Introduction to carboxylic acids
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Learning Objectives
• Identify the general structure of carboxylic
acids
• Give the IUPAC and the common names
• Explain the physical properties of the
carboxylic acids ; e.g solubility , boiling point,
etc
• Give examples and uses of carboxylic acids
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Functional Groups
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R = Alkyl Groups
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Introduction
Carboxylic Acids
A class of organic compounds containing at least
one carboxyl group
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R = alkyl group  Aliphatic acid
R = aryl group  aromatic acid
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Naming Carboxylic Acids
(1) The IUPAC names of carboxylic acids
Replace the ‘e’ with ‘oic acid’ at the end of the name of the
hydrocarbon
H
H
H
C
C H
H H
ethane
H O
H
C C O H
H
ethan oic acid
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IUPAC Names
• Number substituents from the carboxyl
carbon 1.
CH3
O
|
║
CH3—CH—CH2—C—OH
4
3
2
1
3-methylbutanoic acid
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(2) Common Names
The common names of simple carboxylic acids
HCOOH formic acid, CH3—COOH acetic acid
• Locate substituents using greek letters , , γ for
the carbon atoms adjacent to the carboxyl carbon.
CH3
γ

α|
CH3—CH—CH2—COOH
α-methylbutryic acid ( IUPAC ; 2-methylbutanoic acid )
Ph
CH3CH2CH2CHCH2COOH
ε
δ γ  α
-phenylcaproic acid
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Common and IUPAC Names
of Some Carboxylic Acids
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(3) Naming Cyclic Acids
Cycloalkanes bonded to -COOH are named as
cycloalkanecarboxylic acids
COOH
Cyclohexanecarboxylic acid
Br
HO
O
3-bromo-cyclohexane carboxylic acid
O
OH
1-cyclopentene carboxylic acid
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(4) Naming Aromatic Acids
Locates substituent by assigning 1 to the carbon attached to
the carboxyl group. Prefixes ortho, meta, and para for 2
substituents:
ortho 1, 2 location
meta 1, 3 location
para 1, 4 location
COOH
O
O
C OH
C OH
O
C
OH
Benzoic acid
OH
Cl
o-hydroxybenzoic acid
2-hydroxybenzoic acid
(salicylic acid)
NH2
3-chlorobenzoic acid
m-chlorobenzoic acid
4-aminobenzoic acid
p-aminobenzoic acid
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Dicarboxylic Acids
• Aliphatic diacids are usually called by their
common names.
• For IUPAC name, number the chain from the
end closest to a substituent.
Br
HOOCCH2CHCH2CH2COOH
3-bromohexanedioic acid
-bromoadipic acid
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Some examples of dicarboxylic acid
Common name
(Historical name)
HOOC-COOH
oxalic acid
IUPAC name
alkanedioic acid
ethanedioic acid
HOOCCH2COOH
malonic acid
propanedioic acid
HOOC(CH2)2COOH
succinic acid
butanedioic acid
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Common name
IUPAC name
COOH
phthalic acid
O-phthalic acid
COOH
HOOC
COOH
HOOC
1,2-benzenedicarboxylic acid
isophthalic acid
m-phthalic acid
1,3-benzenedicarboxylic acid
terephthalic acid
p-phthalic acid
1,4-benzenedicarboxylic acid
COOH
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PHYSICAL PROPERTIES
Solubility
carboxylic acids are soluble in organic solvents ; e.g alcohols
they are also soluble in water due to hydrogen bonding
small ones dissolve readily in cold water ( up to 4 carbons)
as mass increases, the solubility decreases
benzoic acid is fairly insoluble in cold, but soluble in hot
water
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Boiling Points
The boiling points of carboxylic acids
• Are higher than alcohols, ketones, and aldehydes of
similar mass.
• Are high because they form dimers in which hydrogen
bonds form between the polar groups in the two
carboxyl groups.
O
H—O
||
|
CH3—C
C—CH3
|
||
O—H
O
A dimer of acetic acid
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Melting Points
• Aliphatic acids with more than 8 carbons are solids
at room temperature.
• Double bonds (especially cis) lower the melting
point. The following acids all have 18 carbons:
– Stearic acid (saturated): 72C
CO2H
– Oleic acid (one cis double bond): 16C
CO2H
– Linoleic acid (two cis double bonds): -5C
CO2H
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Polarity of Carboxylic Acids
Polarity is
Carboxylic acids are strongly polar because they have
two polar groups hydroxyl (OH) and carbonyl (C=O)
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Acidity of Carboxylic Acids
• A carboxylic acid may dissociate in water to give a proton and a
carboxylate ion (known as conjugate base).
• The equilibrium constant Ka for this reaction is called the aciddissociation constant, The acidity constant, Ka,, is about 10-5 for
a typical carboxylic acid (pKa ~ 5)
• The acid will be mostly dissociated if the pH of the solution is
higher than the pKa of the acid.
• are weak acids compared to inorganic acids
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Acidity of Carboxylic Acids
Carboxylic acids are significantly more acidic than water
or alcohols.
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This greater acidity of carboxylic acids is attributed to
greater stabilization of carboxylate ion by:
a. Inductive effect of the C=O group
b. Resonance stabilization of the carboxylate ion
C
Od
Od
4 -electrons delocalized
over three p-prbitals
C-O bond length of a
carboxylates are the same
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Electron delocalization; resonance
R
C
•••
O•
••O ••
R
+
C
•••–
O•
••
C
+ O ••
••O ••
H
R
•••–
O•
••
H
H
Electron delocalization stabilizes carbonyl group
Carbon is sp2 hybridized.
Bond angles are close to 120.
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Substituent Effects on Acidity
Electronegative substituents increase acidity
O
X CH2C-OH
X
Ka
pKa
H
F
Cl
1.8 x 10-5
2.5 x 10-3
4.7
2.6
1.4 x 10-3
2.9
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Substituent Effects on Acidity
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Substituent Effects on Acidity
The magnitude of a substituent effect depends on its
distance from the carboxyl group.
• Multiple electronegative substituents have synergistic
effects on acidity
• Fluoroacetic, chloroacetic, bromoacetic, and
iodoacetic acids are stronger acids than acetic acid
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Aromatic Carboxylic Acids
Electron-withdrawing groups (e.g NO2) enhance the acid
strength while electron-donating groups ( e.g alkyl )
decrease the acid strength.
Effects are strongest for substituents in the ortho and para
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positions.
Substituent Effects
• An electronegative group will drive the ionization
equilibrium toward dissociation, increasing acidity
• An electron-withdrawing group (-NO2) increases
acidity by stabilizing the carboxylate anion, and an
electron-donating (activating) group (OCH3)
decreases acidity by destabilizing the carboxylate
anion
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Substituent Effects in Substituted
Benzoic Acids
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So:
• Carboxylic acids are weak acids, average
pH = 5
• Are Polar because of ( the high
electronegative O)
• Soluble in water
• Have high boiling and melting points
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Learning Check
Write the balanced equation for the ionization of butanoic acid
in water and identify the carboxylate ion.
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Solution
Write the balanced equation for the ionization of butanoic acid
in water and identify the carboxylate ion.
O
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O
CH3CH2COH + H2O
CH3CH2CO– + H3O+
carboxylic acid
carboxylate ion
Naming Carboxylic Acid Salts
• First name the cation.
• Then name the anion by replacing the
-ic acid with -ate.
Cl
-
C H 3C H 2C H C H 2C O O K
+
potassium 3-chloropentanoate
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Neutralization of Carboxylic Acids
Carboxylic acid salts
• are a product of the neutralization of a carboxylic acid with a
strong base
CH3—COOH + NaOH
acetic acid
CH3—COO– Na+ + H2O
sodium acetate
(carboxylic acid salt)
• are used as :
• preservatives and flavor enhancers
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Properties of Acid Salts
• Usually solids with no odor.
• Carboxylate salts of Na+, K+, Li+, and NH4+
are soluble in water.
• Soap is the soluble sodium salt of a long
chain fatty acid.
• Salts can be formed by the reaction of an
acid with NaHCO3, releasing CO2.
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Uses of Organic Acids
Alpha hydroxy acids
skin care products.
Acetic acid
in vinegar as preservative and flavourings
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Uses of Organic Acids
Making of drugs, dyes, paints, insecticides, plastics.
Making of esters.
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Other Important Organic Acids
Lactic acid
Citric acid
Malic acid
Limes, lemons
Apples and pears
Sour milk
Tartaric acid
Formic acid
Oxalic acid
Spinch
Insects
Grape juice
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