Hydrogen-exchange reactions, promotion

The dissociation of water coordinated to exchangeable cations of clays results in Brtfnsted acidity. At low moisture content, the Brrfnsted sites may produce extreme acidities at the clay surface-As a result, acid-catalyzed reactions, such as hydrolysis, addition, elimination, and hydrogen exchange, are promoted. Base-catalyzed reactions are inhibited and neutral reactions are not influenced. Metal oxides and primary minerals can promote the oxidative polymerization of some substituted phenols to humic acid-like products, probably through OH radicals formed from the reaction between dissolved oxygen and Fe + sites in silicates. In general, clay minerals promote many of the reactions that also occur in homogenous acid or oxidant solutions. However, rates and selectivity may be different and difficult to predict under environmental conditions. This problem merits further study. 

Divalent copper, cobalt, nickel, and vanadyl ions promote chemiluminescence from the luminol—hydrogen peroxide reaction, which can be used to determine these metals to concentrations of 1—10 ppb (272,273). The light intensity is generally linear with metal concentration of 10 to 10 M range (272). Manganese(II) can also be determined when an amine is added to increase its reduction potential by stabili2ing Mn (ITT) (272). Since all of these ions are active, ion exchange must be used for deterrnination of a particular metal in mixtures (274). 

The use of acetone cyanohydrin (in an exchange reaction) instead of alcoholic hydrogen cyanide affords even higher yields of 17-cyanohydrins and the former reagent has the added advantage of reacting quantitatively and essentially selectively with the 17-ketone of androst-4-ene-3,17-dione. Sodium hydroxide promotes the exchange reaction in some cases. 

Masjedizadeh and coworkers have recently described similar microwave-promoted hydrogen-deuterium exchange reactions in a series of heterocydes using mixtures of deuterium oxide and deuteriomethanol (Scheme 6.173 b) [328], The rapid exchange method was applied to the deuteration of the anti-tumor antibiotic bleomycin A under catalyst-free conditions [328],... 

Scheme 15.4 Deuterium/hydrogen (D/H) exchange reaction in ionic liquids promoted by lr(0) nanoparticles.

Promotion of Hydrogen Exchange The radicals derived from the coal molecules by simple thermal homolyses and by molecule-induced homolyses initiate exchange reactions between tetralin-d, and the donors in the reaction system—principally diphenyl-methane and the hydroaromatic compounds in the macerals (2. ... 

It has been recognized that usually the isomerization of olefins as well as the exchange reaction with deuterium are depressed in the absence of hydrogen (or deuterium) but greatly promoted in the presence of hydrogen. The extent of olefin isomerization depends primarily on the nature of catalyst metal. However, the impurities or additives as well as the reaction conditions may also become factors affecting the isomerization. 

A side reaction may be metallation of hydrogens in positions ortho or a to the halogen. The fast rates of halogen-Li exchanges, especially in cold electron-donating solvents, enable low T to be used and the extent of the slower metallations to be minimized. The presence of tetramethylethylenediamine (TMED) however, can promote metallations more than metal-hydrogen exchanges . 

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