C.S. Venkatachalam

Mechanism of inhibition of electrode reactions at high surface coverages—II

Abstract The inhibition of Cu 2+ , Cd 2+ and Zn 2+ discharge reactions was studied in the presence of C 4 –C 6 alcohols, phenol, benzyl alcohol and cyclohexanol (SAS) in 0.5 M Na 2 SO 4 + 1 mM H 2 SO 4 supporting electrolyte. It is found from the results that at high surface coverages by SAS, the inhibition is due to “sieve effect” which is different from that at low coverages. The true standard rate constant is proportional to exp[(− N Δγ A i )/( RT )] where Δγ A i is the extra energy (in the form of work) that must be done in order to create a free area required for a discharge reaction.

Electrochemical behaviour of 2-nitrobenzidine in N,N-dimethylformamide

Abstract The electrochemical reduction of 2-nitrobenzidine (I) has been investigated in N,N-dimethylformamide solutions. Two well-defined polarographic waves in the ratio 1:3 were obtained. The first wave was found to be a reversible, diffusion-controlled, single electron wave while the second was the result of a diffusion-controlled, irreversible, three-electron process with chemical complications. The nitro and nitroso anion radical intermediates of I formed during the CPE experiments were detected by an ESR technique and confirmed later by cyclic voltammetry. The “current oscillations” observed in the cyclic voltammograms of I are discussed. An eece-type of sequence is suggested for I reduction leading to the formation of 2-hydroxylaminobenzidine.

Electrochemical reduction of 4-thienylquinazoline in dimethylformamide*

Abstract Electrochemical reduction of 4-thienylquinazoline (4-TQ) at the dropping mercury electrode in dimethylformamide solutions containing 0.1 M tetrabutyl ammonium iodide supporting electrolyte gave two well-defined diffusion-controlled waves. The first wave was found to be quasi-reversible and the second irreversible, on the basis of the usual criteria. The controlled potential electrolysis experiments carried out at the limiting regions of the frist and the total waves of 4-TQ yielded the same number of electrons, namely two, in both cases. Mechanisms corresponding to the total wave and the first wave were proposed based on the results obtained.

Electrochemical reduction of 2,3′-dinitrobenzidine in buffered aqueous methanol

Abstract2,2′-dinitrobenzidine (2,3′-DNB) gives a single diffusion-limited irreversible polarographic wave in buffered aqueous methanol. Microcoulometric experiments indicate a transfer of ten electrons in the reduction and cyclic voltammetric studies show direct proof for the existence of a nitroso intermediate. Based on the results obtained, a suitable mechanism is proposed for the reduction of 2,3′-DNB in buffered methanolic solutions.

Electrochemical reduction of 2,4,6-trinitro diphenylamine in buffered and unbuffered methanolic solutions

Abstract The polarographic reduction of 2,4,6-trinitro diphenylamine was found to give three waves of nearly equal height in all pH regions in buffered solutions and also in unbuffered methanol—water mixtures. The waves were shown to be diffusion-controlled and irreversible on the basis of the usual criteria. Controlled-potential electrolysis at the limiting region of the third wave gave n app = 18 and thus it was shown that n app = 6 for each of the waves. It was concluded that each of the nitrogroups in the compound is reduced to the corresponding amino group, the nitro groups ortho to the  NH group in the parent compound being reduced first followed by the other. Cyclic voltammetric experiments in unbuffered solutions gave evidence for a new redox system which was attributed to amine-quinone diimine intermediate couple. It was inferred that each of the nitro groups in the compound is reduced to the amino function via this intermediate.

Nature of the radical formed during electrolytic reduction of 4-(2′-thienyl)quinazoline in dimethylformamide

Abstract EPR studies and HMO calculations are carried out in order to understand the nature of the electrolytically generated radical of 4(2′-thienyl)quinazoline (4-TQ) in dimethylformamide (DMF). Single broad EPR signal obtained is explained on the basis of theoretical hyperfine splitting constants. By correlating the experimental half-wave potentials and the colour of the radical with HMO energies, it is concluded that 4-TQ protonated at N1-position is getting reduced to give the neutral radical.

Electrochemical reduction of 2-nitrobenzidine in methanol-water mixtures

The electroreduction of 2-nitrobenzidine (2-NB) in buffered methanol-water mixtures in the composition range 20–80% (v/v) and pH range 2.80–12.00 gave a single six-electron diffusion-controlled irreversible wave. On the basis of the results obtained, a diphenoquinone-diimine intermediate is suggested in the mechanism of 2-NB reduction in methanol-water mixtures.

Electrochemical reduction of 4-(2′-thienyl)quinazoline in acid solutions☆

Abstract The electrochemical behaviour of 4(2′-thienyl)quinazoline (4-TQ) was studied in acid solutions of varying ionic strengths using polarography, cyclic voltammetry and controlled-potential electrolysis. 4-TQ is reduced in two single electron steps to give the dihydro-4-TQ product. The two polarographic waves were found to be diffusion-controlled, the first wave being reversible and the second wave irreversible. On the basis of the results obtained, a mechanism is suggested for 4-TQ reduction in which a dimerization reaction goes in parallel with the electron-transfer step.