Bronsted plot/data

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.

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)... 

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strict general base catalysis by buffer bases is observed with a Bronsted coefficient of O 39 (Fife and Benjamin, unpublished data).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-... 

Fig. 17. Bronsted plot for the reactions of nucleophiles with p-nitrophenyl acetate in water at 25°C. These reactions involve simple nucleophilic catalysis of hydrolysis. (Data from ref. 283).

Fig. 18. Bronsted plot for the reactions of nucleophiles with phenyl acetate (PA,D)p-nitrophenyl acetate (PNPA, A) and 2,4-dinitrophenylacetate (DNPA,0) at 25°C. The open symbols (and full lines) represent data for the total reaction, which is in most cases nucleophilic catalysis. The closed symbols (and broken lines) represent general base catalysis of hydrolysis.

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. 

Fig. 20. Bronsted plot of the rate coefficients for the tertiary amine-catalyzed hydrolysis of phenyl (0),p-nitrophenyl (A) and 2,4-din itrophenyl ( ) acetates at 25°C. (Data from ref. 283).

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... 

The Bronsted parameter a varies substantially over the large range of ArG° covered by the experimental data collected in Fig. 10 it ranges from 0.2 for the most reactive enolates (phenylethynol anion) to about 0.8 for the least reactive compound (1-naphthol). The a-values calculated by Equation (20) are in satisfactory agreement with those determined experimentally from Bronsted plots of general acid catalysis (Table 2). 

Bronsted plots with some scatter should always be examined carefully because they might conform with the Eigen equation. A classic case is the acid-catalysed reaction of aniline with HNCO which can be fitted to a shallow Bronsted line (slope = —0.19)[9] however, the data are also consistent with an Eigen dependence (Fig. 11.5B) arising from a mechanism involving trapping of an addition intermediate by proton transfer (Scheme 11.11). 

When NMA+ reacts with phenyl-substituted N-phenyldihydronicotin-amides, X-PhNAH, also in anhydrous acetonitrile (Powell and Bruice, 1983b), rate and equilibrium data yield a Bronsted plot with a slope of 0.51, consistent with a centrally located transition state. The primary k.i.e. s h2/ d2, increase from 3.98 for X = />-methoxy to 4.77 for X = m-trifluoro-methyl at 50° and may indicate a trend to a more symmetrical transition state. Marcus treatment of the substituent dependence of the k.i.e. s yields an intrinsic barrier AG = 22.2 kJ mol - L. The temperature dependence of the k.i.e. for reduction by X-PhNAH with X = / -methyl gives [A ] = 7.68 kJ mol-1, but AJA = 4.3 is unusually large. A tunnelling correction of ca. 2 was estimated so that the semi-classical k.i.e. was in the range 2 to 3. 

Attempts were made to observe a curvature of the Bronsted plots for ketone ionisation. Cohen and Marcus (1968) and Bell (1973) (see also J. R. Jones, 1973 Kresge, 1975b) collected data for the reaction of carbonyl compounds (including ketones, esters and keto-esters) with bases and have observed a slight curvature. The data fit the Marcus equation with AG% = 10 kcal mol-1 and Wr = 4 kcal mol-1 (Hupe and Wu, 1977). 

Fia. XVI. 1. Bronsted plot for the base-catalyzed decomposition of nitramide. (Data from compilation by R. P. Bell, Acid-Base Catalysis, Oxford University Press, New York, 1941.) + Doubly charged bases, B++, mostly aquo-iona, [M(H20) (0I1)]++. o Doubly charged bases, B". o- Uncharged bases, B° all substituted anilines. 

The Bronsted relation is accurately obeyed for the individual ketones in Table 2 with carboxylate ions as catalysts. However, the trend of j3 with reactivity implies that if one ketone could be studied over a wide range of catalyst strength, the Bronsted plot would be curved and the Bronsted exponent would vary. Proton transfer from acetylacetone [17] has been studied with bases covering a pK range of ca. 15 units using the temperature-jump method. The values of fef and kr for reaction between acetylacetone and carboxylate ions, phenolate ions, hydroxide ion, and water are shown in Fig. 3. The data refer to reaction (72)... 

Recently a new method has been developed for analysing rate-equilibrium data for proton transfer reactions (Marcus Theory) [200], Although the theory has not been tested extensively, it seems to have received fairly wide acceptance. This new treatment leads to various parameters which are useful in understanding results for carbon acids and offers an explanation for some anomalies in Bronsted plots such as curvature and Bronsted exponents outside the range 0 < a or j3 < 1. 

The importance of both electronic and steric factors is borne out by the reactions of piperidine in equation (159) at 30 °C in benzene (k, m" s" ) PhCOC=CPh (4-4), PhCOC=CBu-rt (1-9), PhCOC=CBu-/ (0 012), MeCOC=CPh (M5)i . If the steric requirements are made constant, as in a series of anilines in process (159) in 95% ethanol, the resulting rate data yield a satisfactory Bronsted plot and Hammett correlations /3 0-93 and p = —(212 to 2-18) in the range 20-50 °C . Further structure-reactivity variations were made with morpholine as nucleophile and XC6H4CSCCOC6H4Y as electrophile in 95% ethanol, the Hammett p = 1-13 to 0-97 for the X series and F42 to 1-20 for the Y series from 20-50 in r-BuOH at... 

The data and mechanistic conclusions summarized above come from work with aryl phosphomonoesters as predicted by the steep jSlg value, alkyl ester dianions react at very slow rates. A recent study of methyl phosphate found the rate of the dianion hydrolysis to be below the threshold of detectability, with an estimated rate constant of 2 x 10 20 s 1 at 25 °C.3 Since this value is close to the rate predicted from an extrapolation of the Bronsted plot of aryl phosphomonoester dianions, a similar mechanism is likely to be followed for alkyl and aryl esters. 

The data in Table 5 were used (Miles et al., 1966) to construct a Bronsted plot of the variation of the rate coefficient for proton removal with acidity along the series of substituted malonate monoanions the plot is reproduced in Fig. 12. The value of the gradient of the best line (a = ca 0.5) was interpreted (Miles et al., 1966) as indicating that proton removal by hydroxide ion occurs in a single step through a transition state in which the... 

Fig. 28. Bronsted plot of log A oh versus pK of the leaving hydroxyl group in the hydrolysis of hippuric acid esters (PhCONHCHjCOOR). Data from A. Williams (1975) J. Chem. Soc. (Perkin 2) 947.

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