Hydrolysis of an ester

Figure 3-5. The search for a) oxidations of primary alcohols to carboxylic acids will obtain reaction b) as a hit, although this reaction is in reality a hydrolysis of an ester, c) The correct specification of the query to obtain reactions invoivingthe oxidation of aicohols to carboxyiic acids.

Drying by hydrolysis. The production of extremely dry (99 -9-(- per cent.) ethyl alcohol from commercial absolute alcohol (99-f percent.) is possible by taking advantage of the fact that the hydrolysis of an ester consumes water. Thus if the absolute alcohol is treated with a little sodium in the presence of an ester of high boiling point e.g., ethyl... 

In the ease of a seeond order reaetion, an example is the alkaline hydrolysis of an ester as represented by the following equation ... 

Sometimes reduction of a ketone by NaBH4 is accompanied by hydrolysis of an ester elsewhere in the molecule. Norymberski found that a 20-keto-21-acetoxy compound with NaBH4 in methanol at 0° for 1 hour gives the 20/ ,21-diol. 50 % aqueous dimethylformamide has been used as the solvent in an attempt to prevent acetate hydrolysis, but sometimes under these conditions the 21-acetoxy group migrates to the 20-position. The rearrangement is favored by addition of the 20-acetate as seeds or by addition of... 

The kinetics of alkaline hydrolysis of an ester can be followed by the pH-stat method, in which the pH is held constant by adding a solution of strong alkali to the reacting ester solution. The volume of alkali added in order to keep the pH constant is recorded as a function-time. Find the volume function needed to determine the rate constant that is, given the volume-time data for this type of experiment, what plot or calculation will yield the rate constant ... 

In deciding whether to write an elementary reaction as either a reversible or an irreversible reaction, we take the practical view that if the reverse reaction is negligibly slow on the exp>erimental time scale, the reaction is essentially irreversible. Consider the alkaline hydrolysis of an ester, for which the rate equation is... 

Saponification (Section 21.6) An old term for the base-induced hydrolysis of an ester to yield a carboxylic acid salt. 

Esterases. Enzymes that catalyze the hydrolysis of an ester bond. 

C22-0084. In the hydrolysis of an ester, one oxygen atom in the products comes from the ester and the other comes from a water molecule R CO — O — R + H2 O i CO — OH + HO — R Describe experiments that would determine If the oxygen atom In the alcohol, HO — R, comes from water or from the ester. 

Chemical reactions usually occur through collision of at least two molecules, very often in a solution, and the rate of the chemical reaction is proportional to the concentrations of all reacting molecules. For example, the rate of hydrolysis of an ester in an alkaline buffered solution depends upon the concentration of both the ester and hydroxide ion ... 

In another approach, the alcohol moiety, formed by an enzymatic hydrolysis of an ester, can act as a nucleophile. In their synthesis of pityol (8-37a), a pheromone of the elm bark beetle, Faber and coworkers [17] used an enzyme-triggered reaction of the diastereomeric mixture of ( )-epoxy ester 8-35 employing an immobilized enzyme preparation (Novo SP 409) or whole lyophilized cells of Rhodococcus erythro-polis NCIMB 11540 (Scheme 8.9). As an intermediate, the enantiopure alcohol 8-36 is formed via kinetic resolution as a mixture ofdiastereomers, which leads to the diastereomeric THF derivatives pityol (8-37a) and 8-37b as a separable mixture with a... 

The first observation of a significant rate enhancement in the alkaline hydrolysis of an ester by a suitably positioned carbonyl group that was related to prior attack of hydroxide at the carbonyl group was made in 1955 (Djerassi and Lippman, 1955). However, in 1962, a more detailed mechanistic pathway was suggested which involved attack by hydroxide at an o-formyl or benzoyl group, followed by intramolecular nucleophilic attack on a benzoate ester (Newman and Hishida, 1962 Bender and Silver, 1962). 

The very important type of reaction known as enolization is catalyzed by both acids and bases. Also, the hydrolysis of an ester,... 

Carboxylic Esters and of Esterification of Carboxylic Acids. Acid Hydrolysis of an Ester with Heavy Oxygen as Isotopic Indicator. J. chem. Soc. [London] 1939, 838. 

In aqueous solution, the rates of many reactions depend on the hydrogen-ion (H+ or h3o+) concentration and/or on the hydroxyl-ion (OH-) concentration. Such reactions are examples of acid-base catalysis. An important example of this type of reaction is esterification and its reverse, the hydrolysis of an ester. 

Examples of autocatalytic reactions include the decomposition of C2H4I2 either in the gas phase or in solution in CC14 (Arnold and Kistiakowsky, 1933), hydrolysis of an ester, and some microbial fermentation reactions, The first of these may be used to illustrate some observed and mechanistic features. 

The typical metabolic reactions of pyrethroids are hydrolysis of an ester linkage and oxidation of an alkyl group or an aromatic ring in either acid or alcohol moiety, as shown in Fig. 6. The oxidative cleavage of the C=C bond of the prop-l-enyl... 

Mammalian esterases have been classified into three groups according to specificity for substates and inhibitors (110). In terms of overall hydrolytic activity in mammals, the most important class of esterases is that of the B-esterases, which are principally active with aliphatic esters and amides. A-Esterases are important for aromatic esters and organophosphorus esters, and C-esterases are active with acetyl esters. In general, the specificity of mammalian esterases is determined by the nature of substituent groups (acetyl, alkyl, or aryl) rather than the heteroatom (O, N, or S) that is adjacent to the carboxy group. That is, the same esterase would likely catalyze hydrolysis of an ester, amide, or thioester as long as the substituents were identical except for the heteroatom (110). 

FIGURE 6.2 Examples of the relative rates of hydrolysis of an ester, the amide of an aromatic amine, and the amide of an aliphatic amine. 

It is possible to reverse the formation of an ester by a catalyzed hydrolysis reaction. In this reaction, the water reenters the molecule where it was removed. If an acid is the catalyst, the original acid and alcohol will reform. If a base is the catalyst, the alcohol will reform however, the acid will react further to produce its conjugate base. The base-catalyzed hydrolysis of an ester is saponification. 

If the reaction system has a ionic species and if there is a change in number and/or nature of the ionic species during the reaction, the electrical conductance of the solution can be measured as a function of time. Let us consider the hydrolysis of an ester in presence of NaOH... 

Explain, why hydrolysis of an ester follows first-order kinetics in presence of an acid and it follows a second-order kinetics in presence of dilute alkali. 

It has been shown that the catalytic action of acids is proportional to their strength, of which their degree of ionisation is an expression. Conversely, the strength of an acid can be determined by measuring the rate of hydrolysis of an ester (usually methyl acetate) in aqueous solution in the presence of the acid concerned. 

Microorganisms can inactive toxicants by cleavage of a bond by the addition of water. Such reactions may involve a simple hydrolysis of an ester bond, as with the insecticide Malathion by a carboxyesterase enzyme ... 

X. C. Ding, B. F. Rasmussen, G. A. Petsko, D. Ringe, Direct Structural Observation of an Acyl-Enzyme Intermediate in the Hydrolysis of an Ester Substrate by Elastase , Biochemistry 1994, 33, 9285-9293. 

Both esters and amides undergo hydrolysis reactions. In a hydrolysis reaction, the ester or amide bond is cleaved, or split in two, to form two products. As mentioned earlier, the hydrolysis of an ester produces a carboxylic acid and an alcohol. The hydrolysis of an amide produces a carboxylic acid and an amine. There are two methods of hydrolysis acidic hydrolysis and basic hydrolysis. Both methods are shown in Figure 2.9. Hydrolysis usually requires heat. In acidic hydrolysis, the ester or amide reacts with water in the presence of an acid, such as H2SO4. In basic hydrolysis, the ester or amide reacts with the OH ion, from NaOH or water, in the presence of a base. Soap is made by the basic hydrolysis of ester bonds in vegetable oils or animal fats. 

A) The acid hydrolysis of an amide produces a carboxylic acid and an amine. (B) The basic hydrolysis of an ester produces the salt of a carboxylic acid and an alcohol. 

Ding X, Rasmussen BF, Petsko GA, Ringe D. 1994. Direct structural observation of an acyl-enzyme intermediate in the hydrolysis of an ester substrate by elastase. Biochemistry 33 9285-9293. 

Hydrolysis of an ester can be achieved by either base- or acid-catalysed reactions, and the nucleophilic substitution mechanisms follow processes that should... 

A typical role for the histidine imidazole ring is shown below, in the enzyme mechanism for a general base-catalysed hydrolysis of an ester. The imidazole nitrogen acts as a base to remove a proton from water, generating hydroxide that attacks the carbonyl. Subsequently, the alkoxide leaving group is reprotonated by the imidazolium... 

Let us first remind ourselves of the two mechanisms for hydrolysis of an ester, namely acid-catalysed hydrolysis and base-catalysed hydrolysis (see Section 7.9.2). 

This is another exampie of substrate-level phosphorylation, but differs from the earlier example that involved hydrolysis of a mixed anhydride. Here, we have merely the hydrolysis of an ester, and thus a much lower release of energy. In fact, with 1,3-diphosphoglycerate, we specifically noted the difference in reactivity between the anhydride and ester groups. So how can this reaction lead to ATP synthesis The answer lies in the stability of the hydrolysis product, enolpyruvic acid. Once formed, this enol is rapidly isomerized to its keto tautomer, pyruvic acid, with the equilibrium heavily favouring the keto tautomer (see Section 10.1). The driving force for the substrate-level phosphorylation reaction is actually the position of equilibrium in the subsequent tautomerization. 

The mechanistic steps can be deduced by inspection of structures and conditions. Enolate anion formation from diethyl malonate under basic conditions is indicated, and that this must attack the epoxide in an Sn2 reaction is implicated by the addition of the malonate moiety and disappearance of the epoxide. The subsequent ring formation follows logically from the addition anion, and is analogous to base hydrolysis of an ester. Ester hydrolysis followed by decarboxylation of the P-keto acid is then implicated by the acidic conditions and structural relationships. 

The hydrolysis of an ester to alcohol and acid (1) and the esterification of a carboxylic acid with an alcohol (2) are shown here as an example of the Sn2 mechanism. Both reactions are made easier by the marked polarity of the C=0 double bond. In the form of ester hydrolysis shown here, a proton is removed from a water molecule by the catalytic effect of the base B. The resulting strongly nucleophilic OH ion attacks the positively charged carbonyl C of the ester (la), and an unstable sp -hybridized transition state is produced. From this, either water is eliminated (2b) and the ester re-forms, or the alcohol ROH is eliminated (1b) and the free acid results. In esterification (2), the same steps take place in reverse. 

Esters can undergo hydrolysis using either an acid or a base as a catalyst. Hydrolysis always produces an alcohol from the alkyl portion of the ester. During acid hydrolysis, the acid portion of the ester yields a carboxylic acid. During base hydrolysis of an ester, which is called saponification, the acid portion of the ester yields the carboxylate ion. 

The saponification (the base-catalyzed hydrolysis of an ester) of fats has been important since ancient times. This process frees the glycerin and releases the fatty acids as carboxylate ions. The carboxylate ions, along with sodium or potassium ions from the base, create a soap (refer to Figure 16-28). 

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