Carboxylic esters, base basic hydrolysis

Ester hydrolysis in base is called saponification, which means soap making Over 2000 years ago the Phoenicians made soap by heating animal fat with wood ashes Animal fat is rich m glycerol triesters and wood ashes are a source of potassium car bonate Basic hydrolysis of the fats produced a mixture of long chain carboxylic acids as their potassium salts... 

In base the tetrahedral intermediate is formed m a manner analogous to that pro posed for ester saponification Steps 1 and 2 m Figure 20 8 show the formation of the tetrahedral intermediate m the basic hydrolysis of amides In step 3 the basic ammo group of the tetrahedral intermediate abstracts a proton from water and m step 4 the derived ammonium ion dissociates Conversion of the carboxylic acid to its corresponding carboxylate anion m step 5 completes the process and renders the overall reaction irreversible... 

In HO -catalyzed hydrolysis (specific base catalyzed hydrolysis), the tetrahedral intermediate is formed by the addition of a nucleophilic HO ion (Fig. 3.1, Pathway b). This reaction is irreversible for both esters and amides, since the carboxylate ion formed is deprotonated in basic solution and, hence, is not receptive to attack by the nucleophilic alcohol, phenol, or amine. The reactivity of the carboxylic acid derivative toward a particular nucleophile depends on a) the relative electron-donating or -withdrawing power of the substituents on the carbonyl group, and b) the relative ability of the -OR or -NR R" moiety to act as a leaving group. Thus, electronegative substituents accelerate hydrolysis, and esters are more readily hydrolyzed than amides. 

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. 

Amides may be hydrolysed to carboxylic acids by either acids or bases, though hydrolysis is considerably slower than with esters. Although amines are bases and become protonated on nitrogen via the lone pair electrons, we know that amides are not basic (see Section 4.5.4). This is because the lone pair on the nitrogen in amides is able to overlap into the carbonyl... 

Carboxylic acid derivatives are compounds that possess an acyl group (R—C=0) linked to an electronegative atom, e.g. —Cl, —CO2 R, —OR or —NH2. They can be converted to carboxylic acids via simple acidic or basic hydrolysis. The important acid derivatives are acid chlorides, acid anhydrides, esters and amides. Usually nitriles are also considered as carboxylic acid derivatives. Although nitriles are not directly carboxylic acid derivatives, they are conveniently hydrolysed to carboxylic acids by acid or base catalysts. Moreover, nitriles can be easily prepared through dehydration of amides, which are carboxylic acid derivatives. 

Esters, on the other hand, are very common hydrolytic precursors to carboxylic acids. The traditional reaction for the hydrolysis of esters is basic saponification using sodium hydroxide or potassium hydroxide. While acid catalysis can also be employed, preparative methods usually use base catalysis because formation of the carboxylate salt drives the reaction to the right and gives high yields of products. 

It is more common to hydrolyze esters under basic conditions because the equilibrium is favorable. The mechanism for this process, called saponification, is presented in Figure 19.4. The production of the conjugate base of the carboxylic acid, the car-boxylate anion, which is at the bottom of the reactivity scale, drives the equilibrium in the desired direction. To isolate the carboxylic acid the solution must be acidified after the hydrolysis is complete. Some examples are provided in the following equations. We saw another example of this hydrolysis reaction in Chapter 10, where it was... 

Problem-Solving Strategy Proposing Reaction Mechanisms 1007 Mechanism 21-8 Transesterification 1008 21-7 Hydrolysis of Carboxylic Acid Derivatives 1009 Mechanism 21-9 Saponification of an Ester 1010 Mechanism 21-10 Basic Hydrolysis of an Amide 1012 Mechanism 21-11 Acidic Hydrolysis of an Amide 1012 Mechanism 21-12 Base-Catalyzed Hydrolysis of a Nitrile 1014 21-8 Reduction of Acid Derivatives 1014... 

Basic hydrolysis of esters, called saponification, avoids the equilibrium of the Fischer esterification. Hydroxide ion attacks the carbonyl group to give a tetrahedral intermediate. Expulsion of alkoxide ion gives the acid, and a fast proton transfer gives the carboxylate ion and the alcohol. This strongly exothermic proton transfer drives the saponification to completion. A full mole of base is consumed to deprotonate the acid. 

Esters are hydrolyzed in aqueous base to form carboxylate anions. Basic hydrolysis of an ester is called saponification. 

Soap is prepared by the basic hydrolysis or saponification of a triacylglycerol. Heating an animal fat or vegetable oil with aqueous base hydrolyzes the three esters to form glycerol and sodium salts of three fatty acids. These carboxylate salts are soaps, which clean away dirt because of their two structurally different regions. The nonpolar tail dissolves grease and oil and... 

Amides undergo hydrolysis to yield carboxylic acids plus amine on heating in either aqueous acid or aqueous base. The conditions required for amide hydrolysis are more severe than those required for the hydrolysis of acid chlorides or esters, but the mechanisms are similar. The acidic hydrolysis reaction occurs by nucleophilic addition of water to the protonated amide, followed by loss of ammonia. The basic hydrolysis occurs by nucleophilic addition of OH" to the amide carbonyl group, followed by deproto nation of the -OH group and elimination of amide ion NH2). 

Under basic conditions, the hydroxide ion acts as the nucleophile. For example, the mechanism of hydrolysis of ethyl acetate is shown (in fig.K). However, the mechanism does not stop here. The carboxylic acid which is formed reacts with sodium hydroxide to form a water soluble carboxylate ion [Fig.L (a)]. Moreover, the ethoxide ion that is lost from the molecule is a stronger base than water and undergoes protonation [Fig.L (b)]. The basic hydrolysis of an ester is also called saponification and produces a water soluble carboxylate... 

As with hydrolysis of carboxylic esters, amide hydrolysis requires the presence of a strong acid or base, and the nature of the final products depends on which of these reactants is used. Notice in the following examples that under acidic conditions, the salt of the amine is produced along with the carboxylic acid, and under basic conditions, the salt of the carboxylic acid is formed along with the amine. Also, notice that heat is required for the hydrolysis of amides. 

The chemical properties of PVAc are those of an aliphatic ester. Thus, acidic or basic hydrolysis produces poly(vinyl alcohol) and acetic acid or the acetate of the basic cation. Industrially, poly(vinyl alcohol) is produced by a base-catalyzed ester interchange with methanol, where methyl acetate forms in addition to the pol5mieric product. The chemical properties of PVAc can be modified by copolymerization. When a comonomer having a carboxylic acid group or a sulfuric acid group is used, the copolymer becomes soluble in dilute aqueous alkah or... 

Esters are less reactive than acid chlorides and anhydrides in addition reactions, but more reactive than amides. Esters can be converted into their parent carboxylic acids under either basic or acidic aqueous conditions in a process called, logically enough, ester hydrolysis. In base, the mechanism is the familiar addition-elimination one (Fig. 18.31). Hydroxide ion attacks the carbonyl group to form a tetrahedral intermediate. Loss of alkoxide then gives the acid, which is rapidly deproto-nated to the carboxylate anion in basic solution. Notice that this reaction, saponification (p. 862), is not catalytic. The hydroxide ion used up in the reaction is not regenerated at the end. To get the carboxylic acid itself, a final acidification step is necessary. 

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