Chapter 18 Carboxylic Acids

1
Chapter 18Carboxylic Acids
2
Carboxylic Acid Nomenclature
3
Table 18.1

• systematic IUPAC names replace "-e" ending of
  alkane with "oic acid"

Systematic Name
methanoic acid
ethanoic acid
octadecanoic acid
4
Table 18.1

• common names are based on natural origin rather
  than structure

Systematic Name
Common Name
methanoic acid
formic acid
ethanoic acid
acetic acid
octadecanoic acid
stearic acid
5
Table 18.1
Systematic Name
Common Name
2-hydroxypropanoicacid
lactic acid
(Z)-9-octadecenoicacid
oleic acid
6
Structure and Bonding
7
Formic acid is planar
8
Formic acid is planar
O
H
C
O
120 pm
H
134 pm
9
Electron Delocalization
10
Electron Delocalization

• stabilizes carbonyl group

11
Physical Properties
12
Boiling Points
bp
31C
80C
99C

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

13
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.

14
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.

15
Solubility in Water

• carboxylic acids are similar to alcohols in
  respect to their solubility in water
• form hydrogen bonds to water

16
Acidity of Carboxylic Acids

• Most carboxylic acids have a pKa close to 5.

17
Carboxylic acids are weak acids

• but carboxylic acids are far more acidic than
  alcohols

CH3CH2OH
Ka 1.8 x 10-5 pKa 4.7
Ka 10-16 pKa 16
18
Free Energies of Ionization
CH3CH2O H
DG 64 kJ/mol
DG 91 kJ/mol
DG 27 kJ/mol
CH3CH2OH
19
Greater acidity of carboxylic acids is
attributedstabilization of carboxylate ion by
inductive effect of carbonyl group

resonance stabilization of carboxylate ion
20
Figure 19.4 Electrostatic potential maps
ofacetic acid and acetate ion
Acetic acid
Acetate ion
21
Substituents and Acid Strength
22
Substituent Effects on Acidity
standard of comparison is acetic acid (X H)
Ka 1.8 x 10-5pKa 4.7
23
Substituent Effects on Acidity

• alkyl substituents have negligible effect

24
Substituent Effects on Acidity

• electronegative substituents increase acidity

25
Substituent Effects on Acidity

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

26
Substituent Effects on Acidity
X
Ka
pKa

H
1.8 x 10-5
4.7
1.4 x 10-3
2.9
Cl
ClCH2
1.0 x 10-4
4.0
ClCH2CH2
3.0 x 10-5
4.5

• effect of substituent decreases as number of
  bonds between X and carboxyl group increases

27
Ionization ofSubstituted Benzoic Acids
28
Hybridization Effect

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

29
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
30
Salts of Carboxylic Acids
31
Carboxylic acids are neutralized by strong bases


RCOH
HO
RCO
H2O
strongeracid
weakeracid

• equilibrium lies far to the right K is 1011
• as long as the molecular weight of the acid is
  not too high, sodium and potassium carboxylate
  salts are soluble in water

32
Micelles

• unbranched carboxylic acids with 12-18
  carbonsgive carboxylate salts that form micelles
  inwater

ONa
sodium stearate(sodium octadecanoate)

Na
33
Micelles
ONa
polar
nonpolar

• sodium stearate has a polar end (the carboxylate
  end) and a nonpolar "tail"
• the polar end is "water-loving" or hydrophilic
• the nonpolar tail is "water-hating" or
  hydrophobic
• in water, many stearate ions cluster together to
  form spherical aggregates carboxylate ions on
  the outside and nonpolar tails on the inside

34
Micelles
ONa
polar
nonpolar
35
Figure 19.5 A micelle
36
Micelles

• The interior of the micelle is nonpolar and has
  the capacity to dissolve nonpolar substances.
• Soaps clean because they form micelles, which
  are dispersed in water.
• Grease (not ordinarily soluble in water)
  dissolves in the interior of the micelle and is
  washed away with the dispersed micelle.

37
Dicarboxylic Acids
38
Dicarboxylic Acids
pKa
Oxalic acid
1.2
Malonic acid
2.8
Heptanedioic acid
4.3

• one carboxyl group acts as an electron-withdrawin
  g group toward the other effect decreases with
  increasing separation

39
Carbonic Acid
40
Carbonic Acid

H2O
CO2
99.7
0.3
41
Carbonic Acid


H2O
CO2
H
42
Carbonic Acid


H2O
CO2
H
overall K for these two steps 4.3 x 10-7

• CO2 is major species present in a solution of
  "carbonic acid" in acidic media

43
Carbonic Acid
Ka 5.6 x 10-11
Second ionization constant

H
44
Sources of Carboxylic Acids
45
Synthesis of Carboxylic Acids Review

• side-chain oxidation of alkylbenzenes (Chapter
  11)
• oxidation of primary alcohols (Chapter 15)
• oxidation of aldehydes (Chapter 17)

46
Synthesis of Carboxylic Acids by the
Carboxylation of Grignard Reagents
47
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

48
Carboxylation of Grignard Reagents
d
C
O


H3O
49
Example Alkyl Halide
1. Mg, diethyl ether
2. CO2 3. H3O
Cl
CO2H
(76-86)
50
Example Aryl Halide
1. Mg, diethyl ether
2. CO2 3. H3O
(82)
51
Synthesis of Carboxylic Acidsby thePreparation
and Hydrolysis of Nitriles
52
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, i.e. best
  with primary, then secondary tertiary gives
  elimination

53
Example
NaCN
DMSO
(92)
H2O
H2SO4
heat
(77)
54
Example Dicarboxylic Acid
BrCH2CH2CH2Br
NaCN
H2O
(77-86)
NCCH2CH2CH2CN
H2O, HCl
heat
(83-85)
55
via Cyanohydrin
1. NaCN
2. H
H2O
HCl, heat
(60 from 2-pentanone)
56
Reactions of Carboxylic AcidsA Review and a
Preview
57
Reactions of Carboxylic Acids
Reactions already discussed

• Acidity (Chapter 18)
• Reduction with LiAlH4 (Chapter 15)
• Esterification (Chapter 15)
• Reaction with Thionyl Chloride (Chapter 12)

58
Reactions of Carboxylic Acids
New reaction in this chapter

• Decarboxylation
• But first we revisit acid-catalyzed
  esterificationto examine its mechanism.

59
Mechanism of Acid-Catalyzed Esterification
60
Acid-catalyzed Esterification
(also called Fischer esterification)

CH3OH

H2O

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

61
Mechanism of Fischer Esterification

• The mechanism involves two stages
• 1) formation of tetrahedral intermediate (3
  steps)
• 2) dissociation of tetrahedral intermediate
  (3 steps)

62
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
63
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
64
Second stage conversion of tetrahedral
intermediate to ester

H2O
H

• this stage corresponds to an acid-catalyzed
  dehydration

65
Mechanism of formationoftetrahedral intermediate
66
Step 1
67
Step 1

68
Step 1

O
H

C

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

69
Step 2

70
Step 2

71
Step 3
72
Step 3
73
Tetrahedral intermediatetoester stage
74
Step 4
75
Step 4
76
Step 4
77
Step 5

OH


C
OCH3


O
H

H
78
Step 5

79
Step 5
80
Step 6
CH3

H
O

H
H
81
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

82
Intramolecular Ester FormationLactones
83
Lactones

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

84
Examples


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

85
Examples


H2O
4-hydroxybutanoic acid
4-butanolide

H2O
5-pentanolide
5-hydroxypentanoic acid
86
Common names

a
b
a
b
g
g
d
g-butyrolactone
d-valerolactone

• Ring size is designated by Greek letter
  corresponding to oxygenated carbon
• A g lactone has a five-membered ring
• A d lactone has a six-membered ring

87
Lactones

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

88
Example
5-hexanolide (78)
89
Example
via
5-hexanolide (78)
90
Decarboxylation of Malonic Acidand Related
Compounds
91
Decarboxylation of Carboxylic Acids
Simple carboxylic acids do not decarboxylatereadi
ly.

RH
CO2
92
Decarboxylation of Carboxylic Acids
Simple carboxylic acids do not decarboxylatereadi
ly.

RH
CO2
But malonic acid does.
150C

CO2
93
Mechanism of Decarboxylation

• One carboxyl group assists the loss of the other.

94
Mechanism of Decarboxylation
One carboxyl group assists the loss of the other.

• This compound is the enol form of acetic acid.

95
Mechanism of Decarboxylation
One carboxyl group assists the loss of the other.

96
Mechanism of Decarboxylation
One carboxyl group assists the loss of the other.
These hydrogens play no role.

97
Mechanism of Decarboxylation
One carboxyl group assists the loss of the other.
Groups other than H may be present.

R
98
Decarboxylation is a general reaction for
1,3-dicarboxylic acids
CO2H
H
(74)
(96-99)
99
Mechanism of Decarboxylation
One carboxyl group assists the loss of the other.
This OH group plays no role.

100
Mechanism of Decarboxylation
One carboxyl group assists the loss of the other.
Groups other than OH may be present.
R
101
Mechanism of Decarboxylation
This kind of compoundis called a b-keto acid.
a
b

• Decarboxylation of a b-keto acid gives a ketone.

102
Decarboxylation of a b -Keto Acid
25C

CO2
103
Spectroscopic Analysis ofCarboxylic Acids
104
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.

105
Figure 19.8 Infrared Spectrum of
4-Phenylbutanoic acid
C6H5CH2CH2CH2CO2H
OH and CH stretch
CO
monosubstitutedbenzene
Wave number, cm-1
106
1H NMR

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

107
Figure 19.9
Chemical shift (d, ppm)
108
13C NMR

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

109
UV-VIS
Carboxylic acids absorb near 210 nm, butUV-VIS
spectroscopy has not proven to be very useful
for structure determination of carboxylic acids.
110
Mass Spectrometry
Aliphatic carboxylic acids undergo a varietyof
fragmentations. Aromatic carboxylic acids first
form acylium ions,which then lose CO.
111
End of Chapter 18