Rale-limiting step

The nature of the nucleophile plays a major role in the SN2 reaction but does not affect an S l reaction. Because the SN1 reaction occurs through a rale-limiting step in which the added nucleophile has no part, the nucleophile can t affect the reaction rate. The reaction of 2-methyl-2-propanoI with HX, for instance, occurs at the same rate regardless of whether X is Cl, Br, or 1. Furthermore, neutral nucleophiles are just as effective as negatively charged ones, so S 1 reactions frequently occur under neutral or acidic conditions. 

A reaction energy diagram for an S l reaction. The rale-limiting step is spontaneous (fssociation of the alkyl haide to give a... 

Conversion of cholesterol to pregnenolone is the rale-limiting step in steroid hormone biosynthesis. It is not ihccnry-malic transformation itself that is rate limiting, however, the translocation of cholesterol to the inner milochondrui membrane of steroid-synihesi/.ing cells is rate limiting. A key protein involved in the translocation is the 5Kruidugcmc... 

Sec 10.3 Synthesizing a Rate Law. Mechanism, aiiu Rale-Limiting Step... 

A more dramatic difference is seen with ornithine decarboxylase (EC 4.1.1.17). This enzyme catcJyses the rale-limiting step in polyamine synthesis, and its activity in many tissues and organisms correlates well with the rate of DNA synthesis and cell proliferation. Its turnover is one of the most rapid of all enzymes, generally having a half-life of less than 20 min. Bullfield et al. (1988) have found a 20-fold higher activity in the skeletal muscle from the broiler strain compared with that in a layer strain of domestic fowl at one week of age. TTiis increased activity is almost certainly achieved by an increase in fcs with little change in fca-... 

In case 1 diffusion the rale limiting step is the diffusion of aqueous fluid into the delivery system. Accordingly, the relationship between the mass of fluid absorbed by the drug delivery system and time may be defined using Pick s law (Equation 18). 

Bouwmeester, H.J.M., Kruidhof, H., and Buiggraaf, A.J., Importance of the surface exchange kinetics as rale limiting step in oxygen permeation through mixed-conducting oxides. Solid State Ionics, 1994, 72, 185-94. 

We have, up to now, not considered reaction 3 at all. If reaction 1 is rale limiting, of course, we can determine nothing at all about whether it is followed by reaction 2 or reaction 3 indeed, it is a general rule that any kinetic analysis will be unable to shed light on any step following the rate-limiting step. 

Eigurc 6. Schematic diagram showing the rale-limiting acylation step in the hydrolysis of peptide bonds by subtilisin. E.St Michaelis complex, E.Sb transition-state complex, E-Ac acyl enzyme. 

Assignment of Rate-Limiting Steps and Pseudoequilibrium Steps. Once the rale-determining steps for reactions in series have been identified, reaction steps that take place before or after them can be treated as pseudoequilibrium reactions. 

Assume a rale-timhing step. Choose the surface reaction first, since ntrjnp than 75% of alt heterogeneous nracfiori.t that are not diffusion-limited are sufface-reaction-limited. The rate law for the surface reaction step is... 

The Kunii-Levenspiel model for fluidization is given on the CD-ROM along with a comprehensive example problem. The rale limiting transpon steps are also discussed. See Professional Reference Shelf R12.3. 

Becau.se the El proces.s involves the same rale-determining step a.s the S. l reaction, its kinetics are the same first order. EJ elimination almost always accompanies SnI substitution. The difference is simple In S l, the nucleophile attaches to the cationic carbon in El, it attaches to and removes a proton. Thus nucleophiles that are very weak bases will favor S l, with El contributing more and more as basicity increases. For the practical piirpo.ses of synthesis, the prc.sence of the El "side reaction can limit the usefulness of SnI siihsliliition,... 

Cumene reaction rale law if aJsorplinn were the limiting step... 

We need a word of caution at this point. Just because the mechanism and rate-limiting step may fit the rate data does not imply that the mechanism is correct. Usually, spectroscopic measurements are needed to confirm a mechanism absolutely. However, the development of various mechanisms and rate-limiting steps can provide insight into the best way to correlate the data and develop a rale law. 

Closure. After reading this chapter, the reader should be able to discuss the steps in a heterogeneous reaction (adsorption, surface reaction, and desorption) and describe what is meant by a rate-limiting step. The differences between molecular adsorption and dissociated adsorption should be explained by the reader, as should the different types of surface reactions (single site, dual site, and Eley-Rideal). Given heterogeneous reaction rate data, the reader should be able to analyze the data and to develop a rale law for Langmuir-Hinshelwood kinetics. The reader should be able to discrimi-... 

The interpretation of this data on metals in terms of microscopic mechanisms of surface atom transport is not totally understood. The original papers[ 11] proposed that during surface transport the controlling process was adatom terrace diffusion between steps with the adatom concentration being that in local equilibrium with the atomic steps. This may indeed be the case, but in light of other experiments on adatom diffusion[13] and exchange processes at steps[14] the possibility of step attachment/detachment limited kinetics caimot be raled out. 

In general, the following steps can occur in an overall liquid-solid extraction process solvent transfer from tile bulk of the solution to the surface of the solid penetration or diffusion of the solvent into the pores of the solid dissolution of the solvent into the solute solute diffusion lo ihe surface of the particle and solute transfer to the bulk of the solution. Any one of the five basic processes may be responsible for limiting the extraction rale. 

The need to drive the polymerizations to completion is common to all step-growth reactions that are carried out under conditions in which polymerization-depolymerization equilibria are significant (Section 5.4.2). This is accomplished in general by removal of a volatile product such as water or an alcohol. The rale of polymerization is often limited by the rate of transfer of such condensation products into the vapor state. A complete kinetic description of the process must then involve both the chemical reaction rate and the rate of mass transfer. The latter depends on the details of reactor design and stirring and therefore so does the rate of polymer production [1]. 

As noted earlier, some carbonic anhydrases can hydrate carbon dioxide at rates as high as a million times a second (10 s ). The magnitude of this can be understood from the following observations. In the first step of a carbon dioxide hydration reaction, the zinc-bound water molecule must lose a proton to regenerate the active form of the enzyme (Figure 9.27). The rate of the reverse reaction, the protonation of the zinc-bound hydroxide ion. is limited by the rate of proton diffusion. Protons diffuse very rapidly with second-order rate constants near 10 M. Thus, the backward rale constant i must be less than 10 s F Because the equilibrium... 

Most of the competing pathways for racemization and retention in. Scheme 7.1 would not be expected to produce such a limit. The pathway that could best account for it may be the singlet version of 1-3-4. Here partial racemization would occur in step 4, where X" has been lost from the reactant. The rales of racemization and passage from the initial excited stale "R to the ground slate could... 

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