Journal of The Electrochemical Society, 160 (10) H693-H698 (2013) H6930013-4651/2013/160(10)/H693/6/$31.00 © The Electrochemical Society

Experimental and Theoretical Analysis of the ElectrochemicalOxidation of Catechol and Hydroquinone Derivatives in thePresence of Various NucleophilesHadi Beiginejad, Davood Nematollahi,∗,z Mehdi Bayat, Fahimeh Varmaghani,and Ali Nazaripour

Faculty of Chemistry, Bu-Ali-Sina University, Hamedan, Iran

Electrochemical oxidation of some catechol and hydroquinone derivatives has been investigated both experimentally and theoreticallyto bring hints into the connection of thermodynamic and oxidation potential. The theoretical results were calculated at DFT (B3LYP,BP86) levels of theory and 6–311G (p,d) basis sets. In this study we focus on the mechanisms of the electrochemical oxidation ofcatechol in the presence of various nucleophiles. A general thermodynamic cycle, which is proposed to calculate �G of oxidationof the intermediates and products, introduces thermodynamic as one of the important parameters on the reaction mechanisms.The results of this work show that electrochemical oxidation potential of studied compounds is directly dependent on the �Gof electrochemical oxidation. Also it was found that depending on the �G of electrochemical oxidation, the products which areproduced on the surface of electrode will participate in the electrochemical or chemical following reactions. Finally we can considerseveral mechanisms in the electrochemical oxidation of catechol in the presence of different nucleophiles.© 2013 The Electrochemical Society. [DOI: 10.1149/2.037310jes] All rights reserved.

Manuscript submitted July 5, 2013; revised manuscript received July 30, 2013. Published August 7, 2013.

Electrochemical methods are widely applied to study of the re-actions of electroactive compounds.1–3 They can be used to obtainboth thermodynamic and kinetic information.4 Among electrochemi-cal techniques, cyclic voltammetry and controlled-potential coulome-try have been used as a powerful independent route for qualitative andquantitative characterization of complex electrode processes. Becauseelectrochemical oxidation very often parallels to cytochrome P450catalyzed oxidation in liver microsomes,5 it is interesting to study theanodic oxidation of catechols in the presence of various nucleophiles.In this direction, we have synthesized a vast number of valuable deriva-tives of catechol and hydroquinone using electrochemical oxidationof them in the presence of various nucleophiles.6–16 As reported previ-ously, the electrochemical oxidation of catechols and hydroquinonesin the presence of different nucleophiles leads to the formation offinal products via various mechanisms such as EC′,17 EC,6–8 ECE,9–11

ECEC,12,13 ECECE,14,15 ECECECE16 and etc. In these mechanisms,two-electron oxidation of catechols or hydroquinones is followed by aMichael addition reaction of nucleophile on electrogenerated quinone.The mechanism is depending on some parameters such as, nature ofnucleophile (electron withdrawing or donating), electrolysis medium(solvent, acidity or pH) and catechol type.18

The aim of this work is providing an answer to the question of:“why the mechanisms of the electrochemical oxidation of catecholsand hydroquinones are so different in the presence of different nucle-ophiles?” In the present paper, �G of oxidation of the intermediatesand products is considered as one of the most important parameters in-fluenced on the reaction mechanism. So, in this work, the electrochem-ical oxidation of catechol has been studied in the presence of N,N-dimethylethylendiamine, p-toluenesulfinic acid, triphenylphosphine,methyl-Meldrum’s acid and Meldrum’s acid as the nucleophiles. Alsothe effect of �Gtot of the electrochemical oxidation of intermediatesand products on the reaction mechanism was investigated with consid-ering a general thermodynamic cycle (Born- Haber cycle). �Gtot of theoxidation of catechol derivatives, such as intermediates and productswere calculated at DFT (B3LYP, BP86) levels of theory using 6–311G(p,d) basis sets. In addition, Ep0 (oxidation potential of studied speciesin pH = 0.0) and �Gtot of the electrochemical oxidation of catecholand hydroquinone derivatives (1–10) have been calculated from E-pHdiagram and computational method, respectively. The results show agood correlation between �Gtot of electrochemical oxidation and Ep0.

∗Electrochemical Society Active Member.zE-mail: nemat@basu.ac.ir

Experimental

Apparatus and reagents.— Cyclic voltammetry was performed us-ing micro Autolab potentiostat/galvanostat. The working electrodeused in the voltammetry experiments was a glassy carbon disk (1.8 mmdiameter) and platinum wire was used as the counter electrode. Theworking electrode potentials were measured versus standard Ag/AgCl(all electrodes from AZAR Electrode). All chemicals were reagent-grade materials, from Aldrich and E. Merck. These chemicals wereused without further purification.

The phosphate buffer solution (NaH2PO4/Na2HPO4) was preparedby dissolving appropriate amounts of sodium dihydrogen phosphate(NaH2PO4) and the pH was then adjusted to 7.0 using concentratedsodium hydroxide and the volume was completed to 1.0 L with deion-ized water (c = 0.2 M). The same procedure was used for preparationof acetate (pH 4-5) and H3PO4/NaH2PO4 (pH 2-3) buffers. All exper-iments were carried out at room temperature.

Computational methods.— The geometries of all species in the gasphase were fully optimized at 6–311G (p,d) basis sets DFT(B3LYP,BP86) levels of theory using the Gaussian 03.19 Vibrational frequencyanalysis, calculated at the same level of theory, indicates that op-timized structures are at the stationary points corresponding to lo-cal minima without any imaginary frequency. A starting molecular-mechanics structure for the ab initio calculations was obtained usingthe HyperChem 5.02 program20 (MM+ method). To calculate solva-tion energies, apopular continuum model of solvation, the Conductor-like Polarizable Continuum Model (CPCM)21 with the setting ICOMP= 0, have been used at the above levels of theory. The optimized atomicradii were invoked via the solvent keyword RADII = UAHF. Thensolvation free energies were obtained using the SCFVAC keyword.There are several reported values for Gibbs free energy of H+ at gasphase, �G0(g,H+), and its solvation energy, �G0(s,H+), in solution.In this work the values of −6.2822 and −262.523 kcal/mol for latterenergies have been used respectively. In addition, based on what wasreported by Davis and Fry24 the value energy “−0.03766 eV” wasused for a free electron at 298 K in our calculation.

Results and Discussion

Voltammetric studies.— Electrochemical behavior of some cat-echol and hydroquinone derivatives (4-tert-butylcatechol (1), cat-echol (2), hydroquinone (3), 3-methylcatechol (4), 4-nitrocatechol(5), 2,3-dihydroxybenzoic acid (6), 3,4-dihydroxybenzoic acid (7),2-tert-butylbenzene-1,4-diol (8), 2,5-dihydroxybenzoic acid (9) and

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H694 Journal of The Electrochemical Society, 160 (10) H693-H698 (2013)

OH

OH

OH

OH

OH

OH

OH

OH

OH

OH

O2N

OH

OH

OH

OOH

OH

OHOOH

OH

OH

OH

OH

OOH

OH

O

2

3

4

7

5

6

1

8 9

10

Figure 1. The structure of studied catechol and hydroquinone derivatives.

Figure 2. Cyclic voltammogram of 1.0 mM 4-tert-butylcatechol (1) in 0.1 Mperchloric acid at a glassy carbon electrode. Scan rate = 100 mV/s. t = 25± 1◦C.

2,3-dihydroxybenzaldehyde (10)) (Fig. 1), has been studied in aque-ous solution at various pHs using cyclic voltammetry.

Fig. 2 shows the cyclic voltammogram of 1.0 mM of 4-tert-butylcatechol (1) in 0.1 M perchloric acid. The voltammogram showsan anodic peak (A1) in the positive-going scan and a cathodic coun-terpart peak (C1) in the negative-going scan which correspond to thetransformation of 1 to o-benzoquinone (1a) and vice-versa within aquasi-reversible two electron process (Scheme 1).

Cyclic voltammograms of 1.0 mM solution of 1 in aqueous solutionat various pHs are also shown in Fig. 3. It was found that the peakpotential of A1 (EpA1) shifted to the negative potentials by increasingpH. This is expected because of the participation of protons in theoxidation of 1. The anodic peak potential (EpA1), is given by:

EpA1 = Ep0 − (2.303m RT/2F)pH [1]

Where m is the number of protons involved in the reaction and Ep0 isthe anodic peak potential at pH = 0.0, R is the universal gas constant,T is the temperature (in Kelvin) and F is the Faraday constant. AnE-pH diagram is constructed for oxidation of the 1 by plotting theEpA1 values as a function of pH (Fig. 4). As can be seen, EpA1 isshifted to the negative potentials with the slope of 58.5 mV/pH. This

OH

OH1

-2e- -2H+O

O1a

Scheme 1. Electrochemical oxidation of 4-tert-butylcatechol (1).

Figure 3. Cyclic voltammograms of 1.0 mM 4-tert-butylcatechol (1), at aglassy carbon electrode, in buffered solutions at various pHs with same ionicstrength. pHs are: (a) 1.0, (b) 3.0, (c) 4.0, (d) 5.0, (e) 6.0, (f) 6.7 and (g) 8.0.Scan rate: 100 mV/s. t = 25 ± 1◦C.

slope is in agreement with the theoretical slope (2.303 mRT/2F) of59 mV/pH with m = 2. On the basis of the above mentioned slope,it can be concluded that the electrode reaction is a two electron-twoproton process (Scheme 1). In addition, Ep0 was calculated usingintercept of E-pH diagrams.

Similar considerations were performed for other species (2–10)and their Ep0 values were obtained. In addition, the effect of the �Gtot

of the electrochemical oxidation on the electrochemical oxidationpotential of latter compounds was studied. The �Gtot can be relatedwith potential via the Eq. 2.

�Gtot = −nF(Ec − Ea) [2]

Where Ec is the reduction potential of water in cathode and Ea isoxidation potential of studied species in anode. By considering thereduction of water as the cathodic reaction for all studied species,�Gtot is calculated by Eq. 3.

�Gtot = K + nF Ea [3]

Where K is constant and �Gtot is the free energy change for electro-chemical oxidation of studied species (1–10). As can be seen, �Gtot

showed a linear relationship with Ea. The change in the Gibbs free en-ergy, �Gtot, was calculated using 6–311G (p,d) basis set DFT(B3LYP,

Figure 4. Potential-pH diagram of 1.0 mM 4-tert-butylcatechol (1) in aqueoussolution.

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Journal of The Electrochemical Society, 160 (10) H693-H698 (2013) H695

Scheme 2. Thermodynamic cycle used to calculate Gibbs free energy.

BP86) levels of theory according to the Born-Haber cycle (Scheme 2)and Eq. 4.

�Gtot = �Gredoxgas + �Gredox

sol [4]

Where �Ggas is the standard Gibbs energy in gas phase and �Gsol isthe solvation energy and is defined as follows:

�Gredoxsol = �Gox

sol + 2�GH+gsol − �Gred

sol [5]

The gas-phase contribution to the Gibbs energy can be determinedusing DFT calculations. These calculations have been performed atBP86 and B3LYP levels. In this study, Ep0 is an indication of theoxidation potential (Ea) of studied species which is obtained usingpotential-pH diagram.

The calculated �Gtot and the Ep0 of studied species are summarizedin Table I. The plot of calculated �Gtot (using B3LYP level of theoryand 6–311G (p,d) basis sets) versus Ep0 was shown in Fig. 5. Theplot shows a good correlation between the electrochemical oxidationpotential (Ep0) and calculated �Gtot of electrochemical oxidation ofstudied species. Similar diagram was also obtained using BP86 levelof theory. The results show that the �Gtot of the electrochemicaloxidation increases upon the increasing Ep0. These data illustrate thatthermodynamic indicates precedence of electrochemical oxidation ofstudied species.

Effect of �G on the reaction mechanism.— Fig. 6 (curve a) showsa cyclic voltammogram of 1.0 mM catechol (2) in aqueous solutioncontaining 0.2 M phosphate buffer (pH = 7.0). The voltammogramshows an anodic peak (A1) in the positive-direction scan and a ca-thodic counterpart peak (C1) in the negative-direction scan whichcorrespond to the transformation of catechol (2) to o-benzoquinone(2a) and vice versa within a quasi-reversible two-electron process.Fig. 6 (curve b) shows the first cycle voltammogram obtained for a1.0 mM solution of 2 in the presence of 1.0 mM Melderam’s Acid(MA). The voltammogram exhibits two cathodic peaks C1 and C0. Anew peak (A0) appears in the second cycle (curve c) with an Ep value of0.08 V versus Ag/AgCl. This new peak is related to the electrochem-ical oxidation of intermediates 3a or 5a.15

As discussed in our previous published paper,15 controlled-potential coulometry of catechol (2) in the presence of MA was per-

Table I. The calculated �Gtot of electrochemical oxidation processand the Ep0 of studied species.

�Gtot (B3LYP) �Gtot (BP86) Ep0species Kcal/mol Kcal/mol (vs. Ag/AgCl)

1 233.29 232.42 0.562 237.01 236.84 0.603 232.59 232.43 0.524 235.21 234.49 0.565 246.16 245.36 0.766 241.36 241.70 0.697 242.36 241.79 0.718 228.77 228.61 0.489 238.80 239.15 0.6410 240.06 240.83 0.68

Figure 5. Diagram of �Gtot of the electrochemical oxidation versus Ep0 forstudied species (1–10) at DFT (B3LYP) level of theory using 6–311G (p,d)basis sets.

formed in potential 0.2 V versus Ag/AgCl. The anodic peak A1 andits counterpart (C1) decreased and disappeared when the charge con-sumption became about 6e− per molecule of 2. These observationsallowed us to propose the ECECE pathway (E ′′ represents an electrontransfer at the electrode surface, and “C ” represents a homogeneouschemical reaction) illustrated in Scheme 3 for the electrochemicaloxidation of 2 in the presence of MA. According to our previousexperience,15 it seems that the Michael addition reaction of MA as anucleophile to o-benzoquinone (2a) as a Michael acceptor, leads tothe intermediate 3a. The oxidation of this compound (3a) is easierthan the oxidation of the parent-starting molecule (2). In the next step,o-benzoquinone 4a, via an intramolecular Michael reaction, is con-verted to intermediate 5a. Further oxidation converts intermediate 5ainto the final product 6a.

To investigate the parameters which influencing on the reactionmechanism, the effect of the �Gtot values of the electrochemicaloxidation of catechol (2) and intermediates 3a and 5a were calculatedand presented in Scheme 3. As shown in Scheme 3 the trend of

Figure 6. Cyclic voltammograms of: (a) 1.0 mM catechol (2); (b) first and (c)second cycle of 1.0 mM catechol (2) in the presence of 1.0 mM Melderam’sacid (MA), respectively; at a glassy carbon electrode (1.8 mm diameter) insolution containing 0.20 M phosphate buffer (pH 7.0). Scan rate: 100 mV/s. t= 25 ± 1◦C.

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H696 Journal of The Electrochemical Society, 160 (10) H693-H698 (2013)

OH

OH2

-2e- -2H+ O

O2a

3a

-2e- -2H+

-2e- -2H+

O O

OO

O

OO

O

OH

3aO

OO

O

OH

4aO

OO

O

O

5a

O

OO

O

OH

5a

O

OO

O

OH

6a

O

OO

O

O

MA

OH

OOH

OH O

OH

O

O2a

ΔGE1 = +237.01 Kcal/mol

ΔGE2= +223.86 Kcal/mol

4aO

OO

O

O

O

ΔGE3= +232.86 Kcal/mol

E1

C

E2

C

E3

Scheme 3. Proposed mechanism for the electrochemi-cal oxidation of catechol in the presence of MA.

�Gtot is as following: �GE1> �GE3 > �GE2 As can be seen, thechange of Gibbs free energy (�Gtot) of the electrochemical oxidationof catechol is more than intermediates 3a and 5a. This led to theoxidation of intermediates 3a and 5a. So, 3a and 5a can easily oxidizeat the applied potential of 0.2 V during electrolysis. According to theseresults, we can say that the �Gtot of the electrochemical oxidation hasa significant role on the mechanism of reaction pathway.

Figure 7. Cyclic voltammogram of 1.0 mM catechol (2) (a) in the absence and(b) in the presence of 1.0 mM methyl-Meldrum’s acid (MMA) in phosphatebuffer solution (pH = 7.0) at a glassy carbon electrode. Scan rate = 100 mV/s.t = 25 ± 1◦C.

Fig. 7 shows cyclic voltammograms of 1.0 mM catechol (2) in theabsence (curve a) and in the presence of 1.0 mM methyl-Meldrum’sacid (MMA) (curve b) in phosphate buffer solution (pH = 7.0,c = 0.2 M). In the presence of 1.0 mM methyl-Meldrum’s acid(MMA), the cathodic peak current (IpC1) decreases and a new redoxpeak (A2 and C2) appears at more positive potentials.

Controlled-potential coulometry was performed in aqueous solu-tion containing catechol (2) and MMA at 0.2 V versus Ag/AgCl. Thefollowing changes: a) decrease in the anodic peak current A1, b) in-crease in the redox peak A2/C2 and c) disappearance of peak A1 afterconsumption of about 2e− per molecule of 2, were observed in thecyclic voltammograms during the advancement of coulometry. Ourprevious work15 and the present data allow us to propose an EC mech-anism for the electrochemical oxidation of catechol (2) in the presenceof MMA (Scheme 4).

OH

OH2

-2e- -2H+ O

O2a

7a

O O

OO

O

O O

O

OH

OHMMAMe

Me

O

O2a

7a

O

O O

O

OH

OH

Me

-2e- -2H+

7b

O

O O

O

O

O

Me

ΔGE1 = +237.01 Kcal/mol

ΔGE2= +240.16 Kcal/mol

Scheme 4. Proposed mechanism for the electrochemical oxidation of catecholin the presence of MMA.

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Journal of The Electrochemical Society, 160 (10) H693-H698 (2013) H697

OH

OH2

-2e- -2H+ O

O2a

OH

OHNuNu

8a,9a

SO2HP

Nu1 =

O

O2a

OH

OHNu

8a,9a

-2e- -2H+

OH

OHNu

8b,9b

8a and 8b

Nu2 =

9a and 9b

ΔGE1 = +237.01 Kcal/mol

ΔGox-Nu1 = +243.66 Kcal/mol

ΔGox-Nu2 = +238.66 Kcal/mol

Scheme 5. Proposed mechanism for the electrochemical oxidation of catecholin the presence of p-toluenesulfinic acid (TSA) and triphenylphosphine (TPP).

�Gtot of the electrochemical oxidation of catechol (2) and sub-stituted catechol in the presence of MMA (7a) was calculated. Theresults indicate that the �Gtot of the electrochemical oxidation of 7ais more positive than catechol. In other words, increasing the �Gtot

of the electrochemical oxidation of product (7a) led to the increasingof corresponding electrochemical oxidation potential. So, the finalproduct is produced via an EC mechanism.

Similar studies were performed in detail for the electrochemicaloxidation of catechol in the presence of p-toluenesulfinic acid (TSA)and triphenylphosphine (TPP) as nucleophiles. It was reported thatfinal species are produced via EC mechanism in two cases.6–8 �Gtot

of the electrochemical oxidation of products was calculated whichis shown in Scheme 5. As can be seen because the �Gtot of the

Figure 8. Cyclic voltammograms of: (a) 1.0 mM catechol (2); (b) first and(c) second cycle voltammogram of 1.0 mM catechol (2) in the presence of3.0 mM N,N dimethylethylendiamine (DMD), respectively; at a glassy carbonelectrode (1.8 mm diameter) in solution containing 0.20 M phosphate buffer(pH 7.0). Scan rate: 100 mV/s. t = 25 ± 1◦C.

electrochemical oxidation of products is higher than that of catechol,the electrolysis terminates after consumption 2e− per molecule ofcatechol.

Electrochemical oxidation of catechol in the presence of N,N-dimethylethylendiamine (DMD) was studied in previous work.14

Fig. 8 shows the first and the second cycles of cyclic voltammo-gram obtained for a solution of 2 in the presence of DMD. The anodicand cathodic peaks (A0/C0) are related to the oxidation and reductionof the final product which are produced on the surface of electrode.14

Controlled potential coulometry indicates final product is producedvia ECECE mechanism. Mechanism of the electrochemical oxida-tion of catechol in the presence of DMD and �Gtot of intermediatesare shown in Scheme 6. As can be seen thermodynamic allows to

OH

OH

2

-2e- -2H+O

O

2a

NH HN CH3H3C OH

OHNH

N

CH3

CH310a

OH

OHNH

N

CH3

CH310a

-2e- -2H+O

ONH

N

CH3

CH311a

OH

OHN

N

CH3

CH312a

OH

OHN

N

CH3

CH312a

-2e- -2H+ O

ON

N

CH3

CH313a

DMD

ΔGE1 = +237.01 Kcal/mol

ΔGE2 = +218.27 Kcal/mol

ΔGE3 = +210.67 Kcal/mol

Scheme 6. Proposed mechanism for the electrochemical oxidation of catechol in the presence of N,N-dimethylethylendiamine.

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H698 Journal of The Electrochemical Society, 160 (10) H693-H698 (2013)

oxidation of 10a and 12a. According to these data, it can be con-cluded that the reaction proceeds in the thermodynamically favoreddirection.

Conclusions

This investigation has provided an answer to the question of whythe mechanisms of the electrochemical oxidation of catechol are dif-ferent in the presence of various nucleophiles. In this direction, elec-trochemical oxidation of catechol has been studied in the presence ofN,N-dimethylethylendiamine, p-toluenesulfinic acid, triphenylphos-phine, methyl-Meldrum’s acid and Meldrum’s acid as nucleophiles.�Gtot of the oxidation of catechol derivatives, intermediates and prod-ucts was calculated using 6–311G (p,d) basis sets DFT(B3LYP, BP86)levels of theory. The results released that the amount of �Gtot ofthe electrochemical oxidation determines the mechanism of reactionpathway. Also, electrochemical oxidation of some catechol and hydro-quinone derivatives has been studied in the absence of nucleophiles.The results of this work show the electrochemical oxidation potentialis dependent of �Gtot of electrochemical oxidation.

Acknowledgments

We acknowledge Bu-Ali Sina University Research Council andCenter of Excellence in Development of Environmentally FriendlyMethods for Chemical Synthesis (CEDEFMCS) for their support ofthis work.

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