Aac2 mechanism

It appears that the Aac2 mechanism gives a better agreement with experimental results than that of Goldschmidt. This is not in contradiction with the fact that the alcohol is more basic than the acid and that, consequently, the concentration of R OH is higher than that of RC(OH) since in any case a proton transfer from R OH to the acid is possible. 

Reaction orders in alcohol and add were obtained from a study of the first of these reactions under non-stoichiometric conditions. This is not possible for a polyesterification, carried out in an excess of one of the reactants since in this case stoichiometry is required. The orders in acid and alcohol relative to the reaction of 1-dodecanol with dodecanoic add in dodecyl dodecanoate are 2 and 1 respectively which, according to Solomon, corresponds to an Aac2 mechanism. Since the dielectric constant is low, the ions are assumed to be associated as ion pairs ... 

The acid-catalyzed hydrolysis of enol esters (RCOOCR =CR) can take place either by the normal Aac2 mechanism or by a mechanism involving initial protonation on the double-bond carbon, similar to the mechanism for the hydrolysis of enol ethers given in 10-6, ° depending on reaction conditions. In either case, the products are the carboxylic acid RCOOH and the aldehyde or ketone R2" CHCOR. ... 

For acid catalysis, matters are less clear. The reaction is generally second order, and it is known that amides are primarily protonated on the oxygen (Chapter 8, Ref. 24). Because of these facts it has been generally agreed that most acid-catalyzed amide hydrolysis takes place by the Aac2 mechanism. 

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... 

Figure 2.3 Aac2 mechanism for esterification/hydrolysis and transesterification/ glycolysis...

Esterification, hydrolysis, transesterification and glycolysis have equilibrium constants close to unity and proceed via an AAc2 mechanism (Figure 2.3) [9, 18],... 

The transesterification and glycolysis reactions proceed via the Aac2 mechanism described above in Section 2.1. The reactions are acid catalyzed as demonstrated by Chegolya el al. [27], who added TPA to the polycondensation of PET and observed a significant increase of the apparent reaction rate. The industrial polycondensation process is accelerated by the use of metal catalysts, with these being mainly antimony compounds. 

Further evidence for this mechanism is that a small but detectable amount of 180 exchange (see p. 332) has been found in the acid-catalyzed hydrolysis of benzamide.551 (180 exchange has also been detected for the base-catalyzed process,562 in accord with the Bac2 mechanism). Kinetic data have shown that three molecules of water are involved in the ratedetermining step,563 suggesting that, as in the Aac2 mechanism for ester hydrolysis (0-10), additional water molecules take part in a process such as... 

With solutions of low acidity the increase in the concentration of the conjugate acid of the ester outweighs the decrease in activity of water, and the rate of hydrolysis by the A2 mechanism increases with increasing acid concentration, as expected. Most esters are, in fact, hydrolyzed by the Aac2 mechanism in less concentrated solutions of strong acids all the esters in Fig. 1, for ex-... 

In fact this pathway cannot be a great deal more favourable than the normal ester hydrolysis (Aac2) mechanism since this appears in the case of methoxy-methyl formate80, is dominant in the hydrolysis of /3-chloroethoxy methyl acetate81, CICH2CH2OCH2OCOCH3, and is apparently the sole important mechanism for the hydrolysis of methylene diacetate, CH ,COO CH2-OCOCH,82. 

Recently, Noyce and Pollack306 have found kinetic evidence for a changeover from acyl to alkyl-oxygen cleavage in the hydrolysis of a-acetoxystyrenes. At low acidities (1 M H2S04) compounds with electron-withdrawing substituents are hydrolyzed in a reaction which behaves as expected for the Aac2 mechanism. The solvent deuterium isotope effect, kH/kD = 0.75, and the effect of substituents on the rate is small. As the acidity of the medium is increased... 

The two unimolecular mechanisms for acid-catalyzed ester hycrolysis described above represent exceptional behaviour. They are generally observed only with compounds with narrowly defined structural characteristics, or under extreme conditions. The vast majority of esters are hydrolyzed under the vast majority of acidic conditions by the Aac2 mechanism, and since ester hydrolysis has been a traditional proving ground for theories relating structure and reactivity, a wealth of data is available. Yet the detailed mechanism of the reaction is still in dispute. 

A rather similar result has been obtained much more recently by Sadek et a/.108. These authors carried out a detailed investigation of the hydrolysis of benzyl acetate in aqueous dioxan, varying the solvent composition, and also the temperature (from 25 to 45°C). In the low-acidity region used (0.05 M HC1) the ester is hydrolyzed by the Aac2 mechanism, and the solvent effects are probably typical. The rate of hydrolysis decreases steadily as the proportion of dioxan in the medium is increased, as a result of a steady increase in AH, which is only partially offset by an increasingly favourable entropy of activation. However, the increase in AS1 eventually levels out at about 70% w/w dioxan, as illustrated by the data for 30°C ... 

If it is assumed that ester hydrolysis by the AAc2 mechanism involves fast pre-equilibrium protonation of the substrate, followed by rate-determining attack of water on the conjugate acid of the ester, the mechanism can be written as... 

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