Acetals general acid catalysis

Highly-efficient general acid catalysis of acetal hydrolysis is involved in the reactions of glycosidase enzymes such as lysozyme (Dunn and Bruice, 1973)... 

Thioethers of azines are generally oxidizable to sulfones. By analogy (466) can be oxidized to its sulfone and the intermediate sulfoxide may be isolated. The latter, when subjected to the Pummerer rearrangement, yields the 2-acetoxy derivative (467), which on acid catalysis eliminates acetic acid to form the thiazole (79CPB1207). 

General Acid Catalysis of Acetal, Ketal. and Orthoester Hydrolysis. Fife, T. H. Acc. Chem. Res. 1972, 5, 264. 

Intermolecular general acid-catalysis occurs in a very limited number of known hydrolysis reactions. The hydrolysis of simple amides is susceptible to general acid catalysis by acetic acid at elevated temperatures. General acid catalysis in phosphate buffers is observable in one instance, the lactonization of 2-(hydroxymethyl)-benzoic acid (Weeks, 1965). Small intermolecular general acid terms have been observed 11 ... 

A-SE2 General acid catalysis of acetal in which the protonation of the substrate is the rds. 

Estimates of effective molarities in general acid catalysis of acetals require the analogous intermolecular reaction to be observable and therefore, as we have seen in the previous section, for the oxocarbenium ion to be particularly stable. The termolecular processes corresponding to the hydrolysis of salicyl... 

Figure 6.66 More O Ferrall-Jencks diagram for elimination reactions (like the general acid catalysis of acetals, this is a class e reaction and is drawn in the same way as Figure 3.16).

Scheme 8.1. Acetals and Ketals That Exhibit General Acid Catalysis in Hydrolysis...

A mixed acetal of benzaldehyde, methanol, and salicylic acid has also been studied. It, too, shows a marked rate enhancement attributable to intramolecular general acid catalysis ... 

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. 

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