Reaction mechanisms rate-determining step

Catalyst Reaction mechanism Rate.determining step ... 

Rate constant The proportionality constant in the rate equation for a reaction, 288 Rate-determining step The slowest step in a multistep mechanism, 308 Rate expression A mathematical relationship describing the dependence of reaction rate upon the concentra-tion(s) of reactant(s), 288,308-309 Rayleigh, Lord, 190... 

The interpretation of the kinetics is based on the mechanisms proposed by Schechter et al.55) and Smith 57), for the reaction of secondary amines with epoxides, extended to include the primary amine reaction. The rate-determining step is assumed to be the reaction of amine, epoxide and hydroxyl or other proton-donor species, HX, to form a termolecular complex, Eq. (3-13). The proposed reaction scheme is ... 

The assumption that the consumption of the intermediate in the slow step is insignificant relative to its formation and decomposition in the first step is called a pre-equilibrium condition. A pre-equilibrium arises when an intermediate is formed in a rapid equilibrium reaction prior to a slow step in the mechanism. The slowest elementary step in a sequence of reactions—in our example, the reaction between 02 and N202—is called the rate-determining step of the reaction. The rate-determining step is so much slower than the rest that it governs the rate of the overall reaction (Fig. 13.24). A rate-determining step is like a slow ferry on the route between two cities. The rate at which the traffic arrives at its destination is governed by the rate at which it is ferried across the river, because this part of the journey is much slower than any of the others. 

Van Ho and Harriott have postulated a different mechanism involving intermediate adsorbed carbon atoms.188 This is based on their observation that a larger fraction of the surface of the catalyst was covered by adsorbed carbon monoxide during reaction. The rate determining step was assumed to... 

It has been argued that insight into catalytic mechanisms can most readily be obtained by choosing reactions whose rate-determining step is unimolecular. The reactions best known to be unimolecular in homogeneous media are undoubtedly the solvolyses of tertiary butyl halides [161-163]. Their SN1 mechanisms are typified by the solvolysis of f-butyl bromide in an ethanol + water medium... 

This intermediate survives several molecular collisions before passing through a second transition state to form the product [M(H20)(ra 1)L](m xA. Thus, the rate determining step (k is the bond making between Lx with Mm+ and the mechanism is termed associatively (a) activated and the mechanism associative, A. (In the back reaction the rate determining step is characterized by k 2). The rate of approach to equilibrium in the presence of excess [Lx ], characterized by kobs in Equation (8) is dependent on the nature of Lx ... 

Mechanism Rate-determining step Overpotential range Tafel slope Reaction order inH ... 

In solution of perchlorates and tosylates in acetic acid, the primary products of electro-oxidation of aromatic hydrocarbons are shown to be benzylic acetates, i.e., substitution occurs preferentially in the side chain. A large, primary isotope effect, ku/kjy = 2.6, provided strong evidence that in this type of substitution reaction the rate-determining step involved the loss of a proton.Further refinement of the mechanism correlated isomeric product ratios to the distribution of positive charge density in the intermediate cation radical. 

Solution f-Butyl alcohol is a tertiary alcohol thus it reacts by an Sjjl mechanism. As in all Sfjl reactions, the rate-determining step involves formation of a carbocation in this case, the f-butyl carbocation. The rate of this step does not depend on which acid is used, so all of the reactions proceed at equal rates. 

Elementary Reactions The Rate-Determining Step The Mechanism and the Rate Law... 

Mechanism Rate- determining step Over- potential range Tafel slope/mV" Reaction order in H ... 

The above equations can apply when the rate-determining step is first order even though the complete reaction mechanism is complicated. Thus for the reac-... 

Both steps m this general mechanism are based on precedent It is called elec trophilic addition because the reaction is triggered by the attack of an acid acting as an electrophile on the rr electrons of the double bond Using the two rr electrons to form a bond to an electrophile generates a carbocation as a reactive intermediate normally this IS the rate determining step... 

The Stvfl mechanism is an ionization mechanism The nucleophile does not participate until after the rate determining step has taken place Thus the effects of nucleophile and alkyl halide structure are expected to be different from those observed for reactions pro ceedmg by the 8 2 pathway How the structure of the alkyl halide affects the rate of Stvfl reactions is the topic of the next section... 

The mechanism of the synthesis reaction remains unclear. Both a molecular mechanism and an atomic mechanism have been proposed. Strong support has been gathered for the atomic mechanism through measurements of adsorbed nitrogen atom concentrations on the surface of model working catalysts where dissociative N2 chemisorption is the rate-determining step (17). The likely mechanism, where (ad) indicates surface-adsorbed species, is as follows ... 

Generally, the reactions of halopyrazines and haloquinoxalines with nucleophiles are believed to proceed by way of addition/elimination sequences, although there are clear-cut examples where this is not the case (see Section 2.14.2.2) and, consistent with a mechanism which involves bond forming, rather than bond breaking, reactions in the rate-determining step, fluoro derivatives are considerably more reactive ca. xlO ) than the corresponding chloro derivatives. 

The effect of conformation on reactivity is intimately associated with the details of the mechanism of a reaction. The examples of Scheme 3.2 illustrate some of the w s in which substituent orientation can affect reactivity. It has been shown that oxidation of cis-A-t-butylcyclohexanol is faster than oxidation of the trans isomer, but the rates of acetylation are in the opposite order. Let us consider the acetylation first. The rate of the reaction will depend on the fiee energy of activation for the rate-determining step. For acetylation, this step involves nucleophilic attack by the hydroxyl group on the acetic anhydride carbonyl... 

---