Br0nsted plots

Br0nsted plot for base catalysis of the mutarotation of glucose. 

BR0NSTED RELATION Bronsted coefficient or exponent, BR0NSTED RELATION BR0NSTED PLOT BR0NSTED RELATION BR0NSTED THEORY Brownian motion,... 

Fig. 4. Br0nsted-plot of the logarithms of observed rate constants log for the base catalyzed addition of ethanol to di-phenylketene (59a) (s. Scheme 11) under standardized conditions vs. a relative basicity log K [31]...

Figure 2.5 The Br0nsted plot for the general-base catalysis of the hydrolysis of ethyl dichloroacetate. The logarithms of the second-order constants obtained from the plot of Figure 2.4 are plotted against the pAT s of the conjugate acid of the catalytic base. The slope is the /3 value. Note that the points for amine bases ( ) fall on the same line as those for oxyanion bases (O), showing that the catalysis depends primarily on the basic strength of the base and not on its chemical nature.

Figure 19.6 Br0nsted plot for unfolding and folding of CI2 minicore, k is the rate constant for unfolding and kF that for folding. J D N is the equilibrium constant for denaturation.

Figure 19. Br0nsted plots of inhibition constants. Numbers refer to inhibitors...

The Br0nsted plots (Fig. 3) give information on this point. The higher curvature of the plot for DMSO compared to methanol is indicative of a lower intrinsic barrier to proton transfer for the dipolar aprotic solvent. Since in the extended Marcus theory the solvent effect has already been taken into account, one would expect the intrinsic barrier for proton transfer to be identical in the two systems. This is not the case. Therefore it appears that separation of the mechanism into reagent positioning with concomitant solvent reorganization is not warranted. 

Another approach used to interpret curvature of Br0nsted plots has been given by Murdoch (1972). This model, which incorporates Marcus theory, shows that the diffusive steps (10a, c) of the three-stage Eigen mechanism can also influence curvature. It is shown mathematically that increased difficulty of diffusion has the same... 

Fio. 1. Br0nsted plot for azoles reacting with methylating agents in quaternization reactions. Pyridine is the reference substrate. 1, 1-methylimidazole 2, 1-methylbenzimidazoie 3, thiazole 4, 1-methylpyrazoIe 5, 2-methylindazole 6, benzo-thiazole 7, oxazole 8, 1-methylindazole 9, benzoxazole 10, 2,1-benzisothiazole 11, isothiazole 12, 2,1-benzisoxazole 13, isoxazole and 14, 1,2-benzisoxazole. 

FIGURE 6. Extended Br0nsted plot of logiH+ vs for the protonation of some highly activated... 

The Marcus equation is also examined in Chapter 9. As discussed previously regarding the Lewis chapter, the quadratic term of the Marcus equation leads to a dependency of rate on the square of pKa, so that Brpnsted plots would be expected to be curved. Bordwell and co-workers observe curvature in some of their Br0nsted plots but conclude that the curvature is too large to be a Marcus effect and actually results from a solvation effect for some heteroatom substituents. These workers suggest that the curvature observed for Brpnsted plots in water results from differential solvation. 

Figure 2. Marcus-Br0nsted plot AE vs. AE° for the reaction of substituted benzyl anions with methyl bromide.

Figure I. Br0nsted plot of SN2 reactions for substituted fluorenide ion families with benzyl chloride in (CH3)2SO at 25 °C.

Figure 6. Br0nsted plot of anions reacting to n-propyl tosylate in (CH3)2SO...

Figure 4. Br0nsted plots and isotope effects, for proton abstraction from carbon acids, from Pohl and Hupe (30). The upper plot shows rate data for oxyanion and thiol anion catalyzed proton abstraction from 4-(4-nitrophenoxy)-2-butanone (o,v )t acetylacetone and ethyl...

Figure 5. Schematic Br0nsted plot to show how a requirement for partial desolvation can cause a negative deviation from a Br0nsted plot by its effects on the observed pK, reactivity, and concentration of solvated and desolvated ions (31).

The kinetic data from the reactions conducted in buffers were also used to determine the corresponding intrinsic rate constants. They were calculated from statistically corrected Br0nsted plots of log(fef /q) or log(fe /p) versus pAT — pA + log(p/q) where p is the number of equivalent protons on BH and q is the number of equivalent basic sites on B. Interpolation or extrapolation of such plots to pAT — + og p/q) = Q yields log feo- Such log feo values for the reactions with primary aliphatic and secondary alicyclic amines as well as carboxylate ions are summarized in Table 25. It those cases where log Icq could be obtained from interpolation or short extrapolations the log feo values can be assumed to be quite reliable but those obtained from long extrapolations are less accurate the latter ones are again identified by italics. 

This value is close to the rate predicted from an extrapolation of the Br0nsted plot of aryl phosphomonoester dianions, suggesting that the alkyl and aryl esters likely follow a similar hydrolysis mechanism. 

Fig. 2. Hypothetical Br0nsted plot showing how the observed rate of the reaction outlined in Eqn. 7 of text varies with nucleophilicity of the attacking nucleophile and basicity of the catalyst (a) constant nucleophile, increasing basicity of catalyst when proton transfer from the intermediate is thermodynamically favourable increasing the nucleophilicity of the nucleophile increases the rate from (c) to (b).

Br0nsted plots were used to evaluate the relationship of kinetic and thermodynamic acidities of metal carbonyl hydrides.Proton transfer reactions to anilines in acetonitrile were used to generate the rate and equilibrium data. Br0nsted... 

Novel diene complexes of the type [Fe(T7-C5Me5)(CO)(r7 -diene)]" (37) have been synthesized and the kinetics of addition of amines to the diene ligand measured for the 1,3-cyclohexadiene complex/ A Br0nsted plot of log k vs. of amine conjugate acid has a slope of 0.3, which is much lower than that previously reported for amine addition to [Fe(CO)3(T7 -dienyl)]" cations. This was interpreted in terms of an essentially soft character for the complexed diene in (37). However, the low Br0nsted slope may arise from a saturation effect associated with the highly basic nature of the amines (morpholine, N-methylpiperazine, pyrrolidine, piperidine) employed in the more recent study. 

Linear Br0nsted plots were seen for both phenolysis and atninolysis of 5-methyl 0-4-nitrophenyl thiolocarbonate and 5-methyl 0-2,4-dinitrophenyl thiolocarbonate, in accordance with concerted mechanisms. Kinetic studies of the atninolysis by secondary alicyclic amines of (9-aryl 5-4-nitrophenyl dithiocarbonates (41 X = Me, Cl Y = N02) and (9-aryl 5-phenyl dithiocarbonates (41 X=Me, Cl Y = H) in EtOH/H20 at 298 K showed that in some cases, a stepwise mechanism with two tetrahedral intermediates, one zwitterionic (T= =) and the other anionic (T ), is involved. ... 

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