Carboxylic Acids
Carboxylic Acids
 The functional group of carboxylic acids consists of a
C=O with -OH bonded to the same carbon.
 Carboxyl group is usually written -COOH.
 Aliphatic acids have an alkyl group bonded to COOH.
 Aromatic acids have an aryl group.
 Fatty acids are long-chain aliphatic acids.
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Chapter 20
2
Carboxylic Acids
Carboxylic acid, compounds of the type
RCOOH such as Acetic acid ( present in
vinegar), Malonic acid and Benzoic Acid.
COOH
COOH
CH3COOH
H2C
COOH
Acetic acid
Malonic acid
Benzoic acid
1-Carboxylic Acid Nomenclature
Common Names
 Many aliphatic acids have historical names.
 Positions of substituents on the chain are labeled with
Greek letters.
Br
CH3CH2CH2CHCOOH
 bromovaleric acid
CH3
CH3CHCH2COOH
 -methylbutyric acid
isovaleric acid
4
COOH
special names
benzoic acid
COOH
CH3
COOH
COOH
CH3
CH3
o-toluic acid
m-toluic acid
p-toluic acid
IUPAC Names
 Remove -e from alkane (or alkene) name, add -oic
acid.
 The carbon of the carboxyl group is #1.
Cl O
CH3CH2CHC OH
Ph
H
H
C C
COOH
2-chlorobutanoic acid
trans-3-phenyl-2-propenoic acid
(cinnamic acid)
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Chapter 20
6
parent chain = longest, continuous carbon chain that
contains the carboxyl group  alkane, drop –e, add –
oic acid
HCOOH
methanoic acid
CH3CO2H
ethanoic acid
CH3CH2CO2H
propanoic acid
CH3
CH3CHCOOH
2-methylpropanoic acid
Br
CH3CH2CHCO2H
2-bromobutanoic acid
Dicarboxylic Acids
 Aliphatic diacids are usually called by their common
names (to be memorized).
 For IUPAC name, number the chain from the end
closest to a substituent.
 Two carboxyl groups on a benzene ring indicate a
phthalic acid.
Br
HOOCCH2CHCH2CH2COOH
3-bromohexanedioic acid
-bromoadipic acid
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8
dicarboxylic acids:
HOOC-COOH
oxalic acid
HO2C-CH2-CO2H
malonic acid
HO2C-CH2CH2-CO2H
succinic acid
HO2C-CH2CH2CH2-CO2H
glutaric acid
HOOC-(CH2)4-COOH
adipic acid
HOOC-(CH2)5-COOH
pimelic acid
CO2H
CO2H
CO2H
CO2H
CO2H
CO2H
phthalic acid
H
H
C
C
isophthalic acid
terephthalic acid
COOH
H
COOH
HOOC
maleic acid
C
C
COOH
H
fumaric acid
Naming Cyclic Acids
 Cycloalkanes bonded to -COOH are named as
cycloalkanecarboxylic acids.
 Aromatic acids are named as benzoic acids.
COOH
CH(CH3)2
2-isopropylcyclopentanecarboxylic acid
Chapter 20
COOH
OH
o-hydroxybenzoic acid
(salicylic acid)
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11
2-Physical properties
1- Boiling point
Intermolecular forces, especially hydrogen bonding, are
stronger in carboxylic acids than in other compounds
of similar shape and molecular weight.
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.
2-Solubility in Water
Carboxylic acids are similar to alcohols in respect
to their solubility in water
Form hydrogen bonds to water
H
O
H
O
H3CC
H
O
H
O
H
3-Acidity of carboxylic acids
 Most carboxylic acids have a pKa close to 5.
 Carboxylic acids are weak acids but carboxylic acids are
far more acidic than alcohols.
The greater difference in ionization energy (∆ G) of
ethanol and acetic acid reflects a greater stabilization of
acetate ion relative to ethoxide ion.
Greater acidity of carboxylic acids is attributed
stabilization of carboxylate ion by inductive effect of
carbonyl group
And by resonance stabilization of carboxylate
ion.
Salts of Carboxylic Acids
salts of carboxylic acids:
name of cation + name of acid: drop –ic acid, add –
ate
CH3CO2Na
sodium acetate
CH3CH2CH2CO2NH4
or sodium ethanoate
ammonium butyrate
ammonium butanoate
(CH3CH2COO)2Mg
magnesium propionate
magnesium propanoate
4-Substituent and acid strength
Alkyl substituents have negligible effect
Electronegative substituents increase acidity
Electronegative substituents withdraw electrons
from carboxyl group; increase K for loss of H+
Effect of substituent decreases the acidity as
number of bonds between X and carboxyl
group increases.
Substituent Effects
on Acidity
COOH
COOH
COOH
COOH
COOH
NO 2
NO2
OCH3
p-methoxy
benzoic acid
pKa = 4.46
pKa = 4.19
m-nitro
NO 2
p-nitro
o-nitro
pKa = 3.47
pKa = 3.41
pKa = 2.16
Ionization of Substituted Benzoic
Acids
Hybridization Effect
sp2-hybridized carbon is more electron-withdrawing
than sp3, and sp is more electron-withdrawing than sp2
Effect is small unless X is electronegative;
effect is largest for ortho substituent
5-Synthesis of carboxylic acids
1- Oxidation of alkylbenzene
2- Oxidation of primary alcohols
3- Oxidation of aldehydes
4-Carboxylation of Grignard
reagents
5- Hydroylsis of nitriles
6- Reactions of Carboxylic acids
1- Formation of Acyl chlorides
2-Lithium aluminium hydride
reduction
3-Esterification
Mechanism of Esterification
Intramolecular ester formation
(Lactones)
4-α-Halogenation
This reaction of α- bormination of carboxylic acid
is called Hell- Volhard- Zelinasky reaction
5- Decarboxylation of carboxylic
acids