Electrochemical Behaviour of N ’-( p-toluenesulphonyl )-3-methyl-4-( 4 ’-substituted arylhydrazono ) pyrazolin-5-ones

The electrochemical behaviour of N’-(p-toluenesulphonyl)-3methyl-4-(4’-substituted arylhydrazono) pyrazolin-5-ones has been investigated at dme and gc electrodes in buffer solutions of pH 2.0, 4.0, 6.0, 8.0 and 10.0 using dc polarography and cyclic voltammetry and coulometry. The compounds exhibit one well defined wave in the entire pH range of study. The process is irreversible and diffusion controlled. Controlled potential electrolysis indicates the involvement of four electrons in the reduction process. The effect of solvent, cations and anions, temperature and substitutents on the mechanism of reduction has been studied. Based on the results obtained the mechanism of reduction has been suggested.


Introduction
Many pyrazoline derivatives are well known for their biological activity [1][2][3] .The redox characteristics of such biological substances may provide valuable information about the redox behavior in living systems 4 .In view of the biological activity of pyrazolines and the usefulness of electrochemical techniques in studying biological molecules, some N'-(ptoluenesulphonyl)-3-methyl-(4'-substituted arylhydrazono)pyrazolin-5-ones have been studies at dropping mercury electrode and glassy carbon electrodes.

Preparation of solutions
Stock solutions (1 x 10 -2 M) of all the compounds were prepared in dimethylformamide (AR).Britton-Robinson buffer solutions pH 2.0, 4.0, 6.0, 8.0 and 10.0 were prepared and used.Polarograms were recorded for deaerated solution containing 5 ml of the stock solution of the compound, 27.5 ml of dimethylformamide [the minimum volume necessary to keep the compound in solution], 17.5 ml of buffer solution of desired pH.

Instruments
A systronic polarograph model 1632 was used to record the polarograms.The polarograph consists of dropping mercury electrode (dme) assembly, a console and a printer interface.The dme assembly uses a three electrode system where dme is used as a working electrode with saturated calomel electrode (SCE) as reference and platinum counter electrode.The polarographic console (1632) provides an accurately controlled and programmable DC ramp generator, a highly sensitive current monitor and facilities for controlling drop life of the mercury electrode.The capillary having the following characteristics in water: 3.27 mg/sec, t = 2.0 sec, mg 2/3 t 1/6 = 3.66 mg 2/3 s -1/2 at h = 80 cm was employed as the working electrode.The pH measurements were made with ELICO digital pH meter having a glass electrode model LI 127.Cyclic voltammograms were recorded for deaerated solution containing 1 ml of the stock solution of the compound, 5.5 ml dimethylformamide and 3.5 ml of the buffer solution of desired pH with a Bio-analytical Systems CV-27 controller and conventional three electrode, Ag/AgCl reference electrode, glassy carbon working electrode and platinum counter electrode.Nitrogen gas was used as a purge gas.The controlled potential electrolysis was carried out in a Lingane 6 H-type cell.A large pool of mercury is employed as the cathode at the bottom of the large compartment and a similar pool of mercury at the bottom of the smaller compartment is taken to serve as the anode.The cathode compartment contained 55 ml of DMF, 10 ml of 1.0 M KCl and 25 ml of buffer (pH 4).The solution in the cathode compartment is deaerated by bubbling pure nitrogen gas through it for about 15 minutes.The preelectrolysis is carried out for about 15 minutes with cathode potential fixed at a value which is to be used for subsequent reduction of the oxidant.When the back ground current reached a constant value, 10 ml of the 0.01 M N'-(p-toluenesulphonyl)-3-methyl-(arylhydrazono)pyrazolin-5-one, was added to the cathode compartment and the electrolysis continued at -1.40 V.The decrease in the limiting current with time was recorded at regular intervals of time and the number of electrons involved in the reduction was calculated from the i-t curves following the procedure outlined by Lingane 6 .After disconnecting the electrolysis cell 1 ml of the resulting solution was withdrawn and presence of aromatic amine (Aniline) in this solution was revealed by the standard spot test 7 .

Results and Discussion
Wave in acidic pH's N'-( p-toluenesulphonyl)-3-methyl-pyrazolin-5-one failed exhibit a polarographic wave.This is attributed to the stabilization of the pyrazoline ring by keto-enol tautomerism.The waves (Fig. 1) that are observed in compounds 1-5 given in the Scheme 1 are therefore attributed to the polarographic reduction of exocyclic azomethine group(>NH-N=C<).These compounds exhibit a single wave in the entire pH range of study.The electrochemical characteristics of the compounds are presented in Table 1 and 2. The low value of temperature coefficient (below 1.62% K -1 ) and the direct proportionality observed for i d vs concentration and i d vs h 1/2 , indicate the diffusion-controlled nature of the electrode process.The shift in E 1/2 towards more negative potential with increase in concentration of depolarizer and semi-log plots 8 confirmed the irreversible nature of the wave.E 1/2 values were also found to become more negative with increase in pH before reaching a limiting value beyond a pH of 8.0.This clearly shows the participation of protons in the reduction process.The value of αn a (product of transfer coefficient[α] and number of electrons transferred in the rate determining step) and P (the number of protons involved in the rate determining step) are determined using following expression 8 .
The value of diffusion coefficient has been determined by Ilkovic 9 equation.
where n = number of electrons transferred in the process, m = rate of mercury flow in mg/s, D = diffusion constant of depolarizer in cm 2 /s, t = drop time in s, C = depolarizer concentration in millimoles/litre, i d = diffusion current in micro amperes.
The value of heterogeneous rate constant (k 0 f,h ) has been evaluated by Meites and Isreal 10 equation.
In general the value of r 0 is taken as 2 x 10 -8 cm 11 .

Wave in alkaline pH's
Each compound of the series (1-5) exhibits only one wave in the pH range 8 -10.The halfwave potential of the wave in alkaline solutions is not altered with change in hydroxyl ion concentration.The height of the wave decreases with increase in pH and the wave height vs pH plot assumes the form of dissociation curve ( ) in the pH range of study (1 -10).
Literature survey reveals that azo group exhibits azo-hydrazone tautomerism in benzeneazopyrazolin-5-ones 12 .In alkaline solution arylhydrazono-pyrazolin-5-ones (I) exists in the azo-methine anionic form 13 .(II, Cf.Scheme 1).The anionic form is susceptible for chemical cleavage in the presence of hydroxyl ions to the corresponding carbonyl compound 14 .But the wave corresponding to the reduction of carbonyl compound has not been clearly differentiated in the present studies and seems to have been merged with the decomposition of the buffer solution.

Cyclic voltammetry
The compounds (1-5) gave one cathodic peak at all scan rates (0.020-0.200V/s) at glassy carbon (gc) electrode at pH 4.0 and 8.0.No anodic peak is observed in the reverse scan in any media indicating irreversible nature of the electrode process (Fig. 2 and 3).This is further supported by the negative shift in the peak potential with the increasing scan rate (cf.Table 3 and 4).The plots of i pc vs concentration and i pc vs ν 1/2 fulfill the criteria of the diffusion controlled nature 15,16 of the electrode process.The plots of E pc vs pH are similar to E 1/2 vs pH plots and this lends support to the findings of the dc polarography.

Controlled potential electrolysis
Progress of the electrolysis was followed by recording the decreasing current with time and the number of electrons per molecule were computed from i-t curves following the procedure outlined by Lingane 6 and found to be four (3.7 -4.3).The electrolysis is carried out until the electrolyzed solution did not exhibit any polarographic wave or a cyclic voltammetric peak.

Effect of solvent composition
Effect of solvent composition on the polarographic characteristics of compounds 1-5 was studied by recording polarograms in 65 and 75% organic solvent solutions (dimethylformamide, dimethylsulphoxide, aceto nitrile)-water solutions at pH 4 (Table 5).It was observed that half-wave potential shifted to more negative values 17 in the presence of organic co-solvent and the magnitude of the shift depends on the nature of the solvent.The Potential, V Potential, V order of the shift observed in the present study is CH 3 CN>DMF>DMSO.The shift of halfwave potential towards more negative values and decrease in the limiting current values with an increase in the percentage of organic co-solvent, may be due to rise in pH of the solution 17 which results in an increase in the dissociation constant of the protonated species 18 .Both these factors lower the rate of protonation and consequently lead to a shift in E 1/2 of the reduction wave (in case where protonation precedes the electron transfer) towards more negative potential.

Effect of cations and anions
To evaluate the effect of size of the cation and anion, the polarograms were recorded in supporting electrolytes having a common cation (KBr, KI, KCNS and KNO 3 ) and a common anion LiCl, NaCl, KCl and N + (CH 3 ) 4 Br -.No change in limiting current and half-wave potential values were found in the former while lowering of E 1/2 and constancy in limiting current was observed in the later.This can be explained in terms of the change of the structure of the double layer 19 .Size of anion has no effect on E 1/2 and i l since the cations predominate in the electrical double layer at these potentials and increase in the size of cation lead to decrease in the rate making reaction more difficult 20 resulting in shift of E 1/2 towards more negative potentials (cf.Table 6).

Effect of temperature
All the compounds (1-5) exhibit a single well-defined wave at pH 4.0 at all the temperatures studied (300.13 -323.13 o K).The unsubstituted N'-(4-hydroxybenzoyl)-3-methylpyrazolin-5-one fail to exhibit the reduction wave under similar experimental conditions.So, the wave that has been observed in the present studies is attributed to the reduction of azo group in the hydrazone form (-NH-N=C<).The diffusion controlled nature of the reduction wave has been indicated by the linear plots of i d vs h 1/2 passing through the origin.The low temperature coefficient values (1.02 -1.24 percent/degree) further supports the diffusion controlled nature of the wave.This is in agreement with the values predicated by Meites 8 for organic molecules.Semi-logarithmic analysis of the waves 8 confirmed the irreversible nature of the wave.It is seen form the Table 7 that the half-wave potential values of the depolarizer shifted to more negative potentials with increase in temperature.The values of αn a decrease with increase in temperature.But in the present studies it is observed that the values of n a to be equal to 1. Since the decrease in αn a values with increase in temperature is indiscrete and at no stage the two consecutive values vary by a factor of 1, the possibility of decrease in the value of product αn a due to a change in n a may be ruledout.A decrease in the value of α implies that the transfer of electron(s) is made increasingly difficult as the temperature is elevated.In other words, the electrode reaction of arylhydrazono-pyrazolin-5-one is rendered more irreversible.The shift of E 1/2 values to more negative potentials lends support to the above conclusion.Knowing the value of n, the diffusion coefficient(D) values for the depolarizer at different temperatures have been calculated using the Ilkovic 9 equation.The values of heterogeneous rate constant (k 0 f,h ) have been evaluated using Meites and Isreal 10 equation for the irreversible process and are presented in Table 7.The values of k 0 f,h are low and decreases with increase in temperature.This signifies that the electrode reactions are rendered more irreversible with increasing temperature.This observation is in harmony with the conclusion arrived at on the basis of αn a values.The activation free energy change, ∆G* is positive for all the systems suggesting the non-spontaneous nature of the electrode process 17 .The high values of ∆G* also suggests that the reaction rate is slow 21 at all the temperatures of study.

Effect of substituents on reduction
To establish the effect of substituents on the polarographic reduction, E 1/2 was plotted against the Hammett substituent constant (Fig. 4).The substituent constants, σ used were taken from the literature 22 .It is observed from the plot that the electron donating substituent, -CH 3 and -OCH 3 shifts E 1/2 towards more negative values while electron withdrawing groups -Br and -SO 2 NH 2 shifts E 1/2 towards more positive values.The correlation coefficient calculated for the σ vs E 1/2 plot (r = 1.0) show satisfactory application of Hammett's correlation to the system under study.The positive value of the specific reaction constant (ρ = 0.19 at pH 4) suggests that the electron addition step is more important than the addition of protons.The results also suggest that the presence of substituents do not affect the mechanism of reduction but only makes the reduction either easier or more difficult depending on the nature of the substituent.

Mechanism of reduction
Based on the results obtained a mechanism of reduction shown in Scheme 1 may be proposed for the present compounds.

Conclusions
The results show that the polarographic and cyclic voltammetric reduction of N'-(ptoluenesulphonyl)-3-methyl-4-(4'-substituted arylhydrazono)pyrazolin-5-ones occur in a single step at dme and gc electrodes in buffer solutions of pH 2.0, 4.0, 6.0, 8.0 and 10.0.The reduction wave/peak is been found to be diffusion controlled and irreversible.Controlled potential electrolysis studies indicate the involvement of four electrons in the reduction process.The shift of half-wave potential towards more negative values and decrease in the limiting current values with an increase in the percentage of organic cosolvent, may be due to rise in pH of the solution which results in an increase in the dissociation constant of the protonated species.Size of anion has no effect on E 1/2 and i l and increase in the size of cation results in shift of E 1/2 towards more negative potentials.The studies on the effect of temperature on polarographic reduction signify that the electrode reactions are rendered more irreversible with increasing temperature.The study on the effect of substituents on the polarographic reduction reveal that the presence of substituents do not affect the mechanism of reduction but only makes the reduction either easier or more difficult depending on the nature of the substituent.

Table 6 .
Effect of cations and anions on polarographic characteristics of compound 1-5