Bronsted plots

For acyl transfer, phosphoryl transfer, and sulfonyl transfer, the primary kind of evidence in favor of concerted mechanisms for some reactions is a linear Bronsted plot of log k versus for a range of nucleophiles, spanning p/ir/ = - p r s = 0, coupled... 

It is of interest to consider the form of the Bronsted plot or Eigen plot to be expected for reaction of a series of related intramolecularly hydrogen-bonded acids with hydroxide ion by the mechanisms in Schemes 5 and 6. The effect of a substituent on the value of the dissociation constant of an intramolecularly hydrogen-bonded acid (8 log K) will be two-fold. The stability of the undissociated acid will be modified because of a substituent effect on the... 

Fig. 4 A Bronsted plot of log pKz phenol in methanol vs. log fcaa fn( OCH3-) for methanolysis of aryl acetates promoted by 9 Zn2 + ( OCH3), T = 25°C, data in Table 7. Dashed line corresponds to NLLSQ fit of data to Equation (15) encompassing all esters with Pi = -0.023 0.03 and f 2 = -0.690 0.005 with a breakpoint of pJTa H = 14.8. Reproduced from ref. 16f with permission.

The Bronsted plots shown in Figs 4 and 5 assist in visualizing the data of the metal-catalyzed reactions which show definite evidence of a break where the rate constants are quite sensitive to phenols or alcohols with high (pA a values, but almost no sensitivity to the nature of the ROH groups with low (pA a values. In these cases there is no obvious discrepancy between aliphatic and aryl esters, so all... 

Fig. 7 Bronsted plot of the log second-order rate constant for La3 + - and 9 Zn2 + ( OCH3)-catalyzed methanolysis of phosphates 20 (including 1) vs. the JpA a values for the corresponding phenols linear regressions through the La3 + and 9 Zn +( OCH3) data (excluding 20b) give gradients of (1.43 0.08) (solid line, ) and —(1.12 0.13) (dashed line, O), respectively. Note that points on lower right for the p-methoxy derivative (20f) are coincident. Reproduced from ref. 17c with permission.

Fig. 10 Bronsted plots of the second-order rate constants for methoxide and metal ion-catalyzed methanolysis of 22a-f at 25 °C. (La3 + ( OCH3))2 (A, gradient = —1.26 + 0.06), 9 Zn2 + rOCH3) ( , gradient =+ -1.06 0.09), OCH3 (T, gradient = -0.76 0.06). Reproduced with permission from ref. 17e.

Fig. 11 Bronsted plots for logk atalyst vs. pifa of aryl thiol for the methanolysis of 23a-e,...

The (La3+( OCH3))2 and 9 Zn2+( OCH3) promoted methanolyses of the phos-phonates (22)17e and phosphonothioates (23)17g generally follow the same sort of trends as the phosphates and phosphorothioates discussed above so they need not be discussed in great detail here. Analysis of the linear Bronsted plots for the phospho-nates 22 gives the relationships shown in Equations (27)-(29) which shows the common trend that the j8lg observed for the metal-catalyzed reactions are greater than that of the methoxide reactions. Since the Leffler parameter, a, for the La3+-catalyzed... 

The method consists of plotting the forward electron transfer rate constant against the standard potential of a series of redox catalysts as illustrated by Figure 2.29. Three regions appear on the resulting Bronsted plot, which correspond to the following reaction scheme (Scheme 2.14). The... 

FIGURE 3.22. Endergonic homolytic dissociation of ion radicals, a Potential energy curves, b Bronsted plots combining the effect of diffusion and activation. kBT/h= 1013s-1, 4/= 1010 s-1, kdif = 1010M- s-1, Drx-w + 2o = 0.3eV, r = 298K. Dotted line log[Pg.225]

In 20% dioxan (Hegarty et ai, 1974). The reference reaction is nucleophilic attack by alkoxide ions on phenyl N-methyl-N-phenylcarbamate at 25° in water (Hutchins and Fife, 1973). The rate constant for bimolecular attack by an oxyanion of pKt 4.26 (3.6 x 10-13 dm3 mol-1 s-1) is obtained by a long extrapolation of a two point Bronsted plot (which has the reasonable slope of 0.90)... 

Gandour et al., 1979. The reference reaction is general base catalysis of the hydrolysis of phenyl dichloroacetate at 25° by external carboxylate of the given pK,. Rate constants calculated from a two point Bronsted plot using the data of Fersht and Kirby (1967)... 

DR. DALE MARGERUM (Purdue University) You need to be careful because you may not obtain a true Bronsted plot from what you have just described. You don t want to make a plot where each system has a different rearrangement. You want to be certain to have a common type of acid or base with which you are reacting. The literature is full of inverse Bronsted relationships in which there has been a poor choice of reaction acid-base pairs. Much confusion has been generated as a result. 

The Bronsted relationship can be strictly accurate only over a certain range of acid and base strengths. When has diffusion-controlled values, which of course cannot be exceeded, the linear plot of log k/ y vs log must level off to a zero slope, that is a = 0. As well as being reported, although rarely, in simple metal complexes, the resultant curvature in the Bronsted plot is also shown by the zinc enzyme carbonic anhydrase (Chap. 8. Zn(II)). In... 

Figure 2. Bronsted plot of log P a for the catalysing base in the cyclization of...

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