Reaction mechanisms transition state hypothesis

The notion of concurrent SnI and Sn2 reactions has been invoked to account for kinetic observations in the presence of an added nucleophile and for heat capacities of activation,but the hypothesis is not strongly supported. Interpretations of borderline reactions in terms of one mechanism rather than two have been more widely accepted. Winstein et al. have proposed a classification of mechanisms according to the covalent participation by the solvent in the transition state of the rate-determining step. If such covalent interaction occurs, the reaction is assigned to the nucleophilic (N) class if covalent interaction is absent, the reaction is in the limiting (Lim) class. At their extremes these categories become equivalent to Sn and Sn , respectively, but the dividing line between Sn and Sn does not coincide with that between N and Lim. For example, a mass-law effect, which is evidence of an intermediate and therefore of the SnI mechanism, can be observed for some isopropyl compounds, but these appear to be in the N class in aqueous media. 

This study suggests a radically new explanation for the nature of Lewis acid activation in the Simmons-Smith cyclopropanation. The five-centered migration of the halide ion from the chloromethylzinc group to zinc chloride as shown in TS2 and TS4 has never been considered in the discussion of a mechanism for this reaction. It remains to be seen if some experimental support can be found for this unconventional hypothesis. The small energy differences between all these competing transition states demand caution in declaring any concrete conclusions. 

The Aq dependence of the oxidation in strongly acidic media was taken by Rodek as implying that no water molecule is involved in the transition state and that consequently the ester mechanism as portrayed above cannot hold. However, the Zucker-Hammett hypothesis upon which this argument is based, i.e. that a reaction forming a transition state containing a water molecule will follow a [H30 ] dependence, but that otherwise an dependence will be followed, may not be valid, and in any case the ester can be depicted as breaking down as fol-lows - ... 

The observation that the transition state volumes in many Diels-Alder reactions are product-like, has been regarded as an indication of a concerted mechanism. In order to test this hypothesis and to gain further insight into the often more complex mechanism of Diels-Alder reactions, the effect of pressure on competing [4 + 2] and [2 + 2] or [4 + 4] cycloadditions has been investigated. In competitive reactions the difference between the activation volumes, and hence the transition state volumes, is derived directly from the pressure dependence of the product ratio, [4 + 2]/[2 + 2]p = [4 + 2]/[2 + 2]p=i exp —< AF (p — 1)/RT. All [2 + 2] or [4 + 4] cycloadditions listed in Tables 3 and 4 doubtlessly occur in two steps via diradical intermediates and can therefore be used as internal standards of activation volumes expected for stepwise processes. Thus, a relatively simple measurement of the pressure dependence of the product ratio can give important information about the mechanism of Diels-Alder reactions. 

A good deal of this work had no impact in the development of models of molecular structure and the elucidation of reaction mechanisms one reason was Perrin s own coolness to quantum wave mechanics. 108 Another, according to Oxford s Harold Thompson, who studied with Nernst and Fritz Haber, was that researchers like Lecomte "did not know enough chemistry he was a physicist." 109 Perrin, too, approached physical chemistry as a physicist, not as a chemist. He had little real interest or knowledge of organic chemistry. But what made his radiation hypothesis attractive to many chemists was his concern with transition states and the search for a scheme of pathways defining chemical kinetics. 

The mechanism of action of lysozyme has been suggested to involve strain in the substrate as a result of binding to the protein.31 The protein requires that the substrate binds to the protein in a distorted state that is close to the transition state of the reaction. The energy for this distortion can arise from the substrate binding energy. This example differs from the entatic state hypothesis because this strain arises from substrate binding and is not a feature of the protein structure. However, the entatic state... 

Concerted mechanisms have been proposed on the basis of work carried out with soluble Movl, Wvl and TiIV peroxo compounds. The experimental evidence is consistent with the hypothesis that these compounds act as oxidants in stoichiometric epoxidations and that the reactions involve electrophilic attack of the peroxo compound on the organic molecule or, what is equivalent, a nucleophilic attack of the organic molecule on the peroxidic oxygen, in a butterfly transition state. The reaction product is formed and, after desorption, the peroxo compound is regenerated by reaction of TiIV with H202 this accounts for the catalytic nature of the reaction (Amato et al, 1986). The same type of mechanism... 

For reaction (42) in solvent dioxan, the evidence for mechanism SE2(open) rests on the interpretation of sequence (29), and for reaction (42) in solvents acetone and dimethylformamide there is no available evidence for or against mechanism SE2(open). It seems to the author that some substantial evidence is required in order to support the hypothesis that a bimolecular reaction between two neutral molecules in a solvent of as low a dielectric constant as dioxan (e = 2.1) proceeds through a transition state with a degree of charge separation, when an alternative pathway is available for the reaction to proceed through an SE2(cyclic) transition state in which little or no charge separation occurs. 

Retained chemistry, changed substrate specificity (binding) Nature selects protein from a pool of enzymes whose mechanism provide a partial reaction or stabilization strategy for intermediates or transition states. Evolution decreases the proficiency of the reaction catalyzed by the progenitor. The underlying hypothesis states that chemical mechanism dominance starts with a low level of promiscuous activity and that once evolved it is beneficial for nature to utilize it over and over again. 

The inside alkoxy effect is useful for predicting the stereoselectivity of nitrile oxide cycloaddition reactions with chiral lylic ethers. The hypothesis states that allylic ethers adopt the inside position and alkyl substituents prefer the sterically less-crowded anti conformation in transition states for these electrophilic cycloadditions . The terms inside and outside are defined in (17) for a hypothetical nitrile oxide cycloaddition transition state. Both ab initio (Gaussian 80 with 3-2IG basis set) and molecular mechanics calculations agree, each predicting the lowest-energy transition state to be the one described, i.e. (18 H outside) just above it lies one where the alkyl group is anti, OR outside and H inside (19 ). As illustrated, the former leads to a product wherein OR and the nitrile oxide oxygen are anti, the latter to one with them syn (Scheme 19). 

If the equilibrium hypothesis is assumed to be correct and classical mechanics applies to all degrees of freedom, the activated or transition state method for calculating the absolute rate of a chemical reaction with an activation energy would be rigorously valid.10 The extent of the limitations imposed by quantum mechanics has been considered by Wigner and others,12 13 with the conclusion that on the whole these limitations invalidate the method to a much smaller extent than could be presumed, and it is only in the considerations of the relative rates of reaction between... 

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