Elimination reaction, second-order

Treatment of the related [Pt2HMe(//-H)(//-dppm)2]+ cation with dppm, PMe2Ph, or PPh3 causes elimination of dihydrogen (but not methane) and formation of [Pt2MeL(/i-dppm)2]+. The complex where L = rjl-dppm has also been prepared by addition of methyl iodide to [Pt2(dppm)3]. The elimination follows second-order kinetics, and in the reaction with PPh3 the intermediate [ H( Ph 3 P)Pt( fi-H)( -dppm)2 PtMe]+ could be observed by NMR spectroscopy at low temperature (115) ... 

The reaction involves the nucleophilic attack of a peracid anion on the unionized peracid giving a tetrahedral diperoxy intermediate that then eliminates oxygen giving the parent acids. The observed rate of the reaction depends on the initial concentration of the peracid as expected in a second-order process. The reaction also depends on the stmcture of the peracid (specifically whether the peracid can micellize) (4). MiceUization increases the effective second-order concentration of the peracid because of the proximity of one peracid to another. This effect can be mitigated by the addition of an appropriate surfactant, which when incorporated into the peracid micelle, effectively dilutes the peracid, reducing the rate of decomposition (4,90). 

Among the evidence for the existence of the E2 mechanism are (1) the reaction displays the proper second-order kinetics (2) when the hydrogen is replaced by deuterium in second-order eliminations, there is an isotope effect of from 3 to 8, consistent with breaking of this bond in the rate-determining step. However, neither of these results alone could prove an E2 mechanism, since both are compatible with other mechanisms also (e.g., see ElcB p. 1308). The most compelling evidence for the E2 mechanism is found in stereochemical smdies. As will be illustrated in the examples below, the E2 mechanism is stereospecific the five atoms involved (including the base) in the transition state must be in one plane. There are two ways for this to happen. The H and X may be trans to one another (A) with a dihedral angle... 

However, the E2C mechanism has been criticized, and it has been contended that all the experimental results can be explained by the normal E2 mechanism. McLennan suggested that the transition state is that shown as 18. An ion-pair mechanism has also been proposed. Although the actual mechanisms involved may be a matter of controversy, there is no doubt that a class of elimination reactions exists that is characterized by second-order attack by weak bases. " These reactions also have the following general characteristics (1) they are favored by good leaving groups (2) they are favored by polar aprotic solvents (3) the reactivity order is tertiary > secondary > primary, the opposite of the normal E2 order (p. 1319) (4) the elimination is always anti (syn elimination is not found), but in cyclohexyl systems, a diequatorial anti elimination is about as favorable as a diaxial anti elimination (unlike the normal E2 reaction, p. 1302) (5) they follow Zaitsev s rule (see below), where this does not conflict with the requirement for anti elimination. 

This hypothesis has been criticized by Busvine (2,3), Domenjoz (10), Muller (18), and Cahn (4). Domenjoz and Muller have shown that there is no direct correlation between activity toward a variety of insects in a number of compounds of the type Ar2CHCCl3 and the amount of hydrogen chloride liberated under standard conditions. Busvine attempte( 1 a correlation for similar compounds between activity toward lice and bedbugs and this author s reaction-rate constants (2, 5) for second-order elimination with ethanolic alkali and found that no statistically significant correlation exists. 

In the course of work on the mechanism of elimination reactions, the author and his co-workers have measured reaction-rate constants for the second-order elimination of hydrogen chloride from six dichloroethyl compounds of type Ar2CHCHCl2 and three monochloroethyl compounds of type Ar2CHCH2Cl (7). Samples of each of these materials were furnished to the Bureau of Entomology and Plant Quarantine for insecticidal testing, and the author is indebted to C. C. Deonier and I. H. Gilbert for permission to use certain of their data in this paper. The rate constants and larvicidal results are given in Table I. 

Although equation 4.3.29 refers to a second-order reaction between an atom X and a molecule YZ, the theory is readily generalized to other reaction stoichiometries. An expression characterizing the equilibrium between reactants and the activated complex is used to eliminate the latter from equation 4.3.27, and the desired result is obtained. 

We conclude that the reaction is second-order, by the process of elimination. We confirm this conclusion by computing values of the second-order rate constant with the equation... 

Cristol, S. J. (1947) The kinetics of the alkaline dehydrochlorination of the benzene hexachloride isomers. The mechanisms of second-order elimination reactions. Journal of the American Chemical Society, 69, 338-342. 

In the remaining sections of this chapter we will discuss further examples of kinetic isotope effects. The first considers a system in which there is a competition between two mechanisms, Sn2 and E2 and returns to reaction 10.15. (By E2 we refer to a second order elimination reaction, see Fig. 10.6). In Equation 10.15 the hypochlorite... 

The refers to a nucleophilic substitution process where some nucleophile attacks an electrophile and substitutes for some part of the electrophile. The E refers to an elimination process where the nucleophile attacks an electrophile and causes the elimination of something. The 1 and 2 refer to the order of the reaction. A 1 (first order) means only one molecule determines the rate of the reaction, whereas a 2 (second order) means that a combination of two molecules determines the rate of the reaction. In many cases, two or more of these mechanisms are competing and more than one product may result. 

Although this early synthesis of the fluvastatin template (Scheme 12.1) was important for strucmre-activity smdies, a second synthesis (Scheme 12.2) was later developed to facilitate large-scale production of fluvastatin (Repic et al., 2001). As is typical in process development work, this second approach sought to shorten the reaction sequence, eliminate toxic reagents (such as tri-n-butylstannylvinylethoxide), and improve the yield and/or selectivity of reactions in order to reduce the amount of chromatography required to isolate the products. 

The biperoxy radical produced by the ceric ion oxidation of 2,5-di-methylhexane-2,5-dihydroperoxide decays rapidly with first-order kinetics [k = ioio.e exp( -11,500 1000)/RT sec.1 = 180 sec."1 at 30°C. (30)]. After the first-order decay has run to completion, there is a residual radical concentration (—4% of the initial hydroperoxide concentration) which decays much more slowly by a second-order process. The residual second-order reaction cannot be eliminated or changed even by repeated recrystallization of the dihydroperoxide. This suggests that a small fraction of the biperoxy radicals react intermolecularly rather than by an intramolecular process and thus produce monoperoxy radicals. The bimolecular decay constant for this residual species of peroxy radical is similar to that found for the structurally similar radical from 1,1,3,3-tetra-methylbutyl hydroperoxide. Photolysis of the dihydroperoxide gave radicals with second-order decay kinetics which are presumed to be 2,5-hydroperoxyhexyl-5-peroxy radicals. 

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