AacI mechanism

The other mechanism involving acyl cleavage is the AacI mechanism. This is rare, being found only where R is very bulky, so that bimolecular attack is sterically hindered, and only in ionizing solvents. The mechanism has been demonstrated for esters of 2,4,6-trimethylbenzoic acid (mesitoic acid). This acid depresses the freezing point of sulfuric acid four times as much as would be predicted from its molecular weight, which is evidence for the equilibrium... 

AacI mechanism is also found when acetates of phenols or of primary alcohols are hydrolyzed in concentrated (> 90%) H2SO4 (the mechanism under the more usual dilute acid conditions is the normal Aac2)- ... 

The first step is the same as that of the AacI mechanism (10-10)which explains why good results are obtained with hindered substrates. The mechanism with ketones is"" ... 

In a comparable solution of benzoic acid the freezing point is depressed only twice the predicted amount, indicating only a normal acid-base reaction. Further, a sulfuric acid solution of methyl mesitoate when poured into water gave mesitoic acid, while a similar solution of methyl benzoate similarly treated did not.534 The AacI mechanism is also found when acetates of phenols or of primary alcohols are hydrolyzed in concentrated (more than 90%) H2SO4 (the mechanism under the more usual dilute acid conditions is the normal Aac2).535... 

For steric reasons the 2,6-disubstituted benzoic acids and esters are particularly susceptible to this type of cleavage reaction, and also particularly unreactive in the usual bimolecular solvolytic processes, and they have proved very convenient substrates for the study of the AacI mechanism. The kinetic work is discussed in a later section we are concerned at this point only with the qualitative behaviour of protonated esters amt acids, and of the structures of the cationic species. 

Esters from carboxylic acids and alcohols under acid catalysis the Aac2 and AacI mechanism... 

This mechanism (A = acid catalyzed, AC = acyl transfer, 1 = unimolecular) is observed in the esterification of 2,4,6-trisubstituted benzoic acids with R groups of moderate -i-M effect e.g. methyl). The Aac2 mechanism is blocked by the steric interference of the ortho substituents. Therefore, the acylium cation (17) is generated with anhydrous sulfuric acid and then treated with the alcohol (equation 2). R groups with strong -i-M effects, like methoxy, are not tolerable, as the aromatic nucleus undergoes sulfo-nation under the conditions. A variation of the AacI mechanism for aliphatic acids is achieved by using... 

Unlike acetals, for which one mechanism seems to describe most of the hydrolysis reactions that have been studied, the mechanism of acid-catalyzed hydrolysis of an ester depends on the structure of the ester and on the reaction conditions. For example, Yates concluded that hydrolysis of primary esters occurs by the Aac2 mechanism below 90% H2SO4 but changes to the AacI mechanism in solutions with higher concentrations of sulfuric acid (and... 

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