Transcript Reactions of Carboxylic Acids

Carboxylic Acid
Nomenclature
Introduction
 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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Table 18.1
Systematic IUPAC names replace "-e"
ending of alkane with "oic acid"
Systematic Name
O
HCOH
methanoic acid
O
CH3COH
ethanoic acid
O
CH3(CH2)16COH
octadecanoic acid
Table 18.1
Common names are based on natural
origin rather than structure.
Systematic Name Common Name
O
HCOH
methanoic acid
formic acid
ethanoic acid
acetic acid
octadecanoic acid
stearic acid
O
CH3COH
O
CH3(CH2)16COH
Table 18.1
Systematic Name Common Name
O
CH3CHCOH 2-hydroxypropanoic acid
lactic acid
O
OH
CH3(CH2)7
(CH2)7COH
C
H
C
H
(Z)-9-octadecenoic acid
oleic acid
or (Z)-octadec-9-enoic acid
Aromatic Acids
 Aromatic acids are named as derivatives of benzoic
acid.
 Ortho-, meta-, and para- prefixes are used to specify
the location of a second substituent.
 Numbers are used to specify locations when more
than two substituents are present.
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Structure of the Carboxyl Group
 The sp2 hybrid carbonyl carbon atom is planar, with
nearly trigonal bond angles.
 The O—H bond also lies in this plane, eclipsed with
the C═O bond.
 The sp3 oxygen has a C—O—H angle of 106°.
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Boiling Points
 Carboxylic acids boil at considerably higher
temperatures than do alcohols, ketones, or
aldehydes of similar molecular weights.
 The high boiling points of carboxylic acids result from
formation of a stable, hydrogen-bonded dimer.
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Solubility in Water
Carboxylic acids are similar to alcohols in respect
to their solubility in water.
They form hydrogen bonds to water.
H
O
H
O
H3CC
H
O
H
O
H
Solubility
 Water solubility decreases with the length of the
carbon chain.
 Acids with more than 10 carbon atoms are
nearly insoluble in water.
 Very soluble in alcohols.
 Also soluble in relatively nonpolar solvents like
chloroform because the hydrogen bonds of the
dimer are not disrupted by the nonpolar solvent.
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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.
O
CH3COH
CH3CH2OH
pKa = 4.7
pKa = 16
Energy Diagram of Carboxylic
Acids and Alcohols
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Acetate Ion Structure
 Each oxygen atom bears half of the negative charge.
 The delocalization of the negative charge over the
two oxygens makes the acetate ion more stable than
an alkoxide ion.
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Substituent Effects on Acidity
 The magnitude of a substituent effect depends on its distance from
the carboxyl group.
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Aromatic Carboxylic Acids
 Electron-withdrawing groups enhance the acid
strength, and electron-donating groups decrease the
acid strength.
 Effects are strongest for substituents in the ortho and
para positions.
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Hybridization Effect
O
pKa
COH
4.2
O
H2C
HC
CH
COH
O
4.3
C
COH
1.8
sp2-hybridized carbon is more electronwithdrawing than sp3, and sp is more
electron-withdrawing than sp2.
Salts of Carboxylic Acids
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Deprotonation of Carboxylic Acids
 The hydroxide ion completely deprotonates
the acid to form the carboxylate salt.
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Protonation of Carboxylic Acids Salts
 Adding a strong acid, like HCl, regenerates
the carboxylic acid.
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Naming Carboxylic Acid Salts
 First name the cation.
 Then name the anion by replacing the
-ic acid with -ate.
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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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Basic Hydrolysis of Fats and
Oils
• The basic hydrolysis of fat and oils produces
soap (this reaction is known as saponification).
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Micelles
Unbranched carboxylic acids with 12-18 carbons
give carboxylate salts that form micelles in water.
O
ONa
sodium stearate
(sodium octadecanoate)
O
–
CH3(CH2)16CO Na+
Micelles
O
ONa
nonpolar
polar
Micelles
O
ONa
nonpolar
polar
Sodium stearate has a polar end (the carboxylate
end) and a nonpolar "tail“.
The polar end is hydrophilic ("water-loving”).
The nonpolar tail is hydrophobic ("water-hating”).
In water, many stearate ions cluster together to form
spherical aggregates; carboxylate ions are on the
outside and nonpolar tails on the inside.
Figure 18.5: A micelle
Micelles
The interior of the micelle is nonpolar and
has the capacity to dissolve nonpolar
substances.
Soaps clean because they form micelles,
which are dispersed in water.
Grease (not ordinarily soluble in water)
dissolves in the interior of the micelle and is
washed away with the dispersed micelle.
Dicarboxylic Acids
Dicarboxylic Acids
O
HOC
O
COH
Oxalic acid
1.2
Malonic acid
2.8
Heptanedioic acid
4.3
O
HOCCH2COH
O
pKa
O
O
HOC(CH2)5COH
One carboxyl group acts as an electronwithdrawing group toward the other; effect
decreases with increasing separation.
Synthesis of Carboxylic Acids: Review
side-chain oxidation of alkylbenzenes (Section 11.12)
oxidation of primary alcohols (Section 15.9)
oxidation of aldehydes (Section 17.15)
Side Chain Oxidation of
Alkylbenzenes
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Oxidation of Primary Alcohol to
Carboxylic Acids
 Primary alcohols and aldehydes are commonly
oxidized to acids by chromic acid (H2CrO4 formed
from Na2Cr2O7 and H2SO4).
 Potassium permanganate is occasionally used, but
the yields are often lower.
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Oxidation of Aldehydes
Aldehydes are easily oxidized to carboxylic acids.
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Cleavage of Alkenes Using KMnO4
 Warm, concentrated permanganate solutions oxidize
the glycols, cleaving the central C═C bond.
 Depending on the substitution of the original double
bond, ketones or acids may result.
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Alkyne Cleavage Using Ozone or
KMnO4
 With alkynes, either ozonolysis or a vigorous
permanganate oxidation cleaves the triple
bond to give carboxylic acids.
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Synthesis of Carboxylic Acids
by the
Carboxylation of Grignard Reagents
Carboxylation of Grignard
Reagents
 Grignard reagents react with CO2 to produce, after protonation,
a carboxylic acid.
 This reaction is sometimes called “CO2 insertion,” and it
increases the number of carbons in the molecule by one.
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Example: Alkyl Halide
CH3CHCH2CH3
Cl
1. Mg,
diethyl ether
2. CO2
3. H3O+
CH3CHCH2CH3
CO2H
(76-86%)
2-methylbutanoic acid
Example: Aryl Halide
1. Mg,
diethyl
ether
2. CO2
CH3 3. H O+
3
Br
CH3
CO2H
(82%)
Synthesis of Carboxylic Acids
by the
Preparation and Hydrolysis of Nitriles
Hydrolysis of Nitriles
 Basic or acidic hydrolysis of a nitrile (—CN)
produces a carboxylic acid.
 The overall reaction, starting from the alkyl
halide, adds an extra carbon to the molecule.
 A limitation is that the halide must be reactive
toward substitution by SN2 mechanism.
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Example
NaCN
CH2Cl
CH2CN
DMSO
(92%)
O
CH2COH
(77%)
H2O
H2SO4
heat
Example: Dicarboxylic Acid
BrCH2CH2CH2Br
NaCN
H2O
NCCH2CH2CH2CN
H2O, HCl
O
(77-86%)
heat
O
HOCCH2CH2CH2COH
(83-85%)
via Cyanohydrin
OH
O
1. NaCN
CH3CCH2CH2CH3
2. H+
CH3CCH2CH2CH3
CN
H2O
HCl, heat
OH
CH3CCH2CH2CH3
CO2H
(60% from 2-pentanone)
Reactions of Carboxylic Acids:
A Review and a Preview
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Reactions of Carboxylic Acids
Reactions already discussed
Reduction with LiAlH4 (Section 15.3)
Formation of acyl chlorides (Section 12.7)
Esterification (Section 15.8)
LiAlH4 Reduction of Carboxylic
Acids
 LiAlH4 reduces carboxylic acids to primary alcohols.
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Synthesis of Acid Chlorides
 The best reagents for converting carboxylic acids to
acid chlorides are thionyl chloride (SOCl2) and oxalyl
chloride (COCl2).
 They form gaseous by-products that do not
contaminate the product.
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Mechanism of Acid Chloride
Formation
Step 1
Step 2
Step 3
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Fischer Esterification
 Reaction of a carboxylic acid with an alcohol under acidic
conditions produces an ester.
 Reaction is an equilibrium; the yield of ester is not high.
 To drive the equilibrium toward the formation of products, use a
large excess of alcohol.
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Mechanism of the Fischer
Esterification
 Step 1:
 The carbonyl oxygen is protonated to activate the carbon
toward nucleophilic attack.
 The alcohol attacks the carbonyl carbon.
 Deprotonation of the intermediate produces the ester
hydrate.
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Mechanism of the Fischer
Esterification (Continued)
 Step 2:
 Protonation of one of the hydroxide groups creates a good
leaving group.
 Water leaves.
 Deprotonation of the intermediate produces the ester.
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Key Features of Mechanism
Protonation of carbonyl group activates
carbonyl oxygen.
Nucleophilic addition of alcohol to carbonyl group
forms tetrahedral intermediate.
Elimination of water from tetrahedral intermediate
restores carbonyl group.
Intramolecular Ester Formation:
Lactones
Lactones
Lactones are cyclic esters.
Formed by intramolecular esterification in a
compound that contains a hydroxyl group and
a carboxylic acid function
Examples
O
HOCH2CH2CH2COH
4-hydroxybutanoic acid
O
+
O
4-butanolide
IUPAC nomenclature: replace the -oic acid
ending of the carboxylic acid by –olide.
Identify the oxygenated carbon by number.
H2O
Examples
O
HOCH2CH2CH2COH
4-hydroxybutanoic acid
O
+
O
H2O
4-butanolide
O
HOCH2CH2CH2CH2COH
5-hydroxypentanoic acid
O + H2 O
O
5-pentanolide
Common names


O


O
-butyrolactone


O

O
-valerolactone
Ring size is designated by Greek letter
corresponding to oxygenated carbon
A  lactone has a five-membered ring.
A  lactone has a six-membered ring.
Lactones
Reactions designed to give hydroxy acids often
yield the corresponding lactone, especially if the
resulting ring is 5- or 6-membered.
Example
O
O
CH3CCH2CH2CH2COH
1. NaBH4
2. H2O, H+
via:
OH
O
CH3CHCH2CH2CH2COH
O
O
H3C
5-hexanolide (78%)
Decarboxylation of Malonic Acid
and Related Compounds
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Decarboxylation of Carboxylic Acids
Simple carboxylic acids do not decarboxylate
readily.
O
RH + CO2
RCOH
But malonic acid does.
O
O
HOCCH2COH
150°C
O
CH3COH +
CO2
Mechanism of Decarboxylation of Malonic Acid
One carboxyl group assists the loss of the other.
O
O
O H O
HO
OH
H
HO
H
O
H
H
O
OH
This compound is
HO
the enol form of
acetic acid.
H
H
+
C
O
Mechanism of Decarboxylation of Malonic Acid
One carboxyl group assists the loss of the other.
O
O
O H O
HO
OH
H
HO
H
O
H
H
These hydrogens play no role.
O
HOCCH3
O
OH
H
HO
H
+
C
O
Mechanism of Decarboxylation of Malonic Acid
One carboxyl group assists the loss of the other.
O
O
O H O
HO
OH
HO
R’
R
O
R
R’
Groups other than H may be present.
O
HOCCHR'
R
O
OH
R’
HO
R
+
C
O
Decarboxylation is a general reaction
for 1,3-dicarboxylic acids
CO2H
185°C
CO2H
CO2H
H
(74%)
160°C
CH(CO2H)2
CH2CO2H
(96-99%)
Mechanism of Decarboxylation of Malonic Acid
One carboxyl group assists the loss of the other.
O
O
O H O
HO
OH
HO
R’
R
O
R
R’
This OH group plays no role.
O
HOCCHR'
R
O
OH
R’
HO
R
+
C
O
Mechanism of Decarboxylation of Malonic Acid
One carboxyl group assists the loss of the other.
O
O
O H O
R"
OH
R
R"
R'
O
R
R'
Groups other than OH may be present.
O
R"CCHR'
R
O
OH
R'
R"
R
+
C
O
Mechanism of Decarboxylation of Malonic Acid
O
O
R"


OH
R
This kind of compound
is called a -keto acid.
R'
O
R"CCHR'
R
Decarboxylation of a
-keto acid gives a
ketone.
Decarboxylation of a -Keto Acid
O
CH3
O
CH3
25°C
CH3C
C
CH3
CO2H
CH3C
C
H
CH3
+ CO2
Some Important Acids
 Acetic acid is in vinegar and other foods,
used industrially as a solvent, catalyst,
and reagent for synthesis.
 Fatty acids from fats and oils.
 Benzoic acid is found in drugs and
preservatives.
 Adipic acid is used to make nylon 66.
 Phthalic acid is used to make polyesters.
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Spectroscopic Analysis of
Carboxylic Acids
IR Bands of Carboxylic Acids
 There will be two features in the IR spectrum of a
carboxylic acid: the intense carbonyl stretching
absorption (1710 cm–1) and the OH absorption
(2500–3500 cm–1).
 Conjugation lowers the frequency of the C═O band.
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IR Spectroscopy
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NMR of Carboxylic Acids
 Carboxylic acid protons are the most deshielded
protons we have encountered, absorbing between
 10 and  13.
 The protons on the  carbon atom absorb between
 2.0 and  2.5.
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NMR Spectroscopy
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13C
NMR
Carbonyl carbon is at low field ( 160-185
ppm), but not as deshielded as the carbonyl
carbon of an aldehyde or ketone ( 190-215
ppm).
Mass Spectrometry
Aliphatic carboxylic acids undergo a variety
of fragmentations.
Aromatic carboxylic acids first form acylium ions,
which then lose CO.
••
O ••
ArCOH
•+
O ••
ArCOH
ArC
+
O ••
Ar
+