Download CHAPTER 20 CARBOXYLIC ACIDS Organic Chemistry

CHAPTER 20
CARBOXYLIC
ACIDS
Based on McMurry’s Organic Chemistry, 6th edition
Dr.Mohanad Al-Hachamii
The Importance of Carboxylic Acids
(RCO2H)
Starting materials for acyl derivatives (esters, amides,
and acid chlorides)
Abundant in nature from oxidation of aldehydes and
alcohols in metabolism
Acetic acid, CH3CO2H, - vinegar
Butanoic acid, CH3CH2CH2CO2H (rancid butter)
Long-chain aliphatic acids from the breakdown of fats
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
Dr.Mohanad Al-Hachamii
2
20.1 Naming Carboxylic Acids
• Carboxylic Acids, RCO2H
• If derived from open-chain alkanes, replace the terminal -e
of the alkane name with -oic acid
• The carboxyl carbon atom is C1
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
Dr.Mohanad Al-Hachamii
3
Alternative Names
Compounds with CO2H bonded to a ring are named
using the suffix -carboxylic acid
The CO2H carbon is not itself numbered in this system
Use common names for formic acid (HCOOH) and acetic
acid (CH3COOH) – see Table 20.1
Based on McMurry, Organic Chemistry, Chapter 20, 6th edition, (c) 2003
Dr.Mohanad Al-Hachamii
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• Greek letters are used to designate the location of substituents in
common names.
• The carbon adjacent to the COOH is called the  carbon, followed by
the  carbon, followed by the  carbon, the  carbon and so forth down
the chain.
• The last carbon in the chain is sometimes called the  carbon.
• The  carbon in the common system is numbered C2 in the IUPAC
system.
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• Compounds containing two carboxy groups are called
diacids. Diacids are named using the suffix –dioic acid.
• Metal salts of carboxylate anions are formed from carboxylic
acids in many reactions. To name the metal salt of a
carboxylate anion, put three parts together:
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Figure 19.2
Naming the metal salts of carboxylate anions
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Nitriles, RCN
Closely related to carboxylic acids named by adding -nitrile
as a suffix to the alkane name, with the nitrile carbon
numbered C1
Complex nitriles are named as derivatives of carboxylic
acids.
Replace -ic acid or -oic acid ending with -onitrile
Based on McMurry, Organic Chemistry, Chapter 20, 6th edition, (c) 2003
Dr.Mohanad Al-Hachamii
9
20.2 Structure and Physical Properties of
Carboxylic Acids
Carboxyl carbon sp2 hybridized:
carboxylic acid groups
are planar with C–C=O and O=C–O bond angles of
approximately 120°
Carboxylic acids form hydrogen bonds, existing as
cyclic dimers held together by two hydrogen bonds
Strong hydrogen bonding causes much higher boiling
points than the corresponding alcohols
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
Dr.Mohanad Al-Hachamii
10
Carboxylic Acids and the Acidity of the O—H Bond
Structure and Bonding:
• Carboxylic acids are compounds containing a carboxy group
(COOH).
• The structure of carboxylic acids is often abbreviated as
RCOOH or RCO2H, but keep in mind that the central carbon
atom of the functional group is doubly bonded to one oxygen
atom and singly bonded to another.
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Structure and Bonding:
• The C—O single bond of a carboxylic acid is shorter than the
C—O bond of an alcohol.
• This can be explained by looking at the hybridization of the
respective carbon atoms.
• Because oxygen is more electronegative than either carbon
or hydrogen, the C—O and O—H bonds are polar.
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Physical Properties:
• Carboxylic acids exhibit dipole-dipole interactions because
they have polar C—O and O—H bonds.
• They also exhibit intermolecular hydrogen bonding.
• Carboxylic acids often exist as dimers held together by two
intermolecular hydrogen bonds.
Figure 19.3
Two molecules of acetic acid
(CH3COOH) held together by
two hydrogen bonds
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Spectroscopic Properties:
• Carboxylic acids have very characteristic IR and NMR
absorptions.
• In the IR:
-The C=O group absorbs at ~ 1710 cm-1.
-The O—H absorption occurs from 2500-3500 cm-1.
• In the 1H NMR:
-The O—H proton absorbs between 10-12 ppm.
-The  protons absorb between 2-2.5 ppm.
• In the 13C NMR, the C=O appears at 170-210 ppm.
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Figure 19.4
The IR spectrum of butanoic
acid, CH3CH2CH2COOH
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20.3 Dissociation of Carboxylic Acids
• Carboxylic acids are proton donors toward weak and
strong bases, producing metal carboxylate salts, RCO2 +M
• Carboxylic acids with more than six carbons are only
slightly soluble in water, but their conjugate base salts are
water-soluble
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
Dr.Mohanad Al-Hachamii
16
Acidity Constant and pKa
• Carboxylic acids transfer a proton to water to give H3O+
and carboxylate anions, RCO2, but H3O+ is a much
stronger acid
• The acidity constant, Ka,, is about 10-5 for a typical
carboxylic acid (pKa ~ 5)
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
Dr.Mohanad Al-Hachamii
17
Acidity Compared to Alcohols
• Carboxylic acids are better proton donors than are alcohols
(The pKa of ethanol is ~16, compared to ~5 for acetic acid)
• In an alkoxide ion, the negative charge is localized on
oxygen while in a carboxylate ion the negative charge is
delocalized over two equivalent oxygen atoms, giving
resonance stabilization
Based on McMurry, Organic Chemistry, Chapter 20, 6th edition, (c) 2003Dr.Mohanad Al-Hachamii
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20.4 Substituent Effects on Acidity
• Electronegative substituents promote formation of the
carboxylate ion
Based on McMurry, Organic Chemistry, Chapter 20, 6th edition, (c) 2003
Dr.Mohanad Al-Hachamii
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Substituent Effects
• An electronegative group will drive the ionization
equilibrium toward dissociation, increasing acidity
• An electron-donating group destabilizes the carboxylate
anion and decreases acidity
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
Dr.Mohanad Al-Hachamii
20
Examples of Inductive Effects on
Acidity
• Fluoroacetic, chloroacetic, bromoacetic, and iodoacetic
acids are stronger acids than acetic acid
• Multiple electronegative substituents have synergistic
effects on acidity
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
Dr.Mohanad Al-Hachamii
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20.5 Substituent Effects in
Substituted Benzoic Acids
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
Dr.Mohanad Al-Hachamii
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Aromatic Substituent Effects
• 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
• We can use relative pKa’s as a calibration for effects on
relative free energies of reactions with the same
substituents
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
Dr.Mohanad Al-Hachamii
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20.6 Preparation of Carboxylic
Acids
1- Oxidation of arenes
Oxidation of a substituted alkylbenzene with KMnO4 or Na2Cr2O7
or H2Cr2O7 gives a substituted benzoic acid (see Section 16.10)
1° and 2° alkyl groups can be oxidized, but tertiary groups are not
( the alkylbenzene must contain at least one benzylic hydrogen
atom. Therefore, tert-butylbenzene is not oxidized).
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
Dr.Mohanad Al-Hachamii
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Dr.Mohanad Al-Hachamii
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
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CH3
KMnO4, heat
COOH
toluene
CH3
benzoic acid
COOH
KMnO4, heat
HOOC
H3C
terephthalic acid
p-xylene
KMnO4, heat
CH2CH3
ethylbenzene
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
COOH + CO2
benzoic acid
Dr.Mohanad Al-Hachamii
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• Benzene ring affected by donating group such as OH and NH2…
making it easy to oxidizing
• Converting OH or NH2 into weak donating group such as acetamido to
protecting the benzene ring
Dr.Mohanad Al-Hachamii
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
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2- Oxidation of alkenes and alkynes
Oxidative cleavage of an alkene with KMnO4 gives a carboxylic acid
if the alkene has at least one vinylic hydrogen (see Section 7.8)
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
Dr.Mohanad Al-Hachamii
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Also, can be Oxidation of alkene by Ozone
Dr.Mohanad Al-Hachamii
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
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• Oxdation of Alkynes
• Strong oxidizing reagents (O3 or KMnO4) cleave internal alkynes,
producing two carboxylic acids
• Terminal alkynes are oxidized to a carboxylic acid and carbon dioxide
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Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
Dr.Mohanad Al-Hachamii
O3; then Zn, H2O
KMnO4
C C CH3
COOH
+
KMnO4, heat
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
Dr.Mohanad Al-Hachamii
HOOCCH3
3- Oxidation of primary alcohols or aldehyde
• Oxidation of a primary alcohol or an aldehyde with CrO3 in
aqueous acid
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
Dr.Mohanad Al-Hachamii
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Dr.Mohanad Al-Hachamii
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
33
4- Hydrolysis of nitriles:Conversion of Nitriles
into Carboxylic Acids
• Nitriles and carboxylic acids both have a carbon atom with
three bonds to an electronegative atom, and both contain
a  bond
• Both both are electrophiles
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
Dr.Mohanad Al-Hachamii
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Hot acid or base yields carboxylic acids
Conversion of an alkyl halide to a nitrile (with cyanide ion)
followed by hydrolysis produces a carboxylic acid with one
more carbon (RBr  RCN  RCO2H)
Best with primary halides because elimination reactions
occur with secondary or tertiary alkyl halides
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
Dr.Mohanad Al-Hachamii
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• Hydrolyzed in with acid or base catalysis to a carboxylic
acid and ammonia or an amine
Based on McMurry, Organic Chemistry, Chapter 20, 6th edition, (c) 2003Dr.Mohanad AlHachamii
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Mechanism of Hydrolysis of
Nitriles
Nucleophilic addition of
hydroxide to CN bond
Protonation gives a hydroxy
imine, which tautomerizes to
an amide
A second hydroxide adds to
the amide carbonyl group and
loss of a proton gives a dianion
Expulsion of NH2 gives the
carboxylate
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
Dr.Mohanad Al-Hachamii
37
5- Carboxylation of Grignard Reagents
Grignard reagents react with dry CO2 to yield a metal
carboxylate
Limited to alkyl halides that can form Grignard reagents
(see 17.6)
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
Dr.Mohanad Al-Hachamii
38
Mg
R-X
H+
CO2
RMgX
RCO2MgX
RCOOH
Increases the carbon chain by one carbon.
CO2 H+
Mg
CH3CH2CH2-Br
n-propyl bromide
CH3CH2CH2MgBr
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
CH3CH2CH2COOH
butyric acid
Dr.Mohanad Al-Hachamii
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CH3
CH3
Mg
CH3
CO2
Br
H+
MgBr
COOH
p-toluic acid
Br2, hv
CH3
Mg
CH2Br
CH2MgBr
CO2
H+
CH2 COOH
Based on McMurry, Organic Chemistry, Chapter 20, 6th edition, (c) 2003
Dr.Mohanad Al-Hachamii
phenylacetic acid
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Mechanism of Grignard Carboxylation
• The organomagnesium halide adds to C=O of carbon
dioxide
• Protonation by addition of aqueous HCl in a separate step
gives the free carboxylic acid
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
Dr.Mohanad Al-Hachamii
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6- Hydrolysis of : Esters, Acid chloride , Amids , and Acid anhydride
Dr.Mohanad Al-Hachamii
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
42
20.7 Reactions of Carboxylic
Acids: An Overview
Carboxylic acids transfer a proton to a base to give anions,
which are good nucleophiles in SN2 reactions
Like ketones, carboxylic acids undergo addition of
nucleophiles to the carbonyl group
In addition, carboxylic acids undergo other reactions
characteristic of neither alcohols nor ketones
Based on McMurry, Organic Chemistry, Chapter 20, 6th edition, (c) 2003
Dr.Mohanad Al-Hachamii
43
20.8 Reduction of Carboxylic
Acids
• Reduced by LiAlH4 to yield primary alcohols
• The reaction is difficult and often requires heating in
tetrahydrofuran solvent to go to completion
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
Dr.Mohanad Al-Hachamii
44
Reduction with Borane
• Borane in tetrahydrofuran (BH3/THF) converts carboxylic
acids to primary alcohols selectively
• Preferable to LiAlH4 because of its relative ease, safety,
and specificity
• Borane reacts faster with COOH than it does with NO2
Based on McMurry, Organic Chemistry, Chapter 20, 6th edition, (c) 2003
Dr.Mohanad Al-Hachamii
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Dr.Mohanad Al-Hachamii
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
46
Dr.Mohanad Al-Hachamii
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
47
Dr.Mohanad Al-Hachamii
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
48
Dr.Mohanad Al-Hachamii
Based on McMurry, Organic Chemistry, Chapter 20, 6th
edition, (c) 2003
49
Dr.Mohanad Al-Hachamii
Based on McMurry, Organic Chemistry, Chapter 20, 6th edition, (c) 2003
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