Carboxylic Acids
Chapter 18
Carboxylic Acids
• In this chapter, we study carboxylic acids, another
class of organic compounds containing the
carbonyl group.
• The functional group of a carboxylic acid is a
carboxyl group, which can be represented in any
one of three ways.
O
C-OH
COOH
CO2 H
Nomenclature
IUPAC names
• For an acyclic carboxylic acid, take the longest carbon
chain that contains the carboxyl group as the parent
alkane.
• Drop the final -e from the name of the parent alkane
and replace it by -oic acid.
• Number the chain beginning with the carbon of the
carboxyl group.
• Because the carboxyl carbon is understood to be
carbon 1, there is no need to give it a number.
Nomenclature
• In these examples, the common name is given in
O
parentheses. 1O
6
3
OH
Hexanoic acid
(Caproic acid )
1
OH
3-Methylbu tanoic acid
(Isovaleric acid)
• An -OH substituent is indicated by the prefix hydroxy-;
an -NH2 substituent by the prefix amino-.
OH
5
O
1
OH
5-Hydroxyhexan oic acid
H2 N
COOH
4-A min ob enzoic acid
Nomenclature
• To name a dicarboxylic acid, add the suffix -dioic acid to
the name of the parent alkane that contains both
carboxyl groups; thus, -ane becomes -anedioic acid.
• The numbers of the carboxyl carbons are not indicated
because they can be only at the ends of the chain.
O
HO
2
O
1
3
OH
HO
O
1
OH
O
Ethan edioic acid Prop aned ioic acid
(Malonic acid )
(Oxalic acid )
O
HO
4
O
5
1
OH
O
Butaned ioic acid
(Succinic acid)
HO
O
O
1
OH
Pen tanedioic acid
(Glutaric acid)
HO
6
1
OH
O
Hexan edioic acid
(Ad ipic acid)
Nomenclature
IU PAC N ame
Structure
HCOOH
CH3 COOH
CH3 CH2 COOH
CH3 (CH2 ) 2 COOH
CH3 (CH2 ) 3 COOH
CH3 (CH2 ) 4 COOH
CH3 (CH2 ) 6 COOH
CH3 (CH2 ) 8 COOH
CH3 (CH2 ) 1 0 COOH
CH3 (CH2 ) 1 2 COOH
CH3 (CH2 ) 1 4 COOH
CH3 (CH2 ) 1 6 COOH
CH3 (CH2 ) 1 8 COOH
Common
N ame
(acid)
methanoic
formic
ethan oic
acetic
propanoic
propionic
bu tanoic
bu tyric
pen tanoic
valeric
hexan oic
cap roic
octanoic
cap rylic
decanoic
cap ric
dodecanoic
lauric
tetradecan oic myristic
hexad ecanoic palmitic
octadecanoic stearic
eicosan oic
arachid ic
D erivation
Latin : formica, ant
Latin : acet um, vinegar
Greek: propion, firs t fat
Latin : buty rum, b utter
Latin : valere, to be s trong
Latin : caper, goat
Latin : caper, goat
Latin : caper, goat
Latin : laurus , laurel
Greek: my ris tikos, fragrant
Latin : palma, palm tree
Greek: st ear, solid fat
Greek: arachis, p eanut
Nomenclature
For common names, use, the Greek letters alpha (a),
beta (b), gamma (g), and so forth to locate
substituents.
O
C-C-C-C-OH
g b a
4
3 2 1
O
O
H2 N
4
g
2
1
OH
OH
a
OH
4-A min ob utanoic acid
2-Hyd roxypropan oic acid
(g-A min obu tyric acid; GABA) (a-Hydroxyprop ion ic acid;
lactic acid)
Physical Properties
hydrogen bondin g
betw een tw o
molecules
H3 C
O
+
H
O
C
C
O
H
+
O
-
CH 3
Physical Properties
Carboxylic acids are more soluble in water than are alcohols,
ethers, aldehydes, and ketones of comparable molecular
weight.
Boilin g
Solubility
Molecular Poin t
Weigh t
(°C) (g/100 mL H 2O)
Structu re
N ame
CH3 COOH
CH3 CH2 CH2 OH
CH3 CH2 CHO
acetic acid
60.5
1-prop anol
prop anal
CH3 (CH2 ) 2 COOH butan oic acid
CH3 (CH2 ) 3 CH2 OH 1-pentan ol
pentan al
CH3 (CH2 ) 3 CHO
60.1
58.1
118
97
48
infinite
infinite
16
88.1
88.1
86.1
163
137
103
infinite
2.3
slight
HA
+
H2O
Ka
larger Ka
=
A–
+
H3O+
[A–] [H3O+]
[HA]
increased [H3O+]
stronger acid
HA
+
H2O
Ka
larger Ka
=
A–
+
H3O+
[A–] [H3O+]
[HA]
increased [H3O+]
stronger acid
HA
+
H2O
Ka
larger Ka
=
A–
+
H3O+
[A–] [H3O+]
[HA]
increased [H3O+]
stronger acid
HA
+
H2O
Ka
larger Ka
=
A–
+
H3O+
[A–] [H3O+]
[HA]
increased [H3O+]
stronger acid
RCOOH
+
Ka
H2O
=
RCOO–
[RCOO–] [H3O+]
[RCOOH]
+
H3O+
RCOOH
+
Ka
H2O
=
RCOO–
[RCOO–] [H3O+]
[RCOOH]
+
H3O+
Comparative acidities of 0.1 M aqueous solutions of representative acids HA
HCl
HOAc
PhOH
EtSH
EtOH
HOH
[H3O+], M
Ka
% ionized
~1  107
~100
1.8  10–5
3.3  10–10
1.3
0.0036
1.3  10–3
3.6  10–6
2.88
5.44
2.5  10–11
0.0016
1.6  10–6
5.80
1.3  10–16
0.0001
1.0  10–7
7.00
1.8  10–16
0.0001
1.0  10–7
7.00
~0.1
acids > phenols ~ thiols > water ~ alcohols
pH
1.00
Fatty Acids
Table 18.3 The Most Abundant Fatty Acids in Animal Fats,
Vegetable Oils, and Biological Membranes.
Carbon Atoms:
Double Bonds *
Structure
Saturated Fatty Acids
12:0
CH3 ( CH2 ) 1 0 COOH
Common
Name
Melting Point
(°C)
lauric acid
44
14:0
CH3 ( CH2 ) 1 2 COOH
myris tic acid
58
16:0
CH3 ( CH2 ) 1 4 COOH
palmitic acid
63
18:0
CH3 ( CH2 ) 1 6 COOH
s tearic acid
70
arachidic acid
77
CH3 ( CH2 ) 1 8 COOH
20:0
Unsaturated Fatty Acids
16:1
CH3 ( CH2 ) 5 CH= CH( CH2 ) 7 COOH
18:1
CH3 ( CH2 ) 7 CH= CH( CH2 ) 7 COOH
palmitoleic acid
18:2
oleic acid
CH3 ( CH2 ) 4 ( CH= CHCH2 ) 2 ( CH 2 ) 6 COOH linoleic acid
18:3
CH3 CH2 ( CH= CHCH2 ) 3 ( CH 2 ) 6 COOH
20:4
CH3 ( CH2 ) 4 ( CH= CHCH2 ) 4 ( CH 2 ) 2 COOH arachidonic acid
linolenic acid
* The firs t number is the number of carbons in the fatty acid; the s econd is the
number of carbon-carbon double bonds in its hydrocarbon chain.
1
16
-5
-11
-49
Fatty Acids
Unsaturated fatty acids generally have lower melting
points than their saturated counterparts.
COOH S tearic acid (18:0)
(mp 70°C)
COOH Olei c acid (18;1)
(mp 16°C)
COOH Linoleic acid (18:2)
(mp-5°C)
COOH Linolenic acid (18:3)
(mp -11°C)
Fatty Acids
Saturated fatty acids are solids at room temperature.
• The regular nature of their hydrocarbon chains allows
them to pack together in such a way as to maximize
interactions (by London dispersion forces) between their
chains.
COOH
COOH
COOH
COOH
COOH
Fatty Acids
In contrast, all unsaturated fatty acids are liquids
at room temperature because the cis double
bonds interrupt the regular packing of their
hydrocarbon chains.
COOH
COOH
COOH
COOH
COOH
Soaps
• Natural soaps are sodium or potassium salts of
fatty acids.
• They are prepared from a blend of tallow and
palm oils (triglycerides).
• Triglycerides are triesters of glycerol.
• The solid fats are melted with steam and the
water insoluble triglyceride layer that forms on
the top is removed.
21
Soaps
Preparation of soaps begins by boiling the
triglycerides with NaOH. The reaction that takes
place is called saponification (Latin: saponem,
“soap”). Boiling with KOH gives a potassium soap.
22
Soaps
Figure 18.2 In water, soap molecules
spontaneously cluster into micelles, a spherical
arrangement of molecules such that their
hydrophobic parts are shielded from the aqueous
environment, and their hydrophilic parts are in
contact with the aqueous environment.
23
Soaps
Figure 18.3 When soaps and dirt, such as grease,
oil, and fat stains are mixed in water, the nonpolar
hydrocarbon inner parts of the soap micelles
“dissolve” the nonpolar substances.
24
Soaps
• Natural soaps form water-insoluble salts in hard
water.
• Hard water contains Ca2+, Mg2+, and Fe3+ ions.
25
Detergents
The problem of formation of precipitates in hard
water was overcome by using a molecule
containing a sulfonate (-SO3- ) group in the place
of a carboxylate (-CO2-) group.
• Calcium, magnesium and iron salts of sulfonic acids,
RSO3H, are more soluble in water than are their salts of
fatty acids.
• Following is the preparation of the synthetic detergent,
SDS, a linear alkylbenzenesulfonate (LAS), an anionic
detergent.
26
Detergents
• Among the most common additives to detergents
are foam stabilizers, bleaches, and optical
brighteners.
27
Decarboxylation
• Decarboxylation: The loss of CO2 from a carboxyl group.
• Almost all carboxylic acids, when heated to a very high
temperature, will undergo thermal decarboxylation.
O
•
decarboxylation
+ CO2 to moderate
RHresistant
RCOH
Most carboxylic acids,
however,
are
high temperature
heat and melt and even boil without undergoing
decarboxylation.
• An exception is any carboxylic acid that has a carbonyl
group on the carbon b to the COOH group.
Decarboxylation
• Decarboxylation of a b-ketoacid.
O
O
O
b a
OH
3-Oxobutanoic acid
(Acetoacetic acid)
warm
+
CO 2
Aceton e
• The mechanism of thermal decarboxylation involves (1)
redistribution of electrons in a cyclic transition state
followed by (2) keto-enol tautomerism.
enol of
a keton e
O
H
O
(1)
O
(A cyclic six-membered
transition state)
O
H
O
C
O
(2)
O
+
CO 2
Decarboxylation
• An important example of decarboxylation of a b-ketoacid in
biochemistry occurs during the oxidation of foodstuffs in the
tricarboxylic acid (TCA) cycle. Oxalosuccinic acid, one of the
intermediates in this cycle, has a carbonyl group (in this case
a ketone) b to one of its three carboxyl groups.
only this carboxyl
has a C=O beta to it .
HOOC
O
a b
COOH
COOH
Oxalosuccinic acid
O
HOOC
COOH + CO 2
a-Ketoglutaric acid