Solvent effects kinetic

FOTi M and RUBERTO G (2001) Kinetic solvent effects on phenolic antioxidants determined by spectrophosometric measiuements, JAgric Food Chem, 49, 342-8. 

The kinetic solvent effect of the liquid-liquid interface is very sensitive to the location where the adsorbed molecules react [6]. The interfacial solvent effect has to be studied more extensively. 

The quantitative treatment of micellar rate effects upon spontaneous reactions is simple in that the overall effect can be accounted for in terms of distribution of the substrate between water and the micelles and the first-order rate constants in each pseudophase (Scheme 2). The micelles behave as a submicroscopic solvent and to a large extent their effects can be related to known kinetic solvent effects upon spontaneous reactions. It will be convenient first to consider unimolecular reactions and to relate micellar effects to mechanism. 

The effect of micelles on these spontaneous hydrolyses is difficult to explain in terms of kinetic solvent effects on these reactions. Mukerjee and his coworkers have refined earlier methods for estimating apparent dielectric constants or effective polarities at micellar surfaces. For cationic and zwitterionic betaine sulfonate micelles Def is lower by ca 15 from the value in anionic dodecyl sulfate micelles (Ramachandran et al., 1982). We do not know whether there is a direct connection between these differences in effective dielectric constant and the relation between reaction rates and micellar charge, but the possibility is intriguing. 

It is easy to understand the lower reactivity of non-ionic nucleophiles in micelles as compared with water. Micelles have a lower polarity than water and reactions of non-ionic nucleophiles are typically inhibited by solvents of low polarity. Thus, micelles behave as a submicroscopic solvent which has less ability than water, or a polar organic solvent, to interact with a polar transition state. Micellar medium effects on reaction rate, like kinetic solvent effects, depend on differences in free energy between initial and transition states, and a favorable distribution of reactants from water into a micellar pseudophase means that reactants have a lower free energy in micelles than in water. This factor, of itself, will inhibit reaction, but it may be offset by favorable interactions with the transition state and, for bimolecular reactions, by the concentration of reactants into the small volume of the micellar pseudophase. 

Quite generally, kinetic solvent effects on the Diels-Alder reaction are small, and, in fact, the small solvent effects have been taken as evidence for minor charge separation during the activation process, consistent with a concerted mechanism. 

Remarkable kinetic solvent effects reportedly occur in the reaction between 138 and acrylonitrile 2 the observed large differences in the activation parameters between chloroform (A// = 5.4 kcal moP AS = — 50.3 eu) and carbon tetrachloride (A// = 22.4 kcal mol AS = + 10.3 eu) have been criticized. ... 

Base-catalysed ring fission of 3,4-diphenylcyclobut-3-ene-l,2-diones (103) in 50% (v/v) aqueous DMSO proceeds by rapid reversible addition of hydroxide ion followed by rate-determining benzilic acid-type rearrangement to form an intermediate 1-hydroxycyclopropane-1-carboxylic acid which ring opens to the corresponding (Z)-2-oxo-3,4-diphenylbut-3-enoic acid (Scheme 8).173 This is supported by the value of Hammett p = 1.3 (for variation of substituents on one or both rings), the kinetic solvent effects, and the three-oxygen enrichment of (107) from reaction of (103) in 50% H2 180-DMSO. 

Valgimigli L, Banks JT, Ingold Kl, Lusztyk J (1995) Kinetic solvent effects on hydroxylic hydrogen atom abstractions are independent of the nature of the abstracting radical. Two extreme tests using Vitamin Eand phenol. J Am Chem Soc 117 9966-9971 Viehe HG, Janousek Z, Merenyi R, Stella L (1985) The captodative effect. Acc Chem Res 18 148-154 Volkert 0, Schulte-Frohlinde D (1968) Mechanism of homolytic aromatic hydroxylation III. Tetrahedron Lett 2151-2154... 

Nucleophilic vinylic substitutions of 4//-pyran-4-onc and 2,6-dimethyl-4//-pyran-4-one with a hydroxide ion in aqueous solution were calculated by the density functional theory (B3LYP) and ab initio (MP2) methods using the 6-31+G(d) and 6-31G (d) basis sets. The aqueous solution was modelled by a supermolecular approach, where 11 water molecules were involved in the reaction system. The calculations confirmed a different addition-elimination mechanism of the reaction compared with that in the gas phase or non-polar solution. Addition of OH- at the C(2) vinylic carbon of the pyranone ring with an activation barrier of 10-11 kcalmol-1 (B3LYP) has been identified as the rate-determining step, in good quantitative and qualitative agreement with experimental kinetics. Solvent effects increase the activation barrier of the addition step and, conversely, decrease the barrier of the elimination step.138... 

Labels Stereochemistry and Chirality Kinetics Solvent Effects Substituent Effects Primary Deuterium Isotope Effects Barrier Data... 

These authors demonstrated that a kinetic solvent effect (KSE) was observed in other words, that the solvent affects the reaction rate between attacking radical and antioxidant. 

Kinetic solvent effect (KSE), on antioxidant activity 877-881 Knoevenagel condensation 1405 Kohn-Sham method 68 Koilands 1427 Koilates 1427... 

To complete this concise picture, I should mention that spontaneous monomolecular and water-catalyzed reactions are also affected by the presence of aggregates. The aggregates behave as a submicroscopic solvent, and to a large extent their effects can be explained on the basis of the known kinetic solvent effects in these reactions. 

D. W. Snelgrove, J. Lusztyk, J. T. Banks, P. Mulder, K. U. Ingold, Kinetic solvent effects on hydrogen-atom abstractions Reliable, quantitative predictions via a single empirical equation, J. Am. Chem. Soc. 123 (2001) 469. 

The observation that H addition to benzene is frster in aqueous solution than in the gas phase by a factor of ca. 35 [28], as shown in Figure 8, triggered a series of experimental and theoretical studies on the nature of hydrogen atoms in water. The kinetic solvent effect was explained by H atom equilibrium solvation. Atomic hydrogen in water behaves like a noble gas atom of the same polarizability, its solvation is accompanied not only by a solvation enthalpy of -4.1 kJ mol but in particular by a strong decrease in entropy of 51 J moF K at 298 K, which is indicative of the formation of a solvent cage around a hydrophobic atom [28], Since this is equilibrium solvation it should be nearly independent of the mass of the solvated atom. This was confirmed in recent theoretical work [33, 34],... 

The most important step in the interfacial catalysis in complex formation is the adsorption of extractant, which increases the interfacial concentration thus, the interfacial complexation and extraction rate are accelerated. The kinetic solvent effect of the liquid-liquid interface is very sensitive to the location where the ligand molecule is adsorbing. The interfacial solvent effect in the nanometer region has to be studied more extensively. 

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