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85 CHARTER II AIM AND SCOPE The growing global concern in the chemical and allied industrial sector is maneuvering industrial waste management and pollution control. It is the commitment for the researchers in the field of chemistry to design and to evolve alternative synthetic chemical processes, which would be eco-friendly, product selective and economically viable. Generally non-conventional chemical processes, specifically electrochemical processes employ cheap resource material, viz electricity whose cost remained remarkably stable over the years when compared to the cost of the chemicals. In these processes the good yield of product is ascertained. Large amounts of oxidants or reductants are not required and the formation of bye-products is very much avoided. It considers only the current pressure to avoid pollution. The electrochemical reactions make significant contributions to organic synthesis either in the laboratory or on an industrial scale. These methods have
86 the potential for developing more green chemical syntheses. The electrochemical routes may be found more economical as well if a cheaper raw material can be utilized or of, by using electrochemistry, a continuous operation is facilitated. Thus electroorganic research assumes crucial importance when the environmental care is taken into account. With this background, in the present work, alternative electroorganic synthetic technique has been explored to synthesize organic molecules. ORIGIN OF RESEARCH Survey of earlier literature reveals that not much work has been contemplated to unravel eco-friendly techniques by which chemical reactions would be accomplished. Especially in the cathodic processes involving the derivatives of carbonyl compounds, electro reduction of carbonyl groups on Nickel, Copper, Tin and Mercury have been reported. However, only limited and unfinished works have been carried out so far on the cathodic reduction of carbonyl derivatives like oximes, hydrazones and so on. This has provided the scope for exploring the feasibility of cathodic processes on such carbonyl derivatives at comparatively stable electrodes like Platinum and Graphite. SCOPE AND OBJECTIVE In the present work, it is proposed to explore the feasibility of investigating the electrochemical and electroanalytical aspects of some industrially important aromatic organic compounds.
87 Electrochemical synthetic methods, which are effective alternatives to conventional chemical processes for redox reactions are to be tried for benzenoid and heterocyclic aromatic systems. The experimental parameters for the chosen systems are to be established. In other words, the various factors like nature of electrode, polarity of the solvent, ph of the medium, temperature, and electrodeelectrolyte-solvent combinations are to be exhaustively studied. The paths of the electrode processes are to be defined by confirming the nature and quantity of the products formed with the help of spectrophotometric studies and chromatographic techniques. The type of mechanism, whether radical initiated or ion forming, is to be established by direct and indirect methods. The electroanalytical studies like polarography, cyclic voltammetry, coulometry etc, are to be performed so as to get an insight into the electrode mechanism. Electrode kinetic studies, effect of current density and current efficiency calculations are to be conducted. EXPERIMENTAL STUDIES The experimental scheme was so designed as to explore the various aspects on the cathodic reduction of carbonyl derivatives. In this present work aromatic systems alone are considered. To get the required preliminary objectives the following scheme is adopted. [Table-2.1]
88 a. SUBSTRATES To study the effect of substituents on the cathodic reduction of >C=Nfragment the following substrates were employed. i. Oximes of Benzaldehyde, Salicyaldehyde, Cinnamaldehyde, Acetophenone and Benzophenone ii. Hydrazones of Benzaldehyde, Salicyaldehyde, Cinnamaldehyde, Acetophenone and Benzophenone iii. Phenylhydrazones of Benzaldehyde, Salicyaldehyde, Cinnamaldehyde, Acetophenone and Benzophenone iv. Semicarbazones of Benzaldehyde, Salicyaldehyde, Cinnamaldehyde, Acetophenone and Benzophenone b. EFFECT OF ELECTRODE To study the effect of electrode material on the cathodic process, Benzophenone oxime was subjected to electrolyses on Graphite, Platinum and Nickel independently. c. EFFECT OF ph The impact of ph on the cathodic reduction has been attempted with Benzophenone oxime at platinum and Graphite electrodes in solutions of ph ranging from acidic to neutral to basic.[ ph = 3, 5, 7, 9 and 12 ]
89 d. EFFECT OF TEMPERATURE Electrochemical reduction of Benzophenone oxime at temperatures 0, 30 and 60 c have been performed to explore any possible impact on the electrode process. e. EFFECT OF POLARITY OF SOLVENT Studies of electrolyses in both aqueous and non-aqueous media have been undertaken to get insight into the impact of polarity on the electrode process. f. PRODUCT ANALYSIS The reduction products have been isolated by regular solvent extraction method and qualitatively obtained by chromatographic techniques. Subsequently they were characterized by spectrophotometric analysis. The quantitative analysis on the yields of the products has been realized with glc. g. ELECTROANALYTICAL TECHNIQUES In order to arrive at the working potentials for the various systems and to substantiate the proposed mechanistic pathway, the following electroanalytical techniques were exploited. i Current - Potential Studies The variations in current at different potentials have been observed for all systems prior to commencing the electrolyses. The working
90 potentials for various processes have been fixed by these current - potential studies. ii Cyclic Voltammetry To substantiate the proposed mechanism and to characterize the electro chemical processes under study, cyclic voltammetry technique has been effectively and exhaustively implemented for stipulated combinations. (Table-2.2) In the area of electroorganic chemistry, compared to anodic oxidative processes, cathodic reductions have been attempted only to a lesser extent, presumably due to the restricted availability of cathodic materials. In addition, the electrochemical reduction on carbonyl derivatives finds a very limited report in earlier literature. The present study has exploited this situation to explore the cathodic process on the derivatives of aromatic carbonyl compounds. Though substantive preliminary work has been done in the present study, there is more scope to extend the work to further level so as to achieve product selective and hence industrial level electrochemical technique.
91 TABLE 2.1: ELECTROSYNTHETIC STUDIES S.No SUBSTRATE MEDIUM ELECTRODE CELL A. SUBSTITUENT / ELECTRODE EFFECT 1 Oxime of Benzaldehyde ph=3 C / Pt / Ni Divided 2 Oxime of Salicyaldehyde 3 Oxime of Cinnamaldehyde 4 Oxime of Acetophenone 5 Oxime of Benzophenone 6 Hydrazone of Benzaldehyde 7 Hydrazone of Salicyaldehyde 8 Hydrazone of Cinnamaldehyde 9 Hydrazone of Acetophenone 10 Hydrozone of Benzophenone 11 Phenylhydrazone of Benzaldehyde 12 Phenylhydrazone of Salicyaldehyde 13 Phenylhydrazone of Cinnamaldehyde 14 Phenylhydrazone of Acetophenone 15 Phenylhydrazone of Benzophenone 16 Semicarbazone of Benzaldehyde 17 Semicarbazone of Salicyaldehyde 18 Semicarbazone of Cinnamaldehyde 19 Semicarbazone of Acetophenone 20 Semicarbazone of Benzophenone
92 B. EFFECT OF SOLVENT 1. Oxime of Benzophenone Water, ph=3 Graphite Divided 2. Oxime of Benzophenone DMF 3. Oxime of Benzophenone Water; ph=3 Platinum 4. Oxime of Benzophenone DMF 5. Oxime of Benzophenone Water; ph=3 Nickel 6. Oxime of Benzophenone DMF C. EFFECT OF ph 1. Oxime of Benzophenone ph=3 Graphite Divided 2. Oxime of Benzophenone ph=5 3. Oxime of Benzophenone ph=7 4. Oxime of Benzophenone ph=9 5. Oxime of Benzophenone ph=12 D. EFFECT OF TEMPERATURE 1. Oxime of Benzophenone ph=3(0 c) Graphite Divided 2. Oxime of Benzophenone ph=3(30 c) 3. Oxime of Benzophenone ph=3(60 c) E. CELL MODIFICATION 1. Oxime of Benzophenone ph=3 Graphite Divided 2. Hydrazone of Cinnamaldehyde 3. Phenylhydrazone of Salicyaldehyde 4. Semicarbazone of Acetophenone 5. Oxime of Benzophenone Undivided 6. Hydrazone of Cinnamaldehyde 7. Phenylhydrazone of Salicyaldehyde 8. Semicarbazone of Acetophenone
93 TABLE-2.2: ELECTROANALYTICAL STUDIES CYCLIC VOLTAMMETRY S.No SUBSTRATE ph ELECTRODE SCAN RATE mv/sec A. SUBSTRATE VARIATION STUDIES 1. Oxime of Benzophenone 3 Glassy C 50 2. Hydrazone of Cinnamaldehyde 3. Phenylhydrazone of Salicyaldehyde 4. Semicarbazone of Acetophenone B. ph VARIATION STUDIES 1. Oxime of Benzophenone 3,5,7,9,12 Glassy C 50 C. SCAN RATE VARIATION STUDIES 1. Oxime of Benzophenone 3 Glassy C 50-500 2. Hydrazone of Cinnamaldehyde 3. Phenylhydrazone of Salicyaldehyde 4. Semicarbazone of Acetophenone D. MULTIPLE SCAN STUDIES 1. Oxime of Benzophenone 3 Glassy C - 2. Hydrazone of Cinnamaldehyde - 3. Phenylhydrazone of Salicyaldehyde - 4. Semicarbazone of Acetophenone -