Krishnan, V. R.

Reddy, S. Jayarama

English

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Indian .Journal of ChemistryVol. 15A, February 1977, pp. '(1-'9Electrochemical Oxidation of p-Chloroaniline at a Platinum ElectrodeS. JAYARAMA REDDY & V. R. KRISHNAN.Department of Chemistry, Sri Venkateswara University Postgraduate Centre, AnantapurReceived 23 February 1976; accepted 24 June 1976Anodic oxidation of p-chloroaniline in aqueous solutions has been carried out at a platinumelectrode using the techniques of cyclic voltammetry and chronoamperometry. Mechanism forthe oxidation process has been proposed and is consistent with the electrochemical data andthe products of the reaction. The data is also utilized in the evaluation of the transfer anddiffusion coefficients of the electro active species under consideration. The nature of adsorptioninvolved is discussed.LORD and Rogers- have carried out theconventional voltammetric studies on theoxidation of anilines by recording polaro-grams at gold, graphite, platinum and lead dioxideelectrodes. Anodic voltammetry at an impregnatedgraphite electrode has been applied to study theeffect of ring substitution on the half-wave potentialsof phenol and aniline and to determine whetherthe half-wave potentials of disubstituted anilinesand phenols could be reliably predicted from thevalues of monosubstituted compounds". Bacon andAdams" have carried out cyclic voltammetric studiesof a series of substituted anilines, including p-chloro-aniline, in acid media at a carbon paste electrodeand postulated that these compounds undergo ahead to tail coupling giving the corresponding 4'-substituted p-amino diphenylamines. Santhanam!showed that the process involved removal of onlyone electron with the formation of 4,4'-dichloroazo-benzene as the product of oxidation. Wawzonekand McIntyre6 reported that when p-substitutedanilines were oxidized in acetonitrile in the presenceof pyridine, N-N coupling occurred to give hy.lrazo-benzenes which further underwent oxidation to thesubstituted azobenzenes,In the present investigation, an attempt is madeto understand the nature and mechanism of theelectrochemical oxidation of p-chloroaniline at aplatinum electrode using cyclic voltammetry andchronoamperornetry. The data obtained by thesetechniques have been made use of in the evaluationof transfer and diffusion coefficients of the abovespecies in the media of different PH values.The theoretical principles and various applicationsof the cyclic voltammetric and chronoamperometrictechniques have been describe 1in detail in the litera-turee".Materials and MethodsP-Chloroaniline used was a Burgoyne samplemelting sharply at 70°. Solution of p-chloroanilinewas prepared by first dissolving it in the minimumquantity of ethanol and then diluting with waterto the required volume..Present address: Department of Chemistry, Sri Venkate-swara University College, Tirupati.96The supporting electrolytes used Were: (i) Clarksand Lubs buffer of pH 1'3, prepared by mixing 200ml of 0'1N hydrochloric acid and 250 ml of 0·2Npotassium chloride and diluting to 1000 ml.(ii) Phosphate buffer of jJH 6·6, prepared by mixing178 ml of O'lN sodium hydroxide and 500 ml ofO·IN potassium dihydrog en phosphate and makingup the resulting solution up to 1000 ml. Othersupporting electrolytes (O·IN sodium carbonate,O·lN sodium hydroxide and IN sulphuric acid)were prepared using DDH Analar grade samples.A multi-purpose electrochemical instrument whichcould function as a polarograph, a potentiostat, acyclic volt ammeter and a galvanostat, constructedby Bhagnt awl Sant hanam'' at the Tata Instituteof Fundamental Research, was employed in thepresent work. It could provide automatic cyclingbetween the pot entials nee.le.l for cyclic volt am-metric work with tlte facility to hold the instrumentat any desired value. A Mosley-135AM X-V re-corder of Hewlett & Packard was employed forrecording both the cyclic volt ammogram s andchronoamieromelrir; curves. Fortran IV computerIBM system 360/44 was used for the computationof clata.Procedure - Equal volumes C IOO ml) of thesupporting electrolyte were taken separately in a400 ml beaker and in a porous vessel of convenientsize kept in the beaker. The supporting electrolytein the porous vessel was rendered free of dissolvedoxygen by passing nitrogen gas. The solution wasalso kept under nitrog-en atmosphere to avoidatmospheric oxidation. A platinum wire electrodeof radius O· 5 mm was kept immersed in the solutionin the porous vessel and a spiral platinum rodcounter electrode was placed outside in the solutionin the beaker. A known volume of the electroactivespecies under study in solution at known concen-tration was adde.I and cyclic voltammograms re-corded at the required sweep rates. Cyclic volt am-mograms were obtained at different concentrationsof the electroactive species. Before every run thesolution was stirred by bubbling nitrogen gas for5 min. Cyclic voltammograms for the supportingelectrolytes were also recorded in order to eva-luate the exact contributions of the electroactivespecies.REDDY & KRISHNAN: ELECTROCHEMICAL OXIDATION OF p-CHLOROANILINEIn the chronoamperometric experiments, theelectrolytic cell was prepared in the same manneras above and after the supporting electrolyte wassaturated with nitrogen gas, a known amount ofthe electroactive species was added and the current-time curves recorded at the required potentials.The current-time curves were also taken for thesupporting electrolyte at the potentials at whichthe current-time curves were recorded for the species.Saturated calomel electrode was used as a referenceelectrode in all the above measurements.Performance of the working electrode was checkedby carrying out the cyclic voltammetric andchronoamperometric studies using potassium ferro-cyanide as a test substance in 2M potassiumchloride as the supporting electrolyte, in view ofits known behaviour in terms of reversibility andabsence of adsorption at the electrode. Chrono-amperometric data of this system have beenemployed to evaluate the electrochemical area ofthe working electrode which in the present casewas found to be 1·326 sq. cm.Results and DiscussionCyclic voltammetric results - Typical cyclicvolt ammograms of p-chloroaniline in sulphuric acidand Clarks and Lubs buffer are presented in Fig. 1.The electrochemical oxidation of p-chloroanilinetakes place at +0·98 V (vs SCE) in sulphuric acid(Fig. 1a) and at +0'96 V (vs SCE) in Clarks andLubs buffer (Fig. 1b). The anodic peak potentials(vs SCE) observed in phosphate buffer, sodium car-bonate and sodium hydroxide are +0·94 V (Fig.2a), +0'75 V (Fig. 2b) and +0'68 V (Fig. 2c)respectively. The peak potential values given aboverefer to particular sweep rates and concentrationsof the species. Table 1gives the cyclic voltammetricdata for p-chloroaniline with reference to the peakpotential values at different sweep rates and PHvalues. Ep has been found to vary with pH. Theshift of peak potentials with sweep rate in thedifferent media employed and the absence of currentin the reverse scan suggests the process to beirreversible.Besides the regular oxidation peaks discussedearlier, additional redox couples were also noticedin the subsequent sweeps in certain media asrevealed by the data in Table 2. The redox coupleswere not noticed for the potential ranr.e ernj loyedat zoo II' Abt-Z~Q:Q::::>uo.--0·& V I-Zwa:a::::>uFig. 1 - Cyclic voltammograms of p-chloroaniline in (a)HaSO. (1'5 mM, sweep rate = 0·72 V see-I), (b) Clarks andLubs buffer (1·5 mM, sweep rate = 0'28 \' sec=)Q b1500 IJA~wIf:::>u I-Zwa::VOL TAGE a:::::>uc 0-0·2VI-zwcrDc:::>UoVOLTAGEFig. 2 - Cyclic voltammograms of p-chloroaniline in (a)phosphate buffer (4'32 mM), (b) Na.CO (2'9 mM), (c) NaOH(2'9 mM) [sweep rate in all the cases = 0·13 V sec=]TABLE 1 - PEAK POTENTIALVALUES(Ep) AT DIFFERENTSWEEP RATES IN DIFFERENTSUPPORTINGELECTROLYTESINCYCLICVOLTAMMETRYOF P-CHLOROANILINE(Conc. of p-chloroaniline = 1·5 rnM)Sulphuric acid CL buffer Phosphate buffer Sodium carbonate Sodium hydroxidevjV see"! EpjV vjV see'< EpjV vj\' see"! EpjV vjV sec-I EpjV vjV sec-I EpjV0·05 +0·96 0·04 +0'92 0·04 +0'79 0·04 +0'64 0'04 +0'590·14 +0·96 0·14 +0·95 0·13 +0'84 0·13 +0·72 0·13 +0·650·28 +0·97 0·28 +0'96 0·29 +0·88 0·26 +0·72 0·26 +0·660·35 +0·97 0·35 +0'96 0·37 +0'89 0'36 +0'73 0'35 +0·6&0·49 +0·97 0·49 +0'97 0·49 +0·90 0·51 +0·74 0'51 *0·72 +0·98 0·71 +0'99 0·70 +0·92 0·72 +0·78 0·72 **Well-defined peaks are not obtained.97INDIAN J. CHEM., VOL. 15A, FEBRUARY 1977TABLE 2 - ADDITIO~AL PEAK POTENTIALSIN THE CYCLICVOLTA)IMETRYOF P-CHLOROANILINESupporting Sweep width Additional peakelectrolyte employed/'V potentials/VAnodic CathodicSulphuric acid Otol·22 +0·48 +0·40Clarks and Lubs -0·06 to +1·16 +0'40 +0·34bufferPhosphate buffer +0·15 to + 1·18 +0·18 +0·06Sodium carbonate -0·01 to +0'97Sodium hydroxide -0·1 to +0'90TABLE 3 - VALUES OF THE PARAMETERip/v~cSweep Conc.jm Jf Currcnt/uamp ip/v!crate/V see'?S1:LPHURICACID0·04 108010238951·52·94·3320600780CLARKSAND LUBS BUFFER (PH 1·3)0·04 1·52-94·31054933931320560830PHOSPHATEBUFFER (PH 6'6)0·04 1·52·94·31384116510984207009800·04SODIUMCARBONATE(PH 11·4)1·5 4152'9 7104·3 930SODIUMHYDROXIDE(PH 13'0)1·5 4202·9 7904·3 10601427139912641436124110940·04in sodium carbonate and sodium hydroxide support-ing electrolytes. It is interesting to note thatthe product of oxidation is itself involved in theformation of redox couples in the subsequent sweeps.This is supported by the fact that the cyclic volt am-mo zratns of the electro-oxidized product of p-chloro-aniline (electro-oxidize i for 1 hr and then the cyclicvoltamrnograms taken) show the evidence of thepresence of re Iox couple even in the first sweep.In the calculation of Jl value usin ; the basicequation for peak currentip = 2·98 X 105.n(ocna) 1/2 ADl/2V1/2Cthe diffusion coefficient of p-methoxyphenol, a similarsized molecule (reported" in the literature as0'70~ 10-5 ern" sec+) has been employed for thecomputation of ip/V1/2 c values for comparing withthose obtained from the experiment (vide Table3). The OC1la values emoloyed are calculated fromcyclic voltammetric results (Table 4). The numberof electrons involved in the electrochemical oxidationof p-chloroaniline in sulphuric acid and Clarks andLubs buffer is evaluated to be one and in the othermedia to be two.In acidic solutions, the anodic reaction pathwayfor p-chloroaniline may probably be given as shown98TABLE 4 - DIFFUSION COEFFICIENT (D) DATA FORP-CHLOROANILINEAT 24°(Cone. of p-chloroaniline = 1·5 mM)Medium 105D (em- sec")Cyclic voltammetricchronoamperometric1N sulphuric acidClarks and Lubs bufferPhosphate bufferSodium carbonate (O'lN)Sodium hydroxide (O'LV)0·690·800·530·400'441·4501-0150·9841·8831'6630·7391·0180·5730'6670'667TABLE 5 - CHRO:-;[OAMPERO~IETRICDATA FORP-CHLOROANILI:-;[E*(Cone. of p-chloroaniline = 1·5 mJ!)Tirne/s Current (uamp) in(see)Sul- Clarks Phos- Sodium Sodiumphuric and ph ate carbo- hydro-acid Lubs buffer nate xidebuffer2 205 244 355 382 3554 150 172 250 246 2205 136 150 225 213 1896 128 136 205 194 1707 1211 124 192 166 1528 114 112 175 158 1359 110 108 166 140 12111 100 96 152 117 10113 95 88 136 100 8715 93 86 127 90 7416 88 82 124 82 7218 85 78 115 76 5920 80 72 110 66 57·Potentials employed +1'10, +1,0, +0,90, +0·85 and+0·70 V in sulphuric acid, CL buffer, phosphate buffer,Na.C03 and NaOH respectively.in Scheme 1 and the linearity of ip vs V1/2 plot forp-chloroaniline in sulphuric acid suggests that theelectro::le process is diffusion-controlled.Bacon and Adamss had observed a redox couplehaving an anodic peak at +0·25 V and cathodicpeak at +0'275 V (vs SCE) for a sample of 4-amino-4'-chlorodiphenylamine at PH 3·1 at carbon pasteelectrode. This suggests that the additional redoxcouples noticed in the different media, which werefound to shift with PH, are probably due to the4-amino-4'-chlorodiphenylamine, the product ofoxidation of p-chloroaniline.In the alkaline media reaction (1) yielding 4,4'-dichloroazobenzene can be suggested.2(p-ChloroaniIine) -+ 4,4'-dichloroazobenzene+4W+4e ... (1)The formation of 4,4'-dichloroazobenzene as aproduct of oxidation has been confirmed by theabsorption spectrum of the product collected after30 min at the potential +0·70 V (vs SCE).Chronoamperometric results - Current-time data forthe electrochemical oxidation of p-chloroaniline inthe different media and the potential steps employedare presented in Table 5. These values refer to aREDDY & KRISHNAN: ELECTROCHEMICAL OXIDATION OF p-CHLOROANILINEhydroxide (100 to 125 mV cathodic of the regularpeaks) indicate that a strong adsorption of theproduct of oxidation may be responsible for reducingthe effective area of the electrode and hence thediscrepancy in results-v.The diffusion-coefficient data for p-chloroanilineevaluated using both the techniques are presentedin Table 4.The diffusion-coefficient values obtained bychronoamperometric method are preferred to cyclicvoltammetric values for the following reasons:(i) Cyclic voltammetric evaluation of diffusion-coefficientrequires the knowledge of ex?,,, and ip/Vl/2 cvalues. exn" is extremely sensitive to the surfacestate of the electrode and ip/v1f2 c values are alsoinfluenced by adsorption. (ii) Chronoamperornetricevaluation of the diffusion-coefficient requires aknowledge of (it1f2)o values. Since these values arecalculated by extrapolation of it1/2 values to zerotime, the diffusion-coefficient values obtained bythis method are not likely to be affected by thecomplications associated with the surface state ofthe electrode.N 420.......u<IIIII ('a)«=<.N~-+'(b)2.20o e 16 toTIME,seeFig. 3 - itt vs t plots of p-chloroaniline in (a) H2SO •.(b) Clarks and Lubs buffer, (c) phosphate buffer, (d) Na.CO.and (e) NaOHparticular concentration, and as the data followsimilar pattern at other concentrations studied,they are not shown here. it1/2 vs t plots of p-chloro-aniline (Fig. 3) show that the results are affectedin phosphate buffer, sodium carbonate and sodiumhydroxide. The decreasing trend in the variationof itlf2 with time suggests complications due toadsorption on the electrode in these media. Thepre-peaks obtained in sodium carbonate and sodium(d)References1. LORD, S. S. & ROGERS, L. B., Analyt. Chern., 28 (1952),284.2. SAUTONI,J. c., SNYDER,R. E. & CLARK,R. 0., Analyt.Chem., 33 (1961), 1894.3. BACON,J. & ADAMS,R. N., J. Am. chem, Soc., 84 (1968),6596.4. SANTHANAM,K. S. V., Studies in the electrometric methodsof estimations, Ph.D. Thesis, S. V. University, Tirupati,1963.5. WAWZONEK,S. & McINTYRE, T. W., J. electrochem, Soc.,114 (1967), 1025.6. ADAMS, R. N., Electrochemistry at solid electrodes (MarcelDekker, New York), 1969.7. BROWN,E. R. & LARGE,ROBERTF., Techniques of organicchemistry; Part IIA -s--Electrochemicat methods editedby Weissberger and Rossiter (Wiley, New York), 1971,Chapter VI.8. BHAGAT,V. R. & SANTHANAM,K. S. Y, J. scient. indoRes., 30 (1969), 221.9. ADAMS,R. N., Electrochemistry at solid electrodes (MarcelDekker, New York), 1969, 221.10. WOPSCHELL,H. & SHAIN, I., Analyt. Chem., 39 (1967),1514.99

Electrochemical Oxidation of p-Chloroaniline at a Platinum Electrode

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Electrochemical Oxidation of p-Chloroaniline at a Platinum Electrode

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