Hydrogenation rates, substrate dependence

The optoelectronic properties of the i -Si H films depend on many deposition parameters such as the pressure of the gas, flow rate, substrate temperature, power dissipation in the plasma, excitation frequency, anode—cathode distance, gas composition, and electrode configuration. Deposition conditions that are generally employed to produce device-quahty hydrogenated amorphous Si (i -SiH) are as follows gas composition = 100% SiH flow rate is high, --- dO cm pressure is low, 26—80 Pa (200—600 mtorr) deposition temperature = 250° C radio-frequency power is low, <25 mW/cm and the anode—cathode distance is 1-4 cm. 

The reaction rate was calculated from hydrogen consumption, from pressure drop. As the hydrogen consumption rate was dependent on amount and purity of substrate, catalyst weight and metal content, conversion rate was given (%/min mg Pd), the measured total hydrogen consumption was taken equivalent with 100% conversion. 

The catalyzed hydrogenation of an aldehyde- vs. a ketone-carbonyl is invariably faster because of steric effects (23), and the data for 6 vs. 10 are in line with this (eqs. 4 and 5). Thus, conversions of 6a-c after 0.5 h at standard conditions are 86, 47, and 97%, respectively, while corresponding values for lOa-c after 4 h are 78, 36 and 49%, respectively. Indeed, the aldehydes can be reduced at 25 °C under otherwise identical conditions (6b gives 38% conversion after 4 h, and 6c gives 99% after 15 h). The above reactivity trend for the ketones lOa-c shows that the hydrogenation rates depend on the substituent para to the carbonyl functionality and increase in the order H > OMe > OH. For the aldehyde susbtrates, the more limited data (substrate 6 with R = H and R = OMe was not available) suggest a similar para-substitucnt effect (at least OMe > OH). Note that this is the reverse trend to that observed for reduction of the activated C=C systems described above. 

An example of these pressure studies is provided by the studies of Elsevier et al. [31], who investigated the dependence of the hydrogenation rate of 4-octyne by a Pd-catalyst on the dihydrogen pressure, which was varied between 0 and 40 bar. The hydrogenation rate was shown to depend linearly on the dihydrogen pressure. In order to elucidate the reaction mechanism, the dependence of the reaction rate on substrate and catalyst concentration, and on the temperature, was also measured. NMR experiments with deuterium gas as well as PHIP-ex-periments were also carried out. 

The influence of hydrogen pressure, substrate and catalyst concentration has briefly been mentioned. The reaction rate is dependent upon the catalyst concentration and hydrogen pressure, but appears to be independent of substrate concentration. The mechanism is proposed to involve the activation of the parent [Pd(allyl)] species producing an unstable hydrido-Pd(II) species (71), ensued by a fast reaction with the diene to restore the [Pd(allyl)] moiety (72) (Scheme 14.21). The observation that most of the starting material is isolated after the reaction suggests that only a small portion of the catalyst is active under the reaction conditions. Although a complete selectivity for the monoene is observed (even after full conversion), the presence of catalytically active colloidal palladium has not been completely excluded. 

It appears that all these possibilities can be excluded. If reactions (a) or (gf) were rate-limiting the reaction velocity would be independent of the concentration of the substrate, while reaction (e) (identical with (Z)) would predict no catalysis by acids or bases. If reactions (b), (d) or (h) determined the rate the reaction would show specific catalysis by hydrogen or hydroxide ions, in place of the general acid-base catalysis actually observed. Reactions (c), (f) and (m) are unacceptable as rate-limiting processes, since they involve simple proton transfers to and from oxygen. Reactions (j) and (k) might well be slow, but their rates would depend upon the nucleophilic reactivity of the catalyst towards carbon rather than on its basic strength towards a proton as shown in Section IV,D it is the latter quantity which correlates closely with the observed rates. 

As expected, the decrease in the initial alkyne concentration results in less dimerization and higher initial hydrogenation rates. A similar substrate-inhibition effect has also been observed with PhC CH as substrate, revealing a complex dependence of the hydrogenation rate upon the alkyne concentration. To the best... 

Hydrogen atoms and hydroxyl radicals react with aliphatic compounds mainly by H-abstraction from the chain, although reactions with certain substituents are also important. With hydrated electrons the functioned group is the only site of reaction and its nature determines the reactivity. The reactions of hydrated electrons are by definition electron transfer reactions. The rate of reaction of a certain substrate will depend on its ability to accommodate an additional electron. For example, in an unsaturated compound the rate may depend on the presence of a site with a partial positive charge. Thus acrylonitrile and benzonitrile are three orders of magnitude more reactive toward e q than are ethylene and benzene. On the other hand, this large difference does not exist in the case of addition of H and OH. 

The oxidation of thiols in the form of L-cysteine, penicillamine, and thioglycollic acid by [Mo(CN)g] in aqueous acidic solution also formed disulfides as final products 111). The reactions show a second-order substrate dependence, and the rates are found to decrease with increasing hydrogen ion concentration. This is attributed to the deprotonation of the —SH and —COOH groups in these thiols prior to electron transfer. The reactions are interpreted in terms of outer-sphere activation. An explanation for the second-order dependence on thiol concentration involves ion association between the cyano complex and a protonated form of the thiol, followed by reaction of this complex with a second thiol molecule. 

Kinetic data have been reported for cyclohexene reduction with a 1 6 Cr(acac)3- Bu3Al catalyst in heptane at 30 C, which showed a first-order dependence on catalyst and H2. Hydrogenation rates generally decrease with increasing substitution of the alkene substrate. Similar kinetic results were independently obtained for the Cr(acac)3- Bu3Al catalyst. A proposed mechanism involves alkylation of the metal-halide [equation (a)], hydride formation [equation (b)], followed by reversible insertion of the olefin substrate into the metal-hydride bond [equation (c)], and hydrogenolysis of the resulting metal-alkyl bond [equation (d)]. ... 

The introduction of 9,10-disilaanthracenes as the alternative silanes for the reduction of halides and thionocarbonates has been proposed [67, 69[. Two examples are given in Eqs. (30) and (31). The rate constants for hydrogen abstraction from these substrates depend on the number of available hydrogens and can reach values as high as (TMS).-tSiH (Table 1). 

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