Catalytic hydrogenation heterogeneous

The choice of catalyst applied in the hydrogenation of a certain unsaturated hydrocarbon depends on several factors, such as the reactivity of the substrate and the experimental conditions (pressure, temperature, solvent, liquid- or gas-phase reaction). Multiply unsaturated compounds may require the use of a selective catalyst attaining the reduction of only one multiple bond. The use of suitable selective catalysts and reaction conditions is also necessary to achieve stereospecific hydrogenations. 

In the one step addition of two molecules to each other, it is reasonable to suggest that such an addition occurs via a cyclic transition state, with the new bonds being formed simultaneously with the old bonds being broken. It would also be reasonable to suggest that both parts of the molecule that is adding to the unsaturated molecule approach the double bond from the same side. This would be especially true for the addition of small molecules. This is called sy -addition. 

The commonest example is the catalytic hydrogenation of an alkene. Write down a balanced equation for the hydrogenation of ethene. 

The reaction is commonly catalysed heterogeneously by metals such as nickel, platinum or palladium. The alkene and the hydrogen are adsorbed onto the surface of the metal, and this results in the weakening of the bonds so that the species react with each other more easily. Some of the hydrogen will also be absorbed into the metal lattice, but this is of little use in effecting the reduction of the alkene. 

Draw a mechanism that illustrates the movement of the electrons. 

The syn nature of this addition may be more fully appreciated when a cycloalkene such as 1,2-dimethylcyclopentene is hydrogenated. Suggest what 

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Table 3 Heterogeneous Catalytic Hydrogenation of Nitrile Rubber...

It can be summarized from the available data in Table 3 that supported palladium catalysts selectively hydrogenated carbon-carbon double bonds in the presence of the nitrile group in NBR. However, there is no detailed fundamental study on heterogeneous catalytic hydrogenation of nitrile rubber in the literature that can provide an insight into the reaction. The available information is limited since most of the literature is patented. 

The occurrence of hydrogen exchange and double-bond migration in heterogeneous catalytic hydrogenation means that the hydrogenation does not necessarily... 

The frara-effect in co-ordination chemistry is well known however, what has not been examined in any detail is the effect of trons-molecules in heterogeneous catalytic hydrogenation. In this paper we will show that trans molecules hydrogenate more slowly than other isomers and can poison reactions of species that would be expected to be more strongly bound. However, if a c/s/frans-mixture is used this strong adsorption can be disrapted. 

Sabatier reports the first heterogeneous catalytic hydrogenation of an organic molecule [14-17]... 

S. Nishimura, Handbook of Heterogeneous Catalytic Hydrogenation for Organic Synthesis, Wiley, New York, 2001, pg. 589-590. 

The type of catalyst strongly influences the enantioselectivity of heterogeneous catalytic hydrogenations (1). In the enantioselective saturation of the C=C bond of isophorone over (-)-dihydroapovincaminic acid ethyl ester ((-)-DHVIN) modified Pd catalysts (scheme 1) the optical purity strongly depended on the type and properties of the support used (2, 3, 4). 

SOME BASIC ASSUMPTIONS ABOUT HETEROGENEOUS CATALYTIC HYDROGENATION... 

Harada, K. Asymmetric heterogeneous catalytic hydrogenation. In Morrison, J.D. (Ed.), Asymmetric Synthesis. Academic Press, Inc., Orlando, FL, 1985,... 

Fig. 42.1 Mass transfer resistances for hydrogen in heterogeneous catalytic hydrogenation.

Only a few reactions have been studied with N-allenylsulfenimides. Despite the presence of a sulfur function in allene 281, van Vranken and co-workers carried out its heterogeneous catalytic hydrogenation (Eq. 8.39) [148], The reaction occurs only at the terminal C=C bond, providing the Z-configuration enamide 296 in 70% yield. 

In heterogeneous catalytic hydrogenations suprafacial (as) addition of hydrogen would be expected, as the transfer of hydrogen atoms from the catalyst surface to the reactant is usually assumed. However, in some Pt catalyzed reactions antarafacial (trans) addition of hydrogen is also observed. The ratio of diastereomeric products formed is determined by the chemisorption equilibrium of the surface intermediates and by the relative rates of hydrogen entrance to the different unsaturated carbon sites. Both effects are influenced by steric factors. 

Product in both the reactions is CH3CH2CH3. Both reactions are catalytic hydrogenations (addition of H2). (i) is heterogeneous catalytic hydrogenation and (ii) is homogeneous catalytic hydrogenation. 

The lack of the use of catalyst library design tools in the field of heterogeneous catalytic hydrogenation inspired us to describe our approach used in this area. In this presentation we shall depict our complex approach to design, optimize and mapping catalyst libraries. 

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