12 12-1 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Introduction to Organic Chemistry 2 ed William H. Brown.

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12-1Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.

Introduction to Introduction to Organic Organic

ChemistryChemistry2 ed2 ed

William H. BrownWilliam H. Brown

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Carboxylic Carboxylic

AcidsAcids

Chapter 12Chapter 12

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StructureStructure• The functional group of a carboxylic acid is a

carboxyl group

• The general formula of an aliphatic carboxylic acid is RCO2H; that of an aromatic carboxylic acid is ArCO2H

COOH CO2HCO H

O••

••

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Nomenclature - IUPACNomenclature - IUPAC• 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-

Propenoic acid (Acrylic acid)

trans-3-Phenylpropenoic acid(Cinnamic acid)

CH2 =CHCO2H CCO2 H

CH

C6H5 H

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Nomenclature - IUPACNomenclature - IUPAC• the carboxyl group takes precedence over most other

functional groups

5-Oxohexanoic acid4-Aminobutanoic acid

5-Hydroxyhexanoic acid

OH

O

CH3CHCH2CH2CH2 CO2H

CH3CCH2 CH2CH2CO2HH2 NCH2CH2CH2CO2H

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Nomenclature - IUPACNomenclature - IUPAC• Dicarboxylic acids: add the suffix -dioic acid to

the name of the parent alkane containing both carboxyl groups

Butanedioic acid(Succinic acid)

Propanedioic acid(Malonic acid)

Ethanedioic acid(Oxalic acid)

O O O O O O

HOC-COH HOCCH2COH HOCCH2CH2 COH

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Nomenclature - IUPACNomenclature - IUPAC• If the carboxyl group is attached to a ring, name

the ring compound and add the suffix -carboxyliccarboxylic acidacid

32

1

2-Cyclohexene-carboxylic acid

trans-1,3-Cyclopentane-dicarboxylic acid

CO2 HCO2 HHO2C

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Nomenclature - IUPACNomenclature - IUPAC• The simplest aromatic carboxylic acid is benzoic

acid• derivatives are named using numbers to show the

location of substituents relative to the carboxyl group

Benzoic acid

2-Hydroxybenzoic acid

(Salicylic acid)

4-Aminobenzoic acid

CO2H CO2HOH

H2N CO2H

1

2 123

4

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Nomenclature - IUPACNomenclature - IUPAC• Aromatic dicarboxylic acids are named by adding

the words “dicarboxylic acid” to “benzene”

1,2-Benzenedicarboxylic acid (Phthalic acid)

1,4-Benzenedicarboxylic acid (Terephthalic acid)

CO2H

CO2HCO2HHO2C

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Nomenclature-CommonNomenclature-Common• When common names are used, the letters

etc. are often used to locate substituents

Alanine)(- ;Aminopropionic acid(- )Hydroxybutyric acid2- Aminopropanoic acid4- Hydroxybutanoic acid

4 3 2 15

O

HOCH2CH2CH2CO2H

C-C-C-C-C-OH

CH3CHCO2H

NH2

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

associated by hydrogen bonding into dimeric structures

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Physical PropertiesPhysical Properties• Carboxylic acids are polar compounds and form

very strong intermolecular hydrogen bonds through both their C=O and OH groups• they have significantly higher boiling points than other

types of organic compounds of comparable molecular weight

• they are more soluble in water than alcohols, ethers, aldehydes, and ketones of comparable molecular weight

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Physical PropertiesPhysical Properties• Water solubility decreases as the relative size of

the hydrophobic portion of the molecule increases

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Acidity Acidity • Carboxylic acids are weak acids

• values of pKa for most aliphatic and aromatic carboxylic acids fall within the range 4 - 5

CH3CO2H + H2O CH3CO2- + H3O+

Ka = [CH3CO2-] [H3O+]

[CH3CO2H]= 1.74 x 10-5

pKa = 4.76

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AcidityAcidity• The greater acidity of carboxylic acids relative to

alcohols is due to two factors: • the significant resonance stabilization of a carboxylate

anion compared with an alkoxide anion

C

O

O -CH3 C

O -

O

CH3 CH3CH2O-

the negative charge is localized on oxygenthese equivalent contributing

structures delocalize the negative charge and stabilize

the carboxylic anion

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AcidityAcidity• the electron-withdrawing inductive effect of the

adjacent carbonyl group on the O-H bond

C

O

O-H

CH3 +

δ- because electron density iswithdrawn from the O-H bond, the carboxyl hydrogenis lost more easily to a base

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AcidityAcidity• electron-withdrawing substituents near the carboxyl

group increase acidity through their inductive effect

Increasing acid strength

2.903.184.76 2.86 2.59

CH2CO2HCH2CO2H CH2CO2HCH2CO2HCH2CO2H

FClBrH I

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AcidityAcidity• multiple substitution of electron-withdrawing groups

further increases acidity

pKa:


2.86

Chloroacetic acid

0.70

Trichloroacetic acid

1.48

Dichloroacetic acid

ClCH2CO2H Cl2CHCO2H Cl3CCO2H

Acetic acid

CH3CO2H

4.76

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AcidityAcidity• The inductive effect of an electron-withdrawing

substituent falls off rapidly with its distance from the carboxyl group


pKa: 4.52

4-Chlorobutanoic acid

3.98



2.83

Cl Cl Cl

CH2CH2CH2CO2H CH3CH2CHCO2HCH3CHCH2CO2H

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

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

+

+

Benzoic acid(slightly soluble in water)

Sodium benzoate(60 g/100 mL water)

CO2H

CO2-Na+

NaOHH2O

H2O

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Reaction with BasesReaction with Bases• they also form water-soluble salts with ammonia and

amines

+

Ammonium benzoate(20 g/100 mL water)

Benzoic acid(slightly soluble

in water)

CO2H CO2- NH4

+NH3 H2O

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Reaction with BasesReaction with Bases• Carboxylic acids react with sodium bicarbonate

and sodium carbonate to form water-soluble salts and carbonic acid• carbonic acid, in turn, breaks down to carbon dioxide

and water

+

+ +

CH3CO2H NaHCO3

CH3CO2- Na

+CO2

H2O

H2O

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ReductionReduction• The carboxyl group is one of the organic

functional groups most 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, a reagent that readily reduces the carbonyl groups of aldehydes and ketones

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Reduction by LiAlHReduction by LiAlH44• Lithium aluminum hydride reduces a carboxyl

group to a 1° alcohol• reduction is most commonly carried out in diethyl

ether or THF

+ +

1. LiAlH 4, ether2. H2O

LiOH Al(OH)3

COH

O

CH2OH

3-Cyclopentene- carboxylic acid

4-Hydroxymethyl- cyclopentene

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Selective ReductionSelective Reduction• The carboxyl group is not affected by catalytic

hydrogenation under conditions that easily reduce aldehydes and ketones to alcohols.

5-Oxohexanoic acid

5-Hydroxyhexanoic acid

CH3CCH2CH2CH2COH

O O

OOH

CH3CHCH2CH2CH2COH

Pt

25° C, 2 atm

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Selective ReductionSelective Reduction• it is also possible to selectively reduce the carbonyl

group of an aldehyde or ketone using the less reactive NaBH4

5-Oxo-5-phenyl-pentanoic acid

5-Hydroxy-5-phenyl-pentanoic acid

CCH2CH2CH2COH

O O

OOH

CHCH2CH2CH2COH

1. NaBH4

2. H2O

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Fischer EsterificationFischer Esterification• Esters can be prepared by treatment of a

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

Ethyl ethanoate (Ethyl acetate)

Ethanol(Ethyl alcohol)

Ethanoic acid (Acetic acid)

+

+

O

O

CH3COH CH3CH2OHH2SO4

CH3COCH2CH3 H2O

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Fischer EsterificationFischer 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

• if water can be removed from the reaction mixture, the equilibrium is also driven to the right

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Fischer EsterificationFischer Esterification• A key intermediate in Fischer esterification is the

tetrahedral carbonyl addition intermediate formed by addition of ROH to the C=O group

••

••

••

C OCH3

OH

O

R

H

••

••C OH

O

R••

••C OCH3

O

R••

••HOCH3+

••

••HOH+

H+ H+

tetrahedral carbonyl addition intermediate

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DecarboxylationDecarboxylation• DecarboxylationDecarboxylation: loss of CO2 from a carboxyl

group• most carboxylic acids, if heated to a very high

temperature, undergo thermal decarboxylation• most carboxylic acids, however, are quite resistant to

moderate heat and melt or even boil without decarboxylation

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DecarboxylationDecarboxylation• Exceptions are carboxylic acids that have a

carbonyl group beta to the carboxyl group• this type of carboxylic acid undergoes decarboxylation

on mild heating

Acetone3-Oxobutanoic acid(Acetoacetic acid)

+warm O O O

CH3 -C-CH2 -C-OH CH3 -C-CH3 CO2

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DecarboxylationDecarboxylation• thermal decarboxylation of a -ketoacid involves

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

(A cyclic six-membered transition state)

CH

O

C C

OH

H3C OH

+

enol ofa ketone

HO

CC

O

C

O

H3C OH H

CH3 -C-CH3 CO2

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DecarboxylationDecarboxylation• Decarboxylation occurs if there is any carbonyl

group beta to the carboxyl• malonic acid and substituted malonic acids, for

example, also undergo thermal decarboxylation

Propanedioic acid (Malonic acid)

+140-150°C

O O O

HOCCH2COH CH3COH CO2

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DecarboxylationDecarboxylation• thermal decarboxylation of malonic acids also

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

A cyclic six-membered transition state

+

enol of a carboxylic acid

CHH

CH

CO

CHO

HOO

H

CO

CHO

OOH

O

CH3-C-OH CO2

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CarboxylicCarboxylic

AcidsAcids

End Chapter 12End Chapter 12

12 12-1 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Introduction to Organic Chemistry 2 ed William H. Brown.

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