In catalytic hydrogenation

Carboxylic acids are exceedingly difficult to reduce Acetic acid for example is often used as a solvent in catalytic hydrogenations because it is inert under the reaction con ditions A very powerful reducing agent is required to convert a carboxylic acid to a pri mary alcohol Lithium aluminum hydride is that reducing agent... 

M. Ereifelder, in Catalytic Hydrogenation in Organic Synthesis Procedures and Commentay, ]ohrs Wiley Sons, Inc., New York, 1978, Chapts. 5, 7. P. N. Rylander, Inc., Catalytic Hydrogenation in Organic Syntheses, Academic Press, New York, 1979. 

In catalytic hydrogenation, a compound is reduced with molecular hydrogen in the presence of a catalyst. This reaction has found appHcations in many areas of chemistry including the preparation of amines. Nitro, nitroso, hydroxylamino, azoxy, azo, and hydrazo compounds can all be reduced to amines by catalytic hydrogenation under the right conditions. Nitriles, amides, thioamides, and oximes can also be hydrogenated to give amines (1). Some examples of these reactions foUow ... 

The solvent used in catalytic hydrogenation is chosen for its ability to dissolve the alkene and is typically ethanol, hexane, or acetic acid. The metal catalysts are insoluble in these solvents (or, indeed, in any solvent). Two phases, the solution and the metal, aie present, and the reaction takes place at the interface between them. Reactions involving a substance in one phase with a different substance in a second phase aie called heterogeneous reactions. 

Solvents are often used in catalytic hydrogenation (< 7). Solvents may be one of the best means available for markedly altering the selectivity, a fact not sufficiently appreciated. Solvents also help to moderate the heat of hydroge nation, to aid in catalyst handling and recovery, and to permit the use of solid substrates. A convenient solvent may be the product itself or the solvent used in a prior or subsequent step. 

There are several sources of potential danger in catalytic hydrogenations these are failure of equipment because of excessive pressures, solvent fires, explosions and fires from mixtures of hydrogen in air, and, with finely divided carbon supports, dust explosions. None of these should cause concern, for all may be avoided easily. 

Steric influences are important in some cases. In catalytic hydrogenation, where the substrate must be adsorbed onto the catalyst surface, the reaction becomes more difficult with increasing substitution. The hydrocarbon 21, in which the double bond is entombed between the benzene rings, does not react with Br2, H2SO4, O3, BH3, CBr2, or other reagents that react with most double bonds. A similarly inactive... 

Figure 10 Real-time plot of reactor component concentrations in catalytic hydrogenation step illustrating early detection of baseline upset by on-line micro-HPLC.

Rylander in Catalytic Hydrogenation Over Platinum Metals (p. 39, Academic Press, New York, 1967). Nitrobenzene in ethanol was hydrogenated at room temperature and 1 atm over various amounts of 5% Pd on carbon. Four loading levels of catalyst were used. At each level, the reduction was carried out in two different types of batch reactor. 

In a somewhat different approach, supported-aqueous-phase-catalysts (SAPC, see Chapter 5, Section 5.2.5 of this book) have been combined with supercritical CO2 in catalytic hydrogenation [55], Ruthenium was supported on silica and combined with the ligand TPPTS in water, after which a scC02/H2 phase was applied together with the substrate. Better levels of conversion were obtained using scC02 than the equivalent system with toluene for the hydrogenation of cinnamaldehyde. 

The metal may be reduced to its zerovalent state and form small metallic particles on the support. These metal particles may take part in the reaction, especially in catalytic hydrogenation. 

Sokol skii, D. V., B. Y. Nogcrbekov and N. N. Gudeleva. 1986. Investigation of the activity of a palladium/glass membrane in catalytic hydrogenation reactions. Sov. Electrochem. 22(9) 1227-1229. 

Side products of reductive C-C bond formation have been observed in catalytic hydrogenation on rare occasions, see [20, 21]. 

The black solution containing IL and lr(0) nanoparticles (-2.0nm in diameter, as determined by TEM) can also be recycled in catalytic hydrogenation reactions. The catalytic activity of these soluble iridium nanoparticles was also compared to that of the Crabtree catalyst ([lr(COD)(PCy3)py]PF( ) in BMI-PFg. These nanoparticles maintain an efficient activity for at least seven recycles, whereas the Crabtree catalyst suffers a significant reduction in activity during recycling reactions. 

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