Intrinsic rate constants, solvent

The Brpnsted coefficient /3b = 0.52 for deprotonation of 3-phenylcoumaran-2-one (108) by a series of bases in 50% (v/v) water-dioxane, and q bh = 0.48 for reprotonation by the conjugate acid of the buffer, are indicative of a fairly symmetrical transition state for proton transfer, although the primary KE, ku/ku = 3.81, found for proton abstraction by HO is lower than expected. " The moderate intrinsic rate constant for deprotonation of (108) suggests that generation of the charge in the transition state is accompanied by only a small amount of molecular and solvent reorganization. In acidic solution, below pH 5, O-protonation of (110) occurs initially to form (109)... 

Solvation can have a large effect on intrinsic barriers or intrinsic rate constants, especially hydrogen bonding solvation of nitronate or enolate ions in hydroxylic solvents. Table 4 reports intrinsic rate constants in water and aqueous DMSO for a number of representative examples.19,20,23 25,40,54 56 Entries 1-4 which refer to nitroalkanes show large increases in ogka when... 

Table 4 Solvent effects on intrinsic rate constants for the deprotonation of C-H acids by secondary alicyclic amines and carboxylate ions at 20°C... 

More detailed analysis of the solvent effects on intrinsic rate constants which takes into consideration potential contributions from nonsynchronous solva-tion/desolvation of the carbon acid itself as well as of the proton acceptors and... 

Acidity constants and rates of reversible deprotonation of triphenylphosphonium ion (54), (55) and pyridinium ions (56), (57) by amines in water, 50 50 v/v DMSO-water, and 90 10 v/v DMSO-water have been determined.157 The intrinsic rate constants for proton transfer were relatively high for all four carbon acids and showed little solvent dependence. This is in contrast with nitroalkanes, which have much lower intrinsic rate constants and show a strong solvent dependence.158... 

Many reactions become possible only in such superbasic solutions, while others can be carried out under much milder conditions. Only some examples of preparative interest (which depend on the ionization of a C—H or N—H bond) will be mentioned here. The subsequent reaction of the resulting carbanion may involve electrophilic substitution, isomerization, elimination, or condensation [321, 322]. Systematic studies of solvent effects on intrinsic rate constants of proton-transfer reactions between carbon acids and carboxylate ions as well as amines as bases in various dimethyl sulfoxide/ water mixtures have been carried out by Bernasconi et al. [769]. 

In this model, rearrangment motions of solvent molecules are described by a onedimensional coordinate X, with energy described as V X). At each X value reaction takes place with an intrinsic rate constant k X) induced by the much faster intrasolute vibrational fluctuations. Then, the probability P X t) with which the reaction system is found at X is regarded as being governed not by the diffusion equation of Eq. 83, but by a diffusion-reaction equation ... 

In the Sumi-Marcus model of Figure 17, trajectories on the reactant surface are also important. Since molecular-arrangement fluctuations in solvents (the abscissa X) are assumed to be much slower than intrasolute-vibrational fluctuations (the ordinate q), a reactive trajectory should be the one shown in Figure 17 The reaction system is first taken to an X value by slow diffusive motions along the abscissa X, as represented by a zigzag line. Subsequently, when the reaction system is taken to line C by rapid vibrational motions on the ordinate q at this X value, electron tunneling takes place with an Z-dependent intrinsic rate constant k X), as represented by a vertical straight line. For TST to be justified in this model, the most probable X value should be around that of the saddle point S. 

Substitution of CO by phosphines 145 The Dotz reaction 149 Rearrangement reactions with loss of CO 151 Photochemical reactions 153 Reactions at the carbene carbon 158 General features 158 Amine nucleophiles 159 Phospine and phosphite nucleophiles 167 Alcohols and alkoxide ion nucleophiles 171 Thiol and thiolate ion nucleophiles 179 Intramolecular nucleophilic reactions 191 Hydroxide ion and water as nucleophiles 194 Insertion reactions initiated by nucleophilic attack Acid-base reactions at the a-carbon 207 General features and methods 207 Kinetic and thermodynamic acidities 209 Effect of structure on pKa values 210 Intrinsic rate constants for proton transfer 219 Thermodynamic acidities in organic solvents 223 Hydrolysis of ionizable carbene complexes 228 Acknowledgments 232 References 233... 

Table 13.3.28. Effects of the concentrations of NaOCHzCF) and kind of solvent on the apparent intrinsic rate constants, lc.. n. and k, ...

Bimolecular reactions involving radicals typically possess very low activation energies and very high intrinsic rate constants. Diffusion-controlled encounter can occur even in low viscosity solvents. Radicals are known to be involved in some classes of homogeneous catalytic reactions (47). 

To what extent are the variations in the rate constant ratio /cs//cpobserved for changing structure of aliphatic and benzylic carbocations the result of changes in the Marcus intrinsic barriers Ap and As for the deprotonation and solvent addition reactions It is not generally known whether there are significant differences in the intrinsic barriers for the nucleophile addition and proton transfer reactions of carbocations. 

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