Hydrogenation kinetic equations, substrate concentration

Preliminary kinetic data on the catalyzed hydrogenation of acrylamide using HRuCl(diop)2 generally show a first-order dependence on hydrogen, between a first- and a zero-order on both ruthenium and substrate, and an inverse dependence on added diop at lower substrate concentrations. These dependences are consistent with the mechanism outlined below (Reaction 4) and the corresponding rate law (Equation 5). The less than first-order dependence on ru-... 

A special situation may arise if reaction products considerably affect the hydrogen solubility, which then varies during the reaction. Such a phenomenon occurs most often in the hydrogenation of the substrate in bulk, without solvent, mainly where the chemical character of the hydrogenation product markedly differs from that of the initial compound [e.g., hydrogenation of nitrobenzene to aniline and water (72)]. In such a case the hydrogen concentration cannot be drawn into the constant, because its varying concentration in the liquid phase is reflected in the form of the kinetic equation. In many such cases the effect of reaction products is also reflected in the kinetic equation. 

Degradation of kinetic equations to zero-order equations with respect to the concentration of the substrate, which is very frequently met in catalytic hydrogenations in the liquid phase, is evidently due to the validity of the reaction... 

A higher form of interpretation of the effect of solvents on the rate of heterogeneously catalyzed reactions was represented by the Langmuir-Hinshelwood kinetics (7), in the form published by Hougen and Watson (2), where the effect of the solvent on the reaction course was characterized by the adsorption term in the kinetic equation. In catalytic hydrogenations in the liquid state kinetic equations of the Hougen-Watson type very frequently degrade to equations of pseudo-zero order with respect to the concentration of the substrate (the catalyst surface is saturated with the substrate), so that such an interpretation is not possible. At the same time, of course, also in these cases the solvent may considerably affect the reaction. As is shown below, this influence is very adequately described by relations of the LFER type. 

The effect of reaction parameters, such as the concentrations of catalyst and olefin and the partial pressures of CO and hydrogen, on the rate of reaction has been studied at 373 K [24], The rate varies linearly with catalyst concentration, olefin concentration, and partial pressure of hydrogen. A typical substrate-inhibited kinetics was observed with the partial pressure of carbon monoxide. Further, a rate equation [Eq. (4)] to predict the observed rate data has been proposed. 

The kinetic expression for observed isotope effects is the ratio of both entire rate equations describing the disappearance of hydrogen and deuterium substrates. The isotopically sensitive step appears in multiple terms and cannot be factored out. In order to achieve factoring and subsequent simplification to useful kinetic equations, it is necessary to examine the Umits of rate equations at low and high substrate concentrations, where enzyme reactions approach first-order and zero-oider kinetics, respectively. To understand this, we must consider how isotope effects in bisubstrate reactions are measured. 

A recent kinetic study (Wender, Greenfield, Metlin, Markby and Orchin, 21) with benzhydrol as substrate indicates that the rate of the hydrogenation to diphenylmethane increases with increasing concentration of dicobalt octacarbonyl catalyst and is first order with respect to the concentration of substrate. The rate also increases slightly with hydrogen pressure and the correct rate equation probably includes the concentration of hydrogen to a fractional exponent (probably %). 

When 2-deutero-3-methylindole was the substrate, the reaction was base catalysed and a small primary hydrogen-deuterium kinetic isotope effect, which varied between 1.70 and 2.86 and increased as the concentration of the / -nitrobenzenediazonium tetrafluoroborate increased, was observed. These results suggest that the reaction occurs via an substitution-rearrangement mechanism (equation 49), where the (p -o-substitution is reversible and the removal of the proton at carbon-2 is at least partially rate-determining. 

The kinetics of the formation of nitration products in reactions of various alkenes with NO + in CCl, CH Cl and hexane were investigated by stopped-flow spectroscopy [62]. The rates of reactions of 2,3-dimethyl-2-butene, cyclohexene, and 1-hexene were measured over a wide range of NO concentrations, from <0.1 mM to 760 mM. For all of these substrates, the order in NO is 2 at high NO and decreases to 1 as the concentration of NO decreases. These data indicate the presence of at least two reaction pathways. One involves the diamagnetic dimer in an addition mechanism (Equation 5.117), and the other involves monomer NO (Equation 5.116). The reaction of the monomer NO proceeds also through abstraction of allylic hydrogen atoms. 

---