ELECTROCHEMICAL REDUCTION OF

 4- (3- PYRIDYLAZO) -3- AMINO-2- PYRAZOLIN-5- ONE .

 

Refat Abdel – Hamid, Mostafa K .M. Rabia and Nadia A. Abdalla*

Deparment of Chemisty , Faculty of Science , Sohag, Egypt .

* Deparment of Chemisty , Faculty of Science , Aswan, Egypt .

 

ABSTRACT :

 The electrochemical reduction of 4-(3-pyridylazo) -3- amino -2- pyrazolin -5- one in universal buffer solutions with different pH's as studied at 268 K. From the results obtained , it is concluded that the title azo compound is reduced via an ECEC mechanism . The mechanism was confirmed by digital simulation . The heterogeneous electron transfer and homogeneous protonation followingup reactions parameters were evaluated and the electrode mechanism was discussed .

 

INTRODUCTION :

 Numerous azo compounds have important application in the field of medicine ( Modest et al 1957 , Gary et al 1970 , Godoy et al 2001 , Jisun et al 2002 and Rageh et al 1999 )  . The electrochemical behavior plays an important role in its biological activity . A survey of literature reveals that not much work has been performed on the electorochemical behavior of heterocyclic azo compounds such as arylazopyrazolones ( Raivindranath et al 1983 , Jain 1984 and Abdel-Hamid 1988 )   and pyridylazophenol ( Florence et al 1973 )    .

 The objective of the present investingation is to study the electrochemical

Reduction of 4-(3-pyridylazo) -3- amino -2- pyrazolin -5- one in universal buffer solutions with different pH's ( 2.18 – 10.75 ) the study is carried out using two electrochemical techniques , cyclic voltammetry and digital simulation , so that the mechanism for the electrochemical reduction of the title compound could be formulated . The behavior at 298 K was described and

A mechanism consistent with the experimental results was proposed . The mechanism was confirmed and fully characterized by cyclic voltammetric simulation analysis making use the computer programs  CVSIM and CVFIT ( Gosser et al 1991 ) .

 

EXPERIMENTAL :

 4-(3-pyridylazo) -3- amino -2- pyrazolin -5- one was prepared according the method described elsewhere ( El- Naghi et al 1973 ). It was recrystallised from ethanol and characterized by elemental analysis and IR spectra . Stock solution ( 5.0 ×10 -3 mol dm -3 )of the title azo compound were prepared in aqueous/ 50 % of ethanol mixture Britton – Robinson modified universal buffers ( Brritton 1956 )   (prepared from A. R. chemicals ) in the pH range 2.18-10.75 were used as supporting electrolytes . The pH of the buffers was checked with an Orion Research model 601 A/ digital Ionalyzer , using combined electrode .

 Cyclic voltammograms were recorded using an  EG&G PAR model

264A Polarographic Analyzer . The measurements were carried out using a conventional three electrodes configuration .An EG&G PAR model SMDE 303A mercury-drop system in small dropping mode was used as working electrode . the electrode area was

1.05×10 -2 cm -2 . The reference electrode was Ag/Ag Cl electrode .A 1.0 cm2 platinum foil was used as auxiliary electrode throughout the experimental work . Solutions were purged with pure nitrogen before the measurements and an atmosphere of nitrogen was maintained above the working solution . All experiments were performed at 298 K .

 

 Since the pioneering work of Feldberg ( Feldberg 1969 )  , digital simulation  techniques have played an  important role in the analysis of electrochemical data ( cyclic voltammetry). Digital simulation on the basis of general methods developed for treamtment of solution of chemical reaction in the context of the explicit-finite-differences ( Nielsen et al 1987 and Gosser et al 1988 ) , is recently used . For  the deduced mechanism the parameters are evaluated on  comparison of digital  simulations with the experimental Vltammograms . All digital simulations here in were  done by the use of CVSIM And CVFIT Computer program( Gosser et al 1991 ) , These program, are based on the expliticit finite difference method using an expanding spatial grid.

Computations of simulations and treatment of the cyclic voltammetric  data were performed on a COPAM PC 3888-25(80386-25MHz) with a  80387 mathematical 

co-processor. The HARVARD GRAPHICS  Version 2.3  was Used for  plotting the simulated and experimental  cyclic voltammogram .

 

RESUL AND DISCUSSION :

 

1- CYCLIC VOLTAMMETRY : 

 

 The electrochemical reduction of 4-(3-pyridylazo)-3-amino-2-pyrazolin -5- one was studied at hanging mercury electrode in universal buffer solution covering the range of pH 2.18 -10.75 . It gives cyclic voltammograms of a single well defined reduction wave in the potential rang of – 0.4 to – 1.6 versus  Ag / Ag + through out the pH range of study . At pH 2.18, the cyclic voltammetric wave is located at -0.618 Volt at a scan rate of

50 mVs-1 .On increasing the pH of the solution the peak current potential , E p does shift to more negative values and the peak current is varied as well . This indicates the participation of hydrogen ions in the electrode process .

 

 Examination of cyclic voltammograms obtained at different scan rate range of

5-200 m Vs -1at pH 2.18 reveals that no anodic counter part of the CV wave is seen on the reverse sweep (of.Fig. 1) indicating either the reduction is totally irreversible, which is unlikely, or the reduction product is consumed rapidly by another process, e.g. , protonation . The peak current potential, Ep does shift to more negative potentials on increasing the scan rate. A similar behavior is obtained at pH's 7.11 and 9.13 . Fig. 1 shows the effect of scan rate at pH 2.18 as  representative example . A Linear relationship is obtained between Ep the peak current potential , and log v , logarithm of scan rate at

each of the three pH's. The regression lines obtained are :

 

  Ep= -(0.678 ±0.003) – (0.043 ±0.002) log ν r = 0.994 (1) 

 

 Ep= -(1.204 ±0.001) – (0.052 ±0.009) log ν r = 0.991 (2)

 and

 Ep= -(0.397 ±0.002) – (0.029 ±0.002) log ν r = 0.996  (3)

 

For the CV waves at pH's = 2.18 , 7.11 and 9.31 , respectively . The slopes obtained are larger than that expected for a reversible process (Nicholson et al 1964 ). On the other hand , the dependence of the voltammetric peak current , ip of the CV wave on the square root of scan rate , ν 1/2 , is linear with correlation coefficients close to unity at all the pH's of the study . This indicates that the CV wave is diffusion – controlled in nature ( Feldberg 1969) . Moreover , the peak potentials are not symmetrical , as indicated from the peak width, Ep-Ep/2 for the CV wave which is greater than 28.25 mv at 298 K (Bard et al 1980 )   expected for a two –electron reversible wave . From these results it is concluded that the title azo compound is reduced electrochemically in a diffusion –controlled irreversible CV wave involving a transfer of two electrons .

 

 The dependence of the current function ip / ν 1/2, on the scan rate , ν , is an important diagnostic criterion for establishing the type of mechanism by cyclic voltammetry. Table shows the value of ip / ν 1/2for the titl azo compound , as a function of scan rate it decreases as the scan rate increase .  

 

 In order shed more Light nature of the CV wave obtained in the electrochemical reduction of the subject compound the effect of pH on the reduction was investigated It is observed that the peak current potential , Ep, of the CV wave shifts towards more negative, potentials with increasing the pH of the solution . Figure 2 represents the relation of Ep versus pH . The plot shows mainly two intersecting straight lines obtained are :

 

 Ep= - 0.367 – 0.106 pH r = 0.999 at 2.18-7.11 (4)

and

 Ep= - 0.470 – 0.095 pH r = 0.999  at 7.11-10.75 (5)

 

 for the first and second second segments , respectively .

 The number of protons per molecule of the reactant involved the electrode process , P , is determined using the following equations :

 Where α is the transfer conefficient , ∆Ep / ∆pH is the slop obtained from equations (4) and (5) and the other symbols have their usual significance .On substituting the value of α na obtained from eqn (6)(Klingler et al 1981) , the values of P are found to be close to two in all studied pH's .

 

 For understanding the course of the electrode process corresponding to the CV wave obtained , it was necessary to assign the wave to various electroctive groups in the title azo compound .   It has been  concluded that

4- azo -2-pyrazolin -5- ones exist mainly in the soild  state and in non – aqueous solutions in the hydrazoketo,(1),azohydroxy,(2),tautomeric equilibrium( Yasuda et al 1966 , Snavely et al 1968 , Fahmy et al 1980 and Mahmoud et al 1984 )  ..In aqueous medium the equilibrium is shifted

 

 

 

 

 

 

 To the azohydroxy from (2), due to its stabilization though a H-bonding interaction of water molecules with the oxygen atom of its hydroxy group. Thus,4-(3-pyridylazo)-3-amino-2-pyrazolin -5- one is represented by the following structur (3):

 

 

 

 

 

 

 

 

 


Thus, the possible reduction groups in the title azo compound are the cyclic azomethine groups, / N= C / , of the pyrazole and the pyridine nuclei, and the azo / N = N /  groups out of these the azo group is more susceptible to reduction than the cyclic azomethine groups as an endocyclic groups require higher potential for reduction .

 

 From the foregoing results for the reduction of the title azo compound, it is obvious that the rate- determining steps involove the uptake of protons, H+,( chemical  reactions ). Therefore, the following mechanism can be suggested for 2e- and 2H+ reduction of the title azo compound .

  

 

 

 

 

 

 

 


SCHEME I

 Where Az stands for the title azo compound . In Scheme I, the first step is a moderately fast( reversibly) single electron transfer to from radical anion

 (Az .- ).In the second step the radical anion accepts a proton (irreversibly) to from a protonated radical ( AzH.) which after taking another electron

(reversiply) froms a protonated anion(AzH- )in the third step. The product of step 3 readily takes up one more proton (irreversibly) to give the final product

( AzH2) .

 

 A similar two-electrons and two-protons reaction mechanism ( ECEC) for the reduction of some azo compounds has been proposed ( Solder et al 1968 and Hamam et al 1981 ) .The above nechanistic steps is supported from the increase of Ep with pH of the nedium .

 

2- DIGITAL SIMULATION :

 

 The digital simulation at three pH's (2.18, 7.11 and 9.31) are performed to establish the mechanism proposed in Scheme I for the reduction of the title azo compound . The kinetics of the process can be digitally simulated using the method of finite-differences, as described by Feldberg. ( Feldbery et al 1969 and feldbery et al 1972 ) for simulation , the parameters required for the construction of the theoretical cyclic voltammograms according to the proposed mechanism are : Ci,j the initial concentrations of the depolarizer and protons, respectively , the standard electrode potentials, E o , the transfer conefficient , α ,the diffusion coefficients,

D, the standard heterogeneous electron transfer rate constants, ks , and the homogeneous rate constants, kc :

 

 For a solution of 5.0 × 10-4 mol dm-3 4-(3-pyridylazo)-3-amino-2-pyrazolin -5- one in buffer solution at pH 2.18 at scan rate of 0.2 Vs-1 the digital simulated cyclic voltammogram is compared with the experimental one . the best fit digital simulated and the experimental cyclic voltammograms are recorded in Fig 3 as a representative example for the a bove proposed mechanism

(Scheme I ).

 

 Complete characterization for the electrochemical  reduction kinetics for

The title azo compound is obtained . Moreover , the heterogeneous electron transfer parameters as well as the homogeneous rate constants of the follow – up protonatiton reactions are calculated . The data obtained through simulation – fitting for the title azo compound at pH's 2.18 , 7.11 α 9.31 are summarized in Table 1 . It is found that ,the homogeneous rate constant  values obtained decrease with increase in pH showing that the electrode reaction tends to become more irreversible .Moreover . the heterogeneous rate constant values

Are observed to be high in acidic  medium indicating that the rate of the reaction is fast as the protonated from is getting reduced . Thus , the reduction mechanism follows the proposed mechanism (Scheme I ) . It is concluded that the electrode reduction kinetics have the same type overall the entire pH range

This behavior is supported from the small difference in slope of the two segments of the Ep-pH relationship (e.f.Fig. 2 and equations 4& 5 ).

 

 

 

REFERENCE

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Bard, A.J. and Faulkner, L. R., "Electrochemical Method: Fundamentals 

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 Fr., 164(1984).

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Caption of Figures :

 

Fig 1: Cyclic Voltammograms of 5.0 × 10-4 mol dm-3  4-(3-pyridylazo)-3-

 amino-2-pyrazolin -5- one at T=298K and different scan rate, ν=5 (1),

 10(2), 20(3), 50(4), 100(5) and 200(3) mVs-1

 

Fig 2: Ep – pH relationship of 5.0 × 10-4 mol dm-3 4-(3-pyridylazo)-3-

 amino-2-pyrazolin -5- one at T=298K and ν =50 mVs-1

 

Fig 3: Cyclic Voltammograms of 5.0 × 10-4 mol dm-3  4-(3-pyridylazo)-3-

  amino-2-pyrazolin -5- one at T=298K and ν =50 mVs-1 ـــــ beak-ground subtracted expermental, 000 digital simulation of cyclic voltammograms, see Table 1 for values of parameters used, in simulation .

 

 

 

  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


 

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