Transfer of hydrogen

Coemymes effecting transfer of hydrogen. These include the pyridine nucleotides, nicotinamide-adenine dinucleolide and nicotinamide-adenine dinucleolide phosphate the flavin nucleotides such as flavin-adenine dinucleotide and lipoic acid. 

Antioxidants markedly retard the rate of autoxidation throughout the useful life of the polymer. Chain-terminating antioxidants have a reactive —NH or —OH functional group and include compounds such as secondary aryl amines or hindered phenols. They function by transfer of hydrogen to free radicals, principally to peroxy radicals. Butylated hydroxytoluene is a widely used example. 

Another method to hydrogenate butadiene occurs during an oxidation—reduction reaction in which an alcohol is oxidi2ed and butadiene is reduced. Thus copper—chromia or copper—2inc oxide cataly2es the transfer of hydrogen from 2-butanol or 2-propanol to butadiene at 90—130°C (87,88). 

Electrophile Addition. The addition of electrophilic (acidic) reagents HZ involves two steps the slow transfer of hydrogen ion from Z to the butylene to form a carbocation and, a rapid combination of the carbocation with the base Z. 

Direct-Liquefaction Kinetics All direct-liquefac tion processes consist of three basic steps (1) coal slurrying in a vehicle solvent, (2) coal dissolution under high pressure and temperature, and (3) transfer of hydrogen to the dissolved coal. However, the specific reac tion pathways and associated kinetics are not known in detail. Overall reaction schemes and semiempirical relationships have been generated by the individual process developers, but apphcations are process specific and limited to the range of the specific data bases. More extensive research into liquefaction kinetics has been conducted on the laboratory scale, and these results are discussed below. 

Let us now turn to the influence of vibrations on exchange chemical reactions, like transfer of hydrogen between two O atoms in fig. 2. The potential is symmetric and, depending on the coupling symmetry, there are two possible types of contour plot, schematically drawn in fig. 17a, b. The O atoms participate in different intra- and intermolecular vibrations. Those normal skeleton... 

Jaquet and Miller [1985] have studied the transfer of hydrogen atom between neighbouring equilibrium positions on the (100) face of W by using a model two-dimensional chemosorption PES [McGreery and Wolken 1975]. In that calculation, performed for fairly high temperatures (T> rj the flux-flux formalism along with the vibrationally adiabatic approximation (section 3.6) were used. It has been noted that the increase of the coupling to the lattice vibrations and decrease of the frequency of the latter increase the transition probability. 

This particular polymer is a fibre-forming material (Perlon U). Although in many respects this reaction resembles the formation of polyesters and polyamides it is not a condensation reaction but involves a transfer of hydrogen atoms and thus may be considered as an example of rearrangement polymerisation. 

Fluoroimines are reduced to ammes in good yield by lithium aluminum hydride [69, 70] (equations 57 and 58) Transfer of hydrogen from 2-propanol to fluoroal-kylated benzaldimines is efficiently catalyzed by rhodium [71] (equation 59), A... 

The concept of group transfer potential is not particularly novel. Other kinds of transfer (of hydrogen ions and electrons, for example) are commonly... 

J.E. thanks the European Union (Project Localization and Transfer of Hydrogen, No. CHRX CT 940582) and the Spanish Ministry of Education (Project Crystal Engineering, No. PB96-0001-C03-03) for financial support. We thank Dr. Scott Henderson for help with the preparation of the manuscript. 

The reaction between the PMMA and PS model radicals (4 and 5, generated from the unsymmetrical azo-compound 3) has been studied as a model for crosstermination in MMA-S copolymerization (Scheme 7.13).178,179 The value for tcross reaction was 0.56. In disproportionation, transfer of hydrogen from the PS model 5 to the PMMA radical 4 was ca 5.1 times more prevalent than transfer in the reverse direction (from 4 to 5). The value of kJklc(90°C) is between those of Atd/ tc(90oC) for the self-reaction of these radicals... 

The value of ,<)// ,- 80°C) in the cross-reaction between radicals 4 and 8 has been examined.175 This system is a model for cross-termination in MMA-BMA eopolymeri/alion. The value of kjkw (1.22) is similar to that found for the self-reaction of 8(1.17) and much larger than that for the self-reaction of 4 (0.78). There is a small preference (m 1.4 fold) for the transfer of hydrogen from the butyl ester (8) to the methyl ester (4). 

Calderbank et al. (C6) studied the Fischer-Tropsch reaction in slurry reactors of 2- and 10-in. diameters, at pressures of 11 and 22 atm, and at a temperature of 265°C. It was assumed that the liquid-film diffusion of hydrogen from the gas-liquid interface is a rate-determining step, whereas the mass transfer of hydrogen from the bulk liquid to the catalyst was believed to be rapid because of the high ratio between catalyst exterior surface area and bubble surface area. The experimental data were not in complete agreement with a theoretical model based on these assumptions. 

FIGURE 10.4 In this molecular portrayal of the structure of a solution of ammonia in water at equilibrium, we see that NH, molecules are still present because only a small percentage of them have been protonated by transfer of hydrogen ions from water. In a typical solution, only about 1 in 100 NH, molecules is protonated. The overlay shows only the solute species. 

The racemization mechanism of sec-alcohols has been widely studied [16,17]. Metal complexes of the main groups of the periodic table react through a direct transfer of hydrogen (concerted process), such as aluminum complexes in Meerwein-Ponn-dorf-Verley-Oppenauer reaction. However, racemization catalyzed by transition metal complexes occurs via hydrogen transfer processes through metal hydrides or metal dihydrides intermediates (Figure 4.5) [18]. 

Simandi and Nagy studied the kinetics of the catalyzed hydrogenation of cinnamic acid (S) to dihydrocinnamic acid (SHj) under steady-state conditions 166). They concluded that the kinetically important reactions were the two successive transfers of hydrogen atoms, viz.. 

Transfer of hydrogen from one substrate to another in a coupled oxidation-reduction reaction (Figure 11-3). These dehydrogenases are specific for their substrates but often utilize common coenzymes or hydrogen carriers, eg, NAD". Since the reactions are re-... 

Catalytic hydrogenation transfers the elements of molecular hydrogen through a series of complexes and intermediates. Diimide, HN=NH, an unstable hydrogen donor that can be generated in situ, finds specialized application in the reduction of carbon-carbon double bonds. Simple alkenes are reduced efficiently by diimide, but other easily reduced functional groups, such as nitro and cyano are unaffected. The mechanism of the reaction is pictured as a concerted transfer of hydrogen via a nonpolar cyclic TS. 

Ethanolamine ammonia lyase, L-/ -lysine mutase, D-a-lysine mutase and ornithine mutase are representative of cobamide enzymes in which transfer of hydrogen occurs with cleavage of the C—N bond (Fig. 16). 

Next, a consideration of the kinetics for Step 2 raises some questions. The transfer of hydrogen in similar reactions has been well studied, and Table III presents data for the relative rates of transfer of hydrogen from a number of hydrocarbons to the free radical C130 at 350°C. The donor hydrocarbons are listed in order of increasing ease of H-transfer to the free radical. Tetralin is near the middle of the list. The most reactive donor in the table, 1,4-dihydronaphthalene, is about four times as active as Tetralin. 

The table also shows the results of experiments with the donors and coal in phenanthrene as solvent. Consistent with the transfer of hydrogen in a radical process, those donors less reactive toward C130 than Tetralin are also less effective than Tetralin in conversion of coal to a phenanthrene-soluble product. However, in contrast to the chemistry of Step 2 we see that those donors that are more reactive toward C130 than Tetralin are also less effective in their action with coal. Thus this simple conversion scheme is suspect. 

The transfer of hydrogen from donors to the coal substrate during liquefaction occurs by concerted pericyclic reactions of the type termed group transfers by Woodward and Hoffman (1 ). ... 

The preceding experiments offer preliminary support to our notion that pericyclic pathways might be intimately involved in the mechanism of coal liquefaction. More specifically, the results indicate that pericyclic group transfer reactions constitute a plausible pathway for the transfer of hydrogen from donor solvents to coal during liquefaction. 

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