Aac2 hydrolyse

Further evidence for the Aa11 mechanism was obtained from a solvent kinetic isotope study. The theoretical kinetic isotope effects for intermediates in the three reaction pathways as derived from fractionation factors are indicated in parentheses in Scheme 6.143,144 For the Aa11 mechanism (pathway (iii)) a solvent KIE (/ch2o A d2o) between 0.48 and 0.33 is predicted while both bimolecular processes (pathways (i) and (ii)) would have greater values of between 0.48 and 0.69. Acid-catalysed hydrolysis of ethylene oxide derivatives and acetals, which follow an A1 mechanism, display KIEs in the region of 0.5 or less while normal acid-catalysed ester hydrolyses (AAc2 mechanism) have values between 0.6 and 0.7.145,146... 

The Aac2 mechanism (Figure 6.22) of ester hydrolysis represents an SN reaction at the carboxyl carbon, which follows the general mechanism of Figure 6.5. Acid-catalyzed hydrolyses of carboxyhc esters that are derived from primary or from secondary alcohols take place according to the Aac2 mechanism. The reverse reactions of these hydrolyses follow the same mechanism, namely, the acid-catalyzed esterifications of carboxylic acids with alcohols. In the esterifications, the same intermediates are formed as during hydrolysis, but in the opposite order. 

Die Weiterreaktion nach Weg oder hangt von den elektronischen Verhaltnissen der Reste R ab. Bci der saurekatalysierten Hydrolyse wird iiblicherweise ein AAC2-Mechanismus mit einer primaren Proto-nierung des Carbonyl-O-Atoms gefunden ... 

It is possible to shift ester hydrolyses away from the normal Aac2 or Bac2 mechanisms by structural changes in the substrate. When the ester is derived from a tertiary alcohol, acid-catalyzed hydrolysis often shifts to a mechanism involving alkyl-oxygen fission. This shift is due to the stability of the carbonium ion that can be formed by C-O heterolysis, and probably also to a decrease in the rate of nucleophilic attack at the carbonyl group because of steric factors. Alkenes, as well as alcohols, may be produced from the carbonium ion, since water can function either as a nucleophile or as a Br0nsted base. 

Hydrolyses with maximum curve (type I and II behavior) are easily explained by the Aac2 mechanism. The initial rate increase is attributed to the increasing concentration of the protonated ester. The rate then decreases since concentration of water required for the hydrolysis step decreases rapidly with increasing acid concentration (Scheme 8). The final rate increase suggests a transition to the A1 process (Aac 1 for type I esters and Aal 1 for type II esters). There is also a change in the mechanism for type III esters. Since the participation of vinyl and phenyl cations are unlikely, a change to the Aac 1 mechanism was suggested. Finally, the hydrolysis pathway for type IV esters is the AalI mechanism. 

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