Acid catalysis mechanisms

The relative importance of the potential catalytic mechanisms depends on pH, which also determines the concentration of the other participating species such as water, hydronium ion, and hydroxide ion. At low pH, the general acid catalysis mechanism dominates, and comparison with analogous systems in which the intramolecular proton transfer is not available suggests that the intramolecular catalysis results in a 25- to 100-fold rate enhancement At neutral pH, the intramolecular general base catalysis mechanism begins to operate. It is estimated that the catalytic effect for this mechanism is a factor of about 10. Although the nucleophilic catalysis mechanism was not observed in the parent compound, it occurred in certain substituted derivatives. 

However, it may be that the acid catalysis mechanism is more complex than that described by Scheme 6-5. Broxton and McLeish (1983) investigated the (ii)-diazo ether of a benzene derivative with a carboxy group in the 2-position. They also observed acid catalysis by added carboxylic acids, but, interestingly enough, the reactivity decreased as the strength of the added acid increased. 

The commonly accepted mechanism for the catalytic reaction of lysozyme is the so-called general acid catalysis mechanism. 

In the pH range 2—3.5 the phosphonate (78) hydrolyses with loss of ROH at approximately 10 times the rate of comparable esters lacking the vicinal oxime function or in which this function is methylated on oxygen. An intramolecular general-acid catalysis mechanism was proposed, but it was not possible to exclude entirely an intramolecular nucleophilic attack at phosphorus. Intramolecular attack by the vicinal dimethylamino-group takes place preferentially at carbon rather than phosphorus in the phos-phonofluoridate (79). ... 

These experimental results suggested a hydrogen-bonding mediated cooperative Bronsted acid catalysis mechanism (Scheme 6.28). Thiourea cocatalyst 9 is viewed to coordinate to mandelic acid 20 through double hydrogen-bonding, stabilizes the acid in the chelate-hke cis-hydroxy conformation, and acidifies the a-OH proton via an... 

By avoiding the acid catalysis mechanism of the conventional FCC zeolite catalyst (optimized over the years for high octane gasoline), the novel MAB catalyst will produce substantially lower aromatics in the liquid products than is possible by less extreme FCC catalyst adaptations. By changing the FCC reaction system, it is possible to overcome the MAB catalyst low activity drawback and achieve slurry yields compatible with those observed in maximum distillate operation in today s FCC units. 

One method of deciding between Mechanisms I and II is to look at the trend of a in acid-catalyzed additions of various nucleophiles to a carbonyl group.26 It follows from the reacting bond rules that in true general acid catalysis (Mechanism I), the sensitivity of the rate to acidity of the catalyst, and therefore also a, should decrease as the species adding is made more nucleophilic. The reason is that this variation will cause the change in reaction coordinate shown in... 

Scheme 11.1 Stepwise acid-catalysis mechanism for the hydrolysis of acetals (a) and the concerted alternative (b).

To sum up the acid-catalysis mechanisms, Aac2 and AalI are the common mechanisms, the latter for R that give stable carbocations, the former for practically... 

On the other hand, for both Pt(l 10) and Pt(lll) stepped surfaces, it is observed that the reorientation kinetics of the water adlayer becomes slower as the proton concentration decreases. This behavior could be explained by an acid-catalysis mechanism. In order to check this hypothesis, the laser-induced transients of Pt(llO) in (0.1 - x) M KCIO4 + X M HCIO4 solutions, where x equals 10 ", 10, 10 and 0.1, were fitted with expression (97). The result shows that, as expected for an acid-catalysis mechanism, the reaction rate is proportional to the proton concentration, and hence, the response time is inversely proportional to the proton concentration. Figure 21 compares the simulated and ejqrerimental curves. The good agreement between these curves corroborates the hypotheses of an acid-catalysis mechanism. 

The hydrolysis of acetals normally involves a specific-acid catalysis mechanism. However, the hydrolysis of tropone diethylketal is found to use a general-acid catalysis mechanism. Why is there this difference ... 

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