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21.7 Hydrolysis of Carboxylic Acid Derivatives

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21.7 Hydrolysis of Carboxylic Acid Derivatives 21_BRCLoudon_pgs5-2.qxd 12/15/08 11:44 AM Page 1004 1004 CHAPTER 21 • THE CHEMISTRY OF CARBOXYLIC ACID DERIVATIVES substitution. Acyl substitution can be represented generally as follows, with E an elec- trophilic group and Y a nucleophilic group: = = O O S S R C X E Y R C Y E X (21.6) carboxylicL L acid+ L LanotherL + L derivative carboxylic acid derivative O an acyl group S R C X L L The term acyl substitution comes from the fact that substitution occurs at the carbonyl carbon of an acyl group. In other words, an acyl group is transferred in Eq. 21.6 between an X and a Y group. The group X might be the Cl of an acid chloride, the OR of an ester,L and soL on; this group is substitutedL by another groupL Y. This is precisely theL same type of reac- tion as esterification of carboxylic acids ( X L OH, E Y H OCH3; Sec. 20.8A). Acyl substitution reactions of carboxylic acidL derivatives= L areL the =majorL focus of this chapter. Although nitriles are not carbonyl compounds, the C'N bond behaves chemically much like a carbonyl group. For example, a typical reaction of nitriles is addition. Y RNC ' E Y RN"C A E (21.7) L 3+ L L 2 L (Compare this reaction with addition to the carbonyl group of an aldehyde or ketone.) Although the resulting addition products are stable in some cases, in most situations they react further. Like aldehydes and ketones, carboxylic acid derivatives undergo certain reactions involving the a-carbon. The a-carbon reactions of all carbonyl compounds are grouped together in Chapter 22. The reactivity of amides at nitrogen is discussed in Sec. 23.11D. 21.7 HYDROLYSIS OF CARBOXYLIC ACID DERIVATIVES All carboxylic acid derivatives have in common the fact that they undergo hydrolysis (a cleav- age reaction with water) to yield carboxylic acids. A. Hydrolysis of Esters Saponification of Esters One of the most important reactions of esters is the cleavage re- action with hydroxide ion to yield a carboxylate salt and an alcohol. The carboxylic acid itself is formed when a strong acid is subsequently added to the reaction mixture. O OCH3 O O_ Na| O OH C C C # % # % # % " " " 20% NaOH HCl OH (21.8) _ 5–10 min + %NO2 %NO2 %NO2 methyl 3-nitrobenzoateCH3OH 3-nitrobenzoic acid + (90–96% yield) 21_BRCLoudon_pgs5-2.qxd 12/15/08 11:44 AM Page 1005 21.7 HYDROLYSIS OF CARBOXYLIC ACID DERIVATIVES 1005 Ester hydrolysis in aqueous hydroxide is called saponification because it is used in the production of soaps from fats (Sec. 21.12B). Despite its association with fatty-acid esters, the term saponification can be used to refer to the hydrolysis in base of any carboxylic acid derivative. The mechanism of ester saponification involves the reaction of the nucleophilic hydroxide ion at the carbonyl carbon to give a tetrahedral addition intermediate from which an alkoxide ion is expelled. 1 1 1 1 O 1 O _ O S3 3 3 3 S3 3 R CO CH R" CO CH RCO H OCH (21.9a) 1 3 3 _ 3 LL1 LL1 LL1 + 3 1 OH "OH _ 1 3 1 tetrahedral3 addition intermediate The alkoxide ion, after expulsion as a leaving group (methoxide in Eq. 21.9a), reacts with the acid to give the carboxylate salt and the alcohol. 1 1 1 O O 1 S3 3 S3 3 RCO H _ OCH3 RCO _ H OCH3 (21.9b) LLL1 + 3 1 LL1 3 + L 1 pKa 4.5 pKa 15 = = The equilibrium in this reaction lies far to the right because the carboxylic acid is a much stronger acid than methanol. Le Chaˆtelier’s principle operates: The reaction in Eq. 21.9b removes the carboxylic acid from the equilibrium in Eq. 21.9a as its salt and thus drives the hydrolysis to completion. Hence, saponification is effectively irreversible. Although an excess of hydroxide ion is often used as a matter of convenience, many esters can be saponified with just one equivalent of _OH. Saponification can also be carried out in an alcohol solvent, even though an alcohol is one of the products of the reaction. If saponification were reversible, an alcohol could not be used as the solvent because the equilibrium would be driven toward start- ing materials. Acid-Catalyzed Ester Hydrolysis Because esterification of an acid with an alcohol is a reversible reaction (Sec. 20.8A), esters can be hydrolyzed to carboxylic acids in aqueous so- lutions of strong acids. In most cases, this reaction is slow and must be carried out with an ex- cess of water, in which most esters are insoluble. Saponification, followed by acidification, is a much more convenient method for hydrolysis of most esters because it is faster, it is irre- versible, and it can be carried out not only in water but also in a variety of solvents—even alcohols. As expected from the principle of microscopic reversibility (Sec. 4.9B), the mechanism of acid-catalyzed hydrolysis is the exact reverse of the mechanism of acid-catalyzed esterifica- tion (Sec. 20.8A). The ester is first protonated by the acid catalyst: 1 H OH| 2 1 L 1 H H H | 1 1 1 O 1 O O O % % % S3 3 S3 3 3 RCOCH3 R C OCH3 R "C OCH3 R "C A O|CH3 H2O LL1 LL1 LL| 1 L 1 + 1 (21.10a) 21_BRCLoudon_pgs5-2.qxd 12/15/08 11:44 AM Page 1006 1006 CHAPTER 21 • THE CHEMISTRY OF CARBOXYLIC ACID DERIVATIVES As in other acid-catalyzed reactions at the carbonyl group, protonation makes the carbonyl carbon more electrophilic by making the carbonyl oxygen a better acceptor of electrons. Water, acting as a nucleophile, reacts at the carbonyl carbon and then loses a proton to give the tetra- hedral addition intermediate: H O| OH OH S % 3 3 2 H2O 3 2 (21.10b) R C OCH3 R "C OCH3 2 3 R "C OCH3 H3O| LL LL LL + 2 OH2 | "OH "OH L 3 2 3 tetrahedral3 addition "H intermediate2 Protonation of the leaving oxygen converts it into a better leaving group. Loss of this group gives a protonated carboxylic acid, from which a proton is removed to give the carboxylic acid itself. OH OH H H OH| 2 R "C OCH3 L R "C" OCH3 L L H2O2 LL 2 | ""OH 2 2 OH 2 2 OH OH| OH OH 3 22S3 3 OH 3 2 ||2 R "COH R COH R "COHA 3 2 R "COA H3O LLL| 2 LLL2 LL2 L 2 + 2 2 CH3OH2 2 (21.10c) + 2 Let’s summarize the2 important differences between acid-catalyzed ester hydrolysis and ester saponification. First, in acid-catalyzed hydrolysis, the carbonyl carbon can react with the STUDY GUIDE LINK 21.3 relatively weak nucleophile water because the carbonyl oxygen is protonated. In base, the car- Mechanism of Ester Hydrolysis bonyl oxygen is not protonated; hence, a much stronger base than water—namely, hydroxide ion—is required to react at the carbonyl carbon. Second, acid catalyzes ester hydrolysis, but base is not a catalyst because it is consumed by the reaction in Eq. 21.9b. Finally, acid-cat- alyzed ester hydrolysis is reversible, but saponification is irreversible, again because of the Further Exploration 21.2 ionization in Eq. 21.9b. Cleavage of Tertiary Ester hydrolysis and saponification are both examples of acyl substitution (Sec. 21.6). Esters and Carbonless Carbon Paper Specifically, the mechanisms of these reactions are classified as nucleophilic acyl substitu- tion mechanisms. In a nucleophilic acyl substitution reaction, the substituting group reacts as a nucleophile at the carbonyl carbon. As in the reactions of aldehydes and ketones (Fig. 19.8, p. 909), nucleophiles approach the carbonyl carbon from above or below the plane of the car- bonyl group (Fig. 21.5), first interacting with the p* (antibonding) molecular orbital of the carbonyl group. As the result of this reaction, a tetrahedral addition intermediate is formed. The leaving group is expelled from this intermediate, departing from above or below the plane of the new carbonyl group. In saponification, the nucleophile is _OH, and in acid-catalyzed hydrolysis, the nucleophile is water; in both cases, the OR group of the ester is displaced. With the exception of the reactions of nitriles, most of Lthe reactions in the remainder of this chapter are nucleophilic acyl substitution reactions. They follow the same pattern as saponifi- cation, the only substantial difference being the identity of the nucleophiles and the leaving groups. 21_BRCLoudon_pgs5-2.qxd 12/15/08 11:44 AM Page 1007 21.7 HYDROLYSIS OF CARBOXYLIC ACID DERIVATIVES 1007 sp2-hybridized sp3-hybridized sp2-hybridized carbon carbon carbon ≈109Њ .. 1 .. ≈120Њ .. O.. R O.. ≈120Њ R´O C 1 Nuc Nuc _ Nuc 3 tetrahedral R´O addition intermediate Figure 21.5 The geometry of nucleophilic acyl substitution.The nucleophile (Nuc _) approaches the carbonyl 3 carbon above or below the carbonyl plane (gray) to form a tetrahedral intermediate.The leaving group (R O_) de- parts from above or below the plane of the newly formed carbonyl group (blue).The green arrows show the´ move- ment of the various groups. Hydrolysis and Formation of Lactones Because lactones are cyclic esters, they un- dergo the reactions of esters, including saponification. Saponification converts a lactone com- pletely into the carboxylate salt of the corresponding hydroxy acid. O O L L S L S C C O_ H2C O OH H2C L (21.11) _ L L + L H2C CH2 H2C CH2 OH L L L g-butyrolactone g-hydroxybutyrate Upon acidification, the hydroxy acid forms.
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