Product formation rate limiting step determination

Consider the series reaction A—>B—>C. If the first step is very much slower than the second step, the rate of formation of C is controlled by the rate of the first step, which is called the rate-determining step (rds), or rate-limiting step, of the reaction. Similarly, if the second step is the slower one, the rate of production of C is controlled by the second step. The slower of these two steps is the bottleneck in the overall reaction. This flow analogy, in which the rate constants of the separate steps are analogous to the diameters of necks in a series of funnels, is widely used in illustration of the concept of the rds. 

The extent of a reaction in these measurements is determined by bare metal cluster ion signal depletion. In most cases products are also observed. Some systems show multiple adducts indicating comparable or higher rates for each successive step up to a saturation level. For other systems the fully saturated product is observed almost as soon as the reaction starts. This later behavior is characteri sti c of an early rate-limiting step. Due to this complexity kinetics have only been reported on the formation of the first adduct, i. e. for the initial chemisorption step. 

Due to the time-resolution limitation of the method, FPTRMS can be used to determine the kinetics of only those unimolecular reactions that occur on millisecond time scales or longer. However, even if a unimolecular reaction occurs too rapidly for time resolution of the kinetics, the occurrence of a reaction can be shown by mass spectrometric detection of the products. If the unimolecular reaction is rate limited by a preceding slow step so that the product formation rates are time resolved, then a lower limit to the unimolecular rate coefficient can be estimated. In the case of atmospheric reactions this will frequently be enough information to permit reaction mechanisms to be sorted out. 

We here review the factors that control the kinetics of product formation through reaction at an active surface. This includes consideration of the availability of those adsorbed intermediates which participate in the rate-limiting step (this term is analogous to concentration in a homogeneous reaction) and the mobility of the same species, which may determine, or at least influence, the frequency of occurrence of the reaction situation. The discussion is given under three broadly interpreted general headings, between which there is considerable overlap. 

The theories proposed to explain the formation of passivation film are salt-film mechanism and acceptor mechanism [21]. In the salt-film mechanism, the assumption is that during the active dissolution regime, the concentration of metal ions (in this case, copper) in solution exceeds the solubility limit and this results in the precipitation of a salt film on the surface of copper. The formation of the salt film drives the reaction forward, where copper ions diffuse through the salt film into electrolyte solution and the removal rate is determined by the transport rate of ions away from the surface. As the salt-film thickness increases, the removal rate decreases. In the acceptor mechanism, it is assumed that the metal-ion products remain adsorbed onto the electrode surface until they are complexed by an acceptor species like water or anions. The rate-limiting step is therefore the mass transfer of the acceptor to the surface. Recent studies confirmed that water may act as an acceptor species for dissolving copper ions [22]. 

When a secondary enzyme-catalyzed reaction, known as an indicator reaction, is used to determine the activity of a different enzyme, the primary reaction catalyzed by the enzyme to be determined must be the rate-limiting step. Conditions are chosen to ensure that the rate of reaction catalyzed by the indicator enzyme is directly proportional to the rate of product formation in the first reaction. 

Because the formation of the carbocation is the rate-limiting step of the reaction, the relative rates of formation of the two carbocations determine the relative amounts of the products that are formed. If the difference in the rates is small, both products will be formed, but the major product will be the one formed from reaction of the nucleophile with the more stable carbocation. If the difference in the rates is sufficiently large, the product formed from reaction of the nucleophile with the more stable carbocation will be the only product of the reaction. For example, when HCl adds to... 

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