Boron carbide was produced on tungsten substrate in a dual impinging-jet CVD reactor from a gas mixture of BCl3, CH4, and H2. The experimental setup was designed to minimise the effect of mass transfer on reaction kinetics, which, together with the on-line analysis of the reactor effluent by FTIR, allowed a detailed kinetic investigation possible. The phase and morphology studies of the products were made by XPS, XRD,micro hardness and SEM methods. XPS analysis showed the existence of chemical states attributed to the boron carbide phase, together with the existence of oxy-boron carbide species. SEM pictures revealed the formation of 5-fold icosahedral boron carbide crystals up to 30 micron sizes for the samples produced at 1300oC. Microhardness tests showed change of boron carbide hardness with the temperature of tungsten substrate. The hardness values (Vickers Hardness) observed were between 3850 kg/mm2 and 4750 kg/mm2 corresponding to substrate temperatures of 1100 and 1300 C, respectively. The FTIR analysis of the reaction products proved the formation of reaction intermediate BHCl2, which is proposed to occur mainly in the gaseous boundary layer next to the substrate surface. The experimental parameters are the temperature of the substrate, and the molar fractions of methane and borontrichloride at the reactor inlet. The effects of those parameters on the reaction rates, conversions and selectivities were analysed and such analyses were used in mechanism determination studies. An Arrhenius type of a rate expression was obtained for rate of formation of boron carbide with an energy of activation 56.1 kjoule/mol and the exponents of methane and boron trichloride in the reaction rate expression were 0.64 and 0.34, respectively, implying complexity of reaction. In all of the experiments conducted, the rate of formation of boron carbide was less than that of dichloroborane. Among a large number of reaction mechanisms proposed only the ones considering the molecular adsorption of boron trichloride on the substrate surface and formation of dichloroborane in the gaseous phase gave reasonable fits to the experimental data. Multiple non-linear regression analysis was carried out to predict the deposition rate of boron carbide as well as formation rate of dichloroborane simultaneously.

In our previous publication of Thiele cage diols, we describe the first pinacol rearrangement to occur over a C(sp3)–C(sp3). Two mechanisms were initially proposed: a concerted mechanism and a stepwise mechanism, proceeding through a carbocation intermediate. Interestingly, the rearrangement provides only a single diastereomer. The aim of this thesis is to investigate the nature of the reaction by measuring the relative rates of reaction with varying substituents that either stabilize or impede the formation of carbocations. From the relative rates, we can narrow in on whether the mechanism is stepwise or concerted, based on substituent effects, and determine how the reaction may provide a single diastereomer. In Chapter 1, the pinacol rearrangement is introduced, and each analogue that followed after, including stereoselective pinacol rearrangements and a series of different semipinacol rearrangements that provide useful synthetic pathways for chemists with desirable stereochemical outcomes. In Chapter 2, we describe the isolation and characterization of two analogues of a key side product. The structure of each analogue was determined through a series of spectroscopic techniques including 1H-1H COSY, HSQC, HMBC, and 1D-selective gradient NOE NMR. From the solved structures, we proposed a possible mechanism to describe their formation during the main rearrangement reaction – one that shares a carbocation intermediate with the stepwise mechanism already proposed. In Chapter 3, we prepared five Thiele cage diol analogues with aryl substituents with different electronic properties: two substituents that stabilize carbocation intermediates (p-OCH3 and p-CH3), two that destabilize (p-F and 3,5-diOCH3), and the base tetraphenyl Thiele cage diol. We measured the rates of reaction of each diol with p-toluenesulfonic acid at 25°C, 35°C, 45°C, and 52°C via variable temperature quantitative 1H-NMR over time. From the rates of reaction, we found that diols with carbocation-stabilizing aryl substituents reacted faster than the destabilizing analogues, providing evidence that the rearrangement proceeds through a carbocation intermediate. We also found that the diols with electron-deficient aryl substituents showed an increase in the entropy of activation with increasing electron-deficiency in the aryl groups, suggesting an associative pathway for the electron-deficient substituents toward the rearrangement product. Considering the pathway proposed for the side product, we propose an updated stepwise mechanism. Based on computational studies and a previously-isolated X-ray crystal structure, we determined that the diastereoselectivity of the reaction was facilitated by favourable π-π stacking interactions between two aryl substituents. The interaction of the aryl groups twists the geometry of the molecules, placing the migrating aryl substituent in the ideal position for the rearrangement to occur stereoselectively. / Graduate / 2023-10-31