Reaction rate estimation intramolecular reactions

For these more efficient reactions the rate of the intermolecular reaction (acetate-catalysed detritiation) is too slow relative to that of the intramolecular reaction to be measured accurately. These EM s are therefore based on estimated upper limits for the rates of the reference reactions Harper and Bender, 1965. The reference intermolecular reaction is the benzoate-catalysed enolization of PhCOCHMe2... 

While a carboxylate anion is a potent nucleophile in an intramolecular reaction it is not powerful enough to displace methoxide or an unprotonated amine, unless the nucleophile and the carbonyl group are held even more rigidly than in the phthalate system. Kirby and Lancaster (referred to by Kirby and Fersht, 1971) have found that such displacement can occur in disubstituted maleate monoesters and amides. The estimated rate constant for cyclization of N-methyl dime thy Imaleamic acid [equation (41)] is 16,000 times greater than that for the unsubstituted compound. Below pH 5-6 hydrolysis of the cyclic anhydride becomes rate-determining. 

Indirect evidence concerning intramolecular electron transfer has also been obtained by the observation of low-energy charge transfer absorption bands in mixed-valence complexes (reaction 8)14 even for outer-sphere electron transfer within ion pairs (reaction 9).15 The theoretical work of Hush makes it possible to use the energies and integrated intensities of such bands to estimate rates of intramolecular electron transfer.16... 

Bruice and his colleagues (Bruice and Lightstone, 1998 Bruice and Benkovic, 2000) introduced the term near attack conformation (NAC) to define the requirement of conformation for juxtaposed reactants to enter the transition state. The greater the mole fraction of reactant NAC conformation in the pretransition state, the greater the reaction rate constant. It was demonstrated that in intramolecular enzyme catalysis, changes of the enthalpy activation AH as compare with chemical analoges, essentially predominate over entropic contribution, which was estimated to be TAS = - 4-6 kcal mole. 

Recently Jaworski and coworkers [303] have analyzed mixed solvent influence on surface intramolecular reactions. The authors tried to explain the change in the rate constant with the H2O-DMF mixed solvent composition in terms of the change in the estimated longitudinal relaxation time. A linear correlation was found between k for various H2O-DMF compositions and Tl . 

Section VI can be applied from an estimate of the volume of the confinement region. However, in some cases of interest this limit is not applicable, and a more careful treatment is required. This is done in Section VIIC, in which reaction in a hemispherical pocket is considered. Another type of confinement occurs in the case of intramolecular reactions of sites on a polymer chain. The maximum separation is dictated by the polymer, and the encounter dynamics can be strongly influenced by the relaxation rates of the polymer configurations. This is discussed in Section VIID. In both of these examples the problem is reduced from three dimensions to one using the effective potential. 

The two effects are independent. The entropy change from dehydration, and from other changes involving the solvent, is included in the observed association constant and binding free energy. The important loss of translational and rotational entropy refers to the loss of the quantities needed to reach the transition state. The observed entropy of activation includes both of these contributions, which are not easily distinguishable. The contribution of translational and rotational entropy is estimated by comparison of the rate constants for bimolecular and intramolecular reactions. 

Benzophenone was used as photosensitizer in a study of the photo-decomposition of benzene solutions of polystyrene and poly(a-methyl styrene). Laser photolysis (A 265 nm) of poIy(a-methyl styrene) in chlorofonn and carbon tetrachloride proceeded by attack of solvent radicals in the polymer molecules resulting in main chain cleavage. " A series of papers have been devoted to the study of the photo-induced decomposition of poly(a-methyl styrene) in benzene solutions, using a,a -bisisobutyronitrile, benzophenone, and triphenyl-methyl cations as sensitizers. Spin-trapping techniques were used to detect transient radicals formed during benzophenone-sensitized degradation of poly-(a-methyl styrene). The rate constant for intramolecular reaction of the macroradical was estimated. 

Xylene isomerization is a test reaction which is claimed to require moderately strong Bronsted acid sites to proceed. One reason for this is the very good stabilization of the formed carbenium ions over the benzene ring. The reaction proceeds via a benzenium ion and the rate-Umiting step in this reaction is the intramolecular methyl transfer. Besides the monomolecular mechanism, xylene isomerization can also proceed via a bimolecular reaction pathway as outlined by Morin et al. [ 172]. They determined the contributions of both pathways and determined the contribution of the monomolecular reaction, which they propose to compare activity and acidity in zeolites. These findings emphasize that the reaction pathway should be known in order to properly estimate acidity. Especially for large pore zeolites, this may be a problem. 

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