Bronsted general acid catalysis

It has been proposed (79) that eaq reacts with Bronsted acids following the Bronsted general acid catalysis law. The work was based on the competition of four acids with acetone and with I- for e aq. The acids... 

The ionization of (E)-diazo methyl ethers is catalyzed by the general acid mechanism, as shown by Broxton and Stray (1980, 1982) using acetic acid and six other aliphatic and aromatic carboxylic acids. The observation of general acid catalysis is evidence that proton transfer occurs in the rate-determining part of the reaction (Scheme 6-5). The Bronsted a value is 0.32, which indicates that in the transition state the proton is still closer to the carboxylic acid than to the oxygen atom of the methanol to be formed. If the benzene ring of the diazo ether (Ar in Scheme 6-5) contains a carboxy group in the 2-position, intramolecular acid catalysis is observed (Broxton and McLeish, 1983). 

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. 

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strict general base catalysis by buffer bases is observed with a Bronsted coefficient of O 39 (Fife and Benjamin, unpublished data).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

At the time the mechanism shown in [60] was proposed it was not known whether it was even chemically reasonable. Bimolecular general acid catalysis involving proton transfer in the transition state [equation (45)] had never been observed in the hydrolysis of glycosides or simple acetals. Bronsted and Wynne-Jones (1929) had... 

In general acid catalysis, the reaction rate increases because the transition state for the reaction is lowered by proton transfer from a Bronsted acid in general base catalysis, the reaction rate increases by virtue of proton abstraction by a Bronsted base. 

To determine a or (3 experimentally a plot of log kB or log kHB vs pKa (a Bronsted plot) is made and the slope is measured. Statistical corrections (Chapter 7) should be applied for dicarboxylic acids and for ammonium ions from which one of three protons may be lost from the nitrogen atom. General base or general acid catalysis implies an important feature of any mechanism for which it is observed, namely, that removal of a proton or addition of a proton is involved in the... 

In Figure 1 we present the data plotted according to Bronsted s equation (0). The slope of —0.51 in this plot is in agreement with the theory for general acid catalysis. 

Fig. 9 Variation of the Bronsted parameter a for general acid catalysis of enol ketonization with the free energy change AG° for carbon protonation of enols (o) and enolates ( ). The data are taken from Table 2.

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). 

When a is intermediate in value, most of the flux is taken by the general acid HA, and general acid catalysis is easily observed. Table 11.1 [4] gives the percentage of the flux taken for given values of a. Similar arguments can be made for base catalysis, i.e. general base catalysis is difficult to characterise when Bronsted (t values are close to 1 and 0. 

General base catalysis of the reaction of a nucleophile (HNu) is kinetically equivalent to general acid catalysis of the reaction of the deprotonated nucleophile (Nu ). A distinction can be made employing cross-correlation effects where the value of the Bronsted a is measured as a function of another parameter such as the nucleophilicity of the attacking nucleophile. 

Fig. 11.5 (A) Eigen-type plot for the general base-catalysed reactions of 4-chlorobenzaldehyde with a substituted hydrazine [8] the line is calculated from log kg = —0.341 log(1 + 106 09 pffa). (B) Eigen-type plot for general acid catalysis in the reaction of cyanic acid (HNCO) with aniline, and an alternative interpretation [9] the dashed line has a Bronsted slope of —0.19 and the break at p/fa near 10 in the solid line is consistent with proton transfer to an addition intermediate (PhNH2+CONH ).

General acid catalysis is indicated by the validity of eqn. (19), which means that the reaction rate is increased by all Bronsted acids present in the solution and does not merely depend on the equilibrium concentration of hydrogen ions. The most likely mechanism for general acid catalysis is rate-determining proton transfer [1]. 

The hydrolysis reactions of acetals, ketals, and orthoesters are catalyzed by acids but not by bases. It has been found that these three groups of substrates are hydrolyzed via a common general mechanism — involving similar types of intermediates — though the rate-determining step may vary from case to case. In the hydrolyses of ethyl orthoacetate, orthopropionate, and orthocarbonate, general acid catalysis was unambiguously established for the first time by Bronsted and Wynne-Jones [158]. 

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