Negative Catalysis - Autocatalysis

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. 

Consequently, at intermediate values of conversion, the rate will go through a maximum. At first glance, it might seem that the reaction is auto-catalytic. In reality, it is inhibited by the reactant itself. This unusual result shows the narrow connection between catalysis, autocatalysis and negative catalysis the kinetic patterns are dictated by the reactions with the active centers, their multiplication or their destruction. 

Autocatalysis is a special type of molecular catalysis in which one of the products of reaction acts as a catalyst for the reaction. As a consequence, the concentration of this product appears in the observed rate law with a positive exponent if a catalyst in the usual sense, or with a negative exponent if an inhibitor. A characteristic of an autocat-alytic reaction is that the rate increases initially as the concentration of catalytic product increases, but eventually goes through a maximum and decreases as reactant is used up. The initial behavior may be described as abnormal kinetics, and has important consequences for reactor selection for such reactions. 

A negative catalyst, inhibitor or stopper is a substance which decreases the rate of a reaction without causing a change in the free energy of the reaction (a topic not considered here). Autocatalysis occurs in a chemical reaction in which a product or an intermediate functions as a catalyst. Such catalysis is characterised by the existence of an induction period during the initial stages of the reaction (but, again, this is not considered here). 

In some reactions, the rate increases rather than decreases as conversion progresses. This is loosely called autocatalysis, although no genuine catalysis may be involved. The acceleration may stem from promotion by a product or major early intermediate, or from consumption of a reactant that functions as inhibitor. In product-promoted reactions, the kinetics order with respect to a product (or early intermediate) is positive. This causes the rate to increase to a maximum and then to decline as the effect of consumption of the reactant or reactants begins to overcompensate that of promotion by the product. In reactant-inhibited reactions, the order with respect to a reactant is negative. The rate may increase until the respective reactant is used up and, in some cases, may theoretically approach infinity. The latter behavior is, of course, physically impossible, and another mechanism or event necessarily takes over. 

The chemical catalysis (i.e. the decrease of the activation energy of the particular hydrolysis of the caprylic ester) may in fact be positive or negative, but it is probably negligible with respect to the physical catalysis (increase of the active surface where the hydrolysis takes place). Lack of understanding of this concept has led to confiision and to the search of complicated mechanism to explain micellar autocatalysis. The same type of argument holds of course for vesicles. 

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