Intrinsic rate constants bonding

Here k0 is an intrinsic rate constant for conversion of the transition state into the products and typically depends on the nuclear vibration frequency of a bond that is being formed or broken. 

A Bronsted fl]t value of 0.5 and occh value of 1.31 have been calculated for deprotonation reactions of (3,5-dinitrophenyl)nitromethane promoted by substituted benzoate ions and of substituted (3-nitro-, 4-nitro- and 3,5-dinitro-)phenyhiitro-methanes promoted by benzoate ion, respectively, in methanol.146 The intrinsic rate constants are (2.0-6.3)x 104 tunes lower than for the same reactions in acetonitrile solution, and this has been attributed to commensurate reduction of strength of the hydrogen bond between the carbon acid and benzoate ion in the imbalanced transition state. The transfer activity coefficient (logMyAN) from methanol to acetonitrile solution have been calculated for (// -nitrophenyl)nitromcthyl anion (3.6) and (m-nitrophenyl)-nitromethane (—1.0). 

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... 

Steric effects reduce rate and equilibrium constants of nucleophilic additions but the question how the intrinsic barrier is affected does not always have a clear answer. Comparisons of intrinsic rate constants for the addition of secondary alicyclic amines versus primary aliphatic amines suggest that k0 is reduced by the F-strain. This implies that the development of the F-strain at the transition state is quite far advanced relative to bond formation. The effects of other types of steric hindrance on k0 such as prevention of coplanarity of Y in the adduct or even prevention of jt-overlap between Y and the C=C double bond in the alkene have not been thoroughly examined and hence are less well understood. 

The intrinsic rate constants for thiolate ion addition to 76-Cr and 76-W are substantially larger than those for alkoxide ion addition. This is similar to the previously mentioned higher intrinsic reactivity of thiolate ions compared to amine nucleophiles for the addition to a-nitrostilbene and p-nitrostyrene. It can be understood in terms of the soft-soft interaction of the thiolate ion with the carbene complex which is more advanced than C S bond formation at the transition state.184... 

There exists substantial evidence that in reactions that involve oxyanions or amines as bases or as nucleophiles, their partial desolvation, as they enter the transition state, typically has made greater progress than bond formation. In the context of the PNS, this partial loss of solvation represents the early loss of a reactant stabilizing factor and hence reduces the intrinsic rate constant. As discussed at some length in our 1992 chapter,4 for strongly basic oxyanions this desolvation effect often manifests itself in terms of negative deviations from Br Ansted plots and/or in abnormally low p or pnuc values.58,188 In fact, a number of cases have been reported where the pnuc value was close to zero or... 

M. W. Deprotonation of arylnitromethanes. Higher intrinsic rate constants with thiolate ions than with oxyanions or amines as the proton acceptors. Hydrogen bonding in the transition state and desolvation of the base as competing factors in proton transfer at carbon. 

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]. 

A primary isotope effect results when the breaking of a carbon-hydrogen versus a carbon-deuterium bond is the rate-limiting step in the reaction. It is expressed simply as the ratio of rate constants, i wlky,. The full expression of k /kn measures the intrinsic primary deuterium isotope for the reaction under consideration, and its magnitude is a measure of the symmetry of the transition state, e.g., -C- H- 0-Fe+3 the more symmetrical the transition state, the larger the primary isotope effect. The theoretical maximum for a primary deuterium isotope effect at 37°C is 9. The less symmetrical the transition state, the more product-like or the more substrate-like the smaller the intrinsic isotope effect will be. 

The intrinsic inertness of the peptide bond is demonstrated by a study of the chemical hydrolysis of N-benzoyl-Gly-Phe (hippurylphenylalanine, 6.37) [67], a reference substrate for carboxypeptidase A (EC 3.4.17.1). In pH 9 borate buffer at 25°, the first-order rate constant for hydrolysis of the peptide bond ( chem) was 1-3 x 10-10 s-1, corresponding to a tm value of 168 y. This is a very slow reaction indeed, confirming the intrinsic stability of the peptide bond. Because the analytical method used was based on monitoring the released phenylalanine, no information is available on the competitive hydrolysis of the amide bond to liberate benzoic acid. 

The same relationships apply, in principle, to the concerted pathway (60). However, the intrinsic barrier is now so high, because of the contribution of bond breaking, that the possibility of observing a region that is counterdiffusion controlled is quite unlikely since the rate constant of the forward reaction would then be immeasurably small in most cases. At any rate, even if one conceives that such a situation might occur, and AGq then determined would be so different from that of the outer sphere electron transfer (59) in the stepwise pathway that the confusion would hardly be possible. 

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