Title

THE KINETICS AND MECHANISMS OF THE ACIDIC HYDROLYSIS AND FORMATION OF THIOSEMICARBAZONES IN THE PRESENCE OF MICELLES

Date of Completion

January 1982

Keywords

Chemistry, Pharmaceutical

Degree

Ph.D.

Abstract

Micellar effects on the hydrolysis and formation of thiosemicarbazones were studied in acidic solutions. The applicability of the electrostatic model was tested by determining the effect of differently-charged micelles on the observed rates of reaction. The effects of the ring nitrogen position in a series of pyridinecarboxaldehyde thiosemicarbazones on the reaction rate and the mechanism in the presence of micelles were also investigated. The application of micellar alteration of reaction rates was extended to (a) hydrogen ion catalyzed reactions, (b) reversible reaction, and (c) pseudo bimolecular reactions. This extension required application of the multi-plot approach to catalytic and bimolecular systems in order to evaluate different kinetic parameters which were also checked by independent measurements. The agreement was good with the exception of the isoquinaldehyde thiosemicarbazones. Studies as a function of temperature were also made and interpreted.^ The observed inhibition and catalysis of the hydrolysis reaction in the presence of cationic cetyltrimethyl ammonium bromide and anionic sodium dodecyl sulfate, complied with the electrostatic theory. The log micellar rate constant-pH profiles confirmed that the reactions were hydrogen ion catalyzed in the presence of the above two surfactants. Analysis of this data also led to the conclusion that the substrates resided on the surface of these ionic micelles. The kinetic results found in the presence of non-ionic polyoxyethylene lauryl ether could not be interpreted on the basis of charge effects. Nevertheless, significant catalysis was observed, although micellar rate constants were relatively less sensitive to pH. The results were interpreted to indicate that catalysis occurred due to the interaction of the surfactant chains with the substrate molecules incorporated in the micellar phase.^ Similar to the hydrolysis studies, the kinetic results for formation of all four thiosemicarbazones in the presence of ionic surfactants indicated that the electrostatic model was followed and the reaction was hydrogen ion catalyzed. Thiosemicarbazones were not formed in the presence of non-ionic surfactants as opposed to catalysis observed in the hydrolysis reaction. Based on the solubility experiments the lack of reaction in the micellar phase was attributed to a negligible thiosemicarbazide partition coefficient. ^