~n...oon I.Nmlllg/Cll.t.. D. W1n11l1

The active ingredients in

these two nonprescription pain

relievers are derivatives of aryl­

propanoic acids. INSET: a model

of (S)-ibuprofen. See Chemical

Connections: "From Willow Bark

to Aspirin and Beyond."

OUTLINE

17.1 StructureThe functional group of a carboxylic acid is the carboxyl group (Section 1.3D) , sonamed because it is made up of a carbonyl group and a hydroxyl group. Followingis a Lewis structure of the carboxyl group as well as three alternative representa­tions for it.

-COOH

o~I

-C\..<?,-H

The most important chemical property of carboxylic acids, another classof organic compounds containing the carbonyl group, is their acidity.Furthermore, carboxylic acids form numerous important derivatives, includ­

ing esters, amides, anhydrides, and acid halides. In this chapter, we study carbox­ylic acids and in Chapter 18 we study their derivatives.

Online homework for thischapter may be assignedin Organic OWL.

17.1 Structure

17.2 Nomenclature

17.3 Physical Properties

17.4 Acidity

17.5 Preparation of CarboxylicAcids

17.6 Reduction

17.7 Esterification

17.8 Conversion to AcidChlorides

17.9 Decarboxylation

Alternative representations ofa carboxyl group

The general formula for an aliphatic carboxylic acid is RCOOH; the general for­mula for an aromatic carboxylic acid is ArCOOH.

17.2 Nomenclature

A. IUPAC System

The IUPAC name of a carboxylic acid is derived from that of the longest carbon chainthat contains the carboxyl group by dropping the final-e from the name of the parentalkane and adding the suffix -oicfollowed by the word "acid" (Section 2.3C). The chain

620

~OH3-Methylbutanoic acid

(Isovaleric acid)Ethanoic acid(Acetic acid)

HCOOH

Methanoic acid(Formic acid)

is numbered beginning with the carbon of the carboxyl group. Because the carboxylcarbon is understood to be carbon 1, there is no need to give it a number. The IUPAC ~system retains the common names formic acid and acetic acid, which are always used jto refer to these acids. j..

fIii

1Iiz...,!Q

If the carboxylic acid contains a carbon-carbon double or triple bond, change theinfix from -an- to -en- or -yn-, as the case may be, to indicate the presence of themultiple bond and show the location of the multiple bond by a number.

o

~OHo

~OHo

~OH

Formic acid was first obtained in1670from the destructive distil­lation ofants, whose Latin genusis Formica. It is one ofthe com­ponents ofthe venom injected bystinging ants.

Propenoic acid(Acrylic acid)

trans-2-Butenoic acid(Crotonic acid)

trans-3-Phenylpropenoic acid(Cinnamic acid)

In the IUPAC system, a carboxyl group takes precedence over most other func­tional groups (Table 16.1), including hydroxyl groups, amino groups, and the car­bonyl groups of aldehydes and ketones. As illustrated in the following examples,an -OH group is indicated by the prefix hydroxy-; an -NH2 group, by amino; andthe C=O group of an aldehyde or ketone, by oXO-.

OH 0

~OHo 0

~OH(R)-5-Hydroxyhexanoic acid 5-0xohexanoic acid 4-Aminobutanoic acid

Dicarboxylic acids are named by adding the suffix -dioic acid to the name ofthe carbon chain that contains both carboxyl groups. The numbers of the carbox­yl carbons are not indicated because they can be only at the ends of the parentchain. Following are IUPAC and common names for several important aliphaticdicarboxylic acids.

o

Ho00H

o

o 0

HO~OH

Ethanedioic acid(Oxalic acid)

Propanedioic acid(Malonic acid)

HO_ ./"... 1y ........, -OH

o

o 0

HO~OH HO_ ./"... ./"... 1y ........, ........, -OH

oButanedioic acid

(Succinic acid)Pentanedioic acid

(Glutaric acid)Hexanedioic acid

(Adipic acid)

The name oxalic acid is derived from one of its sources in the biological world, name­ly, plants of the genus Oxalis, one of which is rhubarb. Adipic acid is one of the twomonomers required for the synthesis of the polymer nylon 66 (Section 29.5A). Amnemonic phrase for remembering the common names for the dicarboxylic acidsoxalic through adipic is Oh my, such good apples.

17.2 Nomenclature 621

Decanoic acid(0.2 g/100 mL H20)

HydrophilicFrom the Greek, meaning waterloving.

HydrophobicFrom the Greek, meaning waterfearing.

17.3 Physical PropertiesIn the liquid and solid states, carboxylic acids are associated by hydrogen bondinginto dimers, as shown here for acetic acid in the liquid state.

Hydrogen bondingin the dimer

Electrostatic potential mapof the acetic acid dimer

Carboxylic acids have significantly higher boiling points than other types oforganic compounds of comparable molecular weight, such as alcohols, aldehydes, andketones. For example, butanoic acid (Table 17.2) has a higher boiling point than either1-pentanol or pentanal. The higher boiling points of carboxylic acids result from theirpolarity and from the fact that they form very strong intermolecular hydrogen bonds.

Carboxylic acids also interact with water molecules by hydrogen bonding throughboth the carbonyl and hydroxyl groups. Because of greater hydrogen-bonding inter­actions, carboxylic acids are more soluble in water than alcohols, ethers, aldehydes,and ketones of comparable molecular weight. The solubility of a carboxylic acid inwater decreases as its molecular weight increases. We account for this trend in the fol­lowing way. A carboxylic acid consists of two regions of distinctly different polarity: apolar hydrophilic carboxyl group and, except for formic acid, a nonpolar hydropho­bic hydrocarbon chain. The hydrophilic carboxyl group increases water solubility; thehydrophobic hydrocarbon chain decreases water solubility.

hydrophilic(polar) head

0/OH

Electrostatic potential map of decanoic acid

Table 17.2 Boiling Points and Solubilities in Water of Selected CarboxylicAcids, Alcohols, and Aldehydes of Comparable Molecular Weight

Molecular Boiling SolubilityStructure Name Weight (g/mol) Point (OC) (g/IOO g H 2O)

CH3COOH Acetic acid 60.1 118 Infinite

CH3CH2CH2OH I-Propanol 60.1 97 Infinite

CH3CH2CHO Propanal 58.1 48 16.0

CH3(CH2hCOOH Butanoic acid 88.1 163 Infinite

CH3(CH2hCH2OH I-Pentanol 88.1 137 2.3

CH3(CH2hCHO Pentanal 86.1 103 Slight

CH3(CH2)4COOH Hexanoic acid 116.2 205 1.0

CH3(CH2)5CH20H I-Heptanol 116.2 176 0.2

CH3(CH2>SCHO Heptanal 114.1 153 0.1

624 Chapter 17 Carboxylic Acids

<)-COOH + NaOH~<)-COO- Na+ + H20

Sodiwn benzoate(60 g/IOO mL water)

Benzoic acid(slightly soluble in water)

~ Sodium benzoate, a fungal growth inhibitor, is often added to baked goods "to~~~~ln i retard spoilage." Calcium propanoate is also used for the same purpose. Carboxylic

i acids also form water-soluble salts with ammonia and amines:'j!

j ( )-COOH + NH, -,;;0>( )-COO- NILe--~~---~----~ .... @

Carboxylic acids react with sodium bicarbonate and sodium carbonate to formwater-soluble sodium salts and carbonic acid (a weaker acid). Carbonic acid, inturn, decomposes to give water and carbon dioxide, which evolves as a gas.

CH3COOH + NaHC03~ CH3COO-Na+ +........_ ....H 2C03~ CO2 + H20

Sodium benzoate and calcium pro­panoate are used as preservativesin baked goods.

Benzoic acid(slightly soluble in water)

Ammoniwn benzoate(20 g/IOO mL water)

CH3COOH + NaHC03~ CH3COO-Na+ + CO2 + H20

Salts of carboxylic acids are named in the same manner as the salts of inorgan­ic acids; the cation is named before the anion. The name of the anion is derivedfrom the name of the carboxylic acid by dropping the suffix -ic acid and adding thesuffix -ate.

Example 17.3

Complete each acid-base reaction and name the carboxylic salt formed.

~H

(a) ~COOH + NaOH~ (b) /"-COOH + NaHC03~

(S)-Lactic acid

Solution

Each carboxylic acid is converted to its sodium salt. In (b), carbonic acid is formed;it decomposes to carbon dioxide and water.

(a) ~COOH + NaOH~~COO-Na++ H20

Butanoic acid Sodiwn butanoate

~H ~H

(b) ~COOH+ NaHC03~ ~COO-Na+ + H20 + CO2

(S)-Lactic acid Sodium (S)-lactate

Problem 17.3

Write equations for the reaction of each acid in Example 17.3 with ammonia, andname the carboxylic salt formed.

A consequence of the water solubility of carboxylic acid salts is that water-insolublecarboxylic acids can be converted to water-soluble ammonium or alkali metal salts andextracted into aqueous solution. The salt, in turn, can be transformed back to the freecarboxylic acid by addition of HCI, H2S04, or other strong acid. These reactions allowan easy separation of carboxylic acids from water-insoluble nonacidic compounds.

628 Chapter 17 Carboxylic Acids

Connections to>K~ ~~~ Biological Chemistry

Ketone Bodies and Diabetes Mellitus

3-0xobutanoic acid (acetoacetic acid) and its reductionproduct, 3-hydroxybutanoic acid, are synthesized in theliver from acetyl-CoA, a product of the metabolism offatty acids and certain amino acids. 3-Hydroxybutanoicacid and 3-oxobutanoic acid are known collectively asketone bodies.

o 0

~OH3-0xobutanoic acid(Acetoacetic acid)

OH 0

~OH3-Hydroxybutanoic acid(,a-Hydroxybutyric acid)

The concentration of ketone bodies in the blood ofhealthy, well-fed humans is approximately 0.01 mM/L.However, in persons suffering from starvation or dia­betes mellitus, the concentration of ketone bodies mayincrease to as much as 500 times normal. Under theseconditions, the concentration of acetoacetic acid in­creases to the point where it undergoes spontaneousdecarboxylation to form acetone and carbon dioxide.Acetone is not metabolized by humans and is excretedthrough the kidneys and the lungs. The odor of acetoneis responsible for the characteristic "sweet smell" of thebreath of severely diabetic patients.

including that of a carboxyl group or ester. Malonic acid and substituted malo­nic acids, for example, undergo thermal decarboxylation, as illustrated by thedecarboxylation of malonic acid when it is heated slightly above its meltingpoint of 135-137°C.

o 0II II

HOCCH2COH

Propanedioic acid(Malonic acid)

The mechanism of decarboxylation of malonic acids is very similar to what we havejust seen for the decarboxylation of ,B-ketoacids. In each case, formation of a cyclic,six-membered transition state involving rearrangement of three electron pairsgives the enol form of a carboxylic acid, which is tautomerized to the carboxy­lic acid.

_ Decarboxylation of a {3-Dicarboxylic Acid

Step 1: Redistribution of six electrons in a cyclic six-membered transition stategives carbon dioxide and the enol form of a carboxyl group.

Step 2: Keto-enol tautomerism (Section 16.9B) of the enol gives the more stableketo form of the carboxyl group.

../H:0 '0' '0'

HO"~ +'f ~), ..~II HO

:0:

640 Chapter 17 Carboxylic Acids

[

H.. J:1~(0:

H§J<>lg:A cyclic six-membered

transition state

(1)-Enolofa

carboxylic acidA carboxylic

acid

PROBLEMS

• Assignable in OWL

HOyCOOH

(f) l.COOH

COOH

(C)~COOH

(b) Lithium acetate(d) Disodium adipate(f) Calcium butanoate

OH

(b) ~COOH

Online homework for this chapter may be assigned is Organic OWL.• indicates problems assignable in OWL.Red numbers indicate applied problems.

(a) Sodium benzoate(c) Ammonium acetate(e) Sodium salicylate

(a) o-COOH

(d) exCOOH

Chapter 17 Carboxylic Acids

o

Jv~OH 60

U ~ + cO2

Structure and Nomenclature17.7 • Write the IUPAC name of each compound, showing stereochemistry where relevant:

(a) What is its IUPAC name?(b) State the number of stereoisomers possible for this compound.

17.10 • Draw a structural formula for each salt.

17.8 • Draw a structural formula for each compound.

(a) Phenylacetic acid (b) 4-Aminobutanoic acid(c) 3-Chloro-4-phenylbutanoic acid (d) Propenoic acid (acrylic acid)(e) (Z)-3-Hexenedioic acid (f) 2-Pentynoic acid(g) Potassium phenylacetate (h) Sodium oxalate(i) 2-0xocydohexanecarboxylic acid (j) 2,2-Dimethylpropanoic acid

17.9 • Megatomoic acid, the sex attractant of the female black carpet beetle, has the fol­lowing structure.

on the carbonyl carbon to give a tetrahedral carbonyl addition intermediate, whichdecomposes to give the acid chloride, S02, and chloride ion.

o 0

~OH +SOC12~~Cl + S02 + HCl

10. Decarboxylation of p-Dicarboxylic Acids (Section 17.98) The mechanism of decarbox­ylation of a f3-dicarboxylic acid is similar to that for decarboxylation of a f3-ketoacid.

o 0 0II II heat II

HOCCH2COH-- CH3COH + CO2

9. Decarboxylation of p-Ketoacids (Section 17.9A) f3-Ketoacids decarboxylate upon heat­ing. The mechanism involves redistribution of electrons in a six-membered transitionstate to give CO2 and the enol of a ketone, which tautomerizes to give a ketone. Thereaction is facilitated by a hydrogen bond between the carboxyl hydrogen atom andf3-earbonyl oxygen.

644