Carboxylic Acids

1
Chapter 19

• Carboxylic Acids

2
Table 19.1 (page 792)

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

Systematic Name
3
Table 19.1 (page 792)

• 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
4
Table 19.1 (page 792)
Systematic Name
Common Name
2-hydroxypropanoicacid
lactic acid
(Z)-9-octadecenoicacid
oleic acid
5
Formic acid is planar
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
Solubility in Water

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

9
Carboxylic acids are weak acids

• but carboxylic acids are far more acidic than
  alcohols.
• Most have a pKa of about 5.

CH3CH2OH
pKa 4.7
pKa 16
10
Free Energies of Ionization
CH3CH2O H
?G 91 kJ/mol
?G 27 kJ/mol
CH3CH2OH
11
Greater acidity of carboxylic acids is
attributedstabilization of carboxylate ion by
inductive effect of carbonyl group
resonance stabilization of carboxylate ion
12
Figure 19.4 Electrostatic potential maps
ofacetic acid and acetate ion
Acetic acid
Acetate ion
13
Carboxylic acids are neutralized by strong bases


RCOH
HO
RCO
H2O
strongeracid
weakeracid

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

14
Soaps
Micelles

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

sodium stearate(sodium octadecanoate)

Na
15
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

16
Figure 19.6 (page 800) A micelle
17
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.

18
Substituent Effects on Acidity

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

19
Substituent Effects on Acidity
20
Effect of electronegative substituent
decreasesas number of bonds between it and
carboxyl group increases.
pKa

ClCH2CH2CH2CO2H
21
Hybridization Effect

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

22
Table 19.3 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
Dicarboxylic Acids
pKa
Oxalic acid
Malonic acid
Heptanedioic acid

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

24
Carbonic Acid

H2O
CO2
99.7
0.3

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

25
Synthesis of Carboxylic Acids Review

• side-chain oxidation of alkylbenzenes (Section
  11.13)
• oxidation of primary alcohols (Section 15.10)
• oxidation of aldehydes (Section 17.15)

26
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

27
Example Alkyl Halide
1. Mg, diethyl ether
2. CO2 3. H3O
Cl
CO2H
(76-86)
28
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

29
Example
NaCN
DMSO
(92)
30
Reactions of Carboxylic Acids
Reactions already discussed

• Acidity (Sections 19.4-19.9)
• Reduction with LiAlH4 (Section 15.3)
• Esterification (Section 15.8)
• Reaction with Thionyl Chloride (Section 12.7)

31
Reactions of Carboxylic Acids
New reactions in this chapter

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

32
Acid-catalyzed Esterification
(also called Fischer esterification)

CH3OH

H2O

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

33
Mechanism of Fischer Esterification

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

34
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
35
Second stage conversion of tetrahedralintermedi
ate to ester

H2O
H

• this stage corresponds to an acid-catalyzed
  dehydration

36
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

37
Lactones

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

38
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

39
Examples


H2O
4-hydroxybutanoic acid
4-butanolide
40
Common names

?
?
?
?
?
?
?
?-butyrolactone
?-valerolactone

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

41
Lactones

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

42
Example
5-hexanolide (78)
43
?-Halogenation of Carboxylic Acids


X2
HX

• analogous to ?-halogenation of aldehydes and
  ketones
• key question Is enol content of carboxylic
  acids high enough to permit reaction to occur
  at reasonable rate? (Answer is NO)

44
But...
P or PX3


X2
HX

• reaction works well if a small amount
  ofphosphorus or a phosphorus trihalide is added
  tothe reaction mixture
• this combination is called the Hell-Volhard-Zelin
  sky reaction

45
Example

Br2
PCl3
benzene80C
46
Value
Br2
P
(77)
47
Synthesis of ?-Amino Acids
Br2
(CH3)2CHCH2COH
PCl3
(88)
48
Decarboxylation of Carboxylic Acids
Simple carboxylic acids do not decarboxylatereadi
ly.

RH
CO2
49
Mechanism of Decarboxylation of Malonic Acid
One carboxyl group assists the loss of the other.

• This compound is the enol form of acetic acid.

50
Decarboxylation is a general reactionfor
1,3-dicarboxylic acids
51
Mechanism of Decarboxylation of Malonic Acid
This kind of compoundis called a ?-keto acid.
?
?

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

52
Decarboxylation of a ?-Keto Acid
25C

CO2
53
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.
54
Figure 19.9 Infrared Spectrum of 4-Phenylbutanoic
acid
C6H5CH2CH2CH2CO2H
OH and CH stretch
CO
monosubstitutedbenzene
Wave number, cm-1
55
1H NMR
proton of OH group of a carboxylic acid is
normallythe least shielded of all of the protons
in a 1HNMR spectrum (? 10-12 ppm broad).
56
Figure 19.10
Chemical shift (?, ppm)
57
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).
58
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).