Chapter 19 Carboxylic Acids

1
Chapter 19Carboxylic Acids
2
19.2Structure and Bonding
3
Formic Acid Is Planar
4
Electron Delocalization

• Stabilizes carbonyl group

5
19.3Physical Properties
6
Boiling Points
bp (1 atm)
31C
80C
99C

• Intermolecular forces, especially hydrogen
  bonding, are stronger in carboxylic acids than in
  other compounds of similar shape and molecular
  weight

7
Hydrogen-Bonded Dimers

• Acetic acid exists as a hydrogen-bonded dimer in
  the gas phase. The hydroxyl group of each
  molecule is hydrogen-bonded to the carbonyl
  oxygen of the other.

8
Hydrogen-Bonded Dimers

• Acetic acid exists as a hydrogen-bonded dimer in
  the gas phase. The hydroxyl group of each
  molecule is hydrogen-bonded to the carbonyl
  oxygen of the other.

9
Solubility in Water

• Carboxylic acids are similar to alcohols with
  respect to their solubility in water.
• Form hydrogen bonds to water.

10
19.4Acidity of Carboxylic Acids

• Most carboxylic acids have a pKa close to 5.

11
Carboxylic Acids Are Weak Acids

• But carboxylic acids are far more acidic than
  alcohols.

CH3CH2OH
pKa 4.7
pKa 16
12
Free Energies of Ionization
CH3CH2O H
?G 91 kJ/mol
?G 27 kJ/mol
CH3CH2OH
13
Greater Acidity of Carboxylic Acids Is Due
toStabilization of Carboxylate Ion
Inductive effect of carbonyl group
Resonance stabilization of carboxylate ion
14
Electrostatic Potential Maps ofAcetic Acid and
Acetate Ion
Acetic acid
Acetate ion
15
19.6Substituents and Acid Strength
16
Substituent Effects on Acidity
Standard of comparison is acetic acid (X H)
pKa 4.7
17
Substituent Effects on Acidity
18
Substituent Effects on Acidity

• Electronegative substituents withdraw electrons
  from carboxyl group increase K for loss of H.

19
Effect of Electronegative Substituent
Decreasesas Number of Bonds between It and
Carboxyl Group Increases.
pKa

ClCH2CH2CH2CO2H
20
19.7Ionization ofSubstituted Benzoic Acids
21
Hybridization Effect

• sp2-Hybridized carbon is more electron-withdrawin
  g than sp3, and sp is more electron-withdrawing
  than sp2.

22
Ionization of Substituted Benzoic Acids

• Effect is small unless X is electronegative
  effect is largest for ortho substituent.

pKa Substituent Ortho Meta Para H 4.2 4.2 4.2 CH
3 3.9 4.3 4.4 F 3.3 3.9 4.1 Cl 2.9 3.8 4.0 CH3O 4.
1 4.1 4.5 NO2 2.2 3.5 3.4
23
19.10Sources of Carboxylic Acids
24
Synthesis of Carboxylic Acids Review

• Side-chain oxidation of alkylbenzenes (11.13)
• Oxidation of primary alcohols (15.10)
• Oxidation of aldehydes (17.15)

25
Synthesis of Carboxylic Acids Review
26
19.11Synthesis of Carboxylic Acidsby
theCarboxylation of Grignard Reagents
27
Carboxylation of Grignard Reagents
Mg
CO2
RMgX
RX
diethylether
H3O

• Converts an alkyl (or aryl) halide to a
  carboxylic acid having one more carbon atom than
  the starting halide.

28
Carboxylation of Grignard Reagents
?
C
O


29
Example Alkyl Halide
1. Mg, diethyl ether
2. CO2 3. H3O
Cl
CO2H
(76-86)
30
Example Aryl Halide
1. Mg, diethyl ether
2. CO2 3. H3O
(82)
31
19.12Synthesis of Carboxylic Acidsby
thePreparation and Hydrolysis of Nitriles
32
Preparation and Hydrolysis of Nitriles
H3O
RX
heat
SN2
NH4

• Converts an alkyl halide to a carboxylic acid
  having one more carbon atom than the starting
  halide.
• Limitation is that the halide must be reactive
  toward substitution by SN2 mechanism.

33
Example
NaCN
DMSO
(92)
34
Example Dicarboxylic Acid
BrCH2CH2CH2Br
NaCN
H2O
(77-86)
NCCH2CH2CH2CN
35
Example ?-Hydroxy Acid from Ketone via
Cyanohydrin
1. NaCN
2. H
H2O,
HCl, heat
2-Hydroxy-2-methylpentanoic acid, an ?-hydroxy
acid (60 from 2-pentanone)
36
19.13Reactions of Carboxylic AcidsA Review and
a Preview
37
Reactions of Carboxylic Acids
Reactions already discussed

• Acidity (19.4, 19.5-19.7)
• Reaction with thionyl chloride (12.7 will also
  see in 20.4)
• Reduction with LiAlH4 (15.3)
• Acid-catalyzed esterification (15.8)

38
Reactions of Carboxylic Acids Review
39
Reactions of Carboxylic Acids Review
40
19.14Mechanism of Acid-Catalyzed
Esterification(Fischer Esterification)
41
Acid-Catalyzed Esterification

CH3OH

H2O

• Important fact the oxygen of the alcohol
  isincorporated into the ester as shown.

42
Mechanism of Fischer Esterification

• The mechanism involves two stages
• 1) Formation of tetrahedral intermediate (3
  steps)
• 2) Dissociation of tetrahedral intermediate
  (3 steps)

43
Mechanism of Fischer Esterification

• The mechanism involves two stages
• 1) Formation of tetrahedral intermediate (3
  steps)
• 2) Dissociation of tetrahedral intermediate
  (3 steps)

Tetrahedral intermediate in esterification of
benzoic acid with methanol
44
First Stage Formation of Tetrahedral Intermediate

CH3OH

• Methanol adds to the carbonyl group of the
  carboxylic acid.
• The tetrahedral intermediate is analogous to a
  hemiacetal.

H
45
Second Stage Conversion of TetrahedralIntermedia
te to Ester

H2O

• This stage corresponds to an acid-catalyzed
  dehydration.

H
46
Mechanism of FormationofTetrahedral Intermediate
47
Step 1
48
Step 1

• Carbonyl oxygen is protonated because cation
  produced is stabilized by electron delocalization
  (resonance).

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Step 2

50
Step 3
51
Tetrahedral IntermediatetoEster Stage
52
Step 4
53
Step 5
54
Step 6
55
Key Features of Mechanism

• Activation of carbonyl group by protonation of
  carbonyl oxygen.
• Nucleophilic addition of alcohol to carbonyl
  groupforms tetrahedral intermediate.
• Elimination of water from tetrahedral
  intermediate restores carbonyl group.

56
Review of Fischer Esterification
57
Industrial Use of Fischer Esterification Synthesi
s of Dacron Polyester
http//heritage.dupont.com/touchpoints/tp_1950/ove
rview.shtml
58
A Waste of Good Chemistry
59
19.15Intramolecular Ester FormationLactones
60
Lactones

• Lactones are cyclic esters.
• Formed by intramolecular esterification in
  acompound that contains a hydroxyl group anda
  carboxylic acid function.

61
Example


H2O
4-Hydroxybutanoic acid
4-Butanolide

• IUPAC nomenclature replace the -oic acid ending
  of the carboxylic acid by -olide.
• Identify the oxygenated carbon by number.

62
Examples


H2O
4-Hydroxybutanoic acid
4-Butanolide
63
Common names

?
?
?
?
?
?
?
?-Butyrolactone
?-Valerolactone

• Ring size is designated by Greek letter.
• A ? lactone has a five-membered ring.
• A ? lactone has a six-membered ring.

64
Lactones

• Reactions designed to give hydroxy acids often
  yield the corresponding lactone, especially if
  theresulting ring is 5- or 6-membered.

65
Example
5-hexanolide (78)
66
Section 19.18Spectroscopic Analysis
ofCarboxylic Acids
67
Infrared Spectroscopy

• A carboxylic acid is characterized by peaks due
  toOH and CO groups in its infrared spectrum.
• CO stretching gives an intense absorptionnear
  1700 cm-1.
• OH peak is broad and overlaps with CH
  absorptions.

68
Infrared Spectrum of 4-Phenylbutanoic Acid
69
1H NMR

• Proton of OH group of a carboxylic acid is
  normallythe least shielded of all of the protons
  in 1HNMR spectra (? 10-12 ppm broad).

70
4-Phenylbutanoic Acid
Chemical shift (?, ppm)
71
13C NMR

• Carbonyl carbon is at low field (? 160-185 ppm),
  but not as deshielded as the carbonyl carbon of
  an aldehyde or ketone (? 190-215 ppm).