KINETICS OF OXIDATION OF SOME AMINES, AMINOALCOHOLS AND DIOLS BY DIPERIODATOARGENTATE(III)

Abstract

Transition metal ions and their complexes are known to play the catalytic role in a variety of homogenous and heterogeneous redox reactions. The unusual oxidation state species of these metal ions makes them highly effective in redox catalysis. Mechanisms of the reaction involving stable oxidation state couples viz., Co(ll)/Co(lll), TI(II)/TI(III), Fe(ll)/Fe(lll), Pt(ll)/Pt(IV), etc. have been studied widely. Acomplete elucidation of the reaction scheme of the short ijVed higher oxidation state species requires to generate them in varied experimental conditions and then monitor their reactivity kinetically. These species being the transients, investigation of their reactions would need to employ sophisticated fast kinetic techniques. Lately, there has been a growing interest in the study of reactions of unusual oxidation state species of silver because of their high redox potentials and involvement in Ag+-catalyzed reactions of different oxidizing agents. Reactions of silver(lll) species have generated further interest mechanistically being a two electron oxidant. Reactions of different silver complexes viz., tetrahydroxoargentate(lll) ([Ag(OH)4]"), [ethylenebis(biguanide)]silver(lll), diperiodatoargentate(lll) (DPA) have been investigated with a variety of substrates. [Ag(OH)4]" is produced in the metastable state by anodic oxidation of silver metal in highly basic medium. Its reactions are in general fairly fast and have to be monitored using stopped-flow method. Reactions of [ethylenebis(biguanide)silver(lll) and DPA are comparatively slow. Their reactions with a variety of substrates have been studied in highly acidic and highly basic medium, respectively. In these investigations mechanistic observations about the interaction of silver(lll) species and the mode of electron transfer have been observed to be quite at variance. In view of this complex behavior it is needed to undertake a systematic investigation on the reactions of different class of organics with silver(lll). In the present work kinetics of oxidation of certain diamines, ii aminoalcohols and diols by DPA have been examined in basic and mild basic conditions. DPA is a well established oxygen donor complex and is a strong oxidant. The present thesis comprises six chapters. The first chapter includes the background of the work being done in this area and defines the objectives of the investigation. The second chapter describes experimental details, about the used reagents and their purification, equipment, techniques and methodology. It incorporates the preparation and characterization of DPA. Method and experimental conditions used in kinetic experiments have also been discussed. Studies on the reactions of DPA with certain diamines, viz., 1,2- diaminoethane (1,2-DAE), 1,3-diaminopropane (1,3-DAP), 1,4-diaminobutane (1,4- DAB) have been included in the third chapter. Products of the reaction of silver(lll) with 1,2-DAE have been analyzed by using chromatographic and colorimetric methods. Formaldehyde, glyoxal and free Ag+ have been detected as products of this reaction. Kinetics of this reaction was analyzed at pH 10.5 and 8.0 using stopped-flow technique. At pH 10.5 the reaction depicts three steps - a rapid decrease in absorbance followed by two slower decay processes. The first step is the complexation of silver(lll) with 1,2-DAE and is over in about 10 ms which could not be analyzed kinetically. In the subsequent step silver(lll)-1,2-DAE complex is decomposed through intramolecular electron transfer from 1,2-DAE to the silver(lll) centre with a second-order rate constant of 1.3 x 104dm3 mol"1s"1. Silver(ll) species formed in this reaction reacts rapidly with 1,2-DAE. It oxidizes 1,2-DAE with a second-order rate constant of 6.3 x 104 dm3 mol'V1. This reaction could be followed only at low [1,2-DAE] (1 x 10"3mol dm"3) and thereafter at high [1,2-DAE] the two processes could not be separated. However, at pH 8.0 this reaction involves two steps. The first step in this case also was complete in about 10 ms. This is followed by a slow decay which is assigned to intramolecular electron transfer from the substrate to silver(lll) centre. The second-order rate constants for this step were found to be 1.4 x 104 dm3 mol'1 s"1. At pH 8.0, the reaction rate shows an inverse dependence on [ICV] and [OH] in the low concentration range (< 1 x 10"3 mol dm"3). At higher [OH] (>1 x 10"3 mol dm"3) the rate of reaction starts increasing and attains a limiting value at very high [OH"]. The nature of the intermediates formed has been ascertained by carrying out this reaction in the presence of free radical scavenger, acrylonitrile. To verify the findings of DPA -1,2-DAE reaction, other diamines like 1,3- DAP and 1,4-DAB have also been investigated. In reaction of 1,3-DAP it could become possible to analyze the rate of complexation. Kinetic behavior of DPA-1,4- DAB reaction was quite different to those of DPA - 1,2-DAE and DPA -1,3-DAP reactions. Rates of their complexation and oxidation by silver(lll) follow the order 1,2-DAE > 1,3-DAP > 1,4-DAB. It suggested that increasing separation of -NH2 in 1,2-DAE by-CH2 reduces both the rates of complexation and oxidation. The fourth chapter presents studies on the reaction of DPA with 1-amino- 2-ethanol (2-AE) and 3-amino-1-propanol (3-AP). These reactions depict three kinetically distinguishable steps - the induction period, the uptake of second ligand and oxidation. The initial process leading to the formation of silver(lll) -mono substrate complex occurred rapidly through axial binding of the substrate to DPA. It rearranges to form the corresponding stable complex during the induction period and uptakes another ligand in the following step. The resulting silver(lll) complex is then reduced by free substrate to form silver(ll) species. Both of these processes follow pseudo-first order kinetics. Reaction rates show inverse dependence on [I04] and [OH]. This behavior was very similar to that of DPA-1,4-DAB system. In these reactions rates of complexation and oxidation processes were slower to those of diamines. Rates of uptake of the second ligand and its oxidation by silver(lll) was higher in case of 1,2-AE compared to 3-AP. Possibly the separation of-NH2 and -OH by increased -CH2 contributes to this observation. IV The fifth chapter describes the reactions of DPA with diols. These reactions are fairly slow, therefore, the kinetics of these reactions have been monitored spectrophotometrically. All these reactions consist of two kinetically distinguishable steps - uptake of second ligand and redox process. Electronic spectra in these cases depict a weak complexation compared to diamines and aminoalcohols. Reaction of 1,2-ethanediol (1,2-ED) with DPA is completed within 30 min. Both the steps follow pseudo-first -order kinetics and show inverse dependence on [I04"] and [OH]. Reaction of 1,2-ED and DPA yields the HCHO as a major product. To find out the nature of the intermediate involved, the effect of [acrylonitrile] on the kinetics of reaction of DPA and 1,2-ED is studied. In diols there is no amino group present. The increased separation of two -OH by insertion of -CH2 still reduces the rates of complexation and follow the order - 1,2- ethanediol > 1,3-propanediol > 1,4-butanediol > 1,5-pentanediol >1,6-hexanediol. However, rates of oxidation are not affected appreciably. The sixth chapter gives a brief account of different investigated silver(lll) species. It incorporates a summary of the results of the present investigations. Reactive silver(lll) species of DPA have been identified kinetically in different experimental conditions. On the basis of these data mechanisms of the studied reactions have been discussed.