Bronsted plots linear

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... 

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.

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 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... 

Belke et al. (1971) reported general base and general acid catalysis in cyclization of 2-hydroxymethylbenzamide [equation (18)]. However, with 2-hydroxymethyl-6-aminobenzamide strict general base catalysis by buffer bases is observed with a Bronsted coefficient of O 39 (Fife and Benjamin, unpublished data). In contrast with the unsubstituted amide, the Bronsted plot is nicely linear. An amino-group in the 6-position might assist decomposition of a tetrahedral intermediate as in [37a, b] or a kinetic equivalent. The pH-rate constant profile for spontaneous cyclization at zero buffer concentra-... 

Bruice and Schmir (3) have shown that for a series of imidazole derivatives, klm depends on the base strength of the catalyst and since pKA is an approximate measure of base strength, the value of klm should increase with increase in pKA. Table I shows that this is indeed the case. Imidazole, pKA = 7.08, has a catalytic constant eight times larger than that of benzimidazole, pKA = 5.53. Bronsted and Guggenheim (2) have obtained a linear relationship between log k/ and pKA for a series of carboxylic acids in the pKA range of 2 to 5, where kB is the carboxvlate anion basic catalytic constant for the mutarotation of glucose and Ka is the acid dissociation constant of the acid. Our results for imidazole and benzimidazole fit fairly well into the Bronsted plot. 

Data for the reactions of several cyclic tertiary amines with phenyl, 4-nitro-phenyl and 2,4-dinitrophenyl acetates, at 25°C and ionic strength 1.0, appear in Table 40, and as a Bronsted plot in Fig. 20. The usual irregularities of such plots for nucleophilic attack are evident. Linear relationships between log k and pKa are generally found for groups of compounds of closely similar structure, as for the substituted pyridines in Fig. 20. The data for the two tricyclic amines fall on separate curves, and the points for imidazole clearly fall on neither of the first two sets of lines. The separate lines for the reactions of particular classes of nucleophile are approximately parallel, as is usually found. 

An example of a linear Gibbs energy relationship that is widely used in discussing mechanisms of enzymatic reactions is the Bronsted plot (Eqs. 9-90 and 9-91). 

The reactivity of amines and imidazolide anions with aryl 4-toluenesulfonates (310) in 80% aqueous DMSO has been studied and Bronstcd [lmlc values of ca 0.7 and ca 1.0, respectively, were found.284 The points for botii die amines and die imidazolides can all be accommodated on die same Bronsted plot. S—O rather than C—O cleavage occurs in the reaction, hi a useful aside to tiiis work die authors have shown that plots of pKA data in water are linear witii tiiose in DMSO or 80% DMSO and tiiese can be used to obtain unknown pA, values. The same Ukrainian group has obtained deviations from Bronsted plots for the reaction of (310) witii highly basic nucleophiles such as imidazoles and arenesulfonamides. The /Jnuc value goes from 0.79 for pA/<11 to 0-0.1 for pKa>11.0.285... 

A number of minima corresponding to oxonium ylides and H-bonded structures were found on the potential-energy surface for reaction of singlet carbenes with water and alcohols." Laser flash photolysis revealed that the rates of reaction between cyclopentadienylidene or fluorenylidene and alcohols increased with alcohol acidity and had linear Bronsted plots with slopes of 0.061 and 0.082, respectively.100 These results point to protonation with a very early transition state or to concerted OH insertion. For tetrachlorocyclopentadienylidene, the results showed that ylide formation (100) is predominant. 

The reaction of 2,4-dinitrophenyl benzoate with secondary cyclic amines in DMSO-water shows a break in a Bronstcd plot near pA a 9.1 and the microconstants for the various stages in the reaction have been obtained and these gave good straight line Bronsted plots.46 The a-effect was observed as a positive deviation in the linear log k vs pKa plot for the reaction of /Miitrophcnyl X-substituted benzoates (43) with primary... 

Modern theoretical developments in the theory of proton-transfer reactions suggest that such linear Bronsted plots are only a first approximation when the range of the P-KHA-values is narrow. When a wider range of bases is used, the curve obtained should be such that the Gibbs free energy of activation AG fits the Marcus eqn (6). This equation was derived (Cohen and Marcus, 1968 ... 

Under these restrictive conditions, the Bronsted plot is expected to be slightly parabolic rather than linear. As represented schematically in Fig. 2, a Bronsted slope smaller than, close to, and bigger than 0.5 indicates that the driving force is, respectively, big, close to zero and small. 

The mechanisms of two other reactions described in Sect. 2.2 involve slow proton transfer to unsaturated carbon. The general acid catalysed cleavage of vinyl mercuric halides [42, 50] for example, allyl mercuric iodide, CH2=CHCH2HgI (XXII), gives Bronsted exponents around 0.7. Linear Bronsted plots are obtained with carboxylic acid catalysts but, as observed in other A—SE 2 reactions, general acids of different structural types (for example, hydronium ion or bisulphate ion) show substantial deviations. Bronsted catalysis of the hydrolysis of diazo compounds (N2 =CR X) has been studied by the groups of Albery and Kreevoy. With... 

The reactions of furansulfonyl chlorides with anilines (entry 11)123 in MeOH yield a linear Bronsted plot with fix =0.51, which indicates an Sw2 mechanism rather than a stepwise process. This value is quite similar to those found for the reactions of anilines with benzenesulfonyl chloride (0.63)117, with 2-thiophenesulfonyl chloride (0.53)124 and with 3-thiophenesulfonyl chloride (0.54)125. Thus their anilinolysis mechanism is also expected to be S/y2. The reaction rate therefore depends not only on the nucleophile basicity but also on the substrate reactivity. Comparison of the reaction rates leads to the following reactivity order for the Ar moiety benzene > 3-thiophene > 3-furan > 2-furan > 2-thiophene. This reactivity sequence follows the order of the resonance interaction between the... 

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...

Variation of Substrate Structure. Whereas linear free energy relationships play a central role in the determination of non-enzymic mechanisms, they are much less important for enzymes, for two reasons. First, enzymes have evolved to bind their natural substrates, and substituents introduced in an attempt merely to alter electron demand at the transition state may have many other interactions with the enzyme protein. The result is very noisy Hammett and Bronsted plots. Whereas conclusions can be drawn from non-enzymic rates varying over a factor of 3, with enzyme reactions, to see any trend above the noise it is usually necessary to have rates ranging over several orders of magnitude. 

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