Hydrogen donation

CgH COO from BPO. The first type involves direct radical displacement on the oxygen—oxygen bond and is the preferred mode for nucleophilic radicals, eg, -CH(R)OR7 The second type involves radical addition to, or abstraction from, the hydrocarbyl group adjacent to the peroxide this is the preferred mode for electrophilic radicals, eg, Cl C (eq. 32). In the last type (eq. 33), there is hydrogen donation from certain hydrogen-donating radicals, eg, ketyls (52,187,188,199). 

Direct reaction of oxygen with most organic materials to produce radicals (eq. 13) is very slow at moderate temperatures. Hydrogen-donating antioxidants (AH), particularly those with low oxidation—reduction potentials, can react with oxygen (eq. 14), especially at elevated temperatures (6). 

Radical Scavengers Hydrogen-donating antioxidants (AH), such as hindered phenols and secondary aromatic amines, inhibit oxidation by competing with the organic substrate (RH) for peroxy radicals. This shortens the kinetic chain length of the propagation reactions. 

For the acetoxy radical, the for decarboxylation is about 6.5 kcal/mol and the rate is about 10 s at 60°C and 10 s at —80°C. Thus, only very rapid reactions can compete with decarboxylation. As would be expected because of the lower stability of aryl radicals, the rates of decarboxylation of aroyloxy radicals are slower. The rate for p-methoxybenzoyloxy radical has been determined to be 3 x 10 s near room temperature. Hydrogen donation by very reactive hydrogen-atom donors such as triethylsilane can compete with decarboxylation at moderate temperatures. 

In order to avoid competitive bimolecular photoreactions such as ketone reduction by hydrogen abstraction, poor hydrogen donating solvents are recommended (acetonitrile, acetic acid, tertiary alcohols). In those cases where ketene trapping is desired, solvents must also be miscible with water or other protic nucleophiles. 

In benzonitrile both types of processes occur in parallel with similar efficiencies. In hydrogen-donating solvents (toluene, ether, dioxane, ethanol), hydrogen abstraction processes prevail [toluene (114) -> (123)]. 

In some cases hydrogen donating functions can be protected hydroxyethyl methacrylate and glyceryl methacrylate 16,17) can undergo anionic polymerization as silyl-ether or-acetal, respectively. After polymerization the OH functions are recovered by mild acid hydrolysis. Similarly, p-hydroxyethylstyrene can be polymerized an-... 

Phenyl or mixed alkyl/phenyl substituted silicon hydrides show similar reactivities to trialkylsilanes. Indeed, by replacing one alkyl by a phenyl group the effect on the hydrogen donating ability of SiH moiety increases only slightly. ... 

Transfer hydrogenolysis of benzyl acetate was studied on Pd/C at room temperature using different formate salts.244 Hydrogen-donating abilities were found to depend on the counterion K+ > NH4 + > Na+ > Li+ > H+. Formate ion is the active species in this reaction. Adsorption of the formate ion on the Pd metal surface leads to dissociative chemisorption resulting in the formation of PdH- and C02. The kinetic isotope effect proves that the dissociative chemisorption of formate is the rate-limiting step. The adsorption and the surface reaction of benzyl acetate occurs very rapidly. 

Conversion of coal to benzene or hexane soluble form has been shown to consist of a series of very fast reactions followed by slower reactions (2 3). The fast initial reactions have been proposed to involve only the thermal disruption of the coal structure to produce free radical fragments. Solvents which are present interact with these fragments to stabilize them through hydrogen donation. In fact, Wiser showed that there exists a strong similarity between coal pyrolysis and liquefaction (5). Recent studies by Petrakis have shown that suspensions of coals in various solvents when heated to 450°C produce large quantities of free radicals (. 1 molar solutions ) even when subsequently measured at room temperature. The radical concentration was significantly lower in H-donor solvents (Tetralin) then in non-donor solvents (naphthalene) (6). 

The microautoclave solvent activity tests measure coal conversion in a small batch reactor under carefully controlled conditions. The tests are described as Kinetic, Equilibrium and SRT. The Kinetic and Equilibrium Tests measure coal conversion to tetrahydrofuran solubles at conditions where conversion should be monotonically related to hydrogen transfer. The Kinetic Test is performed at 399°C for 10 minutes at an 8 to 1 solvent to coal ratio. The combination of high solvent ratio and low time provide a measure of performance at essentially constant solvent composition. The measured conversion is thus related to the rate of hydrogen donation from solvent of roughly a single composition. In contrast, the Equilibrium Test is performed at 399°C for 30 minutes at a 2 to 1 solvent to coal ratio. At these conditions, hydrogen donors can be substantially depleted. Thus performance is related to hydrogen donor... 

These reactions proceed smoothly in the presence of hydrogen donating aromatic solvent (1-4) at temperatures from 400°C to 450°C, resulting in the formation of so called solvent refined coal with carbon content of 86-88% on maf basis independent of coalification grade of feed coal. 

Phenolic compounds may enhance the rate of decomposition of aromatic ether, because the phenoxy radical may be stabilized by solvation (18) or hydrogen bonding (19) with phenolic compounds and may result in the subsequent hydrogen transfer reaction from hydrogen donating solvent or phenols (20). 

Wang, Z.-X., Zhang, H.-Y. Guo, A.-j., et al., Study on hydrogen donation ability of residue hydrocracking system catalyzed by oil-soluble and water-soluble catalysts. Preprints of Symposia - American Chemical Society, Division of Fuel Chemistry, 1998. 43(3) pp. 530-533. 

Once again, use of these donors as solvent may shift the reaction equilibrium towards the desired product. Since the reactivity of olefins is lower than that of carbonyl compounds, higher reaction temperatures are usually required to achieve acceptable TOFs, and then the relatively higher boiling hydrogen donating solvents mentioned above may be the best choice. 

For molecules in which intramolecular hydrogen-abstraction (Type II reaction) is structurally prevented, intermolecular abstraction from a hydrogen-donating solvent may take place (see Scheme 6). This type of reaction usually results in reduction of the carbonyl group (see Table VII). 

Ort/io-substituted nitroarenes can be distinguished from meta- or para-substituted isomers due to a characteristic change in their mass spectra. In precence of a hydrogen-donating ortho subtituent, [M-OH] replaces the [M-O] the fragment ion, e.g., in case of the above nitrophenols, the ion at m/z 123 is replaced by one at m/z 122 (Fig. 6.49c). The mechanism of this OH loss is not quite clear, [209] because different proposals [210-212] are in good accordance with the experimental data. Here, the mechanism which resembles the previously discussed ortho eliminations is shown [212]... 

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