General acid-catalysed mechanism

That the reaction was not catalysed by the buffer anion is shown by the data in Table 197, which gives the rate coefficients observed klob, and the rate coefficients ky corr corrected for difference in pH and ionic strength to values of 6.70 and 0.14 respectively. The existence of the general acid-catalysed mechanism for the reaction was demonstrated by the data in Table 198, which gives the rate coeffi-... 

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

In contrast to its effect upon the general mechanism of nitration by the nitronium ion, nitrous acid catalyses the nitration of phenol, aniline, and related compounds. Some of these compounds are oxidised under the conditions of reaction and the consequent formation of more nitrous acids leads to autocatalysis. 

The SB-GA mechanism consists of a rapid equilibrium deprotonation of the ZH intermediate, followed by rate-limiting, general acid-catalysed leaving-group departure from the anionic cr-complex Z via the concerted transition state, 2. The derived expression for this mechanism is equation 4, where fctBH is the rate coefficient for acid-catalyzed expulsion of L from Z and K3 is the equilibrium constant for the reaction ZH Z- + BH. 

The question of protonation sites is one of the basic questions in the behaviour of complex organic molecules in solution, since protonated molecules are intermediates in synthetic organic chemistry, and the knowledge of protonation sites is important for the theory of reaction mechanisms of acid-catalysed reactions. It is also of fundamental importance for structural theory in general, since it is intimately connected with the concepts of mesomerism, electron density and bond polarization. 

The mechanism of the acid-catalysed breakdown of hemiorthoesters Electron releasing substituents when attached to the pro-acyl carbon have a smaller effect on fcH+ for the breakdown of hemiorthoesters than of orthoesters (see p. 67 above) which suggest that these two classes of compounds react by different mechanisms and that the transition state for breakdown of the hemiorthoesters has less carbocationic character. Jencks and his coworkers (Funderburk et al., 1978) proposed mechanism (5) for the breakdown of hemiacetals and a similar mechanism (6) can be written for the breakdown of hemiorthoesters. This would explain (i) the general acid catalysis observed... 

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. 

The kinetics of the nucleophilic monosubstitution of imidazole by /7-nitrophcnolatc in hexaimidazolylcyclotriphosphazene (265) in water-THF have been studied.242 A two-step mechanism involving the cleavage of a pentacoordinated intermediate to (266) in a general acid-catalysed reaction is proposed. As expected for such a mechanism, a change in the slow step occurs at pH >7 when the [buffer] increases and koiK becomes independent of [buffer]. 

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