1.2- Elimination reactions, characteristics kinetics

The chromophoric pyridoxal phosphate coenzyme provides a useful spectrophotometric probe of catalytic events and of conformational changes that occur at the pyridoxal phosphate site of the P subunit and of the aiPi complex. Tryptophan synthase belongs to a class of pyridoxal phosphate enzymes that catalyze /3-replacement and / -elimination reactions.3 The reactions proceed through a series of pyridoxal phosphate-substrate intermediates (Fig. 7.6) that have characteristic spectral properties. Steady-state and rapid kinetic studies of the P subunit and of the aiPi complex in solution have demonstrated the formation and disappearance of these intermediates.73-90 Fig. 7.7 illustrates the use of rapid-scanning stopped-flow UV-visible spectroscopy to investigate the effects of single amino acid substitutions in the a subunit on the rate of reactions of L-serine at the active site of the P subunit.89 Formation of enzyme-substrate intermediates has also been observed with the 012P2 complex in the crystalline state.91 ... 

The discussion of elimination reactions considers the classical E2, El, and Elcb eliminations that involve removal of a hydrogen and a leaving group. We focus on the kinetic and stereochemical characteristics of elimination reactions as key indicators of the reaction mechanism and examine how substituents influence the mechanism and product composition of the reactions, paying particular attention to the nature of transition structures in order to discern how substituent effects influence reactivity. We also briefly consider reactions involving trisubstituted silyl or stannyl groups. Thermal and concerted eliminations are discussed elsewhere. 

The next logical step was the extension of this chemistry to SH derivatives. Attempts to oxidatively fluorinate CF3SSH with monofluoroxenonium hexafluorometalates failed, even at 195 K. Only decomposition into the already mentioned Sg + salts could be observed (24). On the other hand, the synthesis of the fluoro(trifluoromethyl)sulfonium- and fluoro(methyl)sulfonium cations from the SH acidic sulfanes CF3SH and CH3SH proceeded without difficulty at 213 K (Figure 1) (25). The elimination of HF at 213 K is expected to be preferred from a thermodynamic point of view, although HF elimination is probably kinetically inhibited in this case. However, at 233 K decomposition of all of these compoimds into the characteristic blue Sg2+ salts is observed. By reaction with chlorine... 

It may not be appropriate to compare the thermal stability characteristics of VC/VAc copolymer to that of a VC homopolymer (PVC). The copolymerization would involve different kinetics and mechanism as compared to homopolymerization resulting structurally in quite different polymers. Hence, copolymerization of VC with VAc cannot be regarded as a substitution of chlorines in PVC by acetate groups. To eliminate the possibility of these differences Naqvi [45] substituted chlorines in PVC by acetate groups, using crown ethers (18-crown-6) to solubilize potassium acetate in organic solvents, and studied the thermal stability of the modified PVC. Following is the mechanism of the substitution reaction ... 

Dehydration reactions are typically both endothermic and reversible. Reported kinetic characteristics for water release show various a—time relationships and rate control has been ascribed to either interface reactions or to diffusion processes. Where water elimination occurs at an interface, this may be characterized by (i) rapid, and perhaps complete, initial nucleation on some or all surfaces [212,213], followed by advance of the coherent interface thus generated, (ii) nucleation at specific surface sites [208], perhaps maintained during reaction [426], followed by growth or (iii) (exceptionally) water elimination at existing crystal surfaces without growth [62]. 

In this section, methods are described for obtaining a quantitative mathematical representation of the entire reaction-rate surface. In many cases these models will be entirely empirical, bearing no direct relationship to the underlying physical phenomena generating the data. An excellent empirical representation of the data will be obtained, however, since the data are statistically sound. In other cases, these empirical models will describe the characteristic shape of the kinetic surface and thus will provide suggestions about the nature of the reaction mechanism. For example, the empirical model may require a given reaction order or a maximum in the rate surface, each of which can eliminate broad classes of reaction mechanisms. 

Platinum(IV) is kinetically inert, but substitution reactions are observed. Deceptively simple substitution reactions such as that in equation (554) do not proceed by a simple SN1 or 5 2 process. In almost all cases the reaction mechanism involves redox steps. The platinum(II)-catalyzed substitution of platinum(IV) is the common kind of redox reaction which leads to formal nucleophilic substitution of platinum(IV) complexes. In such cases substitution results from an atom-transfer redox reaction between the platinum(IV) complex and a five-coordinate adduct of the platinum(II) compound (Scheme 22). The platinum(II) complex can be added to the solution, or it may be present as an impurity, possibly being formed by a reductive elimination step. These reactions show characteristic third-order kinetics, first order each in the platinum(IV) complex, the entering ligand Y, and the platinum(II) complex. The pathway is catalytic in PtnL4, but a consequence of such a mechanism is the transfer of platinum between the catalyst and the substrate. 10 This premise has been verified using a 195Pt tracer.2011... 

The product of a elimination is a neutral species that resembles a carbocation in having only six carbon valence electrons. The simplest carbene is CH2, methylene. Carbenes are highly reactive, so much so that they cannot be isolated. Their involvement in reactions usually has to be inferred from the nature of the products or the reaction kinetics. The characteristic carbene reactions involve forming an electron-pair bond to the carbene carbon by reacting with cr bonds, it bonds, or unshared pairs ( ), Some of these reactions are illustrated here for methylene ( CH2). ... 

Important characteristics of zero-order reactions are that (1) a constant amount of drug is eliminated per unit time since the system is saturated (maximized) and (2) the half-life is not constant for zero-order reactions but depends on the concentration. The higher the concentration, the longer the half-life. Therefore, the term zero-order half-life has little practical significance since it can change and (3) zero-order kinetics is also known as nonlinear or dose-dependent. For example, if the body can metabolize ethanol at a rate of 10 ml per hour, then if one consumes 60 ml, it will take 3 hours to metabolize half of it (the half-life under these circumstances). However, if 80 ml is consumed the half-life will now become 4 hours. This is particularly significant regarding ethanol toxicity. 

In this context, Berry [277] studied the enzyme reaction using Monte Carlo simulations in 2-dimensional lattices with varying obstacle densities as models of biological membranes. That author found that the fractal characteristics of the kinetics are increasingly pronounced as obstacle density and initial concentration increase. In addition, the rate constant controlling the rate of the complex formation was found to be, in essence, a time-dependent coefficient since segregation effects arise due to the fractal structure of the reaction medium. In a similar vein, Fuite et al. [278] proposed that the fractal structure of the liver with attendant kinetic properties of drug elimination can explain the unusual... 

This review illustrates the above delineated characteristics of electron-transfer activated reactions by analyzing some representative thermal and photoinduced organometallic reactions. Kinetic studies of thermal reactions, time-resolved spectroscopic studies of photoinduced reactions, and free-energy correlations are presented to underscore the unifying role of ion-radical intermediates [29] in—at first glance—unrelated reactions such as additions, insertions, eliminations, redox reactions, etc. (Photoinduced electron-transfer reactions of metal porphyrin and polypyridine complexes are not included here since they are reviewed separately in Chapters 2.2.16 and 2.2.17, respectively.)... 

Characteristic features of this mechanism are that (i) the rate of the reaction does not depend on the concentration of the base and the kinetics are first order (in substrate) (ii) the reaction may not be stereospecific (iii) the elimination/substitution ratio is mostly independent of the leaving group (but in solvents of low ionization energy ion pairs are formed and then the ratio depends upon the leaving group) (iv) by-products are formed via rearrangements (v) the reaction is reversible (vi) generally the most stable alkene is formed (Zaitsev orientation see Section 5.1.2.5)... 

The characteristics of this mechanism are that (i) the attacking base and the substrate both take part in the rate-determining step, which has second-order kinetics overall (first-order in base and first-order in substrate) (ii) a large primary isotope effect is usually observed (iii) since the mechanisms of 5n2 and E2 differ much more than those of 5n1 and 1 reactions, the substitution/elimination ratio can be controlled in most cases by choosing appropriate conditions (iv) no rearrangement reactions are observed (v) the rate of elimination depends upon the strength of the base (vi) the stereospecificity of an E2 reaction depends on the conditions (see Section 5.1.2.4). 

Although about a quarter of these papers discussed or reported aspects of dehydration (33 entries), few described the use of reaction conditions known to eliminate the influence of the reverse process on tiie measured kinetic characteristics or attempted to establish detailed mechanisms. Almost every dehydration study in the set was concerned with a different reactant and the set included various... 

Autocatalysis. The kinetic curves developed by chemists in the U.S.S.R. (Section II,A,l,e) showing the dependence of the rate of gas evolution on time during the Chichibabin reaction revealed an interesting characteristic. It was observed that gas evolution began at a slow rate, followed by a sharp increase. This behavior was interpreted as evidence for the gradual accumulation of some compound in the reaction mixture during the induction period, which later catalyzed the amination process. The compound responsible was assumed to be simply the sodium salt of the aminoheterocyclic product. Indeed, introduction of such a sodium salt prior to the start of amination resulted in a rapid reaction with no observable induction period. The catalysis was theorized to result from a six-membered transition complex (17), which provides the required orientation of proton and hydride ion acceptors for hydrogen elimination. Proton abstraction should take place first, which then positions the transition complex structurally close to the dianionic (T-adduct (II). 

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