chapter 6 carboxylic acids ssters and other derivatives

Chapter 6Carboxylic Acids, Esters and Other Derivatives

General, Organic, and Biological Chemistry, Fifth Edition

H. Stephen Stoker

Brroks/Cole Cengage Learning. Permission required for reproduction or display.

Prepared by:

GIZEL R. SANTIAGO

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Chapter 6 Topics

• Structure of Carboxylic Acids and Their Derivatives• IUPAC Nomenclature for Carboxylic Acids • Common Names for Carboxylic Acids • Polyfunctional Carboxylic Acids • Metabolic Carboxylic Acids • Physical Properties of Carboxylic Acids • Preparation of Carboxylic Acids • Acidity of Carboxylic Acids • Carboxylic Acid Salts • Structure of Esters • Preparation of Esters

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Chapter 6 Topics

• Nomenclature for Esters • Isomerism for Carboxylic Acids and Esters • Physical Properties of Esters • Chemical Reactions of Esters • Sulfur Analogs of Esters • Polyesters • Acid Chlorides and Acid Anhydrides • Esters and Anhydrides of Inorganic Acids

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Structure of Carboxylic Acids and Their Derivatives

A carboxylic acid is an organic compound whosefunctional group is the carboxyl group. A carboxylgroup is a carbonyl group (C=O) with a hydroxylgroup (—OH) bonded to the carbonyl carbonatom. A general structural representation for acarboxyl group is

Or -COOH and -CO2H

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Although we see within a carboxyl group both acarbonyl group (C=O) and a hydroxyl group (—OH), the carboxyl group does not showcharacteristic behavior of either an alcohol or acarbonyl compound (aldehyde or ketone). Rather,it is a unique functional group with a set ofcharacteristics different from those of itscomponent parts.

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The simplest carboxylicacid has a hydrogenatom attached to thecarboxyl group carbonatom.

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Structures for the next two simplestcarboxylic acids, those with methyl andethyl alkyl groups, are

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The structure of thesimplest aromaticcarboxylic acid involvesa benzene ring to whicha carboxyl group isattached.

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Cyclic carboxylic acids do not exist; havingthe carboxyl carbon atom as part of a ringsystem creates a situation where thecarboxyl carbon atom would have fivebonds. The nonexistence of cycliccarboxylic acids parallels the nonexistenceof cyclic aldehydes.

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A carboxylic acid derivative is an organiccompound that can be synthesized from orconverted into a carboxylic acid. Four importantfamilies of carboxylic acid derivatives are esters,acid chlorides, acid anhydrides, and amides. Thegroup attached to the carbonyl carbon atomdistinguishes these derivative types from eachother and also from carboxylic acids.

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IUPAC Nomenclature for Carboxylic Acids

Monocarboxylic AcidsA monocarboxylic acid is a carboxylic acidin which one carboxyl group is present.IUPAC rules for naming such compoundsare:Rule 1: Select as the parent carbon chainthe longest carbon chain that includes thecarbon atom of the carboxyl group.

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Monocarboxylic AcidsRule 2: Name the parent chain bychanging the -e ending of thecorresponding alkane to -oic acid.Rule 3: Number the parent chain byassigning the number 1 to thecarboxyl carbon atom.

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Monocarboxylic AcidsRule 4: Determine the identityand location of any substituents inthe usual manner, and appendthis information to the front of theparent chain name.

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Dicarboxylic AcidsA dicarboxylic acid is a carboxylic acid that contains twocarboxyl groups, one at each end of a carbon chain.Saturated acids of this type are named by appending thesuffix -dioic acid to the corresponding alkane name (the -eis retained to facilitate pronunciation). Both carboxylcarbon atoms must be part of the parent carbon chain,and the carboxyl locations need not be specified withnumbers because they will always be at the two ends ofthe chain.

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Aromatic CarboxylicAcidsThe simplest aromaticcarboxylic acid is calledbenzoic acid.

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Aromatic Carboxylic AcidsOther simple aromatic acids are named asderivatives of benzoic acid.In substituted benzoic acids, the ring carbonatom bearing the carboxyl group is alwayscarbon 1.

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Common Names for Carboxylic Acids

The common names of monocarboxylic acids are the basis for aldehyde common names:

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Because of their abundance in nature,carboxylic acids were among the earliestclasses of organic compounds to be studied,and they acquired names before the advent ofthe IUPAC naming system. These commonnames are usually derived from some Latin orGreek word that is related to a source for theacid.

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Monocarboxylic AcidsThe common name of a monocarboxylicacid is formed by taking the Latin or Greekroot name for the specific number ofcarbon atoms and appending the suffix –icacid. The stinging sensation associatedwith red ant bites is due in part to formicacid (Latin, formica, “ant”).

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Acetic acid gives vinegar its tartness (sourtaste); vinegar contains small amounts ofacetic acid (Latin, acetum, “sour”).Propionic acid is the smallest acid that canbe obtained from fats (Greek, protos,“first,” and pion, “fat”). Rancid buttercontains butyric acid (Latin, butyrum,“butter”).

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Valeric acid, found in valerian root(an herb), has a strong odor (Latin,valere, “to be strong”). The skinsecretions of goats contain caproicacid, which contributes to the odorassociated with these animals (Latin,caper, “goats”).

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Acetic acid is the most widely used ofall carboxylic acids. Its primary use is asan acidulant—a substance that givesthe proper acidic conditions for achemical reaction. In the pure state,acetic acid is a colorless liquid with asharp odor.

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Vinegar is a 4%–8% (v/v) acetic acidsolution; its characteristic odor comesfrom the acetic acid present. Pureacetic acid is often called glacial aceticacid because it freezes on amoderately cold day (f.p. = 17C),producing icy-looking crystals.

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When using common names for carboxylicacids, the positions (locations) of substituentsare denoted by using letters of the Greekalphabet rather than numbers. The first fourletters of the Greek alphabet are alpha ( α ),beta ( β ), gamma (γ ), and delta ( Δ ). The alpha-carbon atom is carbon 2, the beta-carbon atomis carbon 3, and so on.

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Dicarboxylic AcidsOxalic acid, the simplest dicarboxylic acid,is found in plants of the genus Oxalis,which includes rhubarb and spinach, and incabbage. This acid and its salts arepoisonous in high concentrations. Theamount of oxalic acid present in spinach,cabbage, and rhubarb is not harmful.

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DicarboxylicThe amount of oxalic acid present in spinach,cabbage, and rhubarb is not harmful. Oxalicacid is used to remove rust, bleach straw andleather, and remove ink stains. Succinic andglutaric acid and their derivatives playimportant roles in biochemical reactions thatoccur in the human body.

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Polyfunctional Carboxylic Acids

A polyfunctional carboxylic acid is a carboxylicacid that contains one or more additionalfunctional groups besides one or morecarboxyl groups. Such acids occur naturally inmany fruits, are important in the normalfunctioning of the human body (metabolism),and find use in over-the-counter skin-careproducts and in prescription drugs.

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Three commonly encountered types ofpolyfunctional carboxylic acids areunsaturated acids, hydroxy acids, and ketoacids.

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Unsaturated AcidsThe simplest unsaturated monocarboxylic acid ispropenoic acid (acrylic acid), a substance used inthe manufacture of several polymeric materials.Two isomers exist for the simplest unsaturateddicarboxylic acid, butenedioic acid. The twoisomers have separate common names, fumaricacid (trans) and maleic acid (cis),

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Some antihistamines are salts of maleicacid. The addition of small amounts ofmaleic acid to fats and oils prevents themfrom becoming rancid. Fumaric acid is ametabolic acid. Metabolic acids areintermediate compounds in the metabolicreactions that occur in the human body.

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Hydroxy AcidsFour of the simpler hydroxy acids are

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Malic and tartaric acids are derivatives ofsuccinic acid, the four-carbonunsubstituted diacid. Hydroxy acids occurnaturally in many foods. Glycolic acid ispresent in the juice from sugar cane andsugar beets. Lactic acid is present in sourmilk, sauerkraut, and dill pickles.

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Both malic acid and tartaric acid occurnaturally in fruits. The sharp taste of apples(fruit of trees of the genus Malus) is due tomalic acid. Tartaric acid is particularlyabundant in grapes. It is also a component oftartar sauce and an acidic ingredient in manybaking powders. Lactic and malic acids arealso metabolic acids.

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Citric acid, perhaps the best known of allcarboxylic acids, is a hydroxy acid with astructural feature we have not previouslyencountered. It is a hydroxy tricarboxylic acid.Besides there being acid groups at both ends ofa carbon chain, a third acid group is present asa substituent on the chain. An acid group as asubstituent is called a carboxy group. Thuscitric acid is a hydroxycarboxy diacid.

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Citric acid, Citric acid gives citrus fruits their“sharp” taste; lemon juice contains 4%–8%citric acid, and orange juice is about 1%citric acid. Citric acid is used widely inbeverages and in foods. In jams, jellies, andpreserves, it produces tartness and pHadjustment to optimize conditions forgelation.

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In fresh salads, citric acid preventsenzymatic browning reactions, and infrozen fruits it prevents deteriorationof color and flavor. Addition of citricacid to seafood retards microbialgrowth by lowering pH. Citric acid isalso a metabolic acid.

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Keto AcidsKeto acids, as the designation implies, containa carbonyl group within a carbon chain. Pyruvicacid, with three carbon atoms, is the simplestketo acid that can exist.In the pure state, pyruvic acid is a liquid withan odor resembling that of vinegar (acetic acid.Pyruvic acid is a metabolic acid.

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Metabolic Carboxylic Acids

Numerous polyfunctional acids are intermediates in themetabolic reactions that occur in the human body asfood is processed. These three simple acids and themetabolic acids related to them are1. Propionic acid (3-carbon monoacid): lactic, glyceric,

and pyruvic acids2. Succinic acid (4-carbon diacid): fumaric, oxaloacetic,

and malic acids3. Glutaric acid (5-carbon diacid): -ketoglutaric and

citric acids

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Metabolic acids derived from the diacids succinicand glutaric are encountered in the citric acid cycle,a series of reactions in which C2 units obtainedfrom all types of foods are further processed for thepurpose of obtaining energy. Glyceric and pyruvicacid (propionic acid derivatives) are encountered inglycolysis, a series of reactions in which glucose isprocessed. Lactic acid (a propionic derivative) is aby-product of strenuous exercise.

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Physical Properties of Carboxylic Acids

Carboxylic acids are the most polar organiccompounds. Both the carbonyl part (C=O)and the hydroxyl part (—OH) of the carboxylfunctional group are polar. The result is veryhigh melting and boiling points for carboxylicacids, the highest of any type of organiccompound yet considered.

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Unsubstituted saturated monocarboxylicacids containing up to nine carbon atoms areliquids that have strong, sharp odors. Acidswith 10 or more carbon atoms in anunbranched chain are waxy solids that areodorless (because of low volatility). Aromaticcarboxylic acids, as well as dicarboxylic acids,are also odorless solids.

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The high boiling points of carboxylic acidsindicate the presence of strongintermolecular attractive forces. A uniquehydrogen-bonding arrangement contributesto these attractive forces. A given carboxylicacid molecule forms two hydrogen bonds toanother carboxylic acid molecule, producinga “complex” known as a dimer.

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Because dimers have twice the mass of asingle molecule, a higher temperature isneeded to boil a carboxylic acid thanwould be needed for similarly sizedaldehyde and alcohol molecules wheredimerization does not occur.

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Carboxylic acids readily hydrogen-bond to watermolecules. Such hydrogen bonding contributes towater solubility for short-chain carboxylic acids.The unsubstituted C1 to C4 monocarboxylic acidsare completely miscible with water. Solubility thenrapidly decreases with carbon number. Short-chain dicarboxylic acids are also water-soluble. Ingeneral, aromatic acids are not waters soluble.

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Preparation of Carboxylic Acids

Oxidation of primary alcohols or aldehydes,using an oxidizing agent such as CrO3 orK2Cr2O7, produces carboxylic acids.

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Aromatic acids can be prepared by oxidizing acarbon side chain (alkyl group) on a benzenederivative. In this process, all the carbon atoms ofthe alkyl group except the one attached to the ringare lost. The remaining carbon becomes part of acarboxyl group.

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Acidity of Carboxylic Acids

Carboxylic acids, as the name implies, areacidic. When a carboxylic acid is placed inwater, hydrogen ion transfer (proton transfer)occurs to produce hydronium ion (the acidicspecies in water) and carboxylate ion.

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A carboxylate ion is the negative ion producedwhen a carboxylic acid loses one or more acidichydrogen atoms.Carboxylate ions formed from monocarboxylic acidsalways carry a 1 charge; only one acidic hydrogenatom is present in such molecules. Dicarboxylicacids, which possess two acidic hydrogen atoms(one in each carboxyl group), can producecarboxylate ions bearing a 2 charge.

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A carboxylate ion is the negative ion producedwhen a carboxylic acid loses one or more acidichydrogen atoms.Carboxylate ions formed from monocarboxylic acidsalways carry a 1 charge; only one acidic hydrogenatom is present in such molecules. Dicarboxylicacids, which possess two acidic hydrogen atoms(one in each carboxyl group), can producecarboxylate ions bearing a 2 charge.

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Carboxylate ions are named by dropping the -ic acidending from the name of the parent acid andreplacing it with -ate.

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Carboxylic acids are weak acids (Section 10.4). Theextent of proton transfer is usually less than 5%;that is, an equilibrium situation exists in which theequilibrium lies far to the left.

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Carboxylic Acids Salts

Carboxylic acids react completely with strongbases to produce water and a carboxylic acidsalt.

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A carboxylic acid salt is an ionic compound inwhich the negative ion is a carboxylate ion.Carboxylic acid salts are named similarly toother ionic compounds. The positive ion isnamed first, followed by a separate word givingthe name of the negative ion. The salt formed inthe preceding reaction contains sodium ionsand acetate ions (from acetic acid); hence thesalt’s name is sodium acetate.

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The interconversion reactions between carboxylicacid salts and their “parent” carboxylic acids are soeasy to carry out that organic chemists considerthese two types of compounds interchangeable.

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Uses for Carboxylic Acid SaltsThe solubility of carboxylic acid salts in water ismuch greater than that of the carboxylic acids fromwhich they are derived. Drugs and medicines thatcontain acid groups are usually marketed as thesodium or potassium salt of the acid. This greatlyenhances the solubility of the medication,increasing the ease of its absorption by the body.

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Uses for Carboxylic Acid SaltsMany antimicrobials, compounds used as foodpreservatives, are carboxylic acid salts. Particularlyimportant are the salts of benzoic, sorbic, andpropionic acids.

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Carboxylic Acids SaltsUses for Carboxylic Acid SaltsThe benzoate salts of sodium and potassium areeffective against yeast and mold in beverages, jams andjellies, pie fillings, ketchup, and syrups. Concentrationsof up to 0.1% (m/m) benzoate are found in suchproducts.

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Uses for Carboxylic Acid SaltsSodium and potassium sorbates inhibit mold and yeastgrowth in dairy products, dried fruits, sauerkraut, andsome meat and fi sh products. Sorbate preservativeconcentrations range from 0.02% to 0.2% (m/m).

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Uses for Carboxylic Acid SaltsCalcium and sodium propionates are used in bakedproducts and also in cheese foods and spreads.Benzoates and sorbates cannot be used in yeast-leavened baked goods because they affect the activity ofthe yeast.

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Uses for Carboxylic Acid SaltsCarboxylate salts do not directly killmicroorganisms present in food. Rather,they prevent further growth andproliferation of these organisms byincreasing the pH of the foods in whichthey are used.

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Structure of Esters

An ester is a carboxylic acid derivative in which the —OHportion of the carboxyl group has been replaced with an —OR group.

In linear form, the ester functional group can berepresented as —COOR or —CO2R.

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Structure of Esters

The simplest ester, which has two carbon atoms, has ahydrogen atom attached to the ester functional group.

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Structure of Esters

The structure of the simplest aromatic ester isderived from the structure of benzoic acid, thesimplest aromatic carboxylic acid.

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Preparation of Esters

Esters are produced through esterifi cation. Anesterification reaction is the reaction of a carboxylicacid with an alcohol (or phenol) to produce an ester.A strong acid catalyst (generally H2SO4) is neededfor esterification.

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In the esterification process, a —OH group is lostfrom the carboxylic acid, a —H atom is lost from thealcohol, and water is formed as a by-product. Thenet effect of this reaction is substitution of the —ORgroup of the alcohol for the —OH group of the acid.

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A specific example of esterifi cation is the reaction ofacetic acid with methyl alcohol.

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It is often useful to thinkof the structure of anester in terms of its“parent” alcohol andacid molecules; the esterhas an acid part and analcohol part.

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It is easy to identify the acid and alcohol fromwhich a given ester can be produced; just add a —OH group to the acid part of the ester and a —Hatom to the alcohol part to generate the parentmolecules.

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Cyclic Esters (Lactones)Hydroxy acids—compounds which containboth a hydroxyl and a carboxyl group—havethe capacity to undergo intermolecularesterification to form cyclic esters. Suchinternal esterification easily takes place insituations where a five- or six-membered ringcan be formed.

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Cyclic Esters (Lactones)

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Cyclic esters are formally called lactones.A lactone is a cyclic ester. The ring size in alactone is indicated using a Greek letter. Alactone with a five-membered ring is a g-lactone and one with a six-membered ringis a d-lactone.

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Chemical reactions that are expected toproduce a hydroxy carboxylic acid often yielda lactone instead if a five- or six-memberedring can be formed.

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Nomenclature for Esters

Visualizing esters as having an “alcohol part” andan “acid part” is the key to naming them in boththe common and the IUPAC systems ofnomenclature. The rules are as follows:Rule 1: The name for the alcohol part of theester appears fi rst and is followed by a separateword giving the name for the acid part of theester.

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Rule 2: The name for the alcohol part ofthe ester is simply the name of the R group(alkyl, cycloalkyl, or aryl) present in the —OR portion of the ester.Rule 3: The name for the acid part of theester is obtained by dropping the -ic acidending for the acid’s name and adding thesuffix -ate.

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Consider the ester derived from ethanoicacid (acetic acid) and methanol (methylalcohol). Its name will be methyl ethanoate(IUPAC) or methyl acetate (common).

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Dicarboxylic acids can form diesters, witheach of the carboxyl groups undergoingesterifi cation. An example of such amolecule and how it is named is

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Further examples of ester nomenclature, forcompounds in which substituents are present,are

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IUPAC names for lactones are generated byreplacing the -oic ending of the parenthydroxycarboxylic acid name with -olide andidentifying the hydroxyl-bearing carbon by number.

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Selected Common Esters

Flavor/Fragrance AgentsEsters are largely responsible for the flavor andfragrance of fruits and flowers. Generally, anatural flavor or odor is caused by a mixture ofesters, with one particular compound beingdominant. The synthetic production of these“dominant” compounds is the basis for theflavoring agents used in ice cream, gelatins,soft drinks, and so on.

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Numerous lactones are common inplants. Two examples are 4-decanolide, a compound partiallyresponsible for the taste and odor ofripe peaches, and coumarin (commonname), the compound responsible forthe pleasant odor of newly mown hay.

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PheromonesA number of pheromones contain esterfunctional groups. The compound isoamylacetate, is an alarm pheromone for the honeybee.

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PheromonesThe compound methyl p-hydroxybenzoate, is asexual attractant for canine species. It issecreted by female dogs in heat and evokesattraction and sexual arousal in male dogs.

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PheromonesThe compound nepetalactone, alactone present in the catnip plant, isan attractant for cats of all types. It isnot considered a pheromone,however, because different species areinvolved.

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MedicationsNumerous esters have medicinal value, includingbenzocaine (a local anesthetic), aspirin, and oil ofwintergreen (a counterirritant). The structure ofbenzocaine is

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MedicationsBoth aspirin and oilof wintergreen areesters of salicylicacid, an aromatichydroxyacid.

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Medications

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Because this acid has both an acid groupand a hydroxyl group, it can form twodifferent types of esters: one by reaction ofits acid group with an alcohol, the other byreaction of its alcohol group with acarboxylic acid. Reaction of acetic acid withthe alcohol group of salicylic acid producesaspirin.

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Aspirin has a mode of action in thehuman body.

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Reaction of methanol with the acid group ofsalicylic acid produces oil of wintergreen.

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Oil of wintergreen, also called methylsalicylate, is used in skin rubs and linimentsto help decrease the pain of sore muscles.It is absorbed through the skin, where it ishydrolyzed to produce salicylic acid.Salicylic acid, as with aspirin, is the actualpain reliever.

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The macrolide antibiotics are a family oflarge-ring lactones. Erythromycin, the bestknown member of this antibiotic family, hasan antimicrobial spectrum similar to that ofpenicillin and is often used for people whohave an allergy to penicillin. Structurally,this antibiotic contains a 14-memberedlactone ring.

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Selected Common EstersErythromycin is anaturally occurringsubstance first isolatedfrom a red-pigmentedsoil bacterium. Thelaboratory synthesis ofthis compound has nowbeen achieved. Itschemical formula isC37H67NO13.

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Isomerism for Carboxylic Acids and Esters

Constitutional isomers based ondifferent carbon skeletons and ondifferent positions for the functionalgroup are possible for carboxylic acidsand esters as well as other types ofcarboxylic acid derivatives.

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Carboxylic acids and esters with the same numberof carbon atoms and the same degree of saturationare functional group isomers. The ester ethylpropanoate and the carboxylic acid pentanoic acidboth have the molecular formula, C5H10O2, and arethus functional group isomers.

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Physical Properties Esters

Ester molecules cannot form hydrogen bondsto each other because they do not have ahydrogen atom bonded to an oxygen atom.Consequently, the boiling points of esters aremuch lower than those of alcohols andcarboxylic acids of comparable molecular mass.Esters are more like ethers in their physicalproperties.

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Water molecules can hydrogen-bond to estersthrough the oxygen atoms present in the esterfunctional group. Because of such hydrogenbonding, low-molecular-mass esters are soluble inwater. Solubility rapidly decreases with increasingcarbon chain length; borderline solubility situationsare reached when three to five carbon atoms are ina chain. Low- and intermediate-molecular-massesters are usually colorless liquids at roomtemperature. Most have pleasant odors.

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Chemical Properties Esters

Ester HydrolysisIn ester hydrolysis, an ester reacts withwater, producing the carboxylic acid andalcohol from which the ester was formed.

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Ester hydrolysis requires the presence of a strong-acidcatalyst or enzymes. Ester hydrolysis is the reverse ofesterification, the formation of an ester from acarboxylic acid and an alcohol.

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Ester SaponificationA saponification reaction is the hydrolysis of an organiccompound, under basic conditions, in which a carboxylicacid salt is one of the products. Esters, amides, and fatsand oils all undergo saponification reactions. In estersaponification either NaOH or KOH is used as the baseand the saponification products are an alcohol and acarboxylic acid salt. (Any carboxylic acid product formedis converted to its salt because of the basic reactionconditions.)

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Sulfur Analogs Esters

Just as alcohols react with carboxylic acids to produceesters, thiols react with carboxylic acids to producethioesters. A thioester is a sulfur-containing analog of anester in which an —SR group has replaced the —OR group.

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The thioester methyl thiobutanoate is used as anartificial flavoring agent. It generates the taste wecall strawberry.

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The most importantnaturally occurringthioester is acetylcoenzyme A, whoseabbreviatedstructure is

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Coenzyme A, the parent molecule for acetylcoenzyme A, is a large, complex thiol whosestructure, for simplicity, is usually abbreviatedas CoA—S—H. The formation of acetylcoenzyme A (acetyl CoA) from coenzyme Acan be envisioned as a thioesterificationreaction between acetic acid and coenzyme A.

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Acetyl coenzyme A plays a central role in themetabolic cycles through which the body obtainsenergy to “run itself”.

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Polyesters

A condensation polymer is a polymer formed byreacting difunctional monomers to give a polymerand some small molecule (such as water) as a by-product of the process. Polyesters are an importanttype of condensation polymer. A polyester is acondensation polymer in which the monomers arejoined through ester linkages. Dicarboxylic acids anddialcohols are the monomers generally used informing polyesters.

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Polyesters

The best known of the many polyesters nowmarketed is poly(ethylene terephthalate), which isalso known by the acronym PET. The monomers usedto produce PET are terephthalic acid (a diacid) andethylene glycol (a dialcohol).

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Polyesters

The reaction of one acid group of thediacid with one alcohol group of thedialcohol initially produces an estermolecule, with an acid group left overon one end and an alcohol group leftover on the other end.

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Polyesters

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Polyesters

This species can react further. The remainingacid group can react with an alcohol groupfrom another monomer, and the alcohol groupcan react with an acid group from anothermonomer. This process continues until anextremely long polymer molecule called apolyester is produced.

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Polyesters

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Polyesters

About 50% of PET production goes into textile products,including clothing fibers, curtain and upholstery materials,and tire cord. The trade name for PET as a clothing fiber isDacron. The other 50% of PET production goes intoplastics applications. As a film-like material, it is calledMylar. Mylar products include the plastic backing foraudio and video tapes and computer diskettes. Itschemical name PET is applied when this polyester is usedin clear, flexible soft-drink bottles and as the wrappingmaterial for frozen foods and boil-in-bag foods.

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PolyestersPlastic bottles made of PET cannot be reusedbecause they cannot withstand the hightemperatures needed to sterilize them for reuse.Also such bottles cannot be used for any food itemsthat must be packaged at high temperatures, suchas jams and jellies. For these uses, the polyesterPEN (polyethylene naphthalate), a polymer that canwithstand higher temperatures, is available. Themonomers for this polymer are ethylene glycol andone or more naphthalene dicarboxylic acids.

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Polyesters

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Polyesters

A polymerization reaction in which lactic acidand glycolic acid (both hydroxyacids, aremonomers produces a biodegradable material(trade name Lactomer) that is used as surgicalstaples in several types of surgery. Traditionalsuture materials must be removed later on,after they have served their purpose.Lactomer staples start to dissolve (hydrolyze)after a period of several weeks.

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Polyesters

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Polyesters

Another commercially available biodegradablepolyester is PHBV, a substance that fi nds use inspecialty packaging, orthopedic devices, andcontrolled drug-release formulations. Themonomers for this polymer are 3-hydroxybutanoicacid (b-hydroxybutyrate) and 3-hydroxypentanoicacid (b-hydroxyvalerate). The properties of PHBVvary according to the reacting ratio for the twomonomers.

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Polyesters

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Acid Chlorides and Acid Anhydrides

Acid ChloridesAn acid chloride is a carboxylic acid derivative inwhich the —OH portion of the carboxyl group hasbeen replaced with a —Cl atom. Thus, acid chlorideshave the general formula

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Acid ChloridesAcid chlorides are named in either of two ways:Rule 1: Replace the -ic acid ending of the commonname of the parent carboxylic acid with -yl chloride.

Butyric acid becomes butyryl chloride.

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Acid ChloridesRule 2: R eplace the -oic acid ending of the IUPACname of the parent carboxylic acid with -oylchloride.

3-Methylpentanoic acid becomes 3-methylpentanoyl chloride.

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Acid ChloridesPreparation of an acid chloride from its parent carboxylicacid involves reacting the acid with one of severalinorganic chlorides (PCl3, PCl5, or SOCl2). The generalreaction is

This reactivity with water means that acid chlorides cannot exist in biological systems.

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Acid ChloridesAcid chlorides react rapidly with water, in a hydrolysisreaction, to regenerate the parent carboxylic acid.

This reactivity with water means that acid chlorides cannot exist in biological systems.

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Acid ChloridesAcid chlorides are useful starting materialsfor the synthesis of other carboxylic acidderivatives, particularly esters and amides.Synthesis of esters and amides using acidchlorides is a more efficient process thanester and amide synthesis using acarboxylic acid.

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Acid AnhydridesAn acid anhydride is a carboxylic acidderivative in which the —OH portion of thecarboxyl group has been replaced with

group. Thus, acid anhydrides have thegeneral formula

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Acid AnhydridesThe word anhydride means “withoutwater.” Structurally, acid anhydridescan be visualized as two carboxylicacid molecules bonded together afterremoval of a water molecule from theacid molecules.

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Acid Anhydrides

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Acid AnhydridesSymmetrical acid anhydrides (both R groups are thesame) are named by replacing the acid ending ofthe parent carboxylic acid name with the wordanhydride.

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Acid AnhydridesMixed acid anhydrides (different R groups present)are named by using the names of the individualparent carboxylic acids (in alphabetic order)followed by the word anhydride.

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Acid AnhydridesIn general, acid anhydrides cannot be formed bydirectly reacting the parent carboxylic acidstogether. Instead, an acid chloride is reacted with acarboxylate ion to produce the acid anhydride.

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Acid AnhydridesAcid anhydrides are very reactive compounds,although generally not as reactive as the acidchlorides. Like acid chlorides, they cannot exist inbiological systems, as they undergo hydrolysis toregenerate the parent carboxylic acids.

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Acid AnhydridesReaction of an alcohol with an acid anhydride is auseful method for synthesizing esters.

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Acyl Transfer ReactionsCarboxylic acids contain acyl groups. An acyl groupis that portion of a carboxylic acid that remainsafter the —OH group is removed from the carboxylcarbon atom. Acid chlorides and acid anhydridesalso contain acyl groups.

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Acyl Transfer ReactionsAny compound with the generalized formula

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Acyl Transfer ReactionsAny compound with the generalized formula

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Acyl Transfer ReactionsCompounds that contain acyl groups, when theyreact with an alcohol or phenol, transfer the acylgroup to the oxygen atom of the alcohol or phenol.

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Acyl Transfer ReactionsChemical reactions such as these are called acyl transferreactions. An acyl transfer reaction is a chemical reactionin which an acyl group is transferred from one moleculeto another. Acyl transfer reactions occur frequently inbiochemical systems. The process of protein synthesis isdependent upon acyl transfer reactions, as are manymetabolic reactions. Often in metabolic reactions thethioester acetyl coenzyme A serves as an acyl transferagent.

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Acyl Transfer ReactionsThe acyl group present in carboxylic acids andcarboxylic acid derivatives is named byreplacing the -ic acid ending of the acid namewith the suffix -yl.Common names: -ic acid become -ylIUPAC names: -oic acid becomes -oyl

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Acyl Transfer ReactionsThus, the two- and three-carbonyl acyl groupsare named as follows:

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Esters and Anhydrides of Inorganic AcidsInorganic acids such as sulfuric, phosphoric, and nitricacids react with alcohols to form esters in a mannersimilar to that for carboxylic acids.

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Esters and Anhydrides of Inorganic AcidsInorganic acids such as sulfuric, phosphoric, and nitricacids react with alcohols to form esters in a mannersimilar to that for carboxylic acids.

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Esters and Anhydrides of Inorganic AcidsThe most important inorganic esters, from abiochemical standpoint, are those of phosphoricacid—that is, phosphate esters. A phosphateester is an organic compound formed byreaction of an alcohol with phosphoric acid.Because phosphoric acid has three hydroxylgroups, it can form mono-, di-, and triesters byreaction with one, two, and three molecules ofalcohol, respectively.

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Esters and Anhydrides of Inorganic Acids

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Phosphoric Acid AnhydridesThree biologically important phosphoricacids exist: phosphoric acid, diphosphoricacid, and triphosphoric acid. Phosphoricacid, the simplest of the three acids,undergoes intermolecular dehydration toproduce diphosphoric acid.

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Another intermolecular dehydration,involving diphosphoric acid and phosphoricacid, produces triphosphoric acid.

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In the same manner that carboxylic acids are acidic,phosphoric acid, diphosphoric acid, andtriphosphoric acid are also acidic. The phosphoricacids are, however, polyprotic rather thanmonoprotic acids. The hydrogen atom in each of the—OH groups possesses acidic properties. All threephosphoric acids undergo esterification reactionswith alcohols, producing species such as

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Diphosphoric acid and triphosphoric acid arephosphoric acid anhydrides as well as acids. Notethe structural similarities between a carboxylic acidanhydride and diphosphoric acid.

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Phosphoric acid anhydride systems playimportant roles in cellular processes throughwhich biochemical energy is produced. Thepresence of phosphoric anhydride systems inbiological settings contrasts markedly withcarboxylic acid anhydride systems, which arenot found in biological settings because oftheir reactivity with water.

End of Chapter 6Carboxylic Acids, Esters and Other Derivatives

General, Organic, and Biological Chemistry, Fifth Edition

H. Stephen Stoker

Brroks/Cole Cengage Learning. Permission required for reproduction or display.

chapter 6 carboxylic acids ssters and other derivatives

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