Species and Rate-Determining Steps

The scheme in Fig. 5.5 indicates that the ligand, for example, oxalate, is adsorbed very fast in comparison to the dissolution reaction thus, adsorption equilibrium may be assumed. The surface chelate formed is able to weaken the original Al-oxygen bonds on the surface of the crystal lattice. The detachment of the oxalato-aluminum species is the slow and rate-determining step the initial sites are completely regenerated subsequent to the detachment step provided that the concentrations of the reactants are kept constant, steady state conditions with regard to the oxide surface species are established (Table 5.1). If, furthermore, the system is far from dissolution equilibrium, the back reaction can be neglected, and constant dissolution rates occur. 

Zero order kinetics in monomer was reported [95,97] for the polymerization of vinyl ethers initiated by HI/I2 in hexane and was ascribed to the formation of a complex between iodine (Lewis acid) and monomer. It has been proposed that monomer reversibly forms a complex with the growing chain (.. . -CH2CH(OR)I,l2) which then slowly inserts monomer in the rate-determining step [74]. Another possibility is that the ionization of the dormant species the rate-determining step where the cation subsequently undergoes the rapid reaction with monomer and then soon col-... 

Pyridine (p/fa 5.2) causes more rapid polymerization than dimethyl-tert-butylamine (P a 10.5). It is difficult to reconcile this observation with the idea that the initiating species is a hydroxyl ion formed by hydrolysis of the amine. On the other hand the structure of an amine might be expected to influence its rate of addition to the carbonyl double bond. The first and rate determining step in the formation of a long chain polymer is apparently betaine formation ... 

One could write many other pathways and rate-determining steps, and calculate the kinetic parameters for each, following the same line of reasoning. It is important to note that in a complex reaction sequence there can be more than one type of adsorbed intermediate on the surface, and some steps may involve tlie transformation of one kind of adsorbed species to another, by either an electrochemical or a chemical route. 

Whether the nucleophile attacks the carbon or the heteroatom attacks the electrophilic species, the rate-determining step is usually the one involving nucleophihc attack. It may be observed that many of these reactions can be catalyzed by both acids and bases.Bases catalyze the reaction by converting a reagent of the form YH to the more powerful nucleophile (see p. 490). Acids catalyze it by converting the substrate to an heteroatom-stabilized cation (formation of 3), thus making it more attractive to nucleophilic attack. Similar catalysis can also be found with metallic ions (e.g., Ag ) which act here as Lewis acids. We have mentioned before (p. 242) that ions of type 3 are comparatively stable carbocations because the positive charge is spread by resonance. 

The treatment may be made more detailed by supposing that the rate-determining step is actually from species O in the OHP (at potential relative to the solution) to species R similarly located. The effect is to make fi dependent on the value of 2 and hence on any changes in the electrical double layer. This type of analysis has permitted some detailed interpretations to be made of kinetic schemes for electrode reactions and also connects that subject to the general one of this chapter. 

Since the rate of a chemical reaction only depends on the slowest, or rate-determining step, and any preceding steps, species B will not show up in the rate law. 

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 ... 

The active electrophile is formed by a subsequent reaction, often involving a Lewis acid. As discussed above with regard to nitration, the formation of the active electrophile may or may not be the rate-determining step. Scheme 10.1 indicates the structure of some of the electrophihc species that are involved in typical electrophilic aromatic substitution processes and the reactions involved in their formation. 

NO, the monomer C is CO, and the products are A2 = N2 and CB = CO2. The adsorption probability of C species (Fc) is the parameter of the model. The slow rate-determining step in this sequence is the dissociation of NO which requires a neighboring site to proceed. Since product formation liberates more vacant sites than those necessary for the dissociation of NO, an autocatalytic production of vacant sites takes place. 

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