Ester hydrolysis reaction pathway

Hydrolysis. Esters are cleaved (hydroly2ed) into an acid and an alcohol through the action of water. This hydrolysis is cataly2ed by acids or bases. The mechanistic aspects of ester hydrolysis have received considerable attention and have been reviewed (16). For most esters only two reaction pathways are important. Both mechanisms involve a tetrahedral intermediate and addition-elimination reactions i7i7... 

Acid-catalyzed ester hydrolysis can occur by more than one mechanism, depending on the structure of the ester. The usual pathway, however, is just the reverse of a Fischer esterification reaction (Section 21.3). The ester is first activated toward nucleophilic attack by protonation of the carboxyl oxygen atom, and nucleophilic addition of water then occurs. Transfer of a proton and elimination of alcohol yields the carboxylic acid (Figure 21.8). Because this hydrolysis reaction is the reverse of a Fischer esterification reaction, Figure 21.8 is the reverse of Figure 21.4. 

The lipase-catalyzed fatty acid ester hydrolysis and the lipoxygenation of free polyunsaturated fatty acids are involved in the same lipid degradation pathway. They are respectively the first and second reaction in the lipoxygenase pathway (Fig. 3) [87-91]. The pathway produces volatile products of considerable importance in food technology including Cg[92, 93] or Cg- 94—96 aldehydes and alcohols from polyunsaturated fatty... 

That the major factor responsible for this shift in reaction pathway is indeed a steric one is demonstrated by the observation that the acids (191) and (192), and their simple esters, undergo ready esterification/hydrolysis by the normal Aac2 mode ... 

Considering first the simpler of the two cases, the straight line for the methyl esters (34a) which has a p value of -3-25. From this p value it is apparent that this reaction cannot be proceeding via the normal (Aac2) pathway (p. 241) for acid-catalysed ester hydrolysis which, as we know (reaction 6, p. 364), has a p value of +0-03. That value refers, however, to hydrolysis being carried out with dilute sulphuric acid, while here 99-9% sulphuric acid is being used one... 

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 term acid catalysis is often taken to mean proton catalysis ( specific acid catalysis ) in contrast to general acid catalysis. In this sense, acid-catalyzed hydrolysis begins with protonation of the carbonyl O-atom, which renders the carbonyl C-atom more susceptible to nucleophilic attack. The reaction continues as depicted in Fig. 7. l.a (Pathway a) with hydration of the car-bonium ion to form a tetrahedral intermediate. This is followed by acyl cleavage (heterolytic cleavage of the acyl-0 bond). Pathway b presents an mechanism that can be observed in the presence of concentrated inorganic acids, but which appears irrelevant to hydrolysis under physiological conditions. The same is true for another mechanism of alkyl cleavage not shown in Fig. 7.Fa. All mechanisms of proton-catalyzed ester hydrolysis are reversible. 

Ester hydrolysis, alcoholysis, and aminolysis may be regarded as substitution reactions (via addition-elimination pathways). The activation of (methylthio)methyl esters by S-methylation [51] is an example demonstrating the usefulness of polarity alternation accentuation. 

The observation of lsO-exchange between the carbonyl oxygen atom of an ester and solvent is evidence for the formation of a tetrahedral addition compound, but not proof that this actually lies on the reaction pathway. But the behaviour of the exchange and hydrolysis reactions is so similar that there can be little doubt that this is, in fact, the case. The evidence is most complete for alkaline hydrolysis (see p. 163). One further piece of evidence obtained under acidic conditions is the observation47 that there is no exchange between the solvent and the carbonyl-lsO-labelled ester when methyl 2,4,6-trimethyl-benzoate is hydrolyzed in 3.09,5.78 and 11.5 M sulphuric acid. Other evidence makes it clear that this ester is hydrolyzed by the Aac1 mechanism, and that no reversible addition of water is expected. 

The preceding experiments prove that there is an intermediate on the reaction pathway in each case, the measured rate constants for the formation and decay of the intermediate are at least as high as the value of kcat for the hydrolysis of the ester in the steady state. They do not, however, prove what the intermediate is. The evidence for covalent modification of Ser-195 of the enzyme stems from the early experiments on the irreversible inhibition of the enzyme by organo-phosphates such as diisopropyl fluorophosphate the inhibited protein was subjected to partial hydrolysis, and the peptide containing the phosphate ester was isolated and shown to be esterified on Ser-195.1516 The ultimate characterization of acylenzymes has come from x-ray diffraction studies of nonspecific acylenzymes at low pH, where they are stable (e.g., indolylacryloyl-chymotrypsin),17 and of specific acylenzymes at subzero temperatures and at low pH.18 When stable solutions of acylenzymes are restored to conditions under which they are unstable, they are found to react at the required rate. These experiments thus prove that the acylenzyme does occur on the reaction pathway. They do not rule out, however, the possibility that there are further intermediates. For example, they do not rule out an initial acylation on His-57 followed by rapid intramolecular transfer. Evidence concerning this and any other hypothetical intermediates must come from additional kinetic experiments and examination of the crystal structure of the enzyme. 

The hydrolysis reactions of A -phospho amino acids seen as models for protein dephosphorylation have been studied in Tris-HCl buffer (pH7.5)-DMSO. The reactions were first order and the rates were very much faster than those of simple phosphoamidates. A pentacoordinated phosphorus intermediate is proposed on the reaction pathway.265 The rates of ester exchange reactions of alcohols (nucleoside models) with the oxyphosphorane (299) have been studied and the rates of exchange are much faster for diols than for mono-alcohols.266... 

This tendency to react with a range of nucleophiles is reflected in the general rate equation for reactions of this type, as seen for the hydrolysis of [Co(en)2(H2NCH2C02 Pr)]3+. Typically, a three-term rate equation is obtained. This is indicative of a process in which at least three parallel reaction pathways are being followed. In the case of the hydrolysis of [Co(en)2(H2NCH2C02 Pr)]3+, the kx term refers to attack of the chelated ester by water, the k2 term to attack by hydroxide and the /% term to general base attack by any other nucleophile which is present in solution. The rate is defined in terms of the loss of the starting complex cation, rather than the formation of any specific product of the reaction. 

That a single solvent molecule clustered to a nucleophile can drastically change the reaction pathway has been demonstrated by studying the reaction of phenyl acetate with methoxide ion in the gas phase [671, 672]. Alkaline hydrolysis of esters in solution is known to proceed by attack of the nucleophile at the carbonyl carbon atom to form a tetrahedral intermediate, followed by cleavage of the acyl-oxygen bond (Bac2 mechanism) cf. Eq. (5-138a). 

The reactivity of 02 - with alkyl halides in aprotic solvents occurs via nucleophilic substitution. Kinetic studies confirm that the reaction order is primary > secondary > tertiary and I > Br > Cl > F for alkyl hahdes, and that the attack by 02 - results in inversion of configuration (Sn2). Superoxide ion also reacts with CCI4, Br(CH2)2Br, CeCle, and esters in aprotic media. The reactions are via nucleophilic attack by 02 on carbon, or on chlorine with a concerted reductive displacement of chloride ion or alkoxide ion. As with all oxyanions, water suppresses the nucleophilicity of 02 (hydration energy, lOOkcalmoL ) and promotes its rapid hydrolysis and disproportionation. The reaction pathways for these compounds produce peroxy radical and peroxide ion intermediates (ROO and ROO ). 

As discussed below in detail, first-principles electronic structure calculations have provided accurate predictions of the reaction pathways and the corresponding energy barriers, not only for the first step of hydrolysis of cocaine free base at the benzoyl ester group, but also for the entire reaction processes of hydrolysis of cocaine free base at both the benzoyl ester and methyl ester groups. 

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