Oxygen ion formation

The reverse reaction (ion formation) can occur in two ways internally, by attack of the penultimate polymer oxygen atom, or externally, by attack of a monomer oxygen atom (chain growth). The external process is about 10 times slower than the internal process in bulk THF (1). Since ion formation is a slow process compared to ion chain growth, chain growth by external attack of monomer on covalent ester makes a negligible contribution to the polymerization process. 

Indeed the cobalt(III) ion is sufficientiy unstable in water to result in release of oxygen and formation of cobalt(II) ion. Under alkaline conditions the oxidation is much more facile and in the presence of complexing agents, eg, ammonia or cyanide, the oxidation may occur with ease or even spontaneously. 

Another example of steric selectivity involves the homopoly and heteropoly ions of molybdenum, tungsten, etc. Each molybdenum(VI) and tungsten(VI) ion is octahedraHy coordinated to six oxygen (0x0) ligands. Chromium (VT) is too small and forms only the weU-known chromate-type species having four 0x0 ligands. The abiUty of other cations to participate in stable heteropoly ion formation is also size related. 

An effect which is frequently encountered in oxide catalysts is that of promoters on the activity. An example of this is the small addition of lidrium oxide, Li20 which promotes, or increases, the catalytic activity of dre alkaline earth oxide BaO. Although little is known about the exact role of lithium on the surface structure of BaO, it would seem plausible that this effect is due to the introduction of more oxygen vacancies on the surface. This effect is well known in the chemistry of solid oxides. For example, the addition of lithium oxide to nickel oxide, in which a solid solution is formed, causes an increase in the concentration of dre major point defect which is the Ni + ion. Since the valency of dre cation in dre alkaline earth oxides can only take the value two the incorporation of lithium oxide in solid solution can only lead to oxygen vacaircy formation. Schematic equations for the two processes are... 

In all of these oxide phases it is possible that departures from the simple stoichiometric composition occur dirough variation of the charges of some of the cationic species. Furthermore, if a cation is raised to a higher oxidation state, by the addition of oxygen to tire lattice, a conesponding number of vacant cation sites must be formed to compensate tire structure. Thus in nickel oxide NiO, which at stoichiomen ic composition has only Ni + cations, oxidation leads to Ni + ion formation to counterbalance the addition of extra oxide ions. At the same time vacant sites must be added to the cation lattice to retain dre NaCl sUmcture. This balanced process can be described by a normal chemical equation thus... 

If the movement of ions on the network predominates, the rate will depend on some power of die oxygen pressure, n, which is related to the formation of oxygen ions on the lattice such as... 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

Schematic illustration of shear-plane formation. Structure (a) with aligned oxygen vacancies shears to eliminate these vacancies in favour of an extended planar defect in the cation lattice as in (b). % cations oxygen ions are at the mesh intersections...

Radiochemical studies indicate that the pore base is the actual site of formation of aluminium oxide, presumably by transport of aluminium ions across the barrier-layer, although transport of oxygen ions in the opposite direction has been postulated by some authorities. The downward extension of the pore takes place by chemical solution, which may be enhanced by the heating effect of the current and the greater solution rate of the freshly formed oxide, but will also be limited by diffusion. It has been shown that the freshly formed oxide, y -AljOj, is amorphous and becomes slowly converted into a more nearly crystalline modifipation of y-AljO . 

Formation of C—Nu The second mode of nucleophilic addition, which often occurs with amine nucleophiles, involves elimination of oxygen and formation of a C=Nu bond. For example, aldehydes and ketones react with primary amines, RNH2, to form imines, R2C=NR. These reactions proceed through exactly the same kind of tetrahedral intermediate as that formed during hydride reduction and Grignard reaction, but the initially formed alkoxide ion is not isolated. Instead, it is protonated and then loses water to form an imine, as shown in Figure 3. 

Figure 22.5 Mechanism of enolate ion formation by abstraction of an a proton from a carbonyl compound. The enolate ion is stabilized by resonance, and the negative charge (red) is shared by the oxygen and the a carbon atom, as indicated by the electrostatic potential map.

The factor of seven variation between k2 for the ordinary and benzylic tertiary hydrogen is too small to be associated with carbonium ion formation. The observed degrees of retention and 0 transfer imply a caged radical rapidly reducing Mn(VI) to Mn(V) by accepting oxygen... 

Guilbault GG, Scheide EP. 1970. Chemisorption reactions of diisopropyl methyl phosphonate with various metal salts and the effect of complex-ion formation on the phosphorus-oxygen stretching frequency. Journal of Inorganic and Nuclear Chemistry 32(9) 2959-2962. 

This is the same mechanism as that given above for esters, in equation (42). The difference between esters and amides is apparent from a comparison of the two tetrahedral intermediates [5] and [17], The former contains three oxygens, any of which can be protonated, resulting in much lsO exchange being observed when the reaction takes place in 180-enriched water,275,276 but [17] contains a much more basic nitrogen, which will be protonated preferentially and lead to much less 180 exchange, as observed.274 277,278 Also, ammonium ion formation makes the overall reaction irreversible, unlike ester hydrolysis. The calculated solvent isotope effect for the Scheme 15 process is 1.00,280 exactly in accord with experimental observation.278,279... 

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