Hydrolysis of metal hydrides

Hydrogen can be prepared by the reaction of water or dilute acids on electropositive metals such as the alkali metals, alkaline earth metals, the metals of Groups 3, 4 and the lanthanoids. The reaction can be explosively violent. Convenient laboratory methods employ sodium amalgam or calcium with water, or zinc with hydrochloric acid. The reaction of aluminium or ferrosilicon with aqueous sodium hydroxide has also been used. For small-scale preparations the hydrolysis of metal hydrides is convenient, and this generates twice the amount of hydrogen as contained in the hydride, e.g. ... 

Reaction of Water with Hydrides of Metals. For small-scale preparations, the hydrolysis of metal hydrides is convenient and generates twice the amount of hydrogen as contained in the hydride, for example. 

Hydrogen is a colorless, odorless, and tasteless gas, lighter than air. It liquehes at —252.7°C (—422.86°F). It occurs in the earth s atmosphere in only trace amounts ( 0.00005%). It is produced by reachons of metals with acids reactions of alkali metals with water hydrolysis of metal hydrides electrolysis of water or from coke and steam. It is also produced from methane. 

Asymmetric hydrometallation of ketones and imines with H-M (M = Si, B, Al) catalyzed by chiral transition-metal complexes followed by hydrolysis provides an effective route to optically active alcohols and amines, respectively. Asymmetric addition of metal hydrides to olefins provides an alternative and attractive route to optically active alcohols or halides via subsequent oxidation of the resulting metal-carbon bonds (Scheme 2.1). 

The hydrides phosphine PH3, arsine ASH3 and stibine SbH3 can be prepared by hydrolysis of metal phosphates, or by reduction of molecular compounds like PC13. They are very toxic gases, with decreasing thermal stability P > As > Sb. Unlike ammonia they are not basic in water. The hydrazine... 

We have analyzed, both theoretically and experimentally, the reaction chemistry of a variety of metal hydrides and water, and the chemical stability of the organic carriers in contact with metal hydrides and spent hydrides. Since detailed hydrolysis reaction kinetics of the metal hydride/organic carrier slurry is not known, we conducted experiments using a high pressure (13.790 MPa or 2000 psi) and high temperature (232°C) vessel with temperature, pressure, and magnetic stirrer control capabilities (500 cm3 internal volume). Some of the selection criteria for the hydride follow. 

Table 1. Formation of Hydrides by Base Hydrolysis of Metal-Carbonyl Complexes...

Fluorination is effective not only for protecting surfaces from impurities but also for giving catalytic function and improving both the chemical and electrochemical characteristics of metal hydrides. For example, the surface of Mg2Ni created by a fluorination treatment was found to be effective as a catalyst for the catalytic generation of hydrogen from an aqueous alkaline solution of NaBH4 by hydrolysis (see Chapter 6.8). 

In general, the sensitivity towards hydrolysis of metal phosphides depends on the metal content. Metal-rich compounds undergo relatively straightforward hydrolysis while the behavior of those with higher phosphorus content resembles more that of the elementary phosphorus, specially with respect to their inertness toward water. As observed in Table 4.16, many polynuclear metal phosphide undergo hydrolysis giving rise to corresponding phosphor hydrides or phosphanes. 

Another aspect of the hydrolysis of hydrides is the alkalinity that results, especially from alkaU metal and alkaline-earth hydrides. This alkalinity can cause chemical bums in skin and other tissues. Affected skin areas should be flooded with copious amounts of water. 

Hydrolysis of primary amides cataly2ed by acids or bases is very slow. Even more difficult is the hydrolysis of substituted amides. The dehydration of amides which produces nitriles is of great commercial value (8). Amides can also be reduced to primary and secondary amines using copper chromite catalyst (9) or metallic hydrides (10). The generally unreactive nature of amides makes them attractive for many appHcations where harsh conditions exist, such as high temperature, pressure, and physical shear. 

Hardness on the Mohs scale is often above 8 and sometimes approaches 10 (diamond). These properties commend nitrides for use as crucibles, high-temperature reaction vessels, thermocouple sheaths and related applications. Several metal nitrides are also used as heterogeneous catalysts, notably the iron nitrides in the Fischer-Tropsch hydriding of carbonyls. Few chemical reactions of metal nitrides have been studied the most characteristic (often extremely slow but occasionally rapid) is hydrolysis to give ammonia or nitrogen ... 

Such reactions are discussed at appropriate points throughout the book as each individual compound is being considered. A particularly important set of reactions in this category is the synthesis of element hydrides by hydrolysis of certain sulfides (to give H2S), nitrides (to give NH3), phosphides (PH3), carbides (C Hm), borides (B Hm), etc. Useful reviews are available on hydrometallurgy (the recovery of metals by use of aqueous solutions at relatively low temperatures), hydrothermal syntheses and the use of supercritical water as a reaction medium for chemistry. 

The other way of reducing nitriles to aldehydes involves using a metal hydride reducing agent to add 1 mol of hydrogen and hydrolysis, in situ, of the resulting imine (which is undoubtedly coordinated to the metal). This has been carried out with LiAlH4, LiAlH(OEt)3, LiAlH(NR2)3, and DIBAL-H. The metal hydride method is useful for aliphatic and aromatic nitriles. 

Employing ketones or aldehydes as starting materials, the corresponding silylethers are obtained. Thereafter, the oxidation or hydrolysis of the obtained silylethers gives the corresponding alcohols (Scheme 17). In most cases, a hydride (silyl) metal complex H-M-Si (M = transition-metal), which is generated by an oxidative addition of H-Si bond to the low-valent metal center, is a key intermediate in the hydrosilylation reaction. 

A 20 g sample, prepared and stored in a dry box for several months, developed a thin crust of oxidation/hydrolysis products. When the crust was disturbed, a violent explosion occurred, later estimated as equivalent to 230 g TNT. A weaker explosion was observed with potassium tetrahydroaluminate. The effect was attributed to superoxidation of traces of metallic potassium, and subsequent interaction of the hexahydroaluminate and superoxide after frictional initiation. Precautions advised include use of freshly prepared material, minimal storage in a dry diluent under an inert atmosphere and destruction of solid residues. Potassium hydrides and caesium hexahydroaluminate may behave similarly, as caesium also superoxidises in air. 

The starting material is always the chalcogenol and, consequently, is more used for thiols than selenols and tellurols. There are several types of reactions depending if the starting materials are metal hydrides (hydrogen elimination), complexes with M C (alkane elimination), M-N (transamination), or M-O (hydrolysis) bonds. 

Another synthetic route which gives a good yield of the labelled tin hydride involves the hydrolysis of an organometallic intermediate such as a trialkylstannyllithium46 with deuterated or tritiated water. The trialkylstannyllithium can be prepared by treating the trialkyltin chloride with lithium metal in THF46. This process is shown in equation 42. 

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