Polar molecules, reactions with ions inhibition

Many reactions catalyzed by the addition of simple metal ions involve chelation of the metal. The familiar autocatalysis of the oxidation of oxalate by permanganate results from the chelation of the oxalate and Mn (III) from the permanganate. Oxidation of ascorbic acid [50-81-7] C HgO, is catalyzed by copper (12). The stabilization of preparations containing ascorbic acid by the addition of a chelant appears to be negative catalysis of the oxidation but results from the sequestration of the copper. Many such inhibitions are the result of sequestration. Catalysis by chelation of metal ions with a reactant is usually accomphshed by polarization of the molecule, faciUtation of electron transfer by the metal, or orientation of reactants. 

The process of setting in resin-modified glass-ionomers is complicated because of the variety of reactions involved [6]. In principle it consists of two distinct parts, namely an acid-base reaction as in conventional glass-ionomers, and a free radial polymerization reaction. However, the two reactions influence each other in various ways. The acid-base reaction is slowed down in these materials for two reasons. One is that, as we have seen, the presence of HEMA causes the molecules to adopt a more coiled configuration and this reduces the ease with which they donate protons and behave as acids. The other reason is that the reduced polarity of the HEMA/water mixture compared with pure water inhibits the activation step of the acid-base reaction. This step involves electronically charged species, namely hydrogen ions from the acid and... 

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