Hydride stability considerations

Stability Considerations of AB5 Hydrides in Chemical Heat Pump Applications with Reference to the New LaNi5 xAlx Ternary System... 

There are many compounds in existence which have a considerable positive enthalpy of formation. They are not made by direct union of the constituent elements in their standard states, but by some process in which the necessary energy is provided indirectly. Many known covalent hydrides (Chapter 5) are made by indirect methods (for example from other hydrides) or by supplying energy (in the form of heat or an electric discharge) to the direct reaction to dissociate the hydrogen molecules and also possibly vaporise the other element. Other known endothermic compounds include nitrogen oxide and ethyne (acetylene) all these compounds have considerable kinetic stability. 

Contrary to the normal Michael reaction of the chloro ester 1-Me with (di-phenylmethylene)amine (DPMA-H) in methanol (Scheme 29), reaction of DMPA-H with 1-Me in methylene chloride or tetrachloromethane containing sodium hydride proceeded much more slowly and gave the isoquinoline 126 in 26 and 39% yield, respectively (Scheme 42) [60 a]. The structure of 126 was ascertained by X-ray crystallography [60a]. Taking into consideration the stability of the Michael adducts 94 in refluxing CCI4, the reaction mechanism with intermediacy of a formal [4-1-2] cycloadduct 125 which may be formed in either a concerted or, more probably, a two-step domino Michael reaction, has been postulated [60a]. 

This reaction may account in part for the oligomers obtained in the polymerization of pro-pene, 1-butene, and other 1-alkenes where the propagation reaction is not highly favorable (due to the low stability of the propagating carbocation). Unreactive 1-alkenes and 2-alkenes have been used to control polymer molecular weight in cationic polymerization of reactive monomers, presumably by hydride transfer to the unreactive monomer. The importance of hydride ion transfer from monomer is not established for the more reactive monomers. For example, hydride transfer by monomer is less likely a mode of chain termination compared to proton transfer to monomer for isobutylene polymerization since the tertiary carbocation formed by proton transfer is more stable than the allyl carbocation formed by hydride transfer. Similar considerations apply to the polymerizations of other reactive monomers. Hydride transfer is not a possibility for those monomers without easily transferable hydrogens, such as A-vinylcarbazole, styrene, vinyl ethers, and coumarone. 

The activation energy of this process is only 26 keal., which implies considerable stability for CuH relative to Cu and a hydrogen atom. Thus, it seems that the same species may form in other systems that can transfer hydride ions, and there is indeed evidence in related systems for the formation of analogous hydrido complexes by such hydride transfer—e.g.,... 

Considerations of enthalpy, as illustrated by the rule of reversed stability, and of configurational entropy provided insight into the factors governing the stabilities of the AB5 hydrides. Theoretical understanding to predict dissociation pressures should be developed on heat pump application, for example. Although... 

Primary alcohols possess a considerably less congested environment than secondary ones. Therefore, it may seem contradictory that a certain oxidant could be able to perform the selective oxidation of secondary alcohols. On the other hand, the oxidation potential of aldehydes is generally higher than the one of ketones (see page 257). This means that thermodynamics usually favor the oxidation of secondary alcohols over primary ones and mild oxidants have a tendency to react quicker with secondary alcohols. Other factors that promote the selective oxidation of secondary alcohols include the intermediacy of alkyl hypohalides, which are less stable when derived from secondary alcohols, and the operation of a mechanism involving a hydride transfer, leaving a carbocation located at the a position of an alcohol that possesses a higher stability in secondary alcohols. 

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