Thangavelu, S. (1985) Studies on the electrochemical reduction of carbonyl compounds. PhD thesis, Madurai-Kamaraj University.

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Abstract

This thesis deals with the study of the electrochemical reduction behavior of glyoxylic acid and phenylglyoxylic acid at silver electrode in aqueous medium and phenylglyoxylic acid in non-aqueous medium. The earlier studies of these compounds were confined only with mercury electrode. These bifunctional organic compounds exist in hydrated from in aqueous conditions and only dehydrated molecules are electro active. The above organic acids are in disassociation equilibrium with their anions and due to above mentioned conditions the reduction behavior of these compounds are complex in nature. The electrochemical reduction behavior of glyoxylic acid and phenylglyoxylic acid has been studied using rotating disc electrode voltammetry, cyclic voltammetry, chronoamperometry and controlled potential electrolysis techniques. Reduction of glyoxylic acid : The limiting current was found to be non-linear with W1/2 at neutral and alkaline pH. The current function values, ip/ACV1/2 obtained from cyclic voltammetric study were not constant. These observations along with the current time relationship obtained from chronoamperometric study indicated that the process at the limiting current region is mixed controlled both by diffusion and kinetics of the preceding dehydration reaction even in highly alkaline medium. The overall process was found to be irreversible. Tafel slope value for the reduction of glyoxylic acid was found to be pH dependent. At neutral pH the tafel slope was 60 mV and 30-40 mV at alkaline pH. Reaction orders were fractional with respect to glyoxylic acid and protons. E1/2 were found to shift in the cathodic direction with pH and reactant concentration glyoxylic acid was found to give tartaric acid as dimerized product-both at neutral and alkaline pH conditions. Based on the above observations the reduction mechanism at neutral medium and at alkaline medium involving Temkin type of adsorption was proposed for the reduction of glyoxylic acid. Reduction of phenylglyoxylic acid: Almost similar observations were made with phenylglyoxylic acid. At the limiting current region the process was found to be diffusion controlled irreversible process in the pH range 4-14. The reduction product was mandelic acid. The reduction mechanism at the foot of the wave in alkaline medium was proposed. Reduction of phenylglyoxylate in DMF: The electrochemical reduction of phenylglyoxylate was studied in dimethylformamide containing 0.2 M NaClO4. Single wave was observed with nearly half the wave height of the wave observed for the same concentration of phenylglyoxylic acid in aqueous alkaline medium. This observation gave an indirect indication that one electron transferred radical anion is formed as an intermediate. Addition of proton donors like water and phenol resulted in an additional wave formation at less negative potential. It was also observed that the wave height of the two waves (original wave in pure DMF and additional wave in presence of proton donors) observed in the presence of water was found to increase enormously when n-butyl bromide was added to the system. As n-butyl bromide is known to react with radicals and radical anions to regenerate the reactant, both the waves observed in presence of water were due to one electron reduction and the additional wave was due to the reduction of molecular form of phenylglyoxylic acid and the original wave was due to the reduction of phenylglyoxylate anion. In conclusion the reduction of phenylglyoxylate anion in DMF id found to undergo one electron transfer with coupled fast dimerization reaction which leads to phenyl substituted tartaric acid.

Item Type:	Thesis (PhD)
Uncontrolled Keywords:	Electrochemical Reduction; Carbonyl compounds
Subjects:	Electroorganic
Divisions:	UNSPECIFIED
Depositing User:	Dr. N Meyyappan
Date Deposited:	12 Jun 2012 07:36
Last Modified:	12 Jun 2012 07:36
URI:	http://cecri.csircentral.net/id/eprint/2812

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