Hydride decompositions, reversible

As far as hydride decomposition is concerned, the relations are reversed. The larger the metal crystals are the slower their hydride decomposes (62). Moreover some deposits situated on the exit points of dislocations, for example on the surface of a nickel hydride crystal, inhibit hydrogen desorption and result in prolonging the hydride existence in the crystal (87). 

Effluent from the hydrogenation reactor is depressured to about 400 psig. This level of hydrogen is required to prevent the reverse reaction, diethylaluminum hydride decomposition, which results in plating of aluminum on the process equipment. Product diethylaluminum hydride, unreacted aluminum, and solvent are charged to the ethylation reactor. Ethylene is introduced and undergoes a rapid, exothermic reaction to form triethylaluminum. A tubular reactor with high heat transfer capabilities is required to control this reaction (12). 

Alapati S V, Johnson J K and Sholl D S (2007a), Predicting reaction equilibria for destabilised metal hydride decomposition reactions for reversible hydrogen storage, J. Phys. Chem. C, 111, 1584-1591. 

The higjily water-soluble dienophiles 2.4f and2.4g have been synthesised as outlined in Scheme 2.5. Both compounds were prepared from p-(bromomethyl)benzaldehyde (2.8) which was synthesised by reducing p-(bromomethyl)benzonitrile (2.7) with diisobutyl aluminium hydride following a literature procedure2.4f was obtained in two steps by conversion of 2.8 to the corresponding sodium sulfonate (2.9), followed by an aldol reaction with 2-acetylpyridine. In the preparation of 2.4g the sequence of steps had to be reversed Here, the aldol condensation of 2.8 with 2-acetylpyridine was followed by nucleophilic substitution of the bromide of 2.10 by trimethylamine. Attempts to prepare 2.4f from 2.10 by treatment with sodium sulfite failed, due to decomposition of 2.10 under the conditions required for the substitution by sulfite anion. 

Exercise 31-12 7r-Propenyl(ethyl)nickel decomposes at —70° to give propene and ethene. If the ethyl group is labeled with deuterium as —CH2—CD3, the products are C3H5D and CD2=CH2. If it is labeled as —CD2—CH3, the products are C3H6 + CD2=CH2. Are these the products expected of a radical decomposition, or of a reversible hydride-shift followed by decomposition as in the mechanism of Section 31-2B Suppose the hydride-shift step were not reversible, what products would you expect then ... 

Bicyclobutanes are also obtained from the catalytic decomposition of diazo compound 17492 (equation 51). Copper(I) iodide was the catalyst of choice, whereas rhodium(II) acetate did not show any activity in this case. When the related diazo compound 175 was decomposed, the product pattern depended in an unusually selective manner on the catalyst92. Intramolecular cyclopropanation leading to 176 is obviously less favorable than for carbene 172 and must yield to the 1,2-hydride shift not observed with the former carbene. The configuration of the resulting butadiene 177 can be completely reversed by the choice of the catalyst. 

These data have been rationalized by recognizing that acetic acid plays several roles in the catalytic mechanism (Scheme 10) [80]. In the absence of acetic acid, the Pd(0) intermediate, 49, undergoes competitive decomposition and oxygenation. Low concentrations of acetic acid enhance the rate and minimize catalyst decomposition by trapping the reversibly formed per-oxopalladium(II) intermediate, 50. Acetic acid also can stabilize the catalyst by reversible formation of a Pd-hydride species, 48. At high [AcOH], the reaction rate is slowed because acetic acid inhibits formation of the alkoxide intermediate 47. 

The degradation with alkali, following the reaction path outlined earlier (part-structures LVIIb LVIIIa— -b- c) would result in the formation of the aldehyde LXVI. The production of this compound can be explained very convincingly by reverse Mannich decomposition of apopseudoakuammigine to the intermediate zwitter ion LXIII. The alternative conformation of this molecule is one (LXV) in which the C-17 hydrogen is situated in close proximity to the trigonal C-3 hydride transfer, as postulated by Joule and Smith, would then be expected to... 

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