Cation participation

Further detailed studies in this area are obviously needed to resolve the chemistry involved. Such pyrolysis studies supplemented by hydrogenation experiments with acid-form and salt-form brown coals offer promise of resolving the precise role of pyrolysis in the hydrogenation of these coals and of how the ash-forming cations participate in the hydrogenation reactions. 

In a mixture of liquid ammonia with alcohol, ketoenols and pinacols are still formed along with secondary alcohols. Process selectivity was enhanced on the basis of mechanistic studies (Rautenstrauch et al. 1981). The initial stages of the reaction include the formation of ketone anion-radicals and their dimerization with a metal cation participation. This dimerization results in pina-col formation as shown in Scheme 7.6. 

Radical-cations generated in this way are characterised by their uv spectra (Table 6.2) and their esr spectra. The oxygen lone-pairs in meftoxybenzene radical-cations participate in delocalization of the positive charge so that, on the esr time-scale, phenomena due to hindered rotation about the benzene-oxygen bond appear. The two ortAo-hydrogens in anisole radical-cation have different esr coupling constants. 1,4-Dimethoxybenzene radical-cation is found as a mixture of cis- and fra j-isomers due to this hindered rotation [22],... 

The exact mechanism of lattice oxygen incorporation and second hydrogen abstraction, and the precise sequence of elementary events is still a subject of speculation. Several authors assume that two distinct active sites are involved in the acrolein formation. The first, presumably a cation, participates in the formation of the initial allyl complex, while the second, which may contain a different cation and reactive oxygen anions, is the place where further hydrogen abstraction and oxygen incorporation take place. 

Studies have also been carried out which are more specifically aimed at charge transfer on an atomic scale and deal with the atomic situation within the lattice. This is especially so in the case of binary oxides. Many authors assume that, in these systems, both types of cation participate in electron transfer. The reactivity of the binary oxides is then explained by the hypothesis that the cation on the active site obtains an electron supply from the second type. 

All the reactions of cyclopropenylium metal compounds occur without cleavage of the three-membered ring. Moreover, in all the reactions, these cations participate as o-nucle-... 

Base-catalysed nitrosation of acetone by t-butyl nitrite to give l-hydroxyimino-2-oxopropane [H3C-C(=0)-CH=N0H] has been studied by ab initio methods.89 Using a sodium enolate route, the cation participates to give Z-isomer, whereas a naked enolate calculation results in the -product being favoured. 

Initiation according to reaction (55) is generally accepted, although other mechanisms have also been proposed. Khaitin and Volf [209] consider anion attack on the amide carbon. Further additions would occur with cation participation (Ac+ is the cationic and the anionic part of initiator AcB). 

In all solution environments the bare metal ions are in continuous search of a partner. All metal cations in water are hydrated that is, they form aquo complexes. The coordination reactions in which metal cations participate in aqueous solutions are exchange reactions with the coordinated water molecules exchanged for some preferred ligands. The barest of the metal cations is the fiee hydrogen ion, the proton. Hence in some regards there is little difference in principle between a free metal ion and a proton. 

Cation exchange in groundwater is a multicomponent process in which all the solute cations participate. It can be calculated easily with geochemical models such as PHREEQC-2 (Parkhurst and Appelo, 1999) which have databases with representative values of the exchange constants. An example PHREEQC-2 input file for calculating exchangeable iron is given in Table 1. 

The kinetics of the complexation reaction of Lil with a cryptand in an imidazo-lium-Tf2N IL and one with a functionalized ammonium cation has been studied by Shirai and Ikeda [71]. Here, they found no difference in kinetics for the two ILs, ruling out strong cation participation in the mechanism. 

The decomposition of initiators is induced by heat in the case of molding compounds or by accelerators at temperatures below the decomposition temperature of the initiator in the case of cast polyester resins. Two types of accelerators are used, metal salts—mainly cobalt salts—and amines. The oxidoreduction of metal salts by peroxides produces free radicals. The process is very efficient since both lower and higher valencies of cations participate in the reaction. 

Cyclo-addition Reactions. The tropylium cation participate as a 4 component in a number of cyclo-addition reactions with monoalkenes [96] and polyalkenes [97]. 

Flocculants can be designed on the basis of certain specific interactions, such as chemical reactions of the flocculant with surface sites or ions present on the surface. For example, poly(acrylic acid) (PAA) can react with calcium sites on surfaces. However, such an approach fails when the cations participating in the complexation reactions on the surfaces are the same or chemically similar. 

Stirring a solution of acetonide 256 in a 1 1 mixture of dichloromethane and trifluoroacetic acid for 15 min at room temperature resulted in the stereoselective formation of cycloadduct 260 as a single stereoisomer in approximately 60% yield. The product contains the 5,7-bicyclic carbon skeleton of rameswaralide 255. Presumably, the reaction proceeded by hydrolysis of the acetonide to give diol 257, which then underwent ionization to afford the furfurylic cation 258. This cation can then undergo [4-1-3] cycloaddition, either in a concerted or stepwise manner, with the internal diene, resulting in cation 259. Lastly, rearoma-tization followed by proton loss would afford cycloadduct 260. This is a unique and rare example in which a furfuryl cation participates in [4-1-3] cycloaddition processes and is also completely stereoselective [126]. 

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