Bronsted correlations

In summary, this work which covered seventeen pAi units and nine powers of ten in rate, gave an excellent Bronsted correlation... 

Thomas and Long488 also measured the rate coefficients for detritiation of [l-3H]-cycl[3,2,2]azine in acetic acid and in water and since the rates relative to detritiation of azulene were similar in each case, a Bronsted correlation must similarly hold. The activation energy for the reaction with hydronium ion (dilute aqueous hydrochloric acid, = 0.1) was determined as 16.5 with AS = —11.3 (from second-order rate coefficients (102At2) of 0.66, 1.81, 4.80, and 11.8 at 5.02, 14.98, 24.97, and 34.76 °C, respectively). This is very close to the values of 16.0 and —10.1 obtained for detritiation of azulene under the same condition499 (below) and suggests the same reaction mechanism, general acid catalysis, for each. 

This mechanism is also supported by kinetic isotope effects, Bronsted correlations, and other evidence as described in reference1181. 

The earliest LFER, advanced by Bronsted, correlates the acid dissociation constant and base strength (1/A h) of species with its effectiveness as a catalyst in general acid (At h) and base (Atgl-catalyzed reactions respectively. The relationships take the form... 

In the near future it is expected that the measurement of kinetic basicity of EGB s will become routine and precise. When available for synthetically useful EGB s the data will be used to optimise the choice of probase and carbon acid and to explore factors (e.g. choice of cation) which influence basicity. Where good Bronsted correlations are observed it will be possible to use data on the basicity of EGB s to... 

Steric effects of other substituents such as Et, PhCH2, 2-pyridyl, C02Me, and MeCONH are larger than those five showing the Bronsted correlation. Consequently, they cause lower reactivity. For example, the reactivities of 2-acetylamino- and 2-(2-pyridyl)pyridine are about 10 and 25 times less, respectively, than that suggested by the rate-acidity correlation for the five substrates with an approximately uniform steric impediment.72... 

Comparison of the Bronsted correlation for 3-substituted pyridines [Eq. (12)] and that for the azoles [Eq. (15)1 is most interesting. The slopes are very similar, 0.39 and 0.43, respectively, and represent overlapping values at the uncertainty level of one standard deviation. However, the intercepts, —1.90 and —2.75, respectively, do not overlap. But consideration of the approximations in the plot for the azoles precludes a definite conclusion about whether the differences are truly significant. 

The Bronsted correlation for five-membered rings shows that effects of structure on reactivity and on acidity are related. Variations in rate constants for quaternization and in pKa values (Table III) are understandable in terms of resonance and inductive effects of the heteroatom X.120 The effects on the energy of a transition state leading to quaternized product are similar but smaller than those on the energy of protonated material. The following considers in more detail the influence of benzo-fusion. 

The Bronsted correlation given by Eq. (15) is so extensive that it can be used to predict a pKa value once the rate constant or relative rate constant for N-methylation under the conditions employed to construct this plot is known. This approach is especially valuable for weakly basic heterocyclic compounds, which are protonated only in strongly acidic media. Even for such, it is comparatively easy to determine a rate or relative rate constant for N-methylation. The pKa values of several molecules so estimated are given in Table V. They are expected to be... 

The quantitative effects of a-substituents in decreasing the rates of these reactions are not additive and also depend considerably on solvent and alkylating agent. They are low in liquid sulfur dioxide as a solvent where solvation effects are small and the high dielectric constant increases the bond breaking in the transition state. For 3- and 4-substituted pyridines a Bronsted correlation exists between the rates of quatemization and the pKa values (78AHC(22)86). 

Satisfactory Bronsted correlations for a-substituted azoles offer further evidence of the lesser importance of steric effects in the azole series (78AHC(22)7l). 

Then, in 1973, Streitwieser reported that the polyarylmethane exchange rates measured in cyclohexylamine-cyclohexylamide correlate with equilibrium p/T values with a = 0.31.79 Apparently, when the proton is removed by cyclo-hexylamide, the polyarylmethanes have early transition states, just as the fluorenes did for proton removal by the weaker base methoxide.80 A short extrapolation of the Bronsted correlation led to a pKa for toluene of 40.9, about seven units higher than the value assigned in the MSAD scale. Furthermore, if we... 

If HA is a stronger acid than the ammonium function of 2, the rate constant for proton transfer to 2, kuA, will be for diffusion and the observed rate constant will be independent of the acidity of HA. On the other hand, if HA is a weaker acid than the ammonium function of 2, the proton transfer from general acids, HA, to the nitrogen of 1 in Scheme 11.8 will be given by k K /K, where K A is the acid dissociation constant of HA and kA is the diffusion-controlled rate constant. The observed rate will nowbe dependent upon the acidity of the catalyst HA as described by a Bronsted correlation with slope equal to —1. 

Fla. 5. Bronsted correlation, including substituted pyridines ( ), quinoline ( ), isoquinoline (O), and diazines ( + ). 

In addition to the correlations between rate constants and substituent constants, it is desirable to attempt a Bronsted correlation between rate and equilibrium constants, just as Knoche did for activated arenes (see equation 47)102. Unfortunately there are not as many examples of alkenes for which both kH+ and pK%n are known, as is the case for the arenes many kH+ values have been determined126, but pK%H values are known... 

One of the reasons for the scarcity of rate-equilibrium correlations in non-aqueous solvents is that pK values under these conditions are difficult to obtain and therefore not as readily available as aqueous pK values [186]. In recent years, however, there has been considerable interest in this area and a few Bronsted correlations have been published particularly by the groups of Cram, Streitwieser and Ritchie. 

Fig. 19. Logarithm of the true forward rate constant vs. the reciprocal temperature T and the corrected standard Gibbs energy of transfer (Bronsted correlation projected on the plane of the page) for (O) Pi , ( ) Me3PrP +, (A)Me3EtP, (V) Et4N+, and ( ) M04N+ ion transfer from the solution of 0.05 M LiCl in water to the solution of 0.05 M Bu4NPh4B in nitrobenzene. The numbers on the lines indicate the temperature (K). (After [115]).

Moreover, a Bronsted correlation with a = 0.62 has been obtained for 2-phenyl-amino-l,2-diphenylethanol radical cation [219]. Comparison of this value with that obtained for the C-C bond cleavage of 4-MeOC6H4CH(OH)rBu + catalyzed by a series of bases in aqueous solution f = 0.4) [170], suggests that localization of the positive charge at nitrogen results in a significantly later transition state for fragmentation. 

In view of the substantial use of the extended Bronsted correlation it is always the convention to name the Bronsted exponent as P with a subscript to indicate which reaction is being measured and to distinguish it from the parameters of the standard acid-base correlations. 

Using the data of Table 6 graph the Bronsted correlation for the mutarotation of glucose catalysed by general bases B ( b/M" sec ) 2... 

These equations factor all the acid-base types in a series to that where p = = 1. For example, if triethylamine and diaminoethane monocation are to be interpreted in the same correlation they need corrections because the former has p = 1 and q = 1 whereas the latter has p = 2 and q = 1. The assignment of p and q is not satisfactory for acid-base pairs with uncertain structures (such as H3O and HO ) or species with multiple centres of differing reactivity hydronium and hydroxide ions almost always show anomalous reactivities in corrected Bronsted correlations, but these are partly due to solvation effects. The protons attached to a single atom such as ammonium (R-NHj or oxonium ion (R-OH2 ) are regarded as having p = 1 rather than the number of identical protons. A similar convention selects the q value for hydroxide ion as unity even though there are three lone pairs free to accept a proton. The statistical... 

A reaction studied by this classical method is the formation of aromatic semicarbazones (Scheme 6). The data (Figure 2) fit Equation (4) and Ap is the difference in slope of the two linear portions of the correlation (p i - P2). A similar Equation may be written for the Bronsted correlation or any other free energy relationship including those for concentration changes which might affect 2 or k x. 

The data should be fitted to Equation (4) suitably modified for Bronsted correlations. Comment on the non-linearity of the plot and suggest a mechanism fitting the observations. 

The reaction of substituted phenoxide ions with 2(4 -nitrophenoxy)-4,6-dimethoxy-l,3,5-triazine (Equation 29) exhibits a linear Bronsted correlation over a range of nucleophile from 5.5 to 10 (see Problem 7). ... 

The extended Bronsted correlation for the aminolysis of the 2,4,6-trinitrophenyl ester is linear with an equation ... 

Figure 4 Extended Bronsted correlations. The lines are theoretical...

Figure 15 Extended Bronsted correlation exhibits relatively substantial microscopic medium effects...

Figure 25 Bronsted plot for the hydrolysis of acetoacetate esters as a function of the pK of the conjugate acid of the leaving group. The dashed line is the global fit to parallel mechanisms (see equation in Answer to Problem la) the full lines are linear Bronsted correlations for the individual segments...

---