Fast step

C(ads) + H2 CH2(ads) followed by fast steps. The corresponding rate expression proposed is... 

It has been suggested that the first step, weak coadsorption of NO and O2 on a reduced vanadium site, may represent the slow step in the mechanism. Subsequent formation of a N202-like intermediate could be a fast step, because it is known that in the gas phase the equiUbrium of NO—NO2 and N2O2 is estabflshed within microseconds (35). 

Because proton-transfer reactions between oxygen atoms are usually very fast, step 3 can be assumed to be a rapid equilibrium. With the above mechanism assume4 let us examine the rate expression which would result, depending upon which of the steps is rate-determining. 

Lee, Y.-M. and Lee, L.J., 1987. Effect of mixing and reaction on a fast step growth polymerization. International Polymer Processing, 1, 144-152. 

A-S 2. A slow protonation of substrate followed by fast steps. 

Q The carbocation intermediate reacts with bromide ion in a fast step to yield the neutral substitution product. 

G Loss of a neighboring H+ in a fast step yields the neutral alkene product. The electron pair from the C-H bond goes to form the alkene r bond. 

The simplest case to be analyzed is the process in which the rate of one of the adsorption or desorption steps is so slow that it becomes itself rate determining in overall transformation. The composition of the reaction mixture in the course of the reaction is then not determined by kinetic, but by thermodynamic factors, i.e. by equilibria of the fast steps, surface chemical reactions, and the other adsorption and desorption processes. Concentration dependencies of several types of consecutive and parallel (branched) catalytic reactions 52, 53) were calculated, corresponding to schemes (Ila) and (lib), assuming that they are controlled by the rate of adsorption of either of the reactants A and X, desorption of any of the products B, C, and Y, or by simultaneous desorption of compounds B and C. 

On the basis of these correlations, Gold and Satchell463 argued that the A-l mechanism must apply (see p. 4). However, a difficulty arises for the hydrogen exchange reaction because of the symmetrical reaction path which would mean that the slow step of the forward reaction [equilibrium (2) with E and X = H] would have to be a fast step [equivalent to equilibrium (1) with E and X = H] for the reverse reaction, and hence an impossible contradiction. Consequently, additional steps in the mechanism were proposed such that the initial fast equilibrium formed a 7t-complex, and that the hydrogen and deuterium atoms exchange positions in this jr-complex in two slow steps via the formation of a a-complex finally, in another fast equilibrium the deuterium atom is lost, viz. 

Finally, these results clearly show yet again that attachment of hydrogen to the aromatic is not a fast step, thereby eliminating the A-l mechanism yet the data in mixed solvents followed the prediction of the Gross-Butler cubic equation, and once again this test turns out to be valueless. 

In neutral and alkaline media, the rate of exchange at the 3 and 6 position of 4-aminopyridazine is independent of acidity but decreases markedly when the media become more acidic. This was interpreted in terms of a rate-determining removal of the 6-proton by deuteroxide ion to give the ylid (XXIV), which reacts with deuterium oxide in a fast step. A similar result for the 3 and 6 positions of py-ridazin-4-one suggests the same mechanism. For reaction at the 5 position, the rate-acidity profile indicated reaction on the free base as did that for the 5 position of pyridazin-3-one, though the appearance of a maximum in the rate at — HQ = 0.8 was anomalous and suggested incursion of a further mechanism. 

Just because a rapid prior equilibrium can be invoked does not mean that one has learned anything about the mechanism of the fast step. That is, an interpretation of Eq. (6-20) is given in Eq. (6-23) the data say nothing, however, about the mechanism by which the constituents of Eq. (6-21) and of Eq. (6-22) are interconverted. 

Goering and coworkers201 studied the kinetics of base-promoted dehydrohalogenation of several series of cis- or frans-2-chlorocycloalkyl aryl sulfones. For the trans-2-chlorocyclohexyl series reacting with sodium hydroxide in 80% ethanol at 0 °C the p value was 1.42. The mechanism was considered to involve rate-determining carbanion formation, with the subsequent loss of chloride ion in a fast step. 

This oxidation of DMSO is catalyzed by Ag+ cations. Kinetic and infrared spec-trometric evidence fits a mechanism where DMSO coordinates rapidly with Ag+ through its oxygen atom. The oxidation of this complex by Ce4 + then constitutes the slow step. The Ag2+ adduct would then undergo an intramolecular electron transfer in a fast step resulting in the oxidation of DMSO. 

Notice that the rate law is first order in the catalyst, carbon disulfide, but zero order in triiodide ion, which appears only in the fast step following the slow step. 

Reversible Unimolecular Reactions. The intrinsic reaction steps in heterogeneously catalyzed reactions are usually reversible. The various limiting cases can be found by taking limits before redefining the constants, e.g., take limits on Equation (10.11), not Equation (10.12). However, a more direct route is to assume that the fast steps achieve equilibrium before deriving the counterpart to Equation (10.11). 

The activation energy of the overall reaction equals that of the first step, a,i-Note that fast elementary steps following the one that limits the rate become kine-tically insignificant, whereas fast steps before the rate-determining step do enter the rate equation, as they directly affect the concentration of the intermediate that is converted in the rate-determining step. 

The dehalogenation of the a-haloalkyl radical is a fast step which can take place by several possible routes . Dibromides are reduced much faster than dichlorides and rra j-l,2-dibromocylohexane is reduced 100 times faster than the m-isomer. This accords with neighbouring group assistance which bromine seems particularly capable of offering (see subsection 6.4.10). 

The rates are essentially independent of the distribution of metal in the MT with similar rates between Zn7—MTm (Cd, Zn)7-MT, and Cd7—MT. The values of the rate constants are ks = 6.9( 0.9) x 10 " s and kf=2.7( 1.2) x 10 s for the holo-protein. The slow rate constant is similar in magnitude to the first-order protein-dependent steps observed for reactions of DTNB (5,5 -dithiobis(2-nitrobenzoate)), EDTA (ethylenediamine tetraacetate), cisplatin, and other reagents, which has been attributed to a rearrangement of the protein. The fast step is more rapid by an order of magnitude, which suggests that other mechanisms are prevailing. 

Fast step for mechanism I) (Slow step for mechanism II)... 

Whether substitution or elimination takes place depends on the next step (the fast step). 

The alcohol accepts a proton from the acid in a fast step. 

For reactions in parallel, it is the fast step that governs. Thus, if A B and A C are two competing reactions, and if kAB kAC, the rate of formation of B is much higher than that of C, and very little C is produced. Chemical rates can vary by very large factors, particularly when different catalysts are involved. For example, a metal catalyst favors dehydrogenation of an alcohol to an aldehyde, but an oxide catalyst often favors dehydration. 

This mechanism also illustrates the concept of a rate-determining Step (rds) to designate a slow step (relatively low value of rate constant as opposed to a fast step), which then controls the overall rate for the purpose of constructing the rate law. 

Steps (29) and (30) are those involved also in the decomposition of N205. Initial rates of decomposition are apparently higher than at later stages in the reaction because, initially, the free-radical reaction is not inhibited by the fast step (29). When sufficient nitric oxide is present, either initially added or formed by N02 decomposition, the free-radical reaction path is suppressed. Ashmore et al.212 213 found indeed that the value of the second-order rate coefficient of decomposition kd, depends on the [N0]/[N02] ratio in agreement with the relation... 

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