William H. Brown Thomas Poon

www.wiley.com/college/brown

Chapter FourteenCarboxylic Acids

14-2

Structure• The functional group of a carboxylic acid is a

carboxyl group.

• The general formula of an aliphatic carboxylic acid is RCOOH.

• That of an aromatic carboxylic acid is ArCOOH.

14-3

Nomenclature• IUPAC names: drop the -ee from the parent

alkane and add the suffix -oic acidoic acid.• If the compound contains a carbon-carbon double

bond, change the infix -anan- to -enen-.COOH C6H5

COOH

3-Methylbutanoic acid(Isovaleric acid)

trans-3-Phenylpropenoic acid(Cinnamic acid)

14-4

Nomenclature• The carboxyl group takes precedence over

most other functional groups.

5-Oxohexanoic acid 4-Aminobutanoic acid

(R)-5-Hydroxyhexanoic acidCOOH

OH

H2N COOHCOOHO

Don’t worry about R or S…we didn’t learn about that.

14-5

Nomenclature Dicarboxylic acids: add -dioic aciddioic acid to the name of

the parent alkane containing both carboxyl groups.

• There is no need to use numbers to locate the carboxyl groups; they can only be on the ends of the chain.

HO OH

O

Propanedioic acid(Malonic acid)

Ethanedioic acid(Oxalic acid)

OHO OH

O

O

Hexanedioic acid(Adipic acid)

Pentanedioic acid(Glutaric acid)

Butanedioic acid(Succinic acid)

OOH

OHOOH

O

HO

OHO OH

O

O

14-6

Nomenclature• If the carboxyl group is bonded to a ring, name the

ring compound and add the suffix -carboxylic acidcarboxylic acid.

32

1

2-Cyclohexene-carboxylic acid

trans-1,3-Cyclopentane-dicarboxylic acid

COOH COOHHOOC

Don’t worry about ringswith multiple –COOHs!

14-7

Nomenclature• Benzoic acid is the simplest aromatic carboxylic

acid.• Use numbers to show the location of substituents.

Benzoicacid

2-Hydroxybenzoic acid

(Salicylic acid)

COOH COOHOH

1,2-Benzene-dicarboxylic acid(Phthalic acid)

1,4-Benzene-dicarboxylic acid

(Terephthalic acid)

COOHCOOH

COOH

COOH

Don’t worry about benzene ringswith multiple –COOHs!

14-8

Nomenclature• When common names are used, the letters

etc. are often used to locate substituents.

• In common nomenclature, ketoketo indicates the presence of a ketone, and CH3CO- is named an aceto groupaceto group.

alanine)(-Aminopropionic acid;(-Aminobutyric acid;

GABA)

2-Aminopropanoic acid4-Aminobutanoic acid

4 3 25 H2N OH

O

OH

O

NH2

OH

O

1

OH

OO OCH3C-

3-oxobutanoic acid-Ketobutyric acid;

acetoacetic acid)

Acetyl group(aceto group)

14-9

Physical Properties• In the liquid and solid states, carboxylic acids

are associated by hydrogen bonding into dimeric structures.

14-10

Physical Properties• Carboxylic acids have significantly higher boiling points

than other types of organic compounds of comparable molecular weight.• They are polar compounds and form very strong

intermolecular hydrogen bonds.• Carboxylic acids are more soluble in water than alcohols,

ethers, aldehydes, and ketones of comparable molecular weight.• They form hydrogen bonds with water molecules

through their C=O and OH groups.

14-11

Physical Properties• Water solubility decreases as the relative size of

the hydrophobic portion of the molecule increases.

14-12

Acidity• Carboxylic acids are weak acids

• Values of pKa for most aliphatic and aromatic carboxylic acids fall within the range 4 to 5.

• The greater acidity of carboxylic acids relative to alcohols, both of which contain an OH group, is due to resonance stabilization of the carboxylate anion.

CH3COOH H2O CH3COO- H3O+

[CH3COO-][H3O+][CH3COOH]

+

= 1.74 x 10-5Ka =

pKa = 4.76

+

14-13

Acidity• Electron-withdrawing substituents near the

carboxyl group increase acidity through their inductive effect.

14-14

Acidity• The acid-strengthening effect of a halogen

substituent falls off rapidly with increasing distance from the carboxyl group.

14-15

Reaction with Bases• Carboxylic acids, whether soluble or insoluble in water,

react with NaOH, KOH, and other strong bases to give water-soluble salts.

• They also form water-soluble salts with ammonia and amines.

+ +

Benzoic acid(slightly soluble

in water)

Sodium benzoate(60 g/100 mL water)

COOH COO-Na+NaOH H2 O H2 O

+

Ammonium benzoate(20 g/100 mL water)

Benzoic acid(slightly soluble

in water)

COOH COO- NH4+NH3 H2 O

14-16

Reaction with Bases• Carboxylic acids react with sodium bicarbonate

and sodium carbonate to form water-soluble salts and carbonic acid.• Carbonic acid (H2CO3), in turn, breaks down to

carbon dioxide and water.

CH3COOH Na+HCO3- H2O

H2CO3

CH3COOH Na+HCO3-

CO2

CH3COO-Na+

CH3COO-Na+

H2O

CO2

H2CO3

H2O

+ +

+

+ ++

14-17

Reaction with Bases• Figure 14.1

Flowchart for separation of benzoic acid from benzyl alcohol.

14-18

Reduction• The carboxyl group is very resistant to

reduction.• It is not affected by catalytic hydrogenation under

conditions that easily reduce aldehydes and ketones to alcohols, and reduce alkenes and alkynes to alkanes.

• It is not reduced by NaBH4.

14-19

Reduction• Lithium aluminum hydride reduces a carboxyl

group to a 1° alcohol.• Reduction is carried out in diethyl ether, THF, or

other nonreactive, aprotic solvents.• (Notice that the alkene double bond is

unaffected!)

1. LiAlH4, ether2. H2O

LiOH Al(OH)3+ +H

OOH1. LiAlH4, ether

2. H2OLiOH Al(OH)3+ +H

OOHOH

14-20

Selective Reduction• Catalytic hydrogenation does not reduce a

COOH group!• we can use H2/Metal to reduce an alkene in the

presence of a COOH group.

• we can use NaBH4 to reduce an aldehyde or ketone in the presence of a COOH group.

OH

O

H2+ Pt OH

O

Hexanoic acid5-Hexenoic acid 25°C, 2 atm

C6H5 OH

OO1. NaBH42. H2O

C6H5 OH

OOH

5-Hydroxy-5-phenylpentanoic acid5-Oxo-5-phenylpentanoic acid

14-21

Fischer Esterification• Esters can be prepared by treating a carboxylic

acid with an alcohol in the presence of an acid catalyst, commonly H2SO4 or gaseous HCl.

CH3COHO

CH3CH2OHH2SO4

CH3COCH2CH3O

H2OEthanoic acid(Acetic acid)

++Ethyl ethanoate(Ethyl acetate)

Ethanol(Ethyl alcohol)

14-22

Fischer Esterification• Fischer esterification is an equilibrium

reaction.• By careful control of experimental conditions, it is

possible to prepare esters in high yield.• If the alcohol is inexpensive relative to the

carboxylic acid, it can be used in excess to drive the equilibrium to the right.

• A key intermediate in Fischer esterification is the tetrahedral carbonyl addition intermediate formed by addition of ROH to the C=O group.

C OHRO

HOCH3H+

C OCH3

OH

OR

H

H+

C OCH3RO

HOH+ +

14-23

Acid Chlorides• The functional group of an acid halide is a

carbonyl group bonded to a halogen atom.• Among the acid halides, acid chlorides are by far

the most common and the most widely used.

Functional group of an acid halide

Acetylchloride

Benzoylchloride

-C-X CH3CClO O

C-ClO

14-24

Acid Chlorides• Acid chlorides are most often prepared by

treating a carboxylic acid with thionyl chloride.

+ + +Butanoic

acidThionylchloride

Butanoylchloride

SOCl2 SO2 HClOH

O

Cl

O

14-25

Decarboxylation• Decarboxylation:Decarboxylation: loss of CO2 from a carboxyl

group.• Most carboxylic acids, if heated to a very high

temperature, undergo thermal decarboxylation.

• Most, however, are quite resistant to moderate heat and melt or even boil without decarboxylation.

RCOHO

RH CO2decarboxylation +

heat

14-26

Decarboxylation• Exceptions are carboxylic acids that have a

carbonyl group beta to the carboxyl group.• This type of carboxylic acid undergoes

decarboxylation on mild heating.

OH

OO OCO2

Acetone3-Oxobutanoic acid(Acetoacetic acid)

+warm

14-27

Decarboxylation• Thermal decarboxylation of a -ketoacid involves

rearrangement of six electrons in a cyclic six-membered transition state.

O+

enol ofa ketone

(A cyclic six-membered transition state)

CO2(1) (2)O OH

O

OCOH

O

14-28

Decarboxylation• Thermal decarboxylation of malonic acids also

involves rearrangement of six electrons in a cyclic six-membered transition state.

O OH

HO O

O H

HO CO

O HO

OCO2+

Enol of acarboxylic acid

A cyclic six-membered transition state

(1) (2)

14-29

Decarboxylation• Problem:Problem: Draw the product of decarboxylation.

• Problem:Problem: Draw the -ketoacid that undergoes decarboxylation to give this ketone.

OCOOH heat

O

CO2-ketoacid heat +

14-30

Carboxylic Acids

End Chapter 14End Chapter 14