Ketals hydrolysis mechanism

The second step in acetal and ketal hydrolysis is conversion of the hemiacetal or hemiketal to the carbonyl compound. The mechanism of this step is similar to that of the first step. Usually, the second step is faster than the initial one. Hammett a p plots and solvent isotope effects both indicate that the transition state has less cationic character than... 

A criterion of mechanism based on the Hammett acidity function, H0 (Section 3.2, p. 130),has long been used to decide the type of question raised by the choice between Mechanisms I and II in Scheme 8. Since in strongly acidic media the concentration of the protonated substrate should be proportional to h0, the reaction rate for a unimolecular decomposition of this protonated substrate (Mechanism I) should also be proportional to h0, whereas if a water molecule is required (Mechanism II), the rate should follow H30+ concentration instead. This test, known as the Zucker-Hammett hypothesis,76 when applied to acetal and ketal hydrolysis, appears to confirm the A-l mechanism, since a linear relationship is found between rate constant and h0 at high acidity.77 Inconsistencies have nevertheless been found in application of the Zucker-Hammett hypothesis, for example failure of the plots of log k vs. — H0 to have the theoretical slope of unity in a number of cases, and failure to predict consistent mechanisms for forward and reverse reactions the method is therefore now considered to be of doubtful validity.78 Bunnett has devised a more successful treatment (Equation 8.45), in which the parameter to measures the extent of... 

It was pointed out in the mechanistic discussion concerning acetal and ketal hydrolysis that general acid catalysis occurs only for acetals and ketal having special structural features. Usually, specific acid catalysis operates. The question of whether general acid catalysis could be observed in intramolecular reactions has been of interest because intramolecular general acid catalysis is postulated to play a part in the mechanism of action of the enzyme lysozyme, which hydrolyzes the acetal linkage present in certain polysacharides. One group of molecules that has been examined as a model system are acetals derived from o-hydroxybenzoic acid (sali-... 

Pos twe-Tone Photoresists. The ester, carbonate, and ketal acidolysis reactions which form the basis of most positive tone CA resists are thought to proceed under specific acid catalysis (62). In this mechanism, illustrated in Figure 22 for the hydrolysis of tert-huty acetate (type A l) (63), the first step involves a rapid equihbrium where the proton is transferred between the photogenerated acid and the acid-labile protecting group ... 

The mechanism of this hydrolysis reaction has been studied in great detail. Tb mechanism is the reverse of that for acetal or ketal formation. 

One of the best characterized of solution reactions is the hydrolysis of simple acetals and ketals which almost certainly utilizes the A-l reaction path. The mechanism is as follows (Kreevoy and Taft, 1955 O Gorman and Lucas, 1950) ... 

All the stages in this mechanism are reversible and so it is possible to convert the acetal or ketal back to the original carbonyl compound using water and an aqueous acid as catalyst. Since water is added to the molecule in the reverse mechanism, this is a process known as hydrolysis. 

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]. 

There is no reason to doubt that the sequence of steps in the mechanism of hydrolysis of the ring compounds is the same as in the hydrolyses of dialkyl acetals and ketals. However, it must be expected that carbonium ion formation in the second step is reversible for the reactions of the ring compounds because the intramolecular return step (ring-closure) can successfully compete with the attack of water in the third step, i.e. 

Hydrolysis, of Acetals, Ketals, and Ortho Esters, Mechanism and Catalysis for (Cordes). 

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