Bronsted a-values

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). 

A Bronsted a value close to unity has been found for the pyridinium and anilinium ion catalyzed decarboxylation of 4-aminosalicylate ion. (The point for catalysis by H30+ is not on the line.)... 

A late transition state is expected for the first step of the reaction, since it leads to the formation of an unstable species. An experimental manifestation of this was observed by Hoz and Livneh in the acid-catalyzed addition of MeOH to 35. The reaction was found to be general acid-catalyzed with a Bronsted a value of 0.98. In the second step of the reaction, the cyclobutyl cation is trapped by a nucleophile which usually enters the molecule cis to the proton or to any other attacking electrophile. Since this phenomenon was observed with a variety of substituents at the bridgehead position as well as with a spectrum of electrophiles, a 1,3 steric hindrance to the approaching nucleophiles is not likely. An alternative explanation of this phenomenon is that the cyclobutyl cation is obtained in a bent structure with the equatorial side more exposed to nucleophilic attack. 

The enolisation is catalysed by (relatively unhindered) substituted quinucli-dine bases with a Bronsted (3 value of 0.45 and 0.48 for glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. The elimination reaction of the enediolate (Figure 6.3) gives a clock whereby the relative efficiency of reprotonation can be measured from slopes of the plot of the ratio of reprotonation to elimination product against buffer concentration. This leads to a Bronsted a value of 0.47 for reprotonation to dihydroxyacetone phosphate (reprotonation to glyceraldehyde phosphate could not be observed in the quinuclidinium-catalysed reaction, but in the water-catalysed reaction. 

The theory of absolute reaction rates has not and probably will not, in the foreseeable future, permit the satisfactory a priori calculation of most reaction rates, particularly in condensed phases. What it does do, instead, is to identify an attainable goal toward which speculation, semiempirical calculation, and experimental work may be directed. That is the elucidation of transition state structure. If transition state structure could be determined, by whatever means, as well as the structure of many stable molecules is now known, it would be possible to calculate reaction rates. Further, even well short of enough information to calculate rates, even very approximate and incomplete transition state structures may often suffice for the calculation of such rate-related quantities as isotope effects,Bronsted a values, and Hammett p values. When successful, such partial calculations tend to confirm the partial transition state structures on which they are based and, thus, to permit further progress on a more secure base. We cannot be certain how well pleased Henry Eyring is with this hybrid of theory and empiricism, but it doesn t seem likely that he is too disturbed. The pragmatist, pleased with what works and ready to take information from anywhere, has always seemed to be as well represented in his work as the rigorous theoretician. 

The characterisation of transition states by 2-dimensional structure-reactivity correlations has been dealt with rigorously [131]. The type of behaviour described above characterises a transition state in which the reaction coordinate is in an ENE —SSW direction. The observation of Bronsted a values of > 0.5, and non-linear variation of rate with aglycone acidity, with most substituents giving a negative i value, suggests that qualitatively the free-energy profile looks hke Fig. 9d. ... 

Bronsted a values for a few general-acid-catalyzed reactions are given in Table 10.1. Our discussion in Section 9.3.5 showed that the a value is interpreted as the extent of protonation of the reactant by the acid in the transition state. The fact that the Bronsted plots are linear for carbc nyl hydrations means that the extent of protonation at the transition state remains the same regardless of the strength of the acid. 

Abstract. Alkoxide ion departure from Meisenheimer complexes of the 1,l-dialkoxy-2,6-dinitro-4-X-cyclohexadienate type is catalyzed by pyridinium ions and by the hydronium ion. Bronsted a values which vary between 0.35 and 0.65 decrease when the alkoxy group becomes more electron withdrawing and increase when the X-substituent is made more electron withdrawing. This is inconsistent with a mechanism where proton transfer is rate limiting but in complete agreement with a concerted mechanism of acid catalysis as shown with the help of More O Ferrall-Jencks energy diagrams. 

Table 1. Bronsted a values for the addition and dehydration steps...

There is a relationship between the magnitude of the Bronsted a value (see section IV.B) and the reactivity of the carbonyl... 

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