Anodic Coupling of Aniline

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Aug 27, 2010 - Summary: Electrochemical behavior of aniline at platinum electrode was studied in .... oxidation of 0.66 mM aniline in the presence of 0.44.
GENERAL AND PHYSICAL INAM-UL-HAQUE AND SHAMAILA SADAF

J.Chem.Soc.Pak.,Vol. 34, No. 6, 2012

Anodic Coupling of Aniline 1 INAM-UL-HAQUE* AND 2SHAMAILA SADAF Department of Chemistry University of Engineering and Technology Lahore 54890 Pakistan 1 Present address: Leiden Institute of Chemistry MCBIM Gorlaeus Laboratories Einsteinweg 55 Leiden University P O Box 9502, 2300 RA Leiden The Netherlands 2 Present address: Institute of Chemistry of New Materials University of Osnabrück 49069 Osnabrück Germany [email protected]*

(Received on 27th August 2010, accepted in revised form 28th June 2012) Summary: Electrochemical behavior of aniline at platinum electrode was studied in acetonitrile. The results are compared with those at carbon paste electrode. Little variation was found in the presence and absence of picric acid / picrate in the setup. Mechanism can be preliminarily diagnosed from amongst a number of possible pathways by taking into consideration values of experimentally determined slopes dEp/dlogC, dEp/dlog and dEp/dlogX as judged against theoretical predictions. Some of these parameters were also obtained in distilled water and in 9:1 acetonitrile /water.

Introduction Polymerisation of aniline mainly occurs with head-to-tail coupling in para-position [1-2]. Aniline shows a delayed nucleation with a following autocatalytic polymerisation with hemispherical growth. The existence of a polypara-toluidine (PPTOL) film proves orthocoupling branching reactions during polymerization [2]. The theory for surface redox processes perturbed by a father–son reaction under linear voltammetric conditions and in the absence of mass transport complications has been extended to remove the kinetic restrictions of earlier treatments. The solution of the initial value problem, when reactants and products remain in the adsorbed state, is shown to be closely related to that corresponding to reactants equilibrated with the bulk solution at any potential. Analytical expressions for the voltammetric waves are derived for the fast reversible and irreversible kinetic limits, and empirical equations are developed to relate the peak coordinates with experimental variables when only numerical solutions are available. Bidirectional kinetics of the father–son reaction leads to two voltammetric waves even when a single electron is being exchanged in the redox process [3]. Cyclic voltammetry was used to investigate the mechanism of anodic oxidation of four aromatic amines, together with the effect of the addition of bases, namely lutidine, sym-collidine, quinuclidine and hydroxide. Without a methoxy substituent in the para position of the phenyl group, the coupling of two radical cations occurs on the addition of weak bases but a deprotonation is involved in the presence of hydroxide ion. This deprotonation occurs in all

cases when a methoxy substituent is present on the phenyl group [4]. A comparative study of the electrochemical oxidation of N,N-dimethyl-1-naphthylamine (DMN) in emulsified water-nitrobenzene mixtures with different electrolytes and conditions was performed by quasi-stationary current-potential curves, cyclic voltammetry, controlled potential electrolysis and rotating disk electrode experiments combined with conventional analytical techniques [5]. The anodic oxidation mechanism of DMN in the two-phase system is similar to that in non-aqueous solvents. However due to the ionic migration across the liquidliquid interface, the product N,N,N’,N’-tetramethyll,l’-naphthidine dication (TMN2+) remains in the organic phase while the protons migrate to the aqueous phase, leading to an appreciable increase in the yield of the organic product compared with the one obtained in one-phase non-aqueous electrolysis. The electrochemical oxidation of 4-chloro-, 2,4-dichloro-, 2,4,6-trichloro-anilines and 4-bromo-, 2,4-dibromo-, 2,4,6-tribromo-anilines and 4-iodoaniline were investigated in unbuffered acetonitrile solution. The main reaction during the electrooxidation of these compounds is the dimerization which is accompanied by release of the para-substituent halide and formation of protons. Apparent electron exchange number (napp) determination in the voltammetric time scale has been attempted [6]. This work relates to electrooxidation of aniline [7-36] on platinum in comparison with carbon paste. Results and Discussion

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J.Chem.Soc.Pak.,Vol. 34, No. 6, 2012

Voltammetric Studies Fig. 1 shows cyclic voltammetric anodic peak currents for aniline with and without ferrocene. Peak current in the presence of ferrocene was well within the experimental variation usually associated with cyclic voltammetric measurements.

regression for the entire data set are y = 0.0338x + 1.0608, R2 = 0.9768. Fig. 2 shows two linear portions and the slope, intercept and regression of the first linear portion are y = 0.011x + 1.0429, R2 = 0.9132 and that of second linear portion are y = 0.038x + 1.0607, R2 = 0.9851. 1.08

II

E/V

1

0.8

0.6

0.4

0.2

0

a

1.2

Ep/V

1.06

1.04 I a

1 mA

1.02 -2

b

-1.5

-1

-0.5

0

0.5

log (  / Vs-1)

Fig. 1: Cyclic voltammograms of (a) 0.66 mM aniline in the absence and (b) presence of 0.44 mM ferrocene. Sodium perchlorate 0.1 M in acetonitrile at platinum electrode, scan rate 0.1 Vs-1

Cyclic Voltammetric Scan Rate Dependence of Aniline in the presence of Ferrocene Influence of scan rate on anodic peak current of 0.66 mM aniline in the presence of 0.44 mM ferrocene was studied. Anodic peak potential for aniline is shifted to more positive values when the scan rate in cyclic voltammetry was increased. Such behavior is consistent with the occurrence of chemical reactions following a reversible electron transfer step [27-28]. The influence of square root of scan rate on cyclic voltammetric anodic peak current for the oxidation of 0.66 mM aniline in the presence of 0.44 mM ferrocene shows linear trend indicating the process is diffusion controlled. Influence of log scan rate on cyclic voltammetric half peak potential for the oxidation of 0.66 mM aniline in the presence of 0.44 mM ferrocene exhibits linear trend with the slope, intercept and regression given by y = 0.0221x + 1.0062, R2 = 0.9916. The influence of log scan rate on cyclic voltammetric peak potential for the oxidation of 0.66 mM aniline in the presence of 0.44 mM ferrocene is presented in Fig. 2. The slope, intercept and

Fig. 2:

Influence of log scan rate (0.02-2 Vs-1) on cyclic voltammetric anodic peak potential for the oxidation of 0.66 mM aniline in the presence of 0.44 mM ferrocene containing 0.1 M sodium perchlorate in acetonitrile at platinum electrode. (I) y = 0.011x + 1.0429, R2 = 0.9132 (II) y = -0.0622x + 1.0408, R2 = 0.9894

Table-1 summarizes the slopes obtained from platinum electrode as compared to the corresponding values at carbon paste electrode [27]. Dependence of Aniline Concentration on Cyclic Voltammetric Anodic Peak Current The influence of concentration on cyclic voltammetric anodic peak current for the oxidation of aniline at a scan rate of 0.1 Vs-1 exhibits a linear trend with values of slope, intercept and regression y = 3.6384x + 0.1182, R2 = 0.996. Fig. 3 shows the influence of log concentration on cyclic voltammetric peak potential for the oxidation of aniline. The slope, intercept and regression for the entire data set are y = -0.034x + 1.0457, R2 = 0.9746 (as summarized in Table-1). Fig. 3 shows two linear portions and slope, intercept and regression of the first linear portion are y = -0.0294x + 1.0498, R2 = 0.999 and those of second linear portion are y = -0.0622x + 1.0408, R2 = 0.9894 (as summarized in Table-1).

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J.Chem.Soc.Pak.,Vol. 34, No. 6, 2012

1.1

1.08

mM aniline is shown in Fig. 4 comprising two linear portions and the slope, intercept and regression of the first linear portion are y = 0.0122x + 1.0674, R2 = 0.9761 and that of second linear portion are y = 0.0368x + 1.0833, R2 = 0.9648 (as summarized in Table-1).

Ep/V

I

a

Influence of log of scan rate on cyclic voltammetric half peak potential for the oxidation of 0.66 mM aniline shows the slope, intercept and regression y = 0.0294x + 1.0235, R2 = 0.9839.

1.06

II

1.04 -1.5

-1

-0.5

1.1

0

log (C / mM)

Influence of log concentration (0.049-0.901 mM) on cyclic voltammetric anodic peak potential for the oxidation of aniline at a scan rate of 0.1 Vs-1 in acetonitrile containing 0.1 M sodium perchlorate at platinum electrode. (I) y=-0.0294x + 1.0498, R2 = 0.999 (II) y = -0.0622x + 1.0408, R2 = 0.9894

The influence of log concentration on cyclic voltammetric anodic half peak potential for the oxidation of aniline at a scan rate of 0.1 Vs-1 shows two type of linear behavior one within the concentration range 0.049-0.19 mM, and the other within the concentration range 0.28-0.901mM.

II

1.08

a

E p/ V

Fig. 3:

1.06

I

1.04 -2

-1.5

-1

-0.5

0

0.5

-1

log ( / Vs )

Fig. 4:

Voltammetric Scan Rate Dependence of Aniline The influence of square root of scan rate on cyclic voltammetric anodic peak current for the oxidation of 0.66 mM aniline shows linear trend with y = 6.5509x - 0.0548, R2 = 0.9975 indicating the process is diffusion controlled.

Influence of log scan rate (0.02-2 Vs-1) on cyclic voltammetric anodic peak potential for the oxidation of 0.66 mM aniline containing 0.1 M sodium perchlorate in acetonitrile at platinum electrode. (I) y = 0.0122x + 1.0674, R2 = 0.9761(II) y = 0.0368x + 1.0833, R2 = 0.9648

are y = 0.0368x + 1.0833, R2 = 0.9648

The influence of log of scan rate on cyclic voltammetric peak potential for the oxidation of 0.66 Table-1: Cyclic voltammetric characteristics for anilinea in acetonitrile dEp/dlogC mV -34.0b -29.4c -62.2d

dEp/dlog mV 24.6e 12.2f 36.8g

dEp/2/dlogC mV -30.7h -23.7i -46.5j

Acetonitrile /Platinum Picric Acid:Picrate 1:1

-13.3k -13.3l -28.9m

36.7n 24.8o 63.7p

35.6q 31.5r 20.6s

Acetonitrile /Platinum Picric Acid:Picrate 9:1

115.6t

Phosphate buffer/Carbon paste

-34.0

Medium/Electrode Acetonitrile /Platinum

39.0u 28.9v 55.4w 26.0

132.3t -

dEp/2/dlog mV 24.6e 21.1f 29.4g 28.6n 20.5o 43.5p 27.6u 26.0v 30.5w -

Ref. This Work

This Work

This Work [27]

a

All slopes in mV per 10-fold change of the independent variable; values determined from linear regression analysis. Concentration dependence at 100 mVs-1. Concentration range (0.049-0.90 mM); cConcentration range (0.049-0.19mM); dConcentration range (0.28-0.901mM); eScan rate range (0.01-2 V/s); fScan rate range(0.02-0.2 V/s); gScan rate range (0.2-2 V/s); hConcentration range (0.049-0.90 mM); iConcentration range (0.049-0.19mM); jConcentration range (0.28-0.901mM); kConcentration range (0.005-1.364 mM); lConcentration range (0.918-1.364 mM); mConcentration range (0.005-1.364 mM); nScan rate range (0.05-10 V/s); oScan rate range(0.05 – 1.5 V/s); pScan rate range (1.5-10 V/s); qConcentration range (0.005-1.364 mM); rConcentration range (0.005-1.078 mM); sConcentration range (1.078-1.364 mM); tConcentration range (0.247-1.496 mM); uScan rate range (0.025-10 V/s); vScan rate range (0.025-0.750 V/s); w Scan rate range (0.750-10 V/s). b

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At C< 0.2 mM aniline gives 23.7 mV/logC, y= -0.0237x + 0.9927, R2 = 0.9945 (Table-1). As the concentration of the aniline was increased, dEp/2 shifted to more negative values by an average of 46.5 mV/logC, y= -0.0465x + 0.9901, R2 = 0.9675 (Table-1) for aniline. These shifts are consistent with the occurrence of the EC2 mechanism [27-28]. Voltammetry of Aniline in Picric Acid / Picrate Table-2: Comparison of theoretical and experimental values of slopes for Scheme 1 and Scheme 2; voltammetry of anilinea r.d.s

-dEp/dlogC mV

dEp/dlog mV

dEp/dlogX mV

Theoretical Value 3 29.6 29.6 [30-31] Experimental Value Without Picric 29.4c 24.6e acid:Picrate Experimental Value With Picric -28.9 24.8o acid:Picrate1:1 Experimental Value With Picric 115.6t 28.9v acid:Picrate 9:1 a,,c,e,m,o,t,v,x As per Table-1, x Picric acid:Picrate( 4:6,2:8,1:9)

29.6 --25.3x 25.3 x

Values of slopes for aniline in obtained in distilled water and in 9:1 acetonitrile/water are shown in Table-3 and Table-4.

J.Chem.Soc.Pak.,Vol. 34, No. 6, 2012

Experimental Ferrocene (assay>98% Fluka), sodium perchlorate monohydrate (E-Merck), acetonitrile HPLC grade (assay 99.9 %+water 0.02 % Riedel-de Haen), aniline (Fluka 98%HPLC grade) and picric acid (BDH analytical grade) were used. Potassium picrate was synthesized in the laboratory. Solutions were not purged free of oxygen. All experiments were performed in a threeelectrode cell containing platinum disk working electrode, area 0.00785 cm2, a platinum short-wire or platinum spiral counter electrode and silver/silver chloride, chloride reference electrode. Polishing of the platinum working electrode was done on a nylontexture synthetic cloth pad soaked with -alumina (0.05 micron) slurry in water. Polishing was followed by thorough rinsing with triple-distilled water. Background electrolyte was sodium perchlorate dissolved in acetonitrile. Voltammetry was carried out using EG&G, Princeton Applied Research Corp. Versastat II. Data were acquired using M270 Electrochemistry Research Software on a dedicated PII micro-processor coupled to the potentiostat. All used symbols have their usual meaning in electrochemistry. Other experimental conditions are described elsewhere [37]. Conclusions

Table-3: Cyclic voltammetric characteristics of anilinea in distilled water at platinum electrode Medium Picric Acid:Picrate 9:1

Picric Acid:Picrate 1:1

dEp/dlogC mV -184.46b -44.732c -371.63d

dEp/2/dlogC mV -202.39b -36.06c -411.04d

-150.98b

-161.29b

a

All slopes in mV per 10-fold change of the independent variable. Values given are determined from linear regression analysis. Scan rates for concentration dependences were 100 mV s -1; bConcentration range (0.2-2 mM); cConcentration range (0.2-0.8mM); dConcentration range (0.8-2mM)

The results for aniline at platinum electrode are comparable to those at carbon paste electrode. For non-aqueous set-up instead of considering buffered or unbuffered mechanisms either mechanism can be considered. Mechanism [30-31] can be diagnosed from (Schemes 1-2) by taking into consideration values of slopes dEp/dlogC, dEp/dlog and dEp/dlogX he comparison of theoretical and experimental values for Schemes 1 and 2 is given in Table-2.

Table-4: Cyclic voltammetric characteristics of anilinea in 9:1 acetonitrile/water at platinum electrode dEp/2/dlogC mV -173.86b -223.79b Picric Acid:Picrate 9:1 -21.969c -284.19d -7.731b -13.346b Picric Acid:Picrate 1:1 -11.145e -9.1747g -62.362f -35.281h a All slopes in mV per 10-fold change of the independent variable. Values given are determined from linear regression analysis. Scan rates for concentration dependences were 100 mVs -1;bConcentration range (0.2-2 mM); cConcentration range (0.2-0.8mM); dConcentration range (0.82mM);eConcentration range (0.2-1.4 mM); fConcentration range (1.6-2mM); g Concentration range (0.2-1.2mM) ; hConcentration range (1.2-1.8mM) Medium

dEp/dlogC mV

Scheme 1: Radical-Substrate Coupling, rds Step 3

Scheme 2: Radical-Substrate Coupling, rds Step 4

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Acknowledgments Authors are indebted to University of Engineering and Technology Lahore-Pakistan for facilitation. SS is an HEC scholar. IUH thanks Prof. Dr. E. Bouwman. References 1.

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