Kinetically Significant Hydrogen

Location of Kinetically Significant Hydrogen Westheimer and Nicolaides62 have utilized the deuterium isotope effect to determine whether the C—H or O—H bond rupture is rate controlling in the oxidation of isopropanol by chromic acid. Isopropanol-2-di was synthesized and found to react at approximately one-sixth the rate of the all protium compound. This is conclusive evidence that C—H rather than O—H bond rupture is rate determining. 

It was an early hope that the comparison of acid and base catalyzed reactions in HaO and DaO would prove useful in understanding the general aspects of such reactions. This expectation has been realized only in part. One early observation was that many acid catalyzed reactions, e.g. ester hydrolysis, proceeded more rapidly in DaO than in HaO. The explanation of this result was immediately obvious from the mechanism of the reaction..44 For specific acid catalyzed reactions, one finds 

Location of Kinetically Significant Hydrogen Westheimer and Nicolaides have utilized the deuterium isotope 

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ApA < 1. In Fig. 2 the region of curvature is much broader and extends beyond — 4 < ApA < + 4. One explanation for the poor agreement between the predictions in Fig. 3 and the behaviour observed for ionisation of acetic acid is that in the region around ApA = 0, the proton-transfer step in mechanism (8) is kinetically significant. In order to test this hypothesis and attempt to fit (9) and (10) to experimental data, it is necessary to assume values for the rate coefficients for the formation and breakdown of the hydrogen-bonded complexes in mechanism (8) and to propose a suitable relationship between the rate coefficients of the proton-transfer step and the equilibrium constant for the reaction. There are various ways in which the latter can be achieved. Experimental data for proton-transfer reactions are usually fitted quite well by the Bronsted relation (17). In (17), GB is a... 

Nucleophilic Solvent Assistance—kinetically significant involvement of the solvent as nucleophile or base by partial bonding (as distinct from general electrostatic solvation) to any atom of the substrate (e.g. a-carbon, j3-hydrogen, etc.) ... 

In more recent work on hydrogenation of butadiene polymers and copolymers, the attempt was made to explain the dependence on hydrogen pressure with sole rate control by olefin addition to H2RhClPh2 (X3) and quasi-equilibrium rhodium distribution over the complexes with and without hydrogen [59] instead of kinetic significance of the step Xq + H2 — X3. This gives a rate equation for double-bond disappearance of the form... 

The most intriguing difference is the thermodynamically as well as kinetically facilitated hydrogen addition 11 —> 12 that hampers the last catalytic step, i. e. reductive aldehyde elimination 12 —> 7. In contrast to rhodium, Olefin/CO insertions 8 9/10 —> 11 are thermoneutral/endothermic, whereas Olefm/CO associations 7 8/9 —> 10 are significantly more exothermic. In summary, the total reaction... 

The rates of oil, hydrogen, methane, carbon dioxide and carbon monoxide evolution during the retorting of five Australian oil shales at linear heating rates have been determined and analysed in terms of the Anthony-Howard model for non-iso-thermal kinetics. Significant differences in the retorting properties of these shales were obtained, particularly with respect to the rates of the hydrogen and carbon dioxide evolution. 

The above-mentioned evidence relating carbidic surface carbon to methanation and to FT synthesis raises the question how the reactivities of COajs and compare. This additional information, discussed in the following two sections, is indispensable for deciding whether or not C-O bond breaking is a slow step in the synthesis, and whether or not hydrogenation of carbidic carbon is kinetically significant in the presence of CO, that is, in the FT synthesis proper. 

In the absence of significant electronic interactions with the support, a justifiable assumption for Pt/silica catalysts, a further potential influence of the catalyst support on the reaction kinetics is hydrogen spillover and diffusion on silica. In order to develop a model for the spillover and diffusion of hydrogen on silica a temperature programmed desorption (TPD) study of deuterium desorption from silica was conducted. The TPD experiments were conducted using a narrow bore silica reactor in a small furnace. The catalysts were first reduced in-situ in deuterium at the reduction temperature for 1 hour and then cooled to ambient temperature, again in deuterium. The reactor was then evacuated and heated at 20°C/min. Analysis of the desorbed gases was mass-spectrometer. 

With regards to the apparent kinetic constant, there is experimental evidence that hydrogen inhibits the reaction kinetics significantly, whereas carbon monoxide does not. From this evidence, the apparent kinetic constant is an expression of the Langmuir-Hinshelwood type [32] ... 

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