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Mechanistic Studies On The Oxidative Polymerization Of 8-Hydroxyquinoline And Diiodohydroxyquinoline In Anti-Diarrhea Tablets

Saleh A.Ahmed, Seddique M.Ahmed

Mechanistic studies of the oxidative polymerization reaction of 8-hydroxyquinoline(HQ) were studied experimentally using spectrophotometric technique. Molecular mechanics(MM+) calculations showed that the potential energy(PE) and optimum molecular geometric(OMG) energies(kcal/mol) of the proposed structures formed during polymerization process are very low. These calculations supported the proposed mechanism. The oxidation constant(Kox) of the anionic form(HQ-NaOH) is about twice greater than the protonated form(HQ-H2SO4). Molecular mechanics(MM+) calculations showed that the PE (kcal/mol) of the HQ in the anionic(HQ-NaOH) is about two times greater than the PE in the protonated(HQ-HCl) forms. The proposed procedure was followed successfully for the oxidation of HQ in sterptoquin(SQ) (anti-diarrhea) tablets. The oxidation constant(Kox) of HQ in SQ was found also to be lower than the Kox in synthetic(HQ) solution. Kinetic parameters of the oxidative polymerization of the anionic form of HQ(HQ-NaOH) at different concentration were deduced employing a computer-oriented kinetic analysis of the absorbance(A) at 610nm against the time(t/sec) data. The results obtained indicate that the rate controlling process is governed by the Ginstling-Brounshetin-equation three-dimensional diffusion(D4). It is interesting to note from these calculations that the k value calculated by D4 is about eleven, twenty four, and one hindered five times of magnitude lower than the k values obtained by applying first-(F1), second-(F2), and third-order(F3) kinetics under the same conditions. Activation parameters for the rate of oxidation process of HQ have been computed and discussed.