A Kinetic Equivalency

Do not take these scenarios as the correct mechanisms for these hydration reactions, but consider them, instead, as simply possibilities highlighting our discussion. Equilibria prior to slow steps are assumed to be fast. 

4 Concerted or Sequential General-Acid-General-Base Catalysis 

A bell-shaped pH vs. rate constant profile is found when general-acid-and general-base-catalyzed processes are both involved in a mechanism prior to or at the rate-determining step. 

Using the terminology defined in Chapter 8 to discuss linear free energy relationships, the pKa values are the substituent constants and the a and P values are the sensitivity or reaction constants. These parameters are analogous to the crand p values, respectively, for Hammett plots. All substituent constants are defined by some reference reaction. For example, the (rvalues of Hammett plots are defined by the ionization of various benzoic acids relative to benzoic acid itself. In the Bronsted analysis, the substituent constants are defined by the abi 1 i ty of each acid to protonate water (Eq. 5.5). 

Because HAi is the reference acid to which all evaluated general-acid catalysts are compared, log fci and log Ri act as constants, leading to the Bronsted law (Eq. 9.36). 

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Belke et al. (1971) reported general base and general acid catalysis in cyclization of 2-hydroxymethylbenzamide [equation (18)]. However, with 2-hydroxymethyl-6-aminobenzamide strict general base catalysis by buffer bases is observed with a Bronsted coefficient of O 39 (Fife and Benjamin, unpublished data). In contrast with the unsubstituted amide, the Bronsted plot is nicely linear. An amino-group in the 6-position might assist decomposition of a tetrahedral intermediate as in [37a, b] or a kinetic equivalent. The pH-rate constant profile for spontaneous cyclization at zero buffer concentra-... 

Note that each of the reactions presented in Table 1 may consist of true elemental reactions or of a kinetically equivalent series of elemental steps. 

It cannot be totally excluded that extrinsic species generate also primary radicals (e g. decomposition of structural irregularities or direct oxygen-polymer reaction). But it can be easily demonstrated that their contribution to initiation is very limited the corresponding initiation rate is initially very low and vanishes rapidly (as soon as the extrinsic species concentration vanishes). As a result, in all cases, initiation by Per decomposition rapidly becomes the main source of radicals. From a kinetic modelling point of view, the following approach is usually adopted the initially present extrinsic species are replaced by a kinetically equivalent initial Per concentration [PerJo. Thus, initiation involves two important reactions ... 

Zinc(II) and tris-buffers are effective catalysts for the aminolysis of benzylpenicillin. It is suggested that this is due to formation of a ternary complex in which the metal ion binds both penicillin and tris. Nucleophilic attack of the ionised hydroxyl on bound tris forms a penicilloyl ester which may then react with tris to form a penicilloyl amide (Schwartz, 1982 Tomida and Schwartz, 1983). A kinetically equivalent mechanism, however, would simply involve nucleophilic attack of tris on the zinc-penicillin complex. 

Some of those stable products have a negative influence on the aspect properties (yellowing) [36], Some others may have a further stabilizing role [37]. The chemistry of phenols stabilization is actually complex and has hence aroused a considerable amount of literature (see for example [38, 39, 40]). However, we tentatively showed in recent reviews [41, 42] that this mechanistic complexity can well be represented by a kinetically equivalent scheme ... 

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