Catalysis positive

Chloroanisole and p-nitrophenol, the nitrations of which are susceptible to positive catalysis by nitrous acid, but from which the products are not prone to the oxidation which leads to autocatalysis, were the subjects of a more detailed investigation. With high concentrations of nitric acid and low concentrations of nitrous acid in acetic acid, jp-chloroanisole underwent nitration according to a zeroth-order rate law. The rate was repressed by the addition of a small concentration of nitrous acid according to the usual law rate = AQ(n-a[HN02]atoioh) -The nitration of p-nitrophenol under comparable conditions did not accord to a simple kinetic law, but nitrous acid was shown to anticatalyse the reaction. 

The catalyst which increases the rate of a chemical reaction is called positive catalyst and the phenomenon is known as positive catalysis. Examples are... 

It is known that when the kinetic behaviour cannot be studied in the absence of the molecular complex (and the reactivity of the free reagent is unknown), the kinetic law by itself cannot indicate whether the complex observed is on the reaction pathway or is a non-productive equilibrium267. In S/yAr reactions discussed here, the usual experimental conditions [ArF]o < [RNH2]o are used. In some cases, the k0bs values obtained under experimental conditions [ArF]o > [RNH2]o may be assumed to involve only the uncatalysed process. As a consequence, it is possible to say that the presence of complexes (in apolar solvent) clearly produces an enhancement of the reactivity and a positive catalysis, while in polar solvents256 the presence of complexes depresses the rate of substitution. 

A very interesting (and important) detail of the SnAr reaction showing positive catalysis is the dependence of catalysed and uncatalysed processes on the temperature and was investigated in several instances270-274. There are systems271,272 in which the experimental reaction rate constant (in s 1 mol-1 dm3) is decreased on increasing the temperature. In other instances the increase in the temperature doesn t have an effect on kobs273. 

There is no detectable positive catalysis by added bromide ion, but there is a reduction in rate on adding chloride ions. 

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. 

Although the simplest and most commonly recognized effect of catalysts is to accelerate the rate of a chemical change or exert positive catalysis, it is possible in certain instances for a catalyst to retard a reaction or exert a negative effect. Negative catalysis should be distinguished from the inhibition of positive catalysis brought about by the action of poisons which... 

In contrast to positive catalysis on gold and carbon electrodes, Pb, Tl, and Bi adatoms cause a negative catalytic effect on Pt and Ag electrodes. The four-electron reduction on the bare metals changes to two-electron reduction on the electrodes covered by the modifiers. This partial inhibition occurs at negative potentials, in the mass-transport control region, where almost complete UPD ad-atom coverages are obtained. Finally, the reduction of... 

We observed that increasing the concentration of B, while holding BH+ constant, caused a decrease in the rate of isomerization, while the cleavage went faster. This is expected if the two processes branch from a common intermediate (Scheme 1). The kinetic equations show this, and physically what happens is that the concentration of the steady state phosphorane intermediate I is lower as more B sends it along the cleavage path. If the isomerization branches from this intermediate, but not otherwise, such positive catalysis of the cleavage by B will lower the rate of isomerization. 

In the absence of direct detection of Catam, a skeptic might argue and suggest that the presence of positive catalysis due to tertiary amines could be because of the occurrence of an alternative reaction mechanism as shown by Scheme 2.13, which involves intermolecular general base catalysis instead of nucleophilic catalysis. The formation of reactive intermediate T occurs through the transition state TSg. Although the proposal for the existence of transition states similar to TSg is not uncommon in the vast literature on the related... 

In such catalysis, the reaction or the catalytic site in a catalyst is always electron-deficient and, consequently, it accepts a pair of electrons from an electron donor. In the catalytic reduction of overall free energy of activation for positive catalysis, the predominant destabilization of GS or predominant stabilization of TS must involve the pair of electron transfers (partially or fully) from the reaction site in the reactant (substrate) to the reaction site in the catalyst (neutral/cationic atom/molecule). The interaction between catalyst and reactant should involve... 

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