Hydrogen concentration, effect

Berger, A. et al., Ionic liquid-phase asymmetric catalytic hydrogenation Hydrogen concentration effects on enantioselectivity. Tetrahedron Asymmetry, 12, 1825,2001. 

Figure 4 Hydrogen concentration effect on degree of reduction...

Concentration Effects. The reactivity of ethyl alcohol—water mixtures has been correlated with three distinct alcohol concentration ranges (35,36). For example, the chromium trioxide oxidation of ethyl alcohol (37), the catalytic decomposition of hydrogen peroxide (38), and the sensitivities of coUoidal particles to coagulation (39) are characteristic for ethyl alcohol concentrations of 25—30%, 40—60%, and above 60% alcohol, respectively. The effect of various catalysts also differs for different alcohol concentrations (35). 

Both reactions were carried out under two-phase conditions with the help of an additional organic solvent (such as iPrOH). The catalyst could be reused with the same activity and enantioselectivity after decantation of the hydrogenation products. A more recent example, again by de Souza and Dupont, has been reported. They made a detailed study of the asymmetric hydrogenation of a-acetamidocin-namic acid and the kinetic resolution of methyl ( )-3-hydroxy-2-methylenebu-tanoate with chiral Rh(I) and Ru(II) complexes in [BMIM][BF4] and [BMIM][PFg] [55]. The authors described the remarkable effects of the molecular hydrogen concentration in the ionic catalyst layer on the conversion and enantioselectivity of these reactions. The solubility of hydrogen in [BMIM][BF4] was found to be almost four times higher than in [BMIM][PFg]. 

In addition to the surface physics and chemistry phenomena involved, a further effect may follow the interaction at the hydrogen-metal surface, that is the absorption of hydrogen by the bulk phase of the metal. This absorption leads to the formation of a solid solution within a certain, usually low, range of hydrogen concentrations. However, with several transition metals, exceeding a certain limit of hydrogen concentration results in the formation of a specific crystallographically distinct phase of the... 

NOTE With these types of programs, free sodium hydroxide is not formed, even in the event of complete evaporation thus the damage caused by localized concentration effects (dissolution of iron forming the soluble, nonprotective, sodium ferroate [sodium hypoferrite] salt, together with hydrogen liberation, tube wastage, and ultimate failure) does not occur. 

The variation of enantioselectivities with temperature and pressure was investigated. The effects of these two factors are very substrate dependent and difficult to generalize even in a single substrate serie. However, it seems that enantioselectivities are shghly better at 25-40 °C than at lower temperatures (0 °C or less). The stereoselectivity can be inverted for specific alkenes (formation of the S or R enantiomer preferentially). For several substrates, the reactions tend to proceed to completion with optimal ee s when performed at lower hydrogen pressure (2 bar) instead of 50 bar (Fig. 13). Pronoimced variation of enantioselectivities with hydrogen concentration in solution may indicate the presence of two (or even more) different mechanisms which happen to give opposite enantiomers for some substrates. 

There are three major classes of palladium-based hydrogen sensors [4], The most popular class of palladium-based sensors is based on palladium resistors. A thin film of palladium deposited between two metal contacts shows a change in conductivity on exposure to hydrogen due to the phase transition in palladium. The palladium field-effect transistors (FETs) or capacitors constitute the second class, wherein the sensor architecture is in a transistor mode or capacitor configuration. The third class of palladium sensors includes optical sensors consisting of a layer of palladium coated on an optically active material that transforms the hydrogen concentration to an optical signal. 

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