 (carbonyl carbon). MS m/e (relative intensity) 300 (M+• 25), 258 (23), 241 (5), 216 (21), 199 (21), 171 (21), 143 (5), 115 (10), 77 (8), 43 (100). 3. Results and discussion 3.1. Electrochemical study Fig. 1 shows cyclic voltammogram recorded for 1 mM acetaminophen (1) in aqueous ammonia buffer solution (pH = 9.0). The voltammogram shows an anodic peak (A1 ) in the positive-going scan and a cathodic counterpart peak (C1 ) in the negative-going
Fig. 1. Cyclic voltammogram of 1 mM acetaminophen (1) at glassy carbon electrode, in ammonia buffer solution (pH = 9.0). Scan rate: 25 mV s−1 . t = 25 ± 1 ◦ C.
scan which corresponds to the transformation of acetaminophen (1) to N-acetyl-p-benzoquinone-imine (NAPQI) (2) and vice-versa within a quasi-reversible two-electron process [13,14]. A peak current ratio (IpC1 /IpA1 ) of nearly unity, particularly during the repetitive recycling of potential can be considered as a criterion for the stability of N-acetyl-p-benzoquinone-imine (NAPQI) (2) produced at the surface of electrode under the experimental conditions. Fig. 2 shows the cyclic voltammograms of acetaminophen (1) in various pHs. It was found that the peak potentials for peak A1 shifted to the negative potentials by increasing pH. This is expected because of the participation of proton(s) in the oxidation reaction of 1 to N-acetyl-p-benzoquinone-imine (NAPQI) (2). R O + mH+ + 2e− where R stands for 1; O stands for 2 and m is the number of protons involved in the reaction. The peak potentials for peak A1 (E pA1 ), is given by [15–17]: = EpA1(pH=0) − (2.303mRT/2F)pH EpA1
where E pA1(pH = 0) is the peak potentials for peak A1 at pH = 0.0, R, T, and F have their usual meanings. The values of E pA1 are plotted in Fig. 3. As is seen in Fig. 3, E pA1 was shifted to negative potentials with the slope of 51 mV/pH. This slope is in agreement with the theoretical slope (2.303 mRT/2F) of 59 mV/pH with m = 2.
Fig. 3. Variation of potential of peak A1 (EpA1 ) as a function of the pH.
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Fig. 4. Variation of peak current ratio (IpC1 /IpA1 ) as a function of the pH.
Fig. 4 shows the variation of peak current ratio (IpC1 /IpA1 ) as a function of pH. As can be seen, in time scale of our experiments, IpC1 /IpA1 which can be considered as a criterion for instability of N-acetyl-p-benzoquinone-imine (NAPQI) (2), in more acidic (pHs less than 4) and alkaline (pHs more than 9) media is intensely less than unity. In fact, in pHs less than 4, there is a direct relation between acidity and instability of NAPQI (2). This is in agreeing with those reported by Kissinger [13] and verifies the occurrence of well-defined NAPQI (2) decomposition mechanism in our experiment condition (Scheme 1). On the other hand a relative stability of NAPQI (2) observed in pHs 5 and 9. However the intrinsic instability of NAPQI (2) in this range (pH = 5–9) which can be due to dimerization of NAPQI (2) (Scheme 3) [18–20] is shown in Fig. 4. And finally, in pHs more than 9, there is a strong relation between basicity and instability of NAPQI (2) (Scheme 2). This is expected because of the participation of hydroxide ions in reaction mechanism. 3.2. Electrochemical behaviour in acidic pHs Fig. 5, curve a shows normalized cyclic voltammogram of acetaminophen (1) (1 mM) in acidic media (pH = 2.0), in scan rate 25 mV s−1 . The CV shows only an anodic peak (A1 ) in the positivegoing scan without cathodic counterpart peak. In these conditions, the presence of the cathodic peak C1 strongly depends on the potential sweep rate. In lower sweep rates, the peak current ratio (IpC1 /IpA1 ) is lesser than one and it increases when the sweep rate
Scheme 2.
increases. This indicates the reactivity of electrochemically generated NAPQI (2). These voltammeteric results in accompanied by previously published data [13] allow us to propose the mechanism presented in Scheme 1 for the electrooxidation of acetaminophen (1) in acidic pHs. In this condition, NAPQI (2) under protonation reaction changed to 3 which in subsequent chemical reactions converts to p-benzoquinone (5) (Scheme 1). As described above, in lower sweep rates no cathodic peaks (C1 and C2 ) are observed in the reverse scan and with increasing sweep rate only peak C1 appeared. This means that neither the NAPQI (2) nor the p-benzoquinone (5) is present in appreciable concentrations near the electrode surface during the sweep, this could explained that intermediate (4) is not reducible in the potential range investigated, this voltammetric behaviour could be attributed to a fast transformation of NAPQI (2) into intermediate (4) and to a slow transformation of 4 into p-benzoquinone (5). The reduction peak of p-benzoquinone (C2 ) appeared in extremely acidic conditions and its current (IpC2 ) increases with increasing of solution’s acidity (Fig. 6). Also, generation of p-benzoquinone
Scheme 1.
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Fig. 5. Normalized cyclic voltammograms of 1 mM acetaminophen (1) at glassy carbon electrode, in phosphate buffer solution (pH = 2.0). Scan rate: (a) 25 mV s−1 , (b) 250 mV s−1 and (c) 1000 mV s−1 . t = 25 ± 1 ◦ C.
(5) is confirmed by the addition of 5 to solution containing acetaminophen (1) (Fig. 7). 3.3. Electrochemical behaviour in alkaline pHs Fig. 8 shows normalized cyclic voltammograms of acetaminophen (1) (1 mM) in alkaline solution (pH = 13.0). In scan rate 250 mV s−1 (curve a), the voltammogram exhibits one anodic peak (A1 ) in the positive-going scan and two cathodic peaks C1 (0.09 V versus SCE) and C3 (−0.18 V versus SCE). In addition, in second cycle, a new anodic peak (A3 ) (Fig. 8, inset) appears at less positive potential in comparison with peak A1 . By decreasing
Fig. 6. Cyclic voltammograms of 1 mM acetaminophen (1) at glassy carbon electrode, in the presence of HClO4 (a) 0.1 M, (b) 0.5 M and (c) 1.0 M. Scan rate: 5 mV s−1 . t = 25 ± 1 ◦ C.
Fig. 7. Cyclic voltammograms of 1 mM acetaminophen (1): (a) in the absence and (b) in the presence of p-benzoquinone, at glassy carbon electrode, in HClO4 1.0 M. Scan rate: 10 mV s−1 . t = 25 ± 1 ◦ C.
potential sweep rate from 250 to 10 mV s−1 (curves b and c), the normalized anodic peak current (A1 ) increase from a two electrons peak (in diffusion condition) to a six electrons peak (in kinetic condition), parallel to the decrease in height of C1 . Diagnostic criteria of cyclic voltammetry in accompanied by previously reported papers [21–24], indicated that the reaction mechanism of electrooxidation of acetaminophen (1) in pHs > 9 is an ECECE (Scheme 2). According to obtained results, it seems that the Michael addition reaction of hydroxide ion to electrochemically generated NAPQI (2) is faster than other type of reactions and leads to N-(3,4-dihydroxyphenyl)acetamide (3-hydroxyacetaminophen) (6). The oxidation of intermediate 6 is easier than the oxi-
Fig. 8. Normalized cyclic voltammograms of 1 mM acetaminophen (1) in various scan rates at glassy carbon electrode, in 0.1 M NaOH (pH = 13). Scan rate: (a) 250 mV s−1 , (b) 50 mV s−1 and (c) 10 mV s−1 . (d) Second scan for (c) without solution stirring between the first and the second cycle. t = 25 ◦ C.
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Fig. 9. Cyclic voltammograms of acetaminophen (1) in aqueous solution containing 0.2 M phosphate buffer (pH = 7.0), at glassy carbon electrode, in various concentrations. Concentration from a to d are: 1.0, 2.5, 5.0 and 10.0 mM, respectively. Scan rate: 100 mV s−1 ; t = 25 ± 1 ◦ C. Curves e and f: variation of peak current ratio and EpC1 versus concentration, respectively.
dation of acetaminophen (1) by virtue of the presence of an electron-donating group leads to very reactive o-benzoquinone 7 [25–30]. In the next step, o-benzoquinone 7, via a fast Michael reaction, is converted to N-(2,4,5-trihydroxyphenyl)acetamide (8) [31–33]. The oxidation of this intermediate (8) is easier than the oxidation of acetaminophen (1) and intermediate 6, and further oxidation converts intermediate 8 into the final product 9. According to proposed mechanism, the anodic peaks A1 and A3 in Fig. 8 pertain to the oxidation of acetaminophen (1) to the NAPQI (2) and N-(2,4,5-trihydroxyphenyl)acetamide (8) to the pbenzoquinone 9, respectively. Obviously, the cathodic peaks C1 and C3 in this figure can also correspond to the reduction of NAPQI (2) and p-benzoquinone 9, to acetaminophen (1) and N-(2,4,5trihydroxyphenyl)acetamide (8), respectively. Because of the fast Michael addition reaction of hydroxide ion with o-benzoquinone 7, the anodic and its cathodic counterpart peak of oxidation of 6 was not observed. 3.4. Electrochemical behaviour in pHs 5–9 Cyclic voltammetry of acetaminophen (1) (1 mM) in aqueous solution containing 0.2 M phosphate buffer (pH = 7.0) shows one anodic (A1 ) and the corresponding cathodic peak (C1 ), which corresponds to the transformation of acetaminophen (1) to Nacetyl-p-benzoquinone-imine (NAPQI) (2) and vice-versa within a quasi-reversible two-electron process (Fig. 9, curve a) [13,14]. In a first-order reactions, the characteristic lifetime of a chemical reaction with rate constant k can be taken as t1 = 1/k, but for a second-order reaction (e.g., dimerization) t2 = 1/kCi , where Ci is the initial concentration of reactant, t1 is the time required for the reactant concentration to drop to 37% of its initial value in a firstorder process, and that t2 is the time required for the concentration
to drop to one-half of Ci in a second-order process. Therefore, dimerization can be distinguished from the first-order one by the dependence of the electrochemical response on Ci [34]. As is shown in Fig. 9, the peak current ratio (IpC1 /IpA1 ) is dependent to concentration of acetaminophen (1) and proportional to the augmentation of it, the peak current ratio (IpC1 /IpA1 ) decreases. The dependence of peak current ratio (IpC1 /IpA1 ) on concentration of acetaminophen (1) is indication of dimerization reaction after electron transfer process [34]. Also, cyclic voltammograms mentioned in Fig. 9 show the shift of peak C1 to negative direction proportional to the increasing of concentration. The negative shift of the peak C1 which is enhanced proportional to increasing of concentration of acetaminophen (1) can be due to the formation of a thin film of product (dimer 11) at the surface of the electrode inhibiting to a certain extent the performance of the electrode process [27,26,35]. It is also seen that proportional to the decrease of the potential sweep rate, the peak current ratio (IpC1 /IpA1 ) decreases (Fig. 10). Fig. 10, curve a, shows the cyclic voltammograms of 1 mM of acetaminophen (1) in low scan rate (5 mV s−1 ). In this case, the peak current ratio (IpC1 /IpA1 ) is about 0.45, while, in higher scan rate (250 mV s−1 ) this value reaches to 0.69 (Fig. 10, curve e). Controlled-potential coulometry was performed in an aqueous phosphate buffer solution (c = 0.2 M, pH = 7.0), containing 0.5 mmol of 1 at 0.45 V versus SCE. The monitoring of electrolysis progress was carried out by cyclic voltammetry (Fig. 11). It shows that, proportional to the advancement of coulometry, the anodic peak A1 decreased and disappears when the charge consumption becomes 1.1e− per molecule of 1. Diagnostic criteria of cyclic voltammetry, the consumption of one electron per molecule of 1, and the spectroscopic data of the isolated product (Fig. 12), indicated that the reaction mechanism of electrooxidation of 1 is dimerization reaction (Scheme 3).
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Fig. 10. Cyclic voltammograms of 1 mM acetaminophen (1) in aqueous solution containing 0.2 M phosphate buffer (pH = 7.0) in various scan rates at glassy carbon electrode. Scan rates from a to e are: 5, 10, 25, 100 and 250 mV s−1 , respectively. Inset: variation of peak current ratio (IpC1 /IpA1 ) versus scan rate. t = 25 ± 1 ◦ C.
According to our results, it seems that the Michael addition reaction of anion 1a via C-alkylation to NAPQI (2) leading to dimer 11. The polymerization reaction was circumvented during the preparative reaction because of the insolubility of the dimer 11 in aqueous solution containing 0.2 M phosphate buffer (pH = 7.0). The electrochemical synthesis of dimer 11 has performed according to presented procedure in Section 2. The 1 H NMR spectrum of dimer 11 (Fig. 12) consisted of three singlet peaks with chemical shifts at 1.99 ppm for the acetyl protons, at 9.03 ppm for the hydroxyl proton, and at 9.76 ppm for the amide proton. The protons with resonance at 6.79 ppm (H-2) and 7.34 ppm (H-3) were shown by proton–proton spin coupling constants to be ortho to one another (J2,3 = 9.3). The 13 C NMR spectrum of dimer 11 showed only eight peaks. Among these peaks, peaks with chemical shifts at 24.2 and 168.0 ppm are belonging to acetyl and carbonyl carbons, respectively. Other peaks are pertaining to aromatic carbons. 13 C NMR, 1 H NMR and MS analyses of dimer 11 were consistent with the formation of the symmetric dimer 11. In valuable published work by Potter et al. polymerization of acetaminophen (1) catalyzed by horseradish peroxidase has been discussed [18]. They isolated six types of products from reaction mixture using semi-preparative high pressure liquid chromatography [18]. In mentioned work, two dimers, three trimers and one tetramer were identified. These dimers formed through a covalent bond between carbons ortho to the hydroxyl group, and to a lesser extent, between the carbon ortho to the hydroxyl group and the amino group of another acetaminophen molecule. However in this work, spectroscopic data of final product (Fig. 12) shows that in our experimental conditions, the main product in electrooxidation of
Fig. 11. Part I, cyclic voltammogram of 0.5 mmol acetaminophen (1) in solution containing 0.2 M phosphate buffer (pH = 7.0) at a glassy carbon electrode during controlled-potential coulometry at 0.45 V versus SCE. After the consumption of: (a) 0 C, (b) 15 C, (c) 25 C and (d) 45 C. Part II: variation of peak current (IpA1 ) versus charge consumed. Scan rate 100 mV s−1 . t = 25 ± 1 ◦ C.
acetaminophen (1) in aqueous solution containing 0.2 M phosphate buffer (pH = 7.0) is dimer 11. 3.5. Kinetic evaluation Electrochemical oxidation of acetaminophen (1) in various pHs was tested by digital simulation. The simulation was carried out assuming semi-infinite one-dimensional diffusion and planar electrode geometry. The experimental parameters entered for digital simulation consisted of the following: Estart , Eswitch , Eend , t = 25 ◦ C and analytical concentration of acetaminophen (1). The transfer coefficient (˛) was assumed to be 0.5 and the formal potentials were obtained experimentally as the midpoint potential between the anodic and cathodic peaks (Emid ). The heterogeneous rate constant (0.002 cm s−1 ) for oxidation of acetaminophen (1) was estimated by use of an experimental working curve [36]. All these parameters were kept constant through out the fitting of the digitally simulated voltammogram to the experimental data. The parameter kobs was
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Fig. 12.
1
H NMR and 13 C NMR of dimer 11.
Scheme 3.
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Fig. 13. Cyclic voltammograms of 1 mM acetaminophen (1) in various pHs and scan rates: (a) experimental, (b) simulated. (I) HClO4 1.0 M, (II) pH = 6.0 and (III) pH = 13.0, at glassy carbon electrode. Scan rate from I to III are: 5, 50 and 50 mV s−1 , respectively. t = 25 ± 1 ◦ C.
Scheme 4.
allowed to change through the fitting processes. The simulation was performed based on proposed mechanisms in Schemes 1–3 (Fig. 13). The simplified mechanisms in various pHs are shown in Scheme 4. Table 1 shows the kobs as a function of pH. Table 1 The observed hydrolyzes, hydroxylation and dimerization rate constants (kobs ) for N-acetyl-p-benzoquinoneimine (NAPQI) in various pHs. Solution condition
kobs (s−1 )a , b
pH = 0.0c pH = 0.3d pH = 1.0e pH = 2.0 pH = 3.0 pH = 4.0 pH = 5.0 pH = 6.0 pH = 7.0 pH = 8.0 pH = 9.0 pH = 10.5 pH = 13.0
25.30 17.60 9.67 0.85 0.20 0.12 0.11 0.17 0.16 0.14 0.06 8.25 16.40
± ± ± ± ± ± ± ± ± ± ± ± ±
As shown in Table 1, kobs is strongly dependent to solution’s pH, and decreases with increasing of it in pH range 0–2 (Fig. 14). The observed trend is expected, because of the participation of proton in the electrochemical oxidation of acetaminophen (1) in acidic media (Scheme 1). Also, in pH > 9, kobs increases with increasing pH. This is also, expected because of the contribution of hydroxide ion in the oxidation of 1 in alkaline solutions (Scheme 2). In this study, the least observed homogeneous rate constant (kobs ) belongs to
1.52 0.53 0.19 0.03 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.31 0.67
a Standard deviation of four independent simulations at various scan rates. b For dimerization reaction: M−1 s−1 . c HClO4 (1.0 M). d HClO4 (0.5 M). e HClO4 (0.1 M).
Fig. 14. Variation of the observed homogeneous rate constant (kobs ) as a function of the pH.
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pHs 5.0 and 9.0 (Fig. 14, inset). This is in agreement with Fig. 4 and shows a relative stability of NAPQI (2) in pHs 5.0 and 9.0. 4. Conclusions The results of this work show that acetaminophen (1) is oxidized in aqueous solutions to N-acetyl-p-benzoquinone-imine (NAPQI) (2). The NAPQI (2) participates in dimerization, hydroxylation or hydrolyzes reactions depend on solution’s pH. The reaction mechanisms for anodic oxidation of acetaminophen (1) is presented in Schemes 1–3. We studied the kinetic of the subsequent reactions of the electrochemically generated NAPQI (2) by cyclic voltammetric technique. The cyclic voltammograms were digitally simulated under proposed mechanisms (Schemes 1–3). The simulated cyclic voltammograms show good agreement with those obtained experimentally. The results of the observed homogeneous rate constants (kobs ) are presented in Table 1. The magnitude of observed homogeneous rate constants (kobs ) is dependent on the solution’s pH. The most amounts of kobs are calculated in pHs less than 2 and more than 9. In this study, the least observed homogeneous rate constant (kobs ) belongs to pHs 5.0 and 9.0. Acknowledgments We would like to thank Dr. M. Rudolph for his cyclic voltammogram digital simulation software (DigiElch SB) and the authors acknowledge the Bu-Ali Sina University Research Council and Center of Excellence in Development of Chemical Methods (CEDCM) for support this work. References [1] M. Ebadi, Electrochim. Acta 48 (2003) 4233. [2] L.J. Roberts, J.D. Morrow, in: J.G. Hardman, L.E. Limbird, A. Goodman Gilman (Eds.), The Pharmacological Basis of Therapeutics, 10th ed., McGraw-Hill, London, 2001. [3] R.V. Blanke, W.J. Decker, in: N.W. Tietz (Ed.), Textbook of Clinical Chemistry, W.B. Saunders, Philadelphia, 1986. [4] D. Gunnell, V. Murray, K. Hawton, Suicide Life Threat. Behav. 30 (2000) 313. [5] N. Wangfuengkanagul, O. Chailapakul, J. Pharm. Biomed. Anal. 28 (2002) 841.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical oxidation of acetaminophen in aqueous solutions: Kinetic evaluation of hydrolysis, hydroxylation and dimerization processes D. Nematollahi a,∗ , H. Shayani-Jam a , M. Alimoradi b , S. Niroomand b a b
Faculty of Chemistry, Bu-Ali-Sina University, Mahdiyeh St., Hamadan, Zip Code 65174, Iran Department of Chemistry, Faculty of Science, Arak Branch, Islamic Azad University, Arak, Iran
a r t i c l e
i n f o
Article history: Received 23 June 2009 Received in revised form 20 July 2009 Accepted 26 July 2009 Available online 4 August 2009 Keywords: Acetaminophen Electrochemical oxidation Cyclic voltammetry Observed homogeneous rate constants Dimerization reaction
a b s t r a c t Electrochemical oxidation of acetaminophen (paracetamol) has been studied in various pHs using cyclic voltammetry and controlled-potential coulometry. The results indicate that electrochemically generated N-acetyl-p-benzoquinone-imine (NAPQI) participates in different type reactions based on solution’s pH. It is hydrolyzed in strong acidic media and hydroxylated in strong alkaline media and also, it is dimerized in intermediate pHs. Furthermore, in this work, a simple method for electrochemical synthesis of acetaminophen’s dimer is described. In addition, in various pHs, based on related mechanism, the observed homogeneous rate constants (kobs ) of hydrolysis, hydroxylation and dimerization reactions were estimated by comparing the experimental cyclic voltammetric responses with the digital-simulated results. The most amounts of kobs are calculated in pHs less than 2 and more than 9. In this study, the least observed homogeneous rate constant (kobs ) belongs to pHs 5.0 and 9.0. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Electrochemical methods are more and more widely used for the study of electroactive compounds in pharmaceutical forms and physiological fluids due to their simple, rapid, and economical properties [1]. As an electroactive substance, acetaminophen (paracetamol) has also attracted much interest. Acetaminophen (paracetamol) (N-acetyl-p-aminophenol) is a popular, antipyretic and non-steroidal anti-inflammatory drug [2]. It is the preferred alternative to aspirin, particularly for patients who cannot tolerate aspirin [3], and its use is one of the most common causes of poisoning worldwide [4]. At the recommended dosage, there are no side effects. However, overdoses cause liver and kidney damage. It is suspected that a metabolite of acetaminophen (paracetamol) is the actual hepatotoxic agent [5]. It is reported, in therapeutic doses, 60–90% of the drug is metabolized by conjugation to form acetaminophen glucuronide and sulphate; 5–10% is oxidized by mixed-function oxidase enzymes such as cytochrome P-450 to form highly reactive N-acetyl-p-benzoquinone-imine (NAPQI), which is immediately conjugated with glutathione and subsequently excreted as cysteine and mercapturate conjugates. Only 1–4% of a therapeutic dose of acetaminophen (paracetamol) is excreted unchanged in the urine [6–11]. Because electrochemical oxidation very often parallels the cytochrome P-450 catalyzed oxi-
∗ Corresponding author. Tel.: +98 811 8282807; fax: +98 811 8257407. E-mail addresses: [email protected], [email protected] (D. Nematollahi). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.07.077
dation in liver microsomes, it was interesting to study the anodic oxidation of acetaminophen (1) in various pHs. This encourages us to study electrochemical oxidation of acetaminophen (1) in different pHs and conditions. In addition, the observed homogeneous rate constants (kobs ) of hydrolyses, hydroxylation and dimerization reactions of NAPQI derived from acetaminophen (1) have been estimated by digital simulation of cyclic voltammograms. 2. Experimental 2.1. Apparatus Cyclic voltammetry and controlled-potential coulometry were performed using an Autolab model PGSTAT 20 potentiostat/galvanostat. The cathodic peak current referred to the base line obtained as the extension of the anodic current detected immediately after the inversion potential in the reverse scan. The homogeneous rate constants were estimated by analyzing the cyclic voltammetric responses using the DigiElchSB simulation software [12]. An excellent fit between the experimental and simulated data was obtained over this range of experimental conditions for the following kinetic parameter values. The cell used was a simple and undivided cell. The working electrode used in the voltammetry experiment was a glassy carbon disk (1.8 mm diameter) and a platinum wire was used as the counter electrode. The working electrode used in controlled-potential coulometry was an assembly of four carbon rods (6 mm diameter and 4 cm length) and
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large platinum gauze constituted the counter electrode. The working electrode potentials were measured versus SCE (all electrodes were obtained from AZAR Electrodes). 2.2. Electro-organic synthesis of dimer 11 In a typical procedure, 80 ml of phosphate buffer in water (0.2 M, pH = 7.0) was pre-electrolyzed at 0.45 V versus SCE, in an undivided cell; then, 2 mmol of acetaminophen (1) was added to the cell. The electrolysis was terminated when the decay of the current became more than 95%. The process was interrupted during the electrolysis and the carbon anode was washed in acetone in order to reactivate it. The precipitated solid was collected by filtration and recrystallized from mixture of methanol + acetone. After recrystallization (yield 72%), product was characterized by: IR, 1 H NMR, 13 C NMR and MS.
Fig. 2. Cyclic voltammograms of 1 mM acetaminophen (1) at glassy carbon electrode, in buffer solution with various pHs. pH from a to e are: 1.2, 5.0, 7.0, 9.0 and 13.0. Scan rate: 50 mV s−1 . t = 25 ± 1 ◦ C.
IR(KBr) = 3290, 2823, 1760, 1704, 1664, 1607, 1553, 1501, 1421, 1369, 1314, 1228, 1021, 879, 813 cm−1 . 1 H NMR (300 MHz, DMSOd6 ): ı = 1.99 (s, 6H, acetyl protons), 6.79 (d, 2H, J2,3 = 9.3, H-2 proton in two ring), 7.34 (d, 4H, H-3 and H-5 protons in two ring), 9.03 (s, 2H, two OH proton), 9.76 (s, 2H, two NH proton). 13 C NMR (75 MHz, DMSO-d6 ) ı = 24.2 (acetyl carbon), 116.0 (C-2 carbon), 120.0 (C-5 carbon), 123.1 (C-3 carbon), 126.0 (C-6 carbon), 131.4 (C-4 carbon), 150.6 (C-1 carbon), 168.0 (carbonyl carbon). MS m/e (relative intensity) 300 (M+• 25), 258 (23), 241 (5), 216 (21), 199 (21), 171 (21), 143 (5), 115 (10), 77 (8), 43 (100). 3. Results and discussion 3.1. Electrochemical study Fig. 1 shows cyclic voltammogram recorded for 1 mM acetaminophen (1) in aqueous ammonia buffer solution (pH = 9.0). The voltammogram shows an anodic peak (A1 ) in the positive-going scan and a cathodic counterpart peak (C1 ) in the negative-going
Fig. 1. Cyclic voltammogram of 1 mM acetaminophen (1) at glassy carbon electrode, in ammonia buffer solution (pH = 9.0). Scan rate: 25 mV s−1 . t = 25 ± 1 ◦ C.
scan which corresponds to the transformation of acetaminophen (1) to N-acetyl-p-benzoquinone-imine (NAPQI) (2) and vice-versa within a quasi-reversible two-electron process [13,14]. A peak current ratio (IpC1 /IpA1 ) of nearly unity, particularly during the repetitive recycling of potential can be considered as a criterion for the stability of N-acetyl-p-benzoquinone-imine (NAPQI) (2) produced at the surface of electrode under the experimental conditions. Fig. 2 shows the cyclic voltammograms of acetaminophen (1) in various pHs. It was found that the peak potentials for peak A1 shifted to the negative potentials by increasing pH. This is expected because of the participation of proton(s) in the oxidation reaction of 1 to N-acetyl-p-benzoquinone-imine (NAPQI) (2). R O + mH+ + 2e− where R stands for 1; O stands for 2 and m is the number of protons involved in the reaction. The peak potentials for peak A1 (E pA1 ), is given by [15–17]: = EpA1(pH=0) − (2.303mRT/2F)pH EpA1
where E pA1(pH = 0) is the peak potentials for peak A1 at pH = 0.0, R, T, and F have their usual meanings. The values of E pA1 are plotted in Fig. 3. As is seen in Fig. 3, E pA1 was shifted to negative potentials with the slope of 51 mV/pH. This slope is in agreement with the theoretical slope (2.303 mRT/2F) of 59 mV/pH with m = 2.
Fig. 3. Variation of potential of peak A1 (EpA1 ) as a function of the pH.
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Fig. 4. Variation of peak current ratio (IpC1 /IpA1 ) as a function of the pH.
Fig. 4 shows the variation of peak current ratio (IpC1 /IpA1 ) as a function of pH. As can be seen, in time scale of our experiments, IpC1 /IpA1 which can be considered as a criterion for instability of N-acetyl-p-benzoquinone-imine (NAPQI) (2), in more acidic (pHs less than 4) and alkaline (pHs more than 9) media is intensely less than unity. In fact, in pHs less than 4, there is a direct relation between acidity and instability of NAPQI (2). This is in agreeing with those reported by Kissinger [13] and verifies the occurrence of well-defined NAPQI (2) decomposition mechanism in our experiment condition (Scheme 1). On the other hand a relative stability of NAPQI (2) observed in pHs 5 and 9. However the intrinsic instability of NAPQI (2) in this range (pH = 5–9) which can be due to dimerization of NAPQI (2) (Scheme 3) [18–20] is shown in Fig. 4. And finally, in pHs more than 9, there is a strong relation between basicity and instability of NAPQI (2) (Scheme 2). This is expected because of the participation of hydroxide ions in reaction mechanism. 3.2. Electrochemical behaviour in acidic pHs Fig. 5, curve a shows normalized cyclic voltammogram of acetaminophen (1) (1 mM) in acidic media (pH = 2.0), in scan rate 25 mV s−1 . The CV shows only an anodic peak (A1 ) in the positivegoing scan without cathodic counterpart peak. In these conditions, the presence of the cathodic peak C1 strongly depends on the potential sweep rate. In lower sweep rates, the peak current ratio (IpC1 /IpA1 ) is lesser than one and it increases when the sweep rate
Scheme 2.
increases. This indicates the reactivity of electrochemically generated NAPQI (2). These voltammeteric results in accompanied by previously published data [13] allow us to propose the mechanism presented in Scheme 1 for the electrooxidation of acetaminophen (1) in acidic pHs. In this condition, NAPQI (2) under protonation reaction changed to 3 which in subsequent chemical reactions converts to p-benzoquinone (5) (Scheme 1). As described above, in lower sweep rates no cathodic peaks (C1 and C2 ) are observed in the reverse scan and with increasing sweep rate only peak C1 appeared. This means that neither the NAPQI (2) nor the p-benzoquinone (5) is present in appreciable concentrations near the electrode surface during the sweep, this could explained that intermediate (4) is not reducible in the potential range investigated, this voltammetric behaviour could be attributed to a fast transformation of NAPQI (2) into intermediate (4) and to a slow transformation of 4 into p-benzoquinone (5). The reduction peak of p-benzoquinone (C2 ) appeared in extremely acidic conditions and its current (IpC2 ) increases with increasing of solution’s acidity (Fig. 6). Also, generation of p-benzoquinone
Scheme 1.
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Fig. 5. Normalized cyclic voltammograms of 1 mM acetaminophen (1) at glassy carbon electrode, in phosphate buffer solution (pH = 2.0). Scan rate: (a) 25 mV s−1 , (b) 250 mV s−1 and (c) 1000 mV s−1 . t = 25 ± 1 ◦ C.
(5) is confirmed by the addition of 5 to solution containing acetaminophen (1) (Fig. 7). 3.3. Electrochemical behaviour in alkaline pHs Fig. 8 shows normalized cyclic voltammograms of acetaminophen (1) (1 mM) in alkaline solution (pH = 13.0). In scan rate 250 mV s−1 (curve a), the voltammogram exhibits one anodic peak (A1 ) in the positive-going scan and two cathodic peaks C1 (0.09 V versus SCE) and C3 (−0.18 V versus SCE). In addition, in second cycle, a new anodic peak (A3 ) (Fig. 8, inset) appears at less positive potential in comparison with peak A1 . By decreasing
Fig. 6. Cyclic voltammograms of 1 mM acetaminophen (1) at glassy carbon electrode, in the presence of HClO4 (a) 0.1 M, (b) 0.5 M and (c) 1.0 M. Scan rate: 5 mV s−1 . t = 25 ± 1 ◦ C.
Fig. 7. Cyclic voltammograms of 1 mM acetaminophen (1): (a) in the absence and (b) in the presence of p-benzoquinone, at glassy carbon electrode, in HClO4 1.0 M. Scan rate: 10 mV s−1 . t = 25 ± 1 ◦ C.
potential sweep rate from 250 to 10 mV s−1 (curves b and c), the normalized anodic peak current (A1 ) increase from a two electrons peak (in diffusion condition) to a six electrons peak (in kinetic condition), parallel to the decrease in height of C1 . Diagnostic criteria of cyclic voltammetry in accompanied by previously reported papers [21–24], indicated that the reaction mechanism of electrooxidation of acetaminophen (1) in pHs > 9 is an ECECE (Scheme 2). According to obtained results, it seems that the Michael addition reaction of hydroxide ion to electrochemically generated NAPQI (2) is faster than other type of reactions and leads to N-(3,4-dihydroxyphenyl)acetamide (3-hydroxyacetaminophen) (6). The oxidation of intermediate 6 is easier than the oxi-
Fig. 8. Normalized cyclic voltammograms of 1 mM acetaminophen (1) in various scan rates at glassy carbon electrode, in 0.1 M NaOH (pH = 13). Scan rate: (a) 250 mV s−1 , (b) 50 mV s−1 and (c) 10 mV s−1 . (d) Second scan for (c) without solution stirring between the first and the second cycle. t = 25 ◦ C.
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Fig. 9. Cyclic voltammograms of acetaminophen (1) in aqueous solution containing 0.2 M phosphate buffer (pH = 7.0), at glassy carbon electrode, in various concentrations. Concentration from a to d are: 1.0, 2.5, 5.0 and 10.0 mM, respectively. Scan rate: 100 mV s−1 ; t = 25 ± 1 ◦ C. Curves e and f: variation of peak current ratio and EpC1 versus concentration, respectively.
dation of acetaminophen (1) by virtue of the presence of an electron-donating group leads to very reactive o-benzoquinone 7 [25–30]. In the next step, o-benzoquinone 7, via a fast Michael reaction, is converted to N-(2,4,5-trihydroxyphenyl)acetamide (8) [31–33]. The oxidation of this intermediate (8) is easier than the oxidation of acetaminophen (1) and intermediate 6, and further oxidation converts intermediate 8 into the final product 9. According to proposed mechanism, the anodic peaks A1 and A3 in Fig. 8 pertain to the oxidation of acetaminophen (1) to the NAPQI (2) and N-(2,4,5-trihydroxyphenyl)acetamide (8) to the pbenzoquinone 9, respectively. Obviously, the cathodic peaks C1 and C3 in this figure can also correspond to the reduction of NAPQI (2) and p-benzoquinone 9, to acetaminophen (1) and N-(2,4,5trihydroxyphenyl)acetamide (8), respectively. Because of the fast Michael addition reaction of hydroxide ion with o-benzoquinone 7, the anodic and its cathodic counterpart peak of oxidation of 6 was not observed. 3.4. Electrochemical behaviour in pHs 5–9 Cyclic voltammetry of acetaminophen (1) (1 mM) in aqueous solution containing 0.2 M phosphate buffer (pH = 7.0) shows one anodic (A1 ) and the corresponding cathodic peak (C1 ), which corresponds to the transformation of acetaminophen (1) to Nacetyl-p-benzoquinone-imine (NAPQI) (2) and vice-versa within a quasi-reversible two-electron process (Fig. 9, curve a) [13,14]. In a first-order reactions, the characteristic lifetime of a chemical reaction with rate constant k can be taken as t1 = 1/k, but for a second-order reaction (e.g., dimerization) t2 = 1/kCi , where Ci is the initial concentration of reactant, t1 is the time required for the reactant concentration to drop to 37% of its initial value in a firstorder process, and that t2 is the time required for the concentration
to drop to one-half of Ci in a second-order process. Therefore, dimerization can be distinguished from the first-order one by the dependence of the electrochemical response on Ci [34]. As is shown in Fig. 9, the peak current ratio (IpC1 /IpA1 ) is dependent to concentration of acetaminophen (1) and proportional to the augmentation of it, the peak current ratio (IpC1 /IpA1 ) decreases. The dependence of peak current ratio (IpC1 /IpA1 ) on concentration of acetaminophen (1) is indication of dimerization reaction after electron transfer process [34]. Also, cyclic voltammograms mentioned in Fig. 9 show the shift of peak C1 to negative direction proportional to the increasing of concentration. The negative shift of the peak C1 which is enhanced proportional to increasing of concentration of acetaminophen (1) can be due to the formation of a thin film of product (dimer 11) at the surface of the electrode inhibiting to a certain extent the performance of the electrode process [27,26,35]. It is also seen that proportional to the decrease of the potential sweep rate, the peak current ratio (IpC1 /IpA1 ) decreases (Fig. 10). Fig. 10, curve a, shows the cyclic voltammograms of 1 mM of acetaminophen (1) in low scan rate (5 mV s−1 ). In this case, the peak current ratio (IpC1 /IpA1 ) is about 0.45, while, in higher scan rate (250 mV s−1 ) this value reaches to 0.69 (Fig. 10, curve e). Controlled-potential coulometry was performed in an aqueous phosphate buffer solution (c = 0.2 M, pH = 7.0), containing 0.5 mmol of 1 at 0.45 V versus SCE. The monitoring of electrolysis progress was carried out by cyclic voltammetry (Fig. 11). It shows that, proportional to the advancement of coulometry, the anodic peak A1 decreased and disappears when the charge consumption becomes 1.1e− per molecule of 1. Diagnostic criteria of cyclic voltammetry, the consumption of one electron per molecule of 1, and the spectroscopic data of the isolated product (Fig. 12), indicated that the reaction mechanism of electrooxidation of 1 is dimerization reaction (Scheme 3).
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Fig. 10. Cyclic voltammograms of 1 mM acetaminophen (1) in aqueous solution containing 0.2 M phosphate buffer (pH = 7.0) in various scan rates at glassy carbon electrode. Scan rates from a to e are: 5, 10, 25, 100 and 250 mV s−1 , respectively. Inset: variation of peak current ratio (IpC1 /IpA1 ) versus scan rate. t = 25 ± 1 ◦ C.
According to our results, it seems that the Michael addition reaction of anion 1a via C-alkylation to NAPQI (2) leading to dimer 11. The polymerization reaction was circumvented during the preparative reaction because of the insolubility of the dimer 11 in aqueous solution containing 0.2 M phosphate buffer (pH = 7.0). The electrochemical synthesis of dimer 11 has performed according to presented procedure in Section 2. The 1 H NMR spectrum of dimer 11 (Fig. 12) consisted of three singlet peaks with chemical shifts at 1.99 ppm for the acetyl protons, at 9.03 ppm for the hydroxyl proton, and at 9.76 ppm for the amide proton. The protons with resonance at 6.79 ppm (H-2) and 7.34 ppm (H-3) were shown by proton–proton spin coupling constants to be ortho to one another (J2,3 = 9.3). The 13 C NMR spectrum of dimer 11 showed only eight peaks. Among these peaks, peaks with chemical shifts at 24.2 and 168.0 ppm are belonging to acetyl and carbonyl carbons, respectively. Other peaks are pertaining to aromatic carbons. 13 C NMR, 1 H NMR and MS analyses of dimer 11 were consistent with the formation of the symmetric dimer 11. In valuable published work by Potter et al. polymerization of acetaminophen (1) catalyzed by horseradish peroxidase has been discussed [18]. They isolated six types of products from reaction mixture using semi-preparative high pressure liquid chromatography [18]. In mentioned work, two dimers, three trimers and one tetramer were identified. These dimers formed through a covalent bond between carbons ortho to the hydroxyl group, and to a lesser extent, between the carbon ortho to the hydroxyl group and the amino group of another acetaminophen molecule. However in this work, spectroscopic data of final product (Fig. 12) shows that in our experimental conditions, the main product in electrooxidation of
Fig. 11. Part I, cyclic voltammogram of 0.5 mmol acetaminophen (1) in solution containing 0.2 M phosphate buffer (pH = 7.0) at a glassy carbon electrode during controlled-potential coulometry at 0.45 V versus SCE. After the consumption of: (a) 0 C, (b) 15 C, (c) 25 C and (d) 45 C. Part II: variation of peak current (IpA1 ) versus charge consumed. Scan rate 100 mV s−1 . t = 25 ± 1 ◦ C.
acetaminophen (1) in aqueous solution containing 0.2 M phosphate buffer (pH = 7.0) is dimer 11. 3.5. Kinetic evaluation Electrochemical oxidation of acetaminophen (1) in various pHs was tested by digital simulation. The simulation was carried out assuming semi-infinite one-dimensional diffusion and planar electrode geometry. The experimental parameters entered for digital simulation consisted of the following: Estart , Eswitch , Eend , t = 25 ◦ C and analytical concentration of acetaminophen (1). The transfer coefficient (˛) was assumed to be 0.5 and the formal potentials were obtained experimentally as the midpoint potential between the anodic and cathodic peaks (Emid ). The heterogeneous rate constant (0.002 cm s−1 ) for oxidation of acetaminophen (1) was estimated by use of an experimental working curve [36]. All these parameters were kept constant through out the fitting of the digitally simulated voltammogram to the experimental data. The parameter kobs was
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Fig. 12.
1
H NMR and 13 C NMR of dimer 11.
Scheme 3.
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Fig. 13. Cyclic voltammograms of 1 mM acetaminophen (1) in various pHs and scan rates: (a) experimental, (b) simulated. (I) HClO4 1.0 M, (II) pH = 6.0 and (III) pH = 13.0, at glassy carbon electrode. Scan rate from I to III are: 5, 50 and 50 mV s−1 , respectively. t = 25 ± 1 ◦ C.
Scheme 4.
allowed to change through the fitting processes. The simulation was performed based on proposed mechanisms in Schemes 1–3 (Fig. 13). The simplified mechanisms in various pHs are shown in Scheme 4. Table 1 shows the kobs as a function of pH. Table 1 The observed hydrolyzes, hydroxylation and dimerization rate constants (kobs ) for N-acetyl-p-benzoquinoneimine (NAPQI) in various pHs. Solution condition
kobs (s−1 )a , b
pH = 0.0c pH = 0.3d pH = 1.0e pH = 2.0 pH = 3.0 pH = 4.0 pH = 5.0 pH = 6.0 pH = 7.0 pH = 8.0 pH = 9.0 pH = 10.5 pH = 13.0
25.30 17.60 9.67 0.85 0.20 0.12 0.11 0.17 0.16 0.14 0.06 8.25 16.40
± ± ± ± ± ± ± ± ± ± ± ± ±
As shown in Table 1, kobs is strongly dependent to solution’s pH, and decreases with increasing of it in pH range 0–2 (Fig. 14). The observed trend is expected, because of the participation of proton in the electrochemical oxidation of acetaminophen (1) in acidic media (Scheme 1). Also, in pH > 9, kobs increases with increasing pH. This is also, expected because of the contribution of hydroxide ion in the oxidation of 1 in alkaline solutions (Scheme 2). In this study, the least observed homogeneous rate constant (kobs ) belongs to
1.52 0.53 0.19 0.03 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.31 0.67
a Standard deviation of four independent simulations at various scan rates. b For dimerization reaction: M−1 s−1 . c HClO4 (1.0 M). d HClO4 (0.5 M). e HClO4 (0.1 M).
Fig. 14. Variation of the observed homogeneous rate constant (kobs ) as a function of the pH.
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pHs 5.0 and 9.0 (Fig. 14, inset). This is in agreement with Fig. 4 and shows a relative stability of NAPQI (2) in pHs 5.0 and 9.0. 4. Conclusions The results of this work show that acetaminophen (1) is oxidized in aqueous solutions to N-acetyl-p-benzoquinone-imine (NAPQI) (2). The NAPQI (2) participates in dimerization, hydroxylation or hydrolyzes reactions depend on solution’s pH. The reaction mechanisms for anodic oxidation of acetaminophen (1) is presented in Schemes 1–3. We studied the kinetic of the subsequent reactions of the electrochemically generated NAPQI (2) by cyclic voltammetric technique. The cyclic voltammograms were digitally simulated under proposed mechanisms (Schemes 1–3). The simulated cyclic voltammograms show good agreement with those obtained experimentally. The results of the observed homogeneous rate constants (kobs ) are presented in Table 1. The magnitude of observed homogeneous rate constants (kobs ) is dependent on the solution’s pH. The most amounts of kobs are calculated in pHs less than 2 and more than 9. In this study, the least observed homogeneous rate constant (kobs ) belongs to pHs 5.0 and 9.0. Acknowledgments We would like to thank Dr. M. Rudolph for his cyclic voltammogram digital simulation software (DigiElch SB) and the authors acknowledge the Bu-Ali Sina University Research Council and Center of Excellence in Development of Chemical Methods (CEDCM) for support this work. References [1] M. Ebadi, Electrochim. Acta 48 (2003) 4233. [2] L.J. Roberts, J.D. Morrow, in: J.G. Hardman, L.E. Limbird, A. Goodman Gilman (Eds.), The Pharmacological Basis of Therapeutics, 10th ed., McGraw-Hill, London, 2001. [3] R.V. Blanke, W.J. Decker, in: N.W. Tietz (Ed.), Textbook of Clinical Chemistry, W.B. Saunders, Philadelphia, 1986. [4] D. Gunnell, V. Murray, K. Hawton, Suicide Life Threat. Behav. 30 (2000) 313. [5] N. Wangfuengkanagul, O. Chailapakul, J. Pharm. Biomed. Anal. 28 (2002) 841.
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