Mechanistic Implication of Equation

An extensive kinetic study on methanolysis of ionized phenyl salicylate (PS ), as described elsewhere, reveals that the overall reaction involves PS and CH3OH as the reactants. Thus, the apparent rate law for methanolysis of PS  

For purely mathematical convenience, the assumption of equal association constant for one more addition of CH3OH molecule to an aggregate changes Equation 7.57 to Equation 7.58 

The theoretical kinetic equation Equation 7.60 is similar to the empirical kinetic equation Equation 7.47 with ROH = CH3OH, a = k2, and P = 2K.  

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A deeper perception of the mechanistic implications of equation (9.2) can be had if the rational activity coefficients are described on the molecular level using the methods of statistical mechanics. This approach is the analogue of the statistical mechanical theory of activity coefficients for species in aqueous solution (Sposito, 1983). Fundamental to it is the prescription of surface speciation and the dependence of the rational activity coefficient on surface characteristics. Three representative molecular models of adsorption following this paradigm are summarised in Table 9.8. Each has been applied with success to describe the surface reactions of soil colloids (Goldberg, 1992). 

The kinetic observations reported by Young [721] for the same reaction show points of difference, though the mechanistic implications of these are not developed. The initial limited ( 2%) deceleratory process, which fitted the first-order equation with E = 121 kJ mole-1, is (again) attributed to the breakdown of superficial impurities and this precedes, indeed defers, the onset of the main reaction. The subsequent acceleratory process is well described by the cubic law [eqn. (2), n = 3], with E = 233 kJ mole-1, attributed to the initial formation of a constant number of lead nuclei (i.e. instantaneous nucleation) followed by three-dimensional growth (P = 0, X = 3). Deviations from strict obedience to the power law (n = 3) are attributed to an increase in the effective number of nuclei with reaction temperature, so that the magnitude of E for the interface process was 209 kJ mole-1. 

The process associated with the second equation is favoured by the use of an high aniline concentration. The occurrence of a process that can be described as an aniline oxidative carbonylation using nitrobenzene as an oxidant closely parallels many reactions described in paragraph 3.2.1. and the Pd(OAc)2/DPPP/AcOH-catalysed synthesis of carbamates [125] described in paragraph 3.2.2. The mechanistic implications of these competing stoichiometries are discussed in detail in Chapter 6. 

The implication of the foregoing equations, that stress-corrosion cracking will occur if a mechanism exists for concentrating the electrochemical energy release rate at the crack tip or if the environment in some way serves to embrittle the metal, is a convenient introduction to a consideration of the mechanistic models of stress corrosion. In so far as the occurrence of stress corrosion in a susceptible material requires the conjoint action of a tensile stress and a dissolution process, it follows that the boundary conditions within which stress corrosion occurs will be those defined by failure... 

In alkaline solutions the exchange reaction of equation (3) occurs in hydroxide-independent and hydroxide-dependent paths.54 Rate constants, activation parameters and isotope effects have been reported.54 Although proton transfer is implicated in the rate-determining step of the OH--independent path and an association involving M0O2- and OH- is likely for the OH--dependent pathway, no firm mechanistic conclusions can be drawn. 

Finally a kinetic study of the oxidation of ethylene by palladium (II) acetate gave a rate-[NaOAc] profile similar to Figure 1 which could also be interpreted as conversion of less reactive trimer to more reactive dimer. However at [NaOAc] > 0.2M the decrease in rate with increase in [NaOAc] is much greater than that shown in Figure 1 and corresponds to a 1/[NaOAc] term in the rate expression for reaction of dimer. This difference in rate expression between exchange and olefin oxidation could have very interesting mechanistic implications. For instance, the added acetate inhibition term could result from the need for a vacant coordination site on the Pd (II) before hydride elimination can occur. The scheme is shown in Equations 31 and 32. 

The chemical significance of Eqs. 4.18, 4.26, or 4.27 depends entirely on whether the several mechanistic assumptions (uncoupled reactions, gamma distribution of rate coefficients with the same coefficient of variation, etc.) that were used to derive them can be verified independently by molecular-scale experiments. Unless this kind of substantiation is possible, the adherence of data on adsorption-desorption reactions to these equations has no unique mechanistic implication.2... 

It has been applied mostly to cation-radicals, and some examples are shown in equations (77)-(80). It is stressed that the scheme is only a pictorial representation without mechanistic implications in the absence of a known ion structure. As with other rationalizations, the value of the scheme must rest with its ability to indicate how similar molecules might fragment. One application of the scheme involved a correction of an... 

Vannice s CO partial pressures were 0.08-0.24 atm, and his temperatures were 240°-280°C. Equation 8 gives the observed values for X (1.04) and Y ( —0.20) if b = 2 atm That value of b cannot be directly transferred to the data presented here because of the difference in average reaction temperatures in the two sets of experiments. It does, however, make plausible the implication of the power law coeflBcients reported in Table I that the surface is very nearly saturated with CO (i.e., bFoo >> 1) at 300°C when the CO partial pressure is 50 atm. As shown in Equation 7, Y should approach —X as the surface approaches CO saturation. A mechanistic basis is not obvious for the implied difference in RDS in the two sets of experiments, but this difference is probably not surprising in view of the great differences in experimental conditions. 

The extent of rearrangement of the carbon-14 label in products with both retained [( + )-63ab] and inverted [( —)-63ab) configuration from the thermal decomposition of ( —)-lll was somewhat greater than observed for the deamination of (+)- or (— )-64a in either aqueous solution or in glacial acetic acid [see run 4, Table 4], The data were nicely reproducible, and seemed to indicate that the two reactions were mechanistically similar. The application of equations (19)-(22) to these data in order to calculate and kjk, and the fm ther recalculation of m-values from the ratios of the rate constants so obtained have been discussed earlier, and the results have already been presented in Table 5. The implications of these results (run no. 4, Table 5) clearly are that the deamination of 64 and the thermal decomposition, in acetic acid, of 111 both proceed through similar, open carbonium ion intermediates shown in Chart VI. 

Attempted peroxy acid epoxidation of the bicyclic ketone (31 equation 13) gave the lactone (33), instead of several possible rational alternatives. The epoxide (32) was implicated as an intermediate when it was independently synthesized from the epoxy alcohol, and shown to give (33) on treatment with aqueous acid.- A mechanism involving scission of the acyl bridgehead bond via the hydrated 1,1 -diol form of the ketone was proposed to account for the formation of this unexpected product. The rearrangement of the isolongifolene derivative (34 equation 14) appears to be mechanistically related. The product (35) is formed by brief treatment with dilute HCIO4 in dioxane as a mixture of isomers believed to arise by acid-catalyzed epimerization of the carbinol center. ... 

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