1

Carboxylic Acids

Chapter 20 2

• Adapted from:

Organic Chemistry, 6th Edition;

Chapter 20, Carboxylic Acids

L. G. Wade, Jr.

2

Chapter 20 3

carboxyl group

• (-C=O) + (-OH) on the same carbon.

• -COOH.

• Aliphatic acids: R-COOH.

• Aromatic acids: Ar-COOH.

• Fatty acids: long-chain aliphatic acids.

=>

Chapter 20 4

Common Names

• Many aliphatic acids have historical names.

• Positions of substituents on the chain are

labeled with Greek letters.

=>

3

Chapter 20 5

IUPAC Names• Remove -e from alkane (or alkene)

name, add -oic acid.

• The carbon of the carboxyl group is #1.

CH3CH2CHC

Cl

OH

O

2-chlorobutanoic acid

PhC

H

C

H

COOH

trans-3-phenyl-2-propenoic acid

(cinnamic acid)

=>

Chapter 20 6

order of precedence in the nomenclature

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u/Page/vsc/en/ch/12/oc/c_acid/nomenklatur/nomenklatur.vscml.html

4

Chapter 20 7

Naming Cyclic Acids

• Cycloalkanes bonded to -COOH are named

as cycloalkanecarboxylic acids.

• Aromatic acids are named as benzoic acids.COOH

CH(CH3)2

2-isopropylcyclopentanecarboxylic acid

COOH

OH

o-hydroxybenzoic acid

(salicylic acid)=>

Chapter 20 8

5

Chapter 20 9

219 ºC31.0 ºCdecanoic acidgoats (L. caper)capric acidCH3(CH

2)

8CO

2H

253 ºC12.0 ºCnonanoic acidpelargonium (an herb)pelargonic acidCH3(CH

2)

7CO

2H

239 ºC16.3 ºCoctanoic acidgoats (L. caper)caprylic acidCH3(CH

2)

6CO

2H

223 ºC-7.5 ºCheptanoic acidvines (Gk. oenanthe)enanthic acidCH3(CH

2)

5CO

2H

205 ºC-4.0 ºChexanoic acidgoats (L. caper)caproic acidCH3(CH

2)

4CO

2H

186 ºC-34.5 ºCpentanoic acidvalerian rootvaleric acidCH3(CH

2)

3CO

2H

164 ºC-5.5 ºCbutanoic acidbutter (L. butyrum)butyric acidCH3(CH

2)

2CO

2H

141 ºC-20.8 ºCpropanoic acidmilk (Gk. protus

prion)propionic acidCH

3CH

2CO

2H

118 ºC16.6 ºCethanoic acidvinegar (L. acetum)acetic acidCH3CO

2H

101 ºC8.4 ºCmethanoic acidants (L. formica)formic acidHCO2H

Boiling PointMelting PointIUPAC NameSourceCommon NameFormula

Nomenclature

Chapter 20 10

Dicarboxylic Acids

• Aliphatic diacids are usually called by

their common names (to be memorized).

• For IUPAC name, number the chain from the end closest to a substituent.

• Two carboxyl groups on a benzene ring

indicate a phthalic acid.

HOOCCH2CHCH2CH2COOH

Br

3-bromohexanedioic acid

β-bromoadipic acid

COOH

COOH

1,3-benzenedicarboxylic acid

m-phthalic acid =>

6

Chapter 20 11

Dicarboxylic acids

Oxalic acid (ethanedioic acid)

Malonic acid

(propanedioic acid)

Succinic acid

(butanedioic acid)

found in distillate of succinite

Glutaric acid

(pentanedioic acid)

Adipic acid

(hexanedioic acid)

Chapter 20 12

Dicarboxylic acids

Maleic acid

(cis-2-butenedioic acid)Fumaric acid

(trans-2-butenedioic acid)

isolated from the plant Fumaria officinaliswhich was burnt in the past to drive away evil

spirits

Maleic acid is relatively less stable than fumaric acid.

Upon exposure to light, maleic acid is converted into fumaric acid

7

Chapter 20 13

Saturated fatty acids

76 ºCarachidic acidCH3(CH

2)

18CO

2H

69 ºCstearic acidCH3(CH

2)

16CO

2H

63 ºCpalmitic acidCH3(CH

2)

14CO

2H

55 ºCmyristic acidCH3(CH

2)

12CO

2H

45 ºClauric acidCH3(CH

2)

10CO

2H

Melting PointCommon NameFormula

Chapter 20 14

-49 ºCarachidonic acidCH3(CH2)4(CH=CHCH2)4(CH2)2CO2H

-11 ºClinolenic acidCH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7CO2H

-5 ºClinoleic acidCH3(CH2)4CH=CHCH2CH=CH(CH2)7CO2H

13 ºColeic acidCH3(CH2)7CH=CH(CH2)7CO2H

0 ºCpalmitoleic acidCH3(CH2)5CH=CH(CH2)7CO2H

Melting PointCommon NameFormula

Unsaturated fatty acids

8

Chapter 20 15

Chapter 20 16

Structure of Carboxyl

=>

9

Chapter 20 17

Boiling PointsHigher boiling points than similar alcohols,

due to dimer formation.

Acetic acid, b.p. 118°C =>

Chapter 20 18

Melting Points• Aliphatic acids with more than 8

carbons are solids at room temperature.

• Double bonds (especially cis) lower the melting point. Note these 18-C acids:

�Stearic acid (saturated): 72°C

�Oleic acid (one cis double bond): 16°C

�Linoleic acid (two cis double bonds): -5°C=>

10

Chapter 20 19

Solubility

• Water solubility decreases with the length

of the carbon chain.

• Up to 4 carbons, acid is miscible in water.

• More soluble in alcohol.

• Also soluble in relatively nonpolar

solvents like chloroform because it

dissolves as a dimer.

=>

Chapter 20 20

11

Chapter 20 21

Acidity

=>

Chapter 20 22

Delocalized molecular orbitals

=>

12

Chapter 20 23

Substituent Effects on Acidity

=>

Chapter 20 24

Salts of Carboxylic Acids

• Sodium hydroxide removes a proton to

form the salt.

• Adding a strong acid, like HCl, regenerates the carboxylic acid.

CH3 C

O

OHNaOH

CH3 C

O

O

_Na

+

HCl

=>

13

Chapter 20 25

Naming Acid Salts

• Name the cation.

• Then name the anion by replacing the

-ic acid with -ate.

CH3CH2CHCH2COO- K

+

Cl

potassium 3-chloropentanoate

potassium β-chlorovalerate=>

Chapter 20 26

Properties of Acid Salts

• Usually solids with no odor.

• Carboxylate salts of Na+, K+, Li+, and

NH4+ are soluble in water.

• Soap is the soluble sodium salt of a

long chain fatty acid.

• Salts can be formed by the reaction of

an acid with NaHCO3, releasing CO2.

=>

14

Chapter 20 27

Purifying an Acid

=>

Chapter 20 28

Some Important Acids

• Acetic acid: food, solvent, catalyst, and

reagent.

• Fatty acids: fats and oils.

• Benzoic acid: in drugs, preservatives.

• Adipic: nylon 66.

• Phthalic acid: polyesters.=>

15

Chapter 20 29

IR Spectroscopy

=>

Chapter 20 30

NMR Spectroscopy

=>

16

Chapter 20 31

UV Spectroscopy

• Saturated carboxylic acids absorb very

weakly around 200-215 nm.

• If C=C is conjugated with C=O, molar absorptivity = 10,000 at 200 nm.

• An additional conjugated double bond

increases the absorption wavelength to

250 nm.

=>

Chapter 20 32

Mass Spectrometry

=>

17

Chapter 20 33

Synthesis Review

• Oxidation of primary alcohols and aldehydes with chromic acid.

• Cleavage of an alkene with hot KMnO4

produces a carboxylic acid if there is a hydrogen on the double-bonded carbon.

• Cleavage of an alkyne with ozone or hot permanganate.

• Alkyl benzene oxidized to benzoic acid by hot KMnO4 or hot chromic acid.

=>

Chapter 20 34

18

Chapter 20 35

Chapter 20 36

Organometallic reagents

=>

19

Chapter 20 37

Hydrolysis of Nitriles

Basic or acidic hydrolysis of a nitrile

produces a carboxylic acid.

BrNaCN

CNH

+

H2O

COOH

=>

Chapter 20 38

Arndt-Eistert Synthesis

20

Chapter 20 39

Arndt-Eistert mechanism

Chapter 20 40

β-Homoamino Acids

Synthesis of Fmoc-β-Homoamino Acids by Ultrasound-

Promoted Wolff RearrangementA. Müller, C. Vogt, N. Sewald, Synlett, 2006, 837-841

21

Chapter 20 41

Reactions of Carboxylic Acids

• Salt Formation

• Substitution of the Hydrogen in COOH

• Substitution of the OH

• Reduction

• Oxidation

Chapter 20 42

RCO2δ(-) Agδ(+) + H2OAgOH+RCO2H

RCO2(–) (CH3)3NH(+)(CH3)3N:+RCO2H

RCO2(–)

Na(+) + CO2 + H2ONaHCO3+RCO2H

R > C6 amphiphilicmonolayers

micelles,

22

Chapter 20 43

Acid Derivatives

• The group bonded to the acyl carbon

determines the class of compound:

�-OH, carboxylic acid

�-Cl, acid chloride

�-OR’, ester

�-NH2, amide

• These interconvert via nucleophilic acyl

substitution. =>

Chapter 20 44

from -COOH to -COOR

23

Chapter 20 45

from -COOH to -COOR

Chapter 20 46

24

Chapter 20 47

From -COOHto –COOR, COX, CON

Chapter 20 48

25

Chapter 20 49

Chapter 20 50

26

Chapter 20 51

Chapter 20 52

27

Chapter 20 53

Chapter 20 54

Fischer Esterification

• Acid + alcohol yields ester + water.

• Acid catalyzed for weak nucleophile.

• All steps are reversible.

• Reaction reaches equilibrium.

COOH

+ CH3CH2OHH

+ COCH2CH3

O

+ HOH

=>

28

Chapter 20 55

Diazomethane

• CH2N2 reacts with carboxylic acids to

produce methyl esters quantitatively.

• Very toxic, explosive. Dissolve in ether.

C

O

OH +C

O

OCH3 +CH2N2 N2

=>

Chapter 20 56

Mechanism for Diazomethane

=>

29

Chapter 20 57

Amides from Acids

• Amine (base) removes a proton from

the carboxylic acid to form a salt.

• Heating the salt above 100°C drives off steam and forms the amide.

C

O

OH +C

O

O-

+NH3CH3

+

CH3NH2

heat

C

O

NHCH3

H2O

=>

Chapter 20 58

Reduction to 1° Alcohols

• Use strong reducing agent, LiAlH4.

• Borane, BH3 in THF, reduces carboxylic

acid to alcohol, but does not reduce ketone.

=>

30

Chapter 20 59

Reduction to Aldehyde

• Difficult to stop reduction at aldehyde.

• Use a more reactive form of the acid (an

acid chloride) and a weaker reducing agent, lithium aluminum tri(t-butoxy)hydride.

CCl

O

C

O

HLiAl[OC(CH3)3]3H

=>

Chapter 20 60

Alkylation to Form Ketones

React 2 equivalents of an organolithium

reagent with a carboxylic acid.

COOH 1)

2)

CH3CH2 Li2

H2O

C

O

CH2CH3

=>

31

Chapter 20 61

Oxidation (decarboxylation )

Chapter 20 62

Oxidation

32

Chapter 20 63

Acid Chlorides

• An activated form of the carboxylic acid.

• Chloride is a good leaving group, so

undergoes acyl substitution easily.

• To synthesize acid chlorides use thionyl

chloride or oxalyl chloride with the acid.

C

O

OHC

O

C

O

Cl Cl+

C

O

Cl

+ + +HCl CO CO2

=>

Chapter 20 64

Esters from Acid Chlorides

• Acid chlorides react with alcohols to

give esters in good yield.

• Mechanism is nucleophilic addition of the alcohol to the carbonyl as chloride

ion leaves, then deprotonation.

CCl

O

+ CH3OH

COCH3

O

+ HCl=>

33

Chapter 20 65

Amides from Acid Chlorides

• Acid chlorides react with ammonia and

amines to give amides.

• A base (NaOH or pyridine) is added to remove HCl by-product.

CCl

O

+ CH3NH2

CNHCH3

O

NaOHNaCl H2O+ +

=>

Chapter 20 66

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