Please use this identifier to cite or link to this item: http://hdl.handle.net/123456789/1225
Title: STUDIES ON THE REDOX REACTIONS OF SOME ORGANIC COMPOUNDS OF BIOLOGICAL IMPORTANCE USING VARIOUS ELECTROCHEMICAL TECHNIQUES
Authors: Mathur, Navin Chandra
Keywords: CHEMISTRY
BIOLOGICAL IMPORTANCE
ELECTROCHEMICAL TECHNIQUES
ELUCIDATION
Issue Date: 1988
Abstract: The problem of elucidation of the redox mechanism of biological processes, being fascinating in itself, has been a challenging field because of great complexity of the mechanistic steps and the products formed. Inspite of recent work in this field the mechanistic aspects of many of these reactions are still obscure. Electrochemical techniques such as polarography and cyclic voltammetry etc., together with other instrumental methods such as molecular spectroscopy and mass spectrometry, have the potential to provide useful insight into the possible biological enzyme-catalysed redox reactions. There is a significant body of evidence that suggestes that electrochemical reactions occurring in vitro at electrode-solution interface are superficially similar with that of enzymatic reactions at enzyme - solution interface in vital life processes. Compounds in biological systems, however, are usually complex and the biologically active site is probably deep within the molecular frame work. Hence, the best way to obtain an interpretation, closer to actual situation of the biological phenomena on a certain reaction, would be to start by examining the electrochemical behaviour of a simple compound containing the required group. Simpler electrochemical models, thus, can often be utilized to stimulate more complex biological phenomena, with the inherent advantage that well established electrochemical techniques can be used. 11 The importance of azo, hydrazo and amino compounds has been recognized in numerous biological and pharmaceutical fields, and the aim of the present work is directed to investigate the electrochemical redox behaviour of some organic compounds containing these groups or moieties. Typical compounds selected are fast sulphone black-F, (a bisazo dye), pyridine- 2-carboxaldehyde-2-quinolylhydrazone, sulphacetamide, 5-amino- 3-methylisothiazole and 2-aminoquinoline. The subject matter embodied in this thesis is divided into following Chapters: Chapter I : General Introduction. Chapter II : Electrochemical reduction of fast sulphone black-F; a bisazo dye. Chapter III : Voltammetric redox behaviour of pyridine-2- carboxaldehyde-2-quinolylhydrazone at a pyrolytic graphite electrode. Chapter IV : Mechanistic aspects of electrooxidation of sulphacetamide; .a benzenoid amine and potent antibacterial drug. Chapter V : Studies on the anodic oxidation behaviour of heterocyclic amines. In the first chapter, a brief introduction about the characteristic features and importance of various electrochemical techniques, used in present investigations, has been included. Ill A survey of the mechanism of electrode reactions of selected series of compound has also been presented. The second chapter describes the electroreduction of fast sulphone black-F, (FSBF), a bisazo dye, mainly used as indicator in EDTA titration for the estimation of copper. Bisazo dyes have also been reported to possess antischistosomal, antimycobacterial and antitumor activities. The electrochemical reduction of monoazo compounds has been investigated in detail by various workers earlier but nothing significant is known about the electroreduction of bisazo compounds. Hence the present investigation was undertaken. The purpose of selecting this dye was also to study the effect of electron donating and electron withdrawing groups on the ease of reduction of two azo groups. At dropping mercury electrode, in the pH range 2.0- 4.0, FSBF exhibited one well defined pH dependent wave. Two well defined waves were observed from pH 4.0-9.2, whereas at pH > 9.2, again one wave was observed. The nature of the waves was diffusion controlled as shown by linear i vs /n plots. WPlot of id vs C was linear upto about 1. OmM concentration of the dye. At pyrolytic graphite electrode (PGE), linear sweep voltammetry of FSBF, at a sweep rate of 5mVs_1, exhibited one well defined reduction peak in the pH range 2.0-4.0. At pH 4.0 two well defined peaks were observed. The values of the peak current function (ip/AcV1/2) increased with sweep rate. Total number of 8 electrons were consumed in complete reduction of the dye. The progress of controlled potential electrolysis was monitored spectrophotometrically and by cyclic voltammetry. IV These studies indicate that the cleavage of the -N=N- group attached to the naphthol ring ,takes place at the beginning in a 4e , 4H process and is followed by the reduction of second azo group to give the final products. The products of controlled potential electrolysis were identified and a detailed mechanism of the electro-reduction has been proposed. Electrooxidation of hydrazones is of recent interest and has not been investigated in detail as yet. In the third chapter, electrochemical behaviour of pyridine-2-carboxaldehyde- 2-quinolyl hydrazone (PCQH), an important analytical reagent for the estimation of various metal ions and antineoplastic compound, is studied in phosphate buffers of pH range 3.0-10.8 The experimental evidences point out that this hydrazone undergoes electrooxidation as well as reduction at pyrolytic graphite electrode. The 2e , 2H electrooxidation of PCQH occurs in a single step to give pyridine-2-carboxaldehyde and 2-hydroxyquinoline as final products which have been identified mby IR, UV and melting point determination. The electroreduction behaviour of PCQH follow a different course than reported for pyridine-2-carboxaldehyde-2-pyridylhydrazone in which 2e~, 2H+ were consumed in splitting of N-N bond to give 2-aminopyridine and pyridine-2-carboxaldehyde. Hydrazone understudy, however, undergoes 4e , 4H reduction to yield 2-aminomethylpyridine and 2-aminoquinoline as the final products. The experimental results obtained from voltammetric and spectral studies, for electrooxidation and reduction both, have been v rationalised by proposing tentative mechanisms. Sulpha drugs undergo oxidation in the human metabolism and result in the formation of different oxidation products which are supposed to be responsible for their bacteriostatic action. It was, therefore, considered worthwhile to carry out electrooxidation of sulphacetamide (SA) a benzenoid amine and a potential antibacterial drug, in order co explore the mechanistic aspects of its oxidation products. Fourth chapter deals with the elucidation of electrooxidation mechanism of sulphacetamide using various electrochemical techniques. The linear sweep voltammetry of SA, at a sweep rate of lOmVs , exhibited a well defined anodic peak. The potential of this peak was dependent on pH and shifted to a less positive potential with an increase in pH. Ep vs pH plot exhibited two breaks at around pH 3.5 and 7.1 which may be due to two acidbase centres in SA: the acidic amide group and the basic amino group. In cyclic sweep voltammetry, at a scan rate of lOOmVs , SA exhibited a well defined anodic peak la. After recording la, two cathodic peaks, lie and IIIc were also observed which formed a quasi-reversible couple with peaks Ila and Ilia respectively, on the subsequent sweep in the anodic direction. The peak current of peak la increases with an increase in the concentration of SA upto approximately 1.2mM whereas at concentration 1.2mM the current becomes almost constant. The peak current function (ip/AcV1/2)increased with logarithm of sweep rate confirming thereby strong adsorption of this drug VI at PGE. Under all conditions of concentration and pH, electrolysis at peak la, potential gave a value of n of 2.0 + 0.2. The results obtained form linear and cyclic sweep voltammetry, coulometry and spectral studies indicated that sulphacetamide undergoes 2e , 2H oxidation to give corresponding azo and hydroxyl-amino compounds as the major products. A comparison of electrooxidation mechanism of sulphacetamide with other benzenoid amino compounds has also been presented. Studies on electrochemical oxidation mechanisms of heterocyclic amino compounds are very scarce which may be due to their resemblance with benzeneoid amines. Two heterocyclic amine compounds of diverse structure , viz., 5-amino-3-methylisothiazole. hydrochloride, (5-AMIT) a herbicide, and 2-aminoquinoline (2-AQ) a mutagen and an antibacterial compound, were subjected to detailed anodic oxidation studies and the results of their behaviour have been summarised in the final chapter of the thesis. In linear sweep voltammetry, 5- AMIT and 2-AQ both exhibited one well defined oxidation peak in the pH range 3.0-10.9. Ep vs pH plot of 5-AMIT and 2-AQ exhibited breaks around pH 5.6 and pH 7.4 respectively which correspond to their pKa values. It was interesting to observe that both the compounds follow a 2e~, 2H+ oxidation mechanism to give corresponding azo products. Products of oxidation were characterized by various spectroscopic techniques and on the basis of voltammetric, coulometric and spectral studies, mechanism of electrooxidation, for both compounds, has been suggested.
URI: http://hdl.handle.net/123456789/1225
Other Identifiers: Ph.D
Appears in Collections:DOCTORAL THESES (chemistry)



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