Kinetics and mechanism of the solvolysis of allyl and benzyl arenesulfonates and a study of the preparation of boron-carbide compounds by chemical vapor deposition
Rissmann, Thomas J.
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The first part of this thesis is an analysis of the specific rates of the solvolyses of allyl arenesulfonates (ring substituents are p-CH₃, H, p-Cl, p-N0₂, and m-N0₂) and benzyl p-toluenesulfonate in terms of the extended Grunwald-Winstein equation. Past studies have shown close similarities of allyl and benzyl systems based on analyses using this two-term (four-parameter) equation with incorporation of either N[sub KL] and Y values or N[sub MeOTs] and (Y)^2-AdOTs values. These similarities have led to the suggestion that allyl and benzyl systems should both be slightly more sensitive to solvent nucleophilicity than to solvent ionizing power. However, the analyses from these studies have displayed a significant dependence on both the solvent nucleophilicity (high l values) and the solvent ionizing power (high m values). For the five allyl arenesulfonates studied in the various solvents at 50.0°C, an average value for l was 1.13 and an average value for m was 0.70, based on the use of N[sub Et3O+] and Y[sub 2-AdOTs] scales. A similar analysis of the same rate constants for the five allyl arenesulfonates resulted in 0.83 for l and 0.49 for m, using N[sub Et3O+] and Y[sub 2-AdOTs] scales. For comparison, benzyl tosylate solvolysis was also analyzed using the same two sets of N and Y scales. When using N[sub Et3O+] and Y[sub 2-AdOTs] scales, l = 0.78 and m = 0.84, and for N[sub MeOTs] and Y[sub 2-AdOTs] scales l = 0.53 and m = 0.68. This analysis suggests that the benzyl system reacts with less nucleophilic assistance than the allyl system. The need for variety in the choice of solvents, leading to wide ranges-for the N and Y values, is emphasized. In addition, a mechanistic comparison of the solvolyses of five allyl arenesulfonates in several different solvents based on a Hammett treatment have suggested that the same mechanism occurs throughout. The second part of this thesis is a study of the preparation of boron-carbide compounds produced by chemical vapor deposition (CVD) using boron trichloride (BCI₃) and carbon tetrachloride (CCl₄) in an excess of hydrogen. Initially, experiments were carried out to reproduce material that was obtained from a past study using a hollow-tube (graphite) and following the same procedure. This resulted in obtaining B₅.₆C which is similar to the observed material of the past study and correlates well with B₅.₆₆C obtained from a hot-wire method. By using an impeded flow device for the substrate (graphite muffled tubes) various deposits were discovered with different characteristics for each deposit. A brown crusty material was first observed with an average composition of B₃₀C. The second main deposit was a hard shiny material that analyzed to an average of B₂₀C. The last deposit found was single needle shaped crystals that were seen downstream to the first two which resulted in B₂C suggesting possible B₁₂C₄ character. These findings show changes in the reactant gases as one moves along the muffled tubes, which correspond to BCl₃ reacting faster than CCl₄ at lower temperatures, resulting from the temperature gradient inside the Astro furnace. Attempts to determine "free" graphite by chemically oxidizing the carbon to carbon dioxide suggested that this approach is not promising because free graphite and carbon within boron carbide (B[sub x]c) have apparently equal sensitivities towards oxidation by powerful oxidizing agents.