215 chemistry of acid anhydrides preparation of acid

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21.5 Chemistry of Acid AnhydridesPreparation of Acid Anhydrides: Acid anhydrides are typicallyprepared by nucleophilic acyl substitution reaction of an acidchloride with a carboxylate anion (21.4). Both symmetrical andunsymmetrical acid anhydrides can be prepared in this way.Reactions of Acid Anhydrides: The chemistry ofacid anhydrides is similar to that of acid chlorides,although anhydrides react more slowly. Thus, acidanhydrides react with water to form acids, withalcohols to form esters, with amines to formamides, and with LiAlH4to form primary alcohols.Conversion of Acid Anhydrides into Esters: Aceticanhydride is often used to prepare acetate estersfrom alcohols. Ex. aspirin (acetylsalicylic acid) isprepared commercially by the acetylation of o-hydroxybenzoic acid (salicylic acid) with acetic anhydride.Conversion of Acid Anhydridesinto Amides: Acetic anhydride isalso commonly used to prepareN-substituted acetamides fromamines. Ex.acetaminophen, adrug used in over-the-counter analgesics such as Tylenol, is prepared by reaction ofp-hydroxyanilinewithacetic anhydride. Only the more nucleophilic −NH2 group reacts rather than the less nucleophilic−OH group.
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Notice in both of the previous reactions that only “half” of the anhydride molecule is used, while theother half acts as the leaving group during the nucleophilic acyl substitution step and produces acetateion as a by-product. Thus, anhydrides are inefficient to use, and acid chlorides are normally preferred forintroducing acyl substituents other than acetyl groups.21.6 Chemistry of EstersEsters are among the most widespread of all naturally occurringcompounds. Many simple esters are pleasant-smelling liquids thatare responsible for the fragrant odors of fruits and flowers. Ex.methyl butanoate is found in pineapple oil, and isopentyl acetate isa constituent of banana oil. Theester linkage is also present inanimal fatsand in many biologically important molecules.The chemical industry uses esters for a variety of purposes. Ex.Ethylacetateis acommonly used solvent, and dialkyl phthalates are used asplasticizers to keep polymers from becoming brittle. You may be aware thatthere is current concern about possible toxicity of phthalates at highconcentrations, although a recent assessment by the U.S. FDA found therisk to be minimal for most people, with the possible exception of male infants.Preparation of Esters: Esters are usually prepared from carboxylic acids by the methods discussed. Thus,carboxylic acids are converted directly into esters by SN2 reaction of a carboxylate ion with a primaryalkyl halide or by Fischer esterification of a carboxylic acid with an alcohol in the presence of a mineralacid catalyst. In addition, acid chlorides are converted into esters by treatment with an alcohol in thepresence of base (21.4).

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Term
Spring
Professor
hunt
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pH, Ether, Grignard Reagents
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Chemistry: Principles and Practice
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Chapter 10 / Exercise 10.7
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