Download ch14[1].

William H. Brown
Thomas Poon
www.wiley.com/college/brown
Chapter Fourteen
Carboxylic Acids
Structure
• The functional group of a carboxylic acid is a carboxyl
group.
• The general formula of an aliphatic carboxylic acid is
RCOOH.
• That of an aromatic carboxylic acid is ArCOOH.
14-2
Nomenclature
• IUPAC names: drop the -e from the parent alkane and
add the suffix -oic acid.
• If the compound contains a carbon-carbon double bond,
change the infix -an- to -en-.
COOH
3-Methylbutanoic acid
(Isovaleric acid)
C6 H5
COOH
t rans -3-Phenylpropenoic acid
(Cinnamic acid)
14-3
Nomenclature
• The carboxyl group takes precedence over most other
functional groups.
OH
COOH
Don’t worry about R or S…
we didn’t learn about that.
(R)-5-Hydroxyhexanoic acid
O
COOH
5-Oxohexanoic acid
H2N
COOH
4-Aminobutanoic acid
14-4
Nomenclature
 Dicarboxylic acids: add -dioic acid to the name of the
parent alkane containing both carboxyl groups.
• There is no need to use numbers to locate the carboxyl
groups; they can only be on the ends of the chain.
O
HO
O
OH
O
Ethanedioic acid
(Oxalic acid)
O
HO
O
OH
O
Butanedioic acid
(Succinic acid)
HO
O
HO
OH
Propanedioic acid
(Malonic acid)
O
O
OH
Pentanedioic acid
(Glutaric acid)
HO
OH
O
Hexanedioic acid
(Adipic acid) 14-5
Nomenclature
• If the carboxyl group is bonded to a ring, name the ring
compound and add the suffix -carboxylic acid.
2
3
1
COOH
2-Cyclohexenecarboxylic acid
H OOC
COOH
trans- 1,3-Cyclopentanedicarboxylic acid
Don’t worry about rings
with multiple –COOHs!
14-6
Nomenclature
• Benzoic acid is the simplest aromatic carboxylic acid.
• Use numbers to show the location of substituents.
COOH
COOH
COOH
OH
COOH
COOH
COOH
Benzoic
acid
2-Hydroxybenzoic
acid
(Salicylic acid)
1,2-Benzenedicarboxylic acid
(Phthalic acid)
1,4-Benzenedicarboxylic acid
(Terephthalic acid)
Don’t worry about benzene rings
with multiple –COOHs!
14-7
Nomenclature
• When common names are used, the letters etc.
are often used to locate substituents.
O
O
  

5
1 OH
4 3 2
H2 N 
O

OH
4-A min ob utanoic acid
(-Aminobutyric acid;
GABA )
OH
NH2
2-Aminopropanoic acid
(-Aminop ropionic acid;
alanin e)
• In common nomenclature, keto indicates the presence of
a ketone, and CH3CO- is named an aceto group.
O
O
OH
3-oxobutanoic acid
-Ketobutyric acid ;
acetoacetic acid)
O
CH3 CAcetyl group
(aceto group)
14-8
Physical Properties
• In the liquid and solid states, carboxylic acids are
associated by hydrogen bonding into dimeric
structures.
14-9
Physical Properties
• Carboxylic acids have significantly higher boiling points than other
types of organic compounds of comparable molecular weight.
• They are polar compounds and form very strong
intermolecular hydrogen bonds.
• Carboxylic acids are more soluble in water than alcohols, ethers,
aldehydes, and ketones of comparable molecular weight.
• They form hydrogen bonds with water molecules through
their C=O and OH groups.
14-10
Physical Properties
• Water solubility decreases as the relative size of the
hydrophobic portion of the molecule increases.
14-11
Acidity
• Carboxylic acids are weak acids
• Values of pKa for most aliphatic and aromatic carboxylic
acids fall within the range 4 to 5.
CH3 COOH + H2 O
CH3 COO- + H3 O+
-
+
K a = [CH3 COO ] [H3 O ]
[CH3 COOH]
= 1.74 x 10-5
pK a = 4.76
• The greater acidity of carboxylic acids relative to alcohols,
both of which contain an OH group, is due to resonance
stabilization of the carboxylate anion.
14-12
Acidity
• Electron-withdrawing substituents near the carboxyl
group increase acidity through their inductive effect.
14-13
Acidity
• The acid-strengthening effect of a halogen substituent
falls off rapidly with increasing distance from the carboxyl
group.
14-14
Reaction with Bases
• Carboxylic acids, whether soluble or insoluble in water, react with
NaOH, KOH, and other strong bases to give water-soluble salts.
COOH
+
NaOH
-
H2 O
Benzoic acid
(slightly soluble
in water)
COO Na
+
H2 O
+
Sodium benzoate
(60 g/100 mL water)
• They also form water-soluble salts with ammonia and amines.
COOH + NH3
Benzoic acid
(slightly soluble
in water)
-
H2 O
COO NH4
+
Ammonium benzoate
(20 g/100 mL water)
14-15
Reaction with Bases
• Carboxylic acids react with sodium bicarbonate and
sodium carbonate to form water-soluble salts and
carbonic acid.
• Carbonic acid (H2CO3), in turn, breaks down to carbon
dioxide and water.
+
CH3 COOH + Na HCO3
-
H2 CO3
+
CH3 COOH + Na HCO3
-
H2 O
+
CH3 COO Na + H2 CO3
CO2 + H2 O
+
CH3 COO Na + CO2 + H2 O
14-16
Reaction with Bases
• Figure 14.1
Flowchart for
separation of
benzoic acid
from benzyl
alcohol.
14-17
Reduction
• The carboxyl group is very 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.
14-18
Reduction
• Lithium aluminum hydride reduces a carboxyl group to a
1° alcohol.
• Reduction is carried out in diethyl ether, THF, or other
nonreactive, aprotic solvents.
• (Notice that the alkene double bond is unaffected!)
O O
. LiAlH
er er
4 , eth
4 , eth
H
H. 1LiAlH
OH1
2 . 2H. 2H
O2 O
OHOH
+ Al(OH)
+ Al(OH)
+ LiOH
+ LiOH
3 3
14-19
Selective Reduction
• Catalytic hydrogenation does not reduce a COOH group!
• we can use H2/Metal to reduce an alkene in the presence
of a COOH group.
O
O
OH + H2
5-Hexen oic acid
Pt
25°C, 2 atm
OH
Hexanoic acid
• we can use NaBH4 to reduce an aldehyde or ketone in
the presence of a COOH group.
O
C6 H5
OH
O
OH
5-Oxo-5-p henylpentan oic acid
1 . NaBH4
2 . H2 O
C6 H5
O
OH
5-Hyd roxy-5-phen ylpen tanoic acid
14-20
Fischer Esterification
• Esters can be prepared by treating a carboxylic acid
with an alcohol in the presence of an acid catalyst,
commonly H2SO4 or gaseous HCl.
O
H2 SO4
CH3 COH + CH3 CH2 OH
Ethanoic acid
Eth anol
(A cetic acid ) (Ethyl alcoh ol)
O
CH3 COCH2 CH3 + H2 O
Ethyl ethanoate
(Ethyl acetate)
14-21
Fischer 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.
• A key intermediate in Fischer esterification is the
tetrahedral carbonyl addition intermediate formed by
addition of ROH to the C=O group.
H
O
R C OH + HOCH3
H+
O
R C OCH3
O
H
H+
O
R C OCH3 + HOH
14-22
Acid Chlorides
• The functional group of an acid halide is a carbonyl
group bonded to a halogen atom.
• Among the acid halides, acid chlorides are by far the most
common and the most widely used.
O
- C- X
Functional group
of an acid halide
O
CH3 CCl
A cetyl
chloride
O
C-Cl
Benzoyl
chloride
14-23
Acid Chlorides
• Acid chlorides are most often prepared by treating a
carboxylic acid with thionyl chloride.
O
OH + SOCl 2
Butanoic
Thionyl
acid
chloride
O
Cl + SO 2 + HCl
Butanoyl
chloride
14-24
Decarboxylation
• Decarboxylation: loss of CO2 from a carboxyl group.
• Most carboxylic acids, if heated to a very high
temperature, undergo thermal decarboxylation.
O
RCOH
decarb oxylation
heat
RH + CO2
• Most, however, are quite resistant to moderate heat and
melt or even boil without decarboxylation.
14-25
Decarboxylation
• Exceptions are carboxylic acids that have a carbonyl
group beta to the carboxyl group.
• This type of carboxylic acid undergoes decarboxylation on
mild heating.
O
O
warm
 
OH
3-Oxobu tan oic acid
(Acetoacetic acid)
O
+
CO2
Acetone
14-26
Decarboxylation
• Thermal decarboxylation of a -ketoacid involves
rearrangement of six electrons in a cyclic six-membered
transition state.
enol of
a ketone
O
H
O
(1)
O
O
H
O
C
(2)
O
O
+
CO 2
(A cyclic six-membered
trans ition s tate)
14-27
Decarboxylation
• Thermal decarboxylation of malonic acids also involves
rearrangement of six electrons in a cyclic six-membered
transition state.
O
HO
H
O
O
(1)
O
A cyclic six-membered
trans ition s tate
HO
H
O
C
O
(2)
O
+
HO
CO 2
Enol of a
carboxylic acid
14-28
Decarboxylation
• Problem: Draw the product of decarboxylation.
O
COOH
heat
• Problem: Draw the -ketoacid that undergoes
decarboxylation to give this ketone.
O
-ketoacid
heat
+ CO2
14-29
Carboxylic Acids
End Chapter 14
14-30