Catalysis negative

The anticatalytic effect of nitrous acid in nitration The effect of nitrous acid was first observed for zeroth-order nitrations in nitromethane ( 3.2). The effect was a true negative catalysis the kinetic order was not affected, and nitrous acid was neither consumed nor produced by the nitration. The same was true for nitration in acetic acid. In the zeroth-order nitrations the rate depended on the reciprocal of the square root of the concentration of nitrous acid =... 

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. 

AL Aleksandrov. Negative Catalysis in Chain Reactions Oxidation of N and O-containing Compounds, Doctoral Thesis, Institute of Chemical Physics, Chernogolovka, 1987, pp. 1-40 [in Russian],... 

The catalyst which decreases the rate of reaction is called negative catalyst and phenomenon is called negative catalysis. Examples are... 

Certain constituents when added to the reaction mixture, slow down the rate of reaction. This phenomena is called inhibition and constituent called inhibitor. Such an effect is similar to the negative catalysis. But the constituent usually undergoes chemical change, inhibition is the preferred term. Inhibition may occur in chain reactions, enzyme catalysed reactions, surface reactions or many reversible or irreversible reactions. A trace amount of an inhibitor may cause a marked decrease in the rate of reaction. The inhibitor sometimes combines with a catalyst and prevents it from catalyzing the reaction. 

Negative Catalysis by JfC03 Ions in Chain Oxidation of Cyclohexanol... 

The peroxy radicals of secondary alcohols are oxyperoxide radicals. Experiments using various systems show the relationship between the structure of peroxide radicals and negative catalysis by HC03" (Table 1). 

The explanation of negative catalysis in terms of the chain reaction mechanism was given by Christiansen. ... 

The most direct evidence that negative catalysis sometimes works in this way in ordinary thermal reactions, and, therefore, incidentally that the chain mechanism can operate in such reactions, has been found by Backstrom. In the photochemical oxidation of benzaldehyde, heptalde-hyde, and of solutions of sodium sulphite, there are very large numbers of molecules transformed for each quantum of light absorbed, amounting respectively to 10,000,15,000, and 50,000 for the three reactions. Such deviations from Einstein s law show that the light probably sets up chain reactions. These photochemical changes are markedly subject to the action of inhibitors, which presumably cut short the chains. Backstrom establishes the important... 

Catalysis is the phenomenon in which a relatively small amount of a foreign material, called a catalyst, augments the rate of a chemical reaction without itself being consumed. Cases occur with certain reactants in which the addition of a substance reduces the rate of a particular reaction, for example, the addition of an inhibitor in a chain reaction or a poison in a catalytic reaction. The term negative catalysis has been used for these... 

Breslow, R., How do imidazole groups catalyze the cleavage of RNA in enzyme models and enzymes Evidence from negative catalysis. Acc. Chem. Res. 24 317, 1991. 

The mechanism of negative catalysis could be different for different reactions, e.g.,... 

One of the strongest supports for this theory of negative catalysis is found in photochemical investigations. In these cases the actual quantum yield is reduced enormously by the negative catalyst. In such cases one can see that the negative catalyst functions by breaking the chain, and it is easy to carry over this mechanism to thermal reactions. In fact, several investigations have shown that the effective inhibitors are the same for the thermochemical as for the photochemical reactions. 

The Stationary Velocities of a Branched Sequence Negative Catalysis. 338... 

In 1924 in a note on negative catalysis (12a) Christiansen s views were further substantiated. This paper induced Backstrom to enter on his well-known investigations on inhibition of autoxidations (18). 

Retey, J., 1990, Enzymatic-reaction selectivity by negative catalysis or how do enzymes deal with highly reactive intermediates. Angew. Chem. Inti. Ed. Engl. 29 3559361. 

Figure 2 Possible side reactions for a radical (R ) with protein components and dioxygen energy profile illustrating the concept of negative catalysis.

Effect of additives. A particularly striking solvent effect was one of negative catalysis evident from the results given in Table 4 for reactions carried out with... 

Electrocatalytic processes have received considerable attention in the last decades because of their application in synthesis and sensing. The term catalysis is used for describing the modification in the reaction rate of a given chemical reaction by effect of the addition of a catalyst species. Two essential conditions have to be accomplished by catalytic processes the thermodynamics of the reaction becomes unaltered, and the catalyst stays unchanged. Additionally, a common demand for catalysis is that the catalytic process involves small concentrations of the catalyst. In the most general view, the rate of the reaction can be either increased (positive catalysis) or decreased (negative catalysis), although, obviously, positive catalysis is preferentially desired. 

The last column of table 3 shows the values of k as calculated by the above expression from the values of Vmt C and t given in the same table. It is very probable, therefore, that k expresses the specific inhibitory power of anthraquinone in the reactions studied. Furthermore, anthraquinone seems to behave as a true negative catalyst. Since anthraquinone is not destroyed during the oxidation of anethol, how can its effect be explained in the light of the recently proposed chain mechanism of negative catalysis It is well known that quinones are highly reactive substances and are found to combine with several other organic substances to form imstable and frequently stable complexes. In other words,... 

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