Solvent participation, covalent

Solvent coordination number, 134, 403 Solvent effects, 385, 418 initial and transition state, 418 kinetic measures of, 427 Solvent ionizing power parameter, 430 Solvent isotope effects, 272, 300 Solvent nucleophilicity, 431 Solvent participation, covalent, 429 Solvent polarity, 399, 425 Solvent polarity parameter, 436 Solvent properties, 389 Solvent-separated complex, 152 Solvent sorting, 404 Solvent structure, 402 Solvophobic interaction, 395 Solvophobicity parameter, 427 Sound absorption chemical, 145 classical, 145... 

Similar changes in nucleophilic (or dipole) solvation (Scheme 2.8A) provide a simple explanation for the observation of other correlations between rate constants for solvolysis and solvent nucleophilicity. This interpretation does not require that there be stabilization of the transition state for solvolysis of tertiary derivatives by a partial covalent interaction between nucleophile and electrophile. We have defined this latter interaction as nucleophilic solvent participation (Scheme 2.8B) and have argued that the results of simple and direct experiments to detect stabilization of the transition state for reaction of simple tertiary derivatives by... 

In summary, controversy concerning the mechanism for solvolysis at tertiary carbon is semantic and can be avoided by making a clear distinction between (a) nucleophilic solvation, which is stabilization of the transition state for stepwise solvolysis through carbocation or ion pair intermediates by charge-dipole interactions with nucleophilic solvents (Scheme 2.8A) and, (b) nucleophilic solvent participation, which is stabihzation of the transition state for concerted solvolysis by formation of a partial covalent bond to the solvent nucleophile (Scheme 2.8B). 

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. 

Solvent can play a very significant role in the photoreactions of carbanions, affecting both the covalency of the carbon-metal bond (249) and ion pairing. Some solvents may, furthermore, be active participants, for example, in Tolbert s photo-methylation of arylmethyl anions, eq. 83 (250-253) ... 

Winstein et al. [45] first presented evidence for the concept that different types of electrophilic species, each with distinct reactivities, may participate in reactions involving cationic intermediates. As shown in Eq. (36), Winstein et al. proposed that four species are in equilibrium, including covalent electrophiles, contact ion pairs, solvent-separated ion pairs, and free ions. In addition, ion pairs may aggregate in more concentrated solutions- According to this concept, electrophilic species do not react with a continuous spectrum of charge separation, but rather in well-quantified minima in the potential energy diagram. 

Metal enolate solutions consist of molecular aggregates (6) such as dimers, trimers and tetramers in equilibrium with monomeric covalently bonded species (7), contact ion pairs (8) and solvent-separated ion pairs (9), as shown in Scheme 1. The nature of the metal cation, the solvent and, to a degree, the structure of the enolate anion itself may significantly influence the extent of association between the anion and the metal cation. In general, the factors that favor loose association, e.g. solvent-separated ion pairs, lead to an increase in the nucleophilicity of the enolate toward alkylating agents and also its ability to function as a base, i.e. to participate in proton transfer reactions. 

For the present heterogeneous system (Mn-support-GP-tacn + PO), the structure and the solvent effects in the catalytic experiments resemble most those of the latter, hexadentate complexes. For such hexadentate complexes, a temporary removal of one of the pendant arms is necessary to create a coordinative vacancy on the metal. The particular role of methanol might be to assist in the temporary deligation via hydrogen bond formation with 2-OH-alkyl groups. The system is unique in that the covalent fink to the surface can participate in the metal coordination via the 2-hydroxy group, as indicated by the arrow in Scheme 1. 

Counterions. 1. Sodium-23 Alkali metal MIR is a sensitive >robe of the immediate chemical environment and mobility of alkali metal ions in aqueous and nonaqueous solvents (7, 8). The chemical shifts of alkali metal nuclei will respond to" electronic changes only in the immediate environment of the cation since alkali metals rarely participate in covalent bonding (7). All alkali metal nuclei have spins greater than 1/2 and hence have quadrupole moments. The interaction of these moments with electric field gradients, produced by asymmetries in the electronic environment, is modulated by translation and rotational diffusive motions in the liquid. It is via this relaxation mechanism that the resonance line width is a sensitive probe of ionic mobility. 

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