Mechanistic criteria

Grunwald has shown applications of Eqs. (5-78) and (5-79) as tests of the theory and as mechanistic criteria. One way to do this, for a reaction series, is to estimate AG° and AG from thermodynamic data and from reasonable approximations and then to fit experimental rate data (AG values) to Eq. (5-78) by nonlinear regression. This yields estimates of AGq and AG (which are constants within the reaction series), and these are then used in Eq. (5-79) to obtain the transition state coordinates. 

This account of the kinetics of reactions between (inorganic) solids commences with a consideration of the reactant mixture (Sect. 1), since composition, particle sizes, method of mixing and other pretreatments exert important influences on rate characteristics. Some comments on experimental methods are included here. Section 2 is concerned with reaction mechanisms formulated to account for observed behaviour, including references to rate processes which involve diffusion across a barrier layer. This section also includes a consideration of the application of mechanistic criteria to the classification of the kinetic characteristics of solid-solid reactions. Section 3 surveys rate processes identified as the decomposition of a solid catalyzed by a solid. Section 4 reviews other types of solid + solid reactions, which may be conveniently subdivided further into the classes... 

Since electrophilic and charge-transfer nitrations are both initiated via the same EDA complex and finally lead to the same array of nitration products, we infer that they share the intermediate stages in common. The strength of this inference rests on the variety of aromatic substrates (with widely differing reactivities and distinctive products) to establish the mechanistic criteria by which the identity of the two pathways are exhaustively tested. On this basis, electrophilic nitration is operationally equivalent to charge-transfer nitration in which electron-transfer activation is the obligatory first step. The extent to which the reactive triad in (90) is subject to intermolecu-lar interactions in the first interval (a few picoseconds) following electron transfer will, it is hoped, further define the mechanistic nuances of dissociative electron transfer in adiabatic and vertical systems (Shaik, 1991 Andrieux et al., 1992), especially when inner-sphere pathways are considered (Kochi, 1992). 

Since the hydrolysis of methyl mesitoate conforms with the Zucker-Hammett hypothesis, it is not unexpected that the parameters calculated from the same data should meet Bunnett s more recent mechanistic criteria The value of Bunnett s w for the reaction in sulphuric acid is calculated as —1.1. and in perchloric acid as — 2.55W, both values falling in the region (w belween—2.5 and 0) characteristic of reactions not involving a molecule of solvent in the transition state. Bunnett s is also negative, as expected, values of —0.25 and —0.425 being found for the same two sets of data at 90°C4 . 

In summary, therefore, the detailed mechanism of the hydrolysis of carboxylic anhydrides is still in doubt and we must hope for further experimental evidence to clarify the position. As for the hydrolysis of the other carboxylic acid derivatives dealt with in this chapter, none of the mechanistic criteria, that have been used to interpret the kinetic data, gives an unambiguous interpretation, resulting in a situation where details of mechanism are open to argument. This is particularly the case for solvolysis reactions where uncertainty as to the structure and effect of the solvent preclude a firm assignment of transition state structures. This is not to say that the mechanisms are not... 

In Section VIII we apply the same model in greater detail to Norrish II reactions of ketones. By doing so, a set of similar mechanistic criteria can be viewed in many constraining environments, allowing a more systematic picture of the model to be drawn. 

In summary, rate-acidity function correlations in basic systems appear to be quite informative when the acidity function used is based on an indicator system closely resembling the reactant under study. At present this requirement is only rarely met, and consequently the acidity function technique is best used in conjunction with other mechanistic criteria. 

The above mechanistic criteria almost invariably allow a firm conclusion to be made concerning the species reacting in heteroaromatic nitration. Model compound studies appear to provide the best evidence, and the rate profile approach can be very successful, whereas activation parameters and the encounter rate criterion give, by themselves, less useful information. However, use of a single criterion can often give equivocal evidence, and a firm conclusion frequently requires several criteria used together. 

Aromaticity is a term which requires careful definition in terms of thermodynamic, kinetic, and mechanistic criteria. For the purpose of this review it is defined as an additional stabilization of a molecule specifically associated with delocalization of tt electrons contained in a closed molecular orbital shell. The determination of this specific stabilization or resonance energy may be carried out experimentally or theoretically. Table XVIII contains a number of experimentally accessible parameters and their theoretical counterparts which have all been used to arrive at its value. 

This cautionary attitude toward the stand d enumeration of mechanistic criteria seems fully justified when one reflects on the sorts of evidence advanced in support of 1,4-tetramethylene zwitterionic intermediates in thermal [2 + 2] cycloadditions. 

Integration of the various contributions into mechanistic criteria for the formation of TBP-DNA complexes... 

In discussing the mechanisms of additions and/or substitutions, the sign and magnitude of the CICs are utilized as mechanistic criteria, which have been developed mostly by Lee and coworkers and reviewed in several journals15. 

The tabulated reaction times are those required to cause a 90% or more decrease in CsN infrared absorbance at a reaction temperature of 165°C and demonstrate the marked catalytic effect of each additive. The powerful base, benzyl trimethylammonium hydroxide was employed as the solvent free solid and is by far the most powerful catalyst, however, it gave no detectable quantity (less than one part per million) of phthalocyanine. This is consistent with the proposed mechanistic criteria and dramatically demonstrates that in the absence of a viable redox pathway, reactions other than phthalocyanine formation can occur very rapidly. All other entries gave substantial amounts of phthalocyanine hydroquinone being superior in this regard. 

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