chapter 20 carboxylic acids. chapter 20 2 introduction the functional group of carboxylic acids...
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Chapter 20Carboxylic Acids
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Chapter 20 2
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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Chapter 20 3
Common Names
Many aliphatic acids have historical names. Positions of substituents on the chain are
labeled with Greek letters.
CH3CH2CHC
Cl
OH
O
-chlorobutyric acid
CH3CH2CH2CHCH2COOH
Ph
-phenylcaproic acid
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Chapter 20 4
IUPAC Names
Remove -e from alkane (or alkene) name, add -oic acid.
The carbon of the carboxyl group is #1.
CH3CH2CHC
Cl
OH
O
2-chlorobutanoic acid
PhC
HC
H
COOH
trans-3-phenyl-2-propenoic acid (cinnamic acid)
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Chapter 20 5
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
COOH
OH
o-hydroxybenzoic acid(salicylic acid)
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Chapter 20 6
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.
HOOCCH2CHCH2CH2COOH
Br
3-bromohexanedioic acid -bromoadipic acid =>
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Chapter 20 7
Structure of Carboxyl
Carbon is sp2 hybridized. Bond angles are close to 120. O-H eclipsed with C=O, to get overlap of
orbital with orbital of lone pair on oxygen.
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Chapter 20 8
Boiling Points
Higher boiling points than similar alcohols, due to dimer formation.
Acetic acid, b.p. 118C =>
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Chapter 20 9
Melting Points
Aliphatic acids with more than 8 carbons are solids at room temperature.
Double bonds (especially cis) lower the melting point. Note these 18-C acids: Stearic acid (saturated): 72C Oleic acid (one cis double bond): 16C Linoleic acid (two cis double bonds): -5C
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Chapter 20 10
Solubility
Water solubility decreases with the length of the carbon chain.
Up to 4 carbons, acid is miscible in water. More soluble in alcohol. Also soluble in relatively nonpolar solvents
like chloroform because it dissolves as a dimer. =>
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Chapter 20 11
Acidity
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Chapter 20 12
Resonance Stabilization
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Chapter 20 13
Substituent Effects on Acidity
COOH
OCH3
COOH COOH
NO2
COOH
NO2
COOH
NO2
p-methoxy benzoic acid m-nitro p-nitro o-nitro
pKa = 4.46 pKa = 4.19 pKa = 3.47 pKa = 3.41 pKa = 2.16 =>
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Chapter 20 14
Salts of Carboxylic Acids
Sodium hydroxide removes a proton to form the salt.
Adding a strong acid, like HCl, regenerates the carboxylic acid.
CH3 C
O
OHNaOH
CH3 C
O
O_
Na+
HCl=>
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Chapter 20 15
Naming Acid Salts
Name the cation. Then name the anion by replacing the
-ic acid with -ate.
CH3CH2CHCH2COO- K
+
Cl
potassium -chlorovaleratepotassium 3-chloropentanoate
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Chapter 20 16
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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Chapter 20 17
Purifying an Acid
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Chapter 20 18
Some Important Acids
Acetic acid is in vinegar and other foods, used industrially as solvent, catalyst, and reagent for synthesis.
Fatty acids from fats and oils. Benzoic acid in drugs, preservatives. Adipic acid used to make nylon 66. Phthalic acid used to make polyesters.
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Chapter 20 19
IR Spectroscopy
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Chapter 20 20
NMR Spectroscopy
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Chapter 20 21
UV Spectroscopy
Saturated carboxylic acids absorb very weakly around 200-215 nm.
If C=C is conjugated with C=O, molar absorptivity = 10,000 at 200 nm.
An additional conjugated double bond increases the absorption wavelength to 250 nm. =>
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Chapter 20 22
Mass Spectrometry
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Chapter 20 23
Synthesis Review
Oxidation of primary alcohols and aldehydes with chromic acid.
Cleavage of an alkene with hot KMnO4 produces a carboxylic acid if there is a hydrogen on the double-bonded carbon.
Alkyl benzene oxidized to benzoic acid by hot KMnO4 or hot chromic acid. =>
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Chapter 20 24
Grignard Synthesis
Grignard reagent + CO2 yields a carboxylate salt.
CH3CH3CHCH2MgBr
CH3CO O CH3CH3CHCH2COO
-MgBr
+
CH3H
+
CH3CH3CHCH2COOH
CH3
=>
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Chapter 20 25
Hydrolysis of Nitriles
Basic or acidic hydrolysis of a nitrile produces a carboxylic acid.
BrNaCN
CNH
+
H2O
COOH
=>
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Chapter 20 26
Acid Derivatives
The group bonded to the acyl carbon determines the class of compound: -OH, carboxylic acid -Cl, acid chloride -OR’, ester -NH2, amide
These interconvert via nucleophilic acyl substitution. =>
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Chapter 20 27
Fischer Esterification
Acid + alcohol yields ester + water. Acid catalyzed for weak nucleophile. All steps are reversible. Reaction reaches equilibrium.
COOH
+ CH3CH2OHH
+ COCH2CH3
O
+ HOH
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Chapter 20 28
Fischer Mechanism (1)
Protonation of carbonyl and attack of alcohol, a weak nucleophile.
COH
O+
H+
+COH
OHCOH
OH
+CH3CH2OHCOH
OH
O H
CH2CH3
O
H
R
COH
OH
O
CH2CH3
=>
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Chapter 20 29
Fischer Mechanism (2)
Protonation of -OH and loss of water.
+H
+
COH
OH
O
CH2CH3
C
OH
O
CH2CH3
H
OH+
O
H
RC
O
CH2CH3
OH C
O
CH2CH3
O
=>
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Chapter 20 30
Acid Chlorides
An activated form of the carboxylic acid. Chloride is a good leaving group, so
undergoes acyl substitution easily. To synthesize acid chlorides use thionyl
chloride or oxalyl chloride with the acid.
C
O
OHC
O
C
O
Cl Cl+
C
O
Cl+ + +HCl CO CO2
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Chapter 20 31
Esters from Acid Chlorides
Acid chlorides react with alcohols to give esters in good yield.
Mechanism is nucleophilic addition of the alcohol to the carbonyl as chloride ion leaves, then deprotonation.
CCl
O
+ CH3OH
COCH3
O
+ HCl=>
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Chapter 20 32
Amides from Acid Chlorides
Acid chlorides react with ammonia and amines to give amides.
A base (NaOH or pyridine) is added to remove HCl by-product.
CCl
O
+ CH3NH2
CNHCH3
O
NaOHNaCl H2O+ +
=>
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Chapter 20 33
Diazomethane
CH2N2 reacts with carboxylic acids to produce methyl esters quantitatively.
Very toxic, explosive. Dissolve in ether.
C
O
OH +C
O
OCH3 +CH2N2 N2
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Chapter 20 34
Mechanism for Diazomethane
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Chapter 20 35
Amides from Acids
Amine (base) removes a proton from the carboxylic acid to form a salt.
Heating the salt above 100C drives off steam and forms the amide.
C
O
OH +C
O
O-
+NH3CH3
+
CH3NH2heat
C
O
NHCH3
H2O
=>
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Chapter 20 36
Reduction to 1 Alcohols
Use strong reducing agent, LiAlH4.
Borane, BH3 in THF, reduces carboxylic acid to alcohol, but does not reduce ketone.
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Chapter 20 37
Reduction to Aldehyde
Difficult to stop reduction at aldehyde. Use a more reactive form of the acid (an acid
chloride) and a weaker reducing agent, lithium aluminum tri(t-butoxy)hydride.
CCl
O
C
O
HLiAl[OC(CH3)3]3H
=>
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Chapter 20 38
Alkylation to Form Ketones
React 2 equivalents of an organolithium reagent with a carboxylic acid.
COOH 1)
2)
CH3CH2 Li2
H2O
C
O
CH2CH3
=>
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Chapter 20 39
End of Chapter 20