D.Sc. (Chemistry) / Two different projects with the central theme of oxidation are discussed, namely the ozonation of saturated hydrocarbons and the total synthesis of an unusual steroid glycoside. The oxidation of saturated hydrocarbons by ozone with the so-called dry ozonation technique was investigated. This study led to the development of improved procedures in which the rate, yield and selectivity of the reaction between ozone and various saturated substrates could be controlled. Subsequently the dry ozonation of a broadspectrum of long straight chain saturated hydrocarbons, including waxes, was investigated. Reaction with these highly unreactive compounds gave almost exclusively ketones as products in high yields. The average amount of keto functionalities in the products is dependent on the reaction conditions as well as the chain length of the substrate. Therefore, a reliable method has been developed for the selective oxidation of straight chain saturated hydrocarbons, a process that is of great chemical, industrial as well as commercial importance. The dry ozonation of various straight chain monoketo compounds was also studied. Poliketo compounds are mainly formed in these reactions. This investigation showed that keto functionalities in the substrates exert a deactivating effect on neighbouring methylene groups, so that oxidations do not take place at these positions. Therefore, it was possible to predict the product distribution formed during the dry ozonation of shorter chain length monoketo compounds (Cg) . An effective synthesis of an unusual steroid glycoside similar to orbicusides A-C, compounds that contain extraordinary carbohydrate and A-ring moieties, was developed. Oxidation reactions played a prominent role in this reaction sequence. A substantial effort was also directed towards obtaining the correct stereochemistry at the various stereocentres of the carbohydrate moiety. This facilitated ring closure to a final product that contained carbohydrate and A-ring moieties similar to orbicusides A-C. As a result of this work, a method was developed for the introduction of a vicinal cis-diol on the sterically hindered face of an alkene. This method is especially suited for substrates where acid labile functional groups are present.

The homogeneously catalyzed oxidation of 1-propanol by dioxygen in glacial acetic acid using cobalt(II)acetate and sodium bromide as the catalyst system has been investigated with the view of determining the significance of various experimental variables during the oxidation. The results of this investigation show unequivocally that a number of reaction variables have a direct influence upon catalytic activity and hence the reaction products. It is quite evident that the major product of this autoxidation reaction is propionic acid with the respective esters as side-products. This is an indication that the autoxidation mechanism occurs via a two-stage pathway, namely, the oxidation of 1-propanol to propionaldehyde as the primary product and, subsequently, the further oxidation of the propionaldehyde to propionic acid as the major product. Thus the esterification process of the propionic acid with the substrate 1-propanol could be termed as a side-reaction because its not facilitated by the catalyst system and it consumes the formed product. The catalyst activity has been demonstrated to depend on a number of factors, including the bromide concentration, the cobalt(II)acetate concentration, the water concentration, reaction temperature, and the presence of metal acetates as co-catalysts. There is an observed decrease in catalytic activity at high bromide concentration, which may be explained in terms of cobalt bromide complexes that form at these high concentrations. Subsequently, the same trend of catalyst activity reduction at high cobalt(II)acetate concentration may be ascribed to the “inactive” metal complexes that are susceptible to form at high metal ion concentrations. The catalytic activity increases with increase in total concentration and rapidly decreases at very high concentrations. This can be explained in terms of the observations made during the investigation of the effect of cobalt(II)acetate and bromide concentrations. The high increase in catalytic activity with increasing temperature is ascribed to the Arrhenius law, which relates the rate constant for a particular reaction to temperature. However, there is an observed loss of catalyst selectivity at high temperatures which maybe due to two possible factors. The first is simply related to an increased loss of volatile material from the reactor in the oxygen gas stream as the temperature is increased. The second relates to the increasing activity of the catalyst system for the selective decarboxylation of the carboxylic acid product. The addition of water to the reaction system rapidly reduces the catalyst activity. This detrimental effect is an indication that there is an effective competition by water with bromide for coordination sites on cobalt(II), thereby preventing the formation of the active catalyst species. The introduction of metal acetates as co-catalyst reduces the catalyst activity quite dramatically. This inhibition effect is suggested to relate to the redox potential of the respective metal ions. The results of statistical analysis of the experimentally derived response surface during the oxidation of 1-propanol, show no significant lack of fit, and the residuals obtained by applying the response surface to the design settings show that the data is normally distributed. The response surface is therefore reliable, but keeping in mind that the central composite design used is not rotatable so that its predictive power, especially outside the experimental domain investigated, is quite limited. However, several interesting observations were still possible The oxidative dehydrogenation of ethanol over supported noble-metal catalysts has been investigated with the view of identifying the most active supported noble-metal and also to compare this oxidation procedure with the autoxidation procedure. Secondly, the effect of an acidic resin as a co-catalyst was also investigated during the said oxidation. On the basis of results presented in this study during oxidative dehydrogenation of ethanol, catalysts no.2 (10% Pd/C), 8 (2% Pd/Al – Pb-promoted) and 9 (2% Pt/8% Pd/C) appear to be the most active in terms of relative rates, while catalysts 6 (10% Pd/C- Pbpromoted), 7 (5% Pd/C-shell reduced-Pb -promoted) and 10 (5% Pt 5% Pd on C) are more active based on the comparison of average rates. Two other observations are of interest. Firstly, the promotion of the Pd catalysts with lead appears to improve catalyst activity to some extent as shown by the comparisons between catalysts 1 and 5, 4 and 8, 2 and 6 and 3 and 7. Secondly, the introduction of Pt up to equal amounts with palladium seems to produce the most active catalysts. On its own, platinum appears to be a better catalyst than Pd when supported on activated carbon (catalysts 1 and 12). In comparison with the homogeneous, cobalt-bromide catalyzed oxidation of 1- propanol in the liquid-phase, oxidations over noble-metal catalysts in the liquid-phase appear to be significantly less active. The presence of the resin promoted the formation of ethyl acetate to some extent, the improvements are not as dramatic as expected.

The oxidative coupling of 2,6-di-t-butylphenol under mild reaction conditions is well documented and the subject of many patents. However, the coupling of other monoand di- substituted phenols is not as well documented and thus there is scope for further investigation for providing a convenient, environmentally friendly and economically viable method for the oxidative coupling of these phenols. In this study, the oxidative coupling of a variety of alkylated phenolic substrates, 2-tbutylphenol, 2,6-di-t-butylphenol, 2,4 -di-t-butylphenol and ,4-dimethylphenol, using a range of different oxidizing agents, were investigated by means of experimental and/or theoretical means. The dibutylated aromatics provided the highest selectivities to their respective coupled products, with results obtained with the dimethyl analogue being only satisfactory, and that for 2 -t-butylphenol being totally inefficient. PM3 Molecular orbital (MO) calculations were used to predict the possible modes of coupling for the substrates 2,6 -di-t-butylphenol and 2,4-di-t-butylphenol, and these results were then compared with those obtained experimentally in the laboratory. Preliminarily, the coupling of unsubstituted phenolics was also assessed by means of MO calculations. Much emphasis was placed on Ce(IV) as the oxidant, and the reaction conditions under which it was used and the results that were obtained have not been reported before and are therefore novel. The oxidation of 2,4-di-t-butylphenol using Ce(IV) in the presence of methanesulphonic acid was optimized to afford high yields and selectivities to the desired ortho C-ortho C coupled product under mild reaction conditions. Various reaction parameters were also investigated in this case, such as varying the MeSO3H concentration, the solvent, the reaction temperature, the reaction time, the substrate loading, the rate of oxidant addition and the substrate to oxidant ratio. Ce(IV) also gave a high selectivity to the para C-para C coupled product when IX using 2,6-di-t-butylphenol as the substrate. However, it was not as effective with 2,4- dimethylphenol, and even less so with 2-t-butylphenol. The oxidation reactions of 2-t-butylphenol and 2,4-dimethylphenol with various coupling agents were also investigated with the intention of obtaining high selectivities to the respective desired coupled products. In these studies, 2-t-butylphenol afforded a large number of products, irrespective of the oxidant used. The dimethyl analogue was more selective, but results were not optimal. It was clear that the number of substituents on the phenol ring, their nature and their position with regards to the hydroxyl moiety was of great importance and made a significant impact on the preferred coupling mode of the substrate. It was observed that steric effects also played a major role in the outcome of these reactions: 2,6-di-t-butylphenol never afforded any C-O coupled products whereas 2-t-butylphenol, 2,4-di-t-butylphenol and 2,4-dimethylphenol all appeared to undergo some C-O coupling. Finally, reaction mechanisms were provided for both the K3Fe(CN)6 and Ce(IV) work, these reacting in basic and acidic media, respectively. It was proposed that both of these mechanisms operate through the initial formation of the phenoxyl radical.

No description available.

No description available.

The kinetic behaviour of anhydrous CrCl₃ in gaseous oxidising atmospheres has been examined in the temperature range 350 to 630°C in order to identify optimum reaction conditions and to establish the mechanism by which the reaction is controlled. The reaction under consideration is CrCl₃ (s) + ¾ O₂ (g) → ½ Cr₂O₃ (s) + ³/₂ Cl₂ (g). The main experimental techniques used were isothermal and programmed-temperature thermogravimetry, supplemented by scanning electron-microscopy, surface-area determinations and porosity measurements. The effects of sample pelletisation, gas flowrate, temperature, oxygen partial pressure, pellet porosity and the addition of other oxide species on the rate and extent of reaction have been studied. The reaction is shown to occur in a single step, starting at -350°C under non-isothermal conditions, and exhibiting a deceleratory rate over most of the reaction. Isothermal thermogravimetric curves were fitted to a number of kinetic rate expressions, and a series of statistical analyses used to identify the rate equation which best describes the experimental data. Supporting evidence was provided by scanning electron-microscopic examination of partially-reacted samples. It is concluded that the reaction is under chemical control, and that reaction occurs by means of a linearly-advancing reactant-product interface. The reaction kinetics can be described by a contracting-geometry rate expression. / KMBT_363 / Adobe Acrobat 9.54 Paper Capture Plug-in

The mechanism of the permanganate oxidation of benzylamine and benzylamine-α-d₂ has been investigated in the pH region 2 to 14. The deuterium isotope effect, kH/kD, was found to be 7-0 from pH 8 to 10.7. The absence of salt effects and the observation that the oxidation rate followed the ionization process of the benzylammonium ion show that benzylamine is oxidized via the neutral molecule. Beyond pH 12 the rate of oxidation was found to be directly proportional to the hydroxyl ion concentration. A study of 11 meta- and para-substituted benzylamines shows excellent Hammett correlation with σ+ for 8 of the compounds, with the p-nitro, m-nitro, and m-trifluoromethyl derivatives deviating considerably from the Hammett plot. ρ+ was -0.28. The correlation with σ+ and the very negative ΔS≠ values obtained for the oxidation reaction show that the transition state must be an ionic one involving extensive charge separation. The rate-determining step is consistent with hydrogen transfer from benzylamine to permanganate but the choice of a hydride ion transfer mechanism or a hydrogen atom transfer mechanism cannot be unequivocally made from the results obtained in this thesis. For the oxidations in the highly alkaline regions beyond pH 12, termolecular mechanisms are suggested. Whereas O-alkylation protects alcohols from oxidation by permanganate, N-alkylation greatly increases the oxidation rate of amines. However, N-acylation tends to protect the amine from oxidation. Limited studies of the kinetics of permanganate oxidation of ammonia, cyclohexylamine, N,N-dimethylbenzylamine, and (-)- α-methylbenzylamine were carried out to compare their rates of oxidation with those for benzylamine. The mechanism of the permanganate oxidation of t-butyl -amine to t-nitrobutane was studied from pH 8 to 12. As in the benzylamine oxidation t-butylamine is oxidized via its neutral molecule. The absence of an α-hydrogen and the absence of a substantial isotope effect in t-butylamine-ND₂ suggest that the mechanism involves an oxidative attack on the nitrogen by permanganate. The formation of a quaternary hydroxylamine derivative between t-butylamine and permanganate has been proposed for the rate-determining step. The kinetics of permanganate oxidation of benzylamines at -10° in frozen H₂O and D₂O has been investigated from pH 7.5 to 9.3. The reaction order remains unchanged from that at 25°. The rates of oxidation are more than 10 times greater in ice at -10° than in liquid water at -10°, and this acceleration increases with decreasing pH. For p-nitrobenzylamine there is only a 3-fold increase in rate in going from liquid to frozen water at -10°. An apparent pKBH+ of 8.70 was obtained for benzylamine at -10° in ice. No change in the rate was observed when the oxidation was carried out in frozen D₂O and when the ionic strength of the medium was increased. The accelerating effects observed in the frozen systems are discussed in terms of pH, concentrating effect of the freezing, and the orientating effect of the ice structure. / Science, Faculty of / Chemistry, Department of / Graduate

The mechanism of oxidation of Potassium iodide by Chlorine and Fluorine gas has been studied using powdered KI, pressed pellets and single crystals (pressed pellets only in the case of Fluorine). The electrical conductivity of a KI pellet has also been determined as a function of temperature. The major part of the experimental work concerns changes in D.C. electrical conductivity during reaction. The main conclusions are as follows: (a) The room temperature conductivity of KI is always higher than that obtained by extrapolating the impurity-range, and the activation energy below about 92° C. is very small; this is tentatively attributed to a space-charge effect. (b) In the KI/CI₂ reaction, the products are invariably KCl and I₂ in various forms (including V-Centres), although poly-halides are thermodynamically more stable. Thus the reduction of Chlorine occurs in a manner which does not allow reorganization to ICI₂-. (c) In pressed pellets, the conductivity always increases at the start of oxidation, but the enhanced conductivity may be ionic or electronic in different circumstances, and the initial increase may be followed by several alternative processes which are structure-sensitive. There is an inverse correlation between initial conductance and reactivity which is interpreted in terms of competition between (i) trapping of positive holes at isolated cation vacancies and (ii) nucleation of solid Iodine at grain boundaries. (d) Unpressed powders show no increase in conductance during reaction; this is attributed to preferential oxidation of bound surface states. / Science, Faculty of / Chemistry, Department of / Graduate

A study has been made of the potassium permanganate oxidation of three organic compounds: pivalaldehyde, p-nitrobenzaldehyde and p-nitrophenyltrifluoromethyl-carbinol. The permanganate-pivalaldehyde reaction has been investigated in the pH range 1 to 13. The results show that the reaction is first order in permanganate and aldehyde, that the oxidation by manganate is slower than that of permanganate by a factor of twenty five, and that the oxidation is general acid-catalyzed. The results in alkaline solution show some increase in rate but are not reproducible, which is most likely due to radical decomposition of the pivalaldehyde. Three mechanisms are discussed: Firstly, a typical general acid-catalysis scheme, secondly, one involving a tetragonal transition state and thirdly, one based on a hydride transfer. Of these, only the first two are able to accommodate the experimental evidence found earlier in the permanganate oxidation of benzaldehyde by Wiberg and Stewart (1). The autocatalysis exhibited by the permanganate benzaldehyde reaction has been studied at low pH. The autocatalysis has also been investigated with p-nitrobenzaldehyde in order to see if there is any substituent effect. Results show that the p-nitro aldehyde has a longer induction period. Moreover, it has been shown that overoxidation of the benzene ring rather than true autocatalysis has occurred. A radical chain mechanism has been put forward in an attempt to explain this observation. The permanganate oxidation of p-nitrophenyltri-fluoromethylcarbinol has been studied in 0.1 M sodium hydroxide in order to determine the mechanism of the reaction. A large enhancement in rate observed compared to rates found by Stewart and Van der Linden (2) in the permanganate oxidation of other substituted phenyltrifluoromethylcarbinols is good evidence for a reaction path involving hydrogen atom abstraction from the alkoxide to the permanganate ion. Such a mechanism is consequently favoured. / Science, Faculty of / Chemistry, Department of / Graduate

A study of the Lauth's Violet absorptiometry method for the analysis of sodium sulphide was carried out. The order of adding the reactants in the development of the color, the use of three different methods of titration for standardizing the solutions used for determining the calibration factor, and the effects of varying the concentrations of the reagents were all investigated. In the formation of Lauth's Violet from sodium sulphide, addition of reactants in the order base electrolyte, sodium sulphide, and p-phenylenediamine was shown to be best. The arsenious oxide method of titration was selected for determining the concentration of sodium sulphide in standard solutions used for calibration. If the reagents: base electrolyte containing ferric chloride, and p-phenylenediamine, were used at the concentrations of 0.16% and 5% by weight, respectively, the calibration factor remained constant up to concentrations 16 grams per litre of sodium sulphide. The applicability of the Lambert-Beer law and the effect of sodium thiosulphate on the absorptiometry method also were studied in detail. The oxidation of sodium sulphide was studied under atmospheric pressure at various temperatures by use of two different apparatuses. From the measurements of the amounts of oxygen absorbed, the oxidation of sodium sulphide probably gives disulphide, thiosulphate, sulphite and sulphate. The stoichiometry corresponds to thiosulphate being the main product of this reaction. The rate of absorption of oxygen with simultaneous, oxidation of sodium sulphide was observed to decrease, with increasing temperature from 25° to 45°C Nevertheless, the maximum absorption rate was found at 55°C, and the absorption rate decreases again with temperature at least to 85°C. The reaction orders for the oxidation of sodium sulphide depend upon temperature, being second order at 25°C, order of 1.5 at 35° and 45°C., and first order from 55° to 85°C. The reaction order with respect to oxygen is first order, irrespective of temperature. The maximum rate of oxidation also occurs at 55°C. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate