Reaction mechanisms with slow initial step

When the proposed mechanism for a reaction has a slow initial step— Uke the one shown previously for the reaction between NO2 and CO—the rate law predicted by the mechanism normally contains only reactants involved in the overall reaction. However, when a mechanism begins with a fast initial step, some other subsequent step in the mechanism is the rate-limiting step. In these cases, the rate law predicted by the rate-limiting step may contain reaction intermediates. Since reaction intermediates do not appear in the overall reaction equation, a rate law containing intermediates cannot generally correspond to the experimental rate law. Fortunately, however, we can often express the concentration of intermediates in terms of the concentrations of the reactants of the overall reaction. 

Mechanisms with a Slow Initial Step We ve already seen one mechanism with a rate-determining first step—that for the reaction of NO2 and CO. Another example is the reaction between nitrogen dioxide and fluorine gas ... 

Understand elementary steps and molecularity, and be able to construct a valid reaction mechanism with either a slow or a fast initial step ( 16.7) (SP 16.8) (EPs 16.53-16.64)... 

The slow initiation step can be rationalized by a mechanism involving [2+2] cycloaddition of the olefin at the Ru=C bond of the vinylidene ligand, with generation of a new Ru carbene species able to propagate faster polymerization (Scheme 14). This mechanism has been documented by Ozawa" for ROMP of norbomene with [RuCl2(PPh3)2(=C=CHFc)] (Fc = ferrocenyl) and by Kirchner in a stoichiometric reaction using a Tp-coordi-nated Ru complex (Tp = tris(pyrazolyl)borohydride)." ... 

Mechanisms with a Slow Initial Step The reaction between NO2 and CO that we considered earlier has a mechanism with a slow initial step that is, the first step is rate-determining. The reaction between nitrogen dioxide and fluorine is another... 

Bond energy considerations indicate that the initiation reaction (4.2.2) should be quite slow because its activation energy must be quite high (at least equal to the bond dissociation energy). If one were dealing with an open sequence reaction mechanism, such a step would imply that the overall reaction rate would also be low because in these cases the overall reaction becomes approximately equal to that of the rate limiting step. In the case of a chain reaction, on the other hand, the overall reaction rate is usually much faster because the propagation steps occur many times for each time that an initiation step occurs. 

Studies of the rates of formation of polysilanes have led to the currently accepted mechanism, outlined in scheme 5.1.16 In this model the initiation step, thought to be very slow, is the reaction of RR SiCl2 with sodium to produce the ion pair... 

Examples of catalysts in combustion reaction include the effect of H2O on the carbon monoxide oxidation reaction CO H- 2 C02- Nitric oxide also catalyzes CO oxidation through the mechanism 2 NO 4-O2 2NO2 (overall) and NO2 -f CO NO H- CO2. In both of these examples, an intermediate compound (for example, NO2) is formed and then destroyed. The addition of a small amount of NO2 to an H2 — O2 mixture leads to a branched-chain explosion by introducing the relatively rapid initiation step NO2 H- X NO H- O H- X, with the O atoms so produced generating the usual H2 — O2 chain. The NO2 also participates in the efficient termination step NO2 H- O NO H- O2, which is sufficiently important at large concentrations of NO2 to cause a slow reaction to be... 

The initial step in the mechanism of ethylene polymerization using Phillips catalysts is believed to occur by way of an oxidation-reduction reaction between Cr (VI) and ethylene as depicted in eq 5.1. This generates Cr (II) and vacant coordination sites. As mentioned above, polymerization may be initially slow because of sluggish reduction or desorption of the oxidation by-products which can coordinate with (and block) active centers. 

Selectivity of metal carrier action can, in many cases, be reduced to binding equilibrium properties of the metal ions and ligands involved. Where such a model can be applied the information required relates solely to the free energies of the initial and final states of the complexing process, irrespective of the particular type of reaction mechanism. However, selectivity of ion transport could depend on kinetic behaviour, if the rate of equilibration for the complex is slow compared with the rates of decisive steps of transport. 

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