Esters general acid-catalysed

Now let us see how the imidazole grouping of histidine can be involved in the general acid-catalysed and general base-catalysed hydrolysis of esters by enzymes. In essence, the chemical and enzymic reactions are very similar. This is to be... 

On the basis of the mechanism (6) the alcoholysis (and hydrolysis ) of esters should be general-acid catalysed but as explained above (p. 66) there have been so few investigations that it is not known if this is so. 

If the rate of a reaction is found to be dependent not only on the concentration of the conjugated acid of the solvent, but also on the concentration of any other acid species that might be present, then the reaction is said to be general acid catalysed. In such a case, the catalysis proceeds by way of proton donors in general and is not just limited to (in the case of water as the solvent) hydroxonium ions. General acid catalysis is characteristic of reactions in which the protonation of the substrate is the rate limiting step. An example is the acid catalysed hydrolysis of ortho-esters, RC(OR)3. 

General acid-catalysed hydrolysis of ortho esters (Eqn. 53) has the isotope effect... 

Further examples of the importance of general acid catalysis by water in organic reactions in HTW have been reported. Savage et al. have shown that the observed rate constant for methyl benzoate ester hydrolysis in HTW is comprised of both a specific acid and a general acid-catalysed component (Scheme 3.5(a)). Previous studies on the hydrolysis of esters in HTW had... 

Scheme 3.5 The mechanisms of specific acid-catalysed and general acid-catalysed hydrolysis of (a) ethers and (b) esters.

If we now extend our consideration of base-catalysed (BAC2), and acid-catalysed (Aac2), hydrolysis to esters in general, including aliphatic ones (RC02Et), we see that there is a close similarity between the transition states (42b or 42a) for the rate-limiting step in each of the two pathways they are both tetrahedral and differ... 

In 30% dioxan (Glenn and Kirby, unpublished). The reference intermolecular reaction is the hydrolysis of the methyl ester catalysed by a general acid (RC02H) of pKt 4.72... 

The experimental evidence for ethyl trifluoracetate38 hydrolysis is in accord with transition state XIX and either XIX or XX would be generally applied to other substrates, although transition states of type XXII or XXIII may be possible also. The general acid transition state XXI is unlikely because few intermolecular acid-catalysed ester hydrolyses are known. 

In the synthesis (Expt 8.34), formylacetic acid is formed in situ by the action of a concentrated sulphuric acid-fuming sulphuric acid mixture on malic acid (H02OCH0H CH2 C02H). This a-hydroxydicarboxylic acid undergoes decarbonylation and dehydration under these conditions before the acid-catalysed self-condensation of formylacetic acid. The cyclisation step is generally applicable to / -keto esters. 

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