chapter 18 carboxylic acids copyright © the mcgraw-hill companies, inc. permission required for...
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Chapter 18Chapter 18Carboxylic AcidsCarboxylic Acids
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
18.118.1Carboxylic Acid NomenclatureCarboxylic Acid Nomenclature
Table 18.1
Systematic Name (Common name)
O
HCOHO
CH3COHO
CH3(CH2)16COH
Systematic IUPAC names replace "-e“ ending of alkane with "oic acid ".
methanoic acid (formic acid)
ethanoic acid (acetic acid)
octadecanoic acid (stearic acid)
Systematic Name Common Name
2-hydroxypropanoic acid lactic acid
(Z)-9-octadecenoic acidor (Z)-octadec-9-enoic acid
oleic acid
O
CH3CHCOH
OH O
(CH2)7COH
C C
H H
CH3(CH2)7
Table 18.1
18.218.2Structure and BondingStructure and Bonding
Formic Acid is Planar
CC OO
HH
HH
OO
120 pm
134 pm
Resonance stabilizes carbonyl group.
Electron Delocalization ••
RC
OH
O••
••
••+••
– RC
OH
O••
••
••
+
••
–
••
RC
OH
O••
••
••
Resonance is stronger in the anion.
18.318.3Physical PropertiesPhysical Properties
Boiling Points
Intermolecular forces, especially hydrogen bonding, are stronger in carboxylic acids than in other compounds of similar shape and molecular weight.
bp (1 atm): 31o C 80o C 99o C
OH
141o C
OH
O O
alkene ketone alcohol acid
Hydrogen-bonded Dimers
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.
CH3C
O H O
CCH3
OHO
Hydrogen-bonded Dimers
A space filling model of acetic acid as a hydrogen-bonded dimer in the gas phase.
Carboxylic acids are similar to alcohols in respect to their solubility in water.
They form hydrogen bonds to water.
Solubility in Water
CH3C
O H O
OHO
H
H
H
18.418.4Acidity of Carboxylic AcidsAcidity of Carboxylic Acids
Most carboxylic acids have a pMost carboxylic acids have a pKKaa close to 5. close to 5.
Although they are weak acids (do not ionize completely in solution), carboxylic acids are far more acidic than alcohols.
Carboxylic Acids are Weak Acids
CH3COH
O
CH3CH2OH
pKa = 4.7 pKa = 16
G°= 91 kJ/mol
G°= 27 kJ/mol
G°= 64 kJ/mol
Free Energies of Ionization
CH3CH2O– + H+
CH3CH2OH CH3COH
O
CH3CO– + H+
O
Greater Acidity of Carboxylic Acids is AttributedStabilization of Carboxylate Ion by
Inductive effect of carbonyl group
Resonance stabilization of carboxylate ion
RC
O
O+
–
RC
O
O
–••
••
••
••
•• ••
••
RC
O
O–
••
••••
Figure 18.3(b): Electrostatic Potential Maps ofAcetic Acid and Acetate Ion
Acetic acidAcetate ion
18.518.5Substituents and Acid StrengthSubstituents and Acid Strength
standard of comparison is acetic acid (X = H)
Substituent Effects on Acidity
X CH2COH
O
pKa = 4.7
Electronegative substituents withdraw electrons from carboxyl group; increase K for loss of H+.
X CH2COH
O
Substituent Effects on Acidity
X
H
Cl
F
pKa
4.7
2.9
2.6
Electronegative groups increase acidity
X
H
CH3
CH3(CH2)5
pKa
4.7
4.9
4.9
Alkyl groups have negligible effect
X CH2COH
O
Effect of electronegative substituent decreasesas number of bonds between it and
carboxyl group increases.
pKa
2.8
4.1
4.5
CH3CH2CHCO2H
Cl
CH3CHCH2CO2H
Cl
ClCH2CH2CH2CO2H
18.618.6Ionization ofIonization of
Substituted Benzoic AcidsSubstituted Benzoic Acids
Hybridization Effect
pKa
4.2
4.3
1.8
COH
O
H2C CH COH
O
COH
O
HC C
sp2-hybridized carbon is more electron-withdrawing than sp3, and sp is more electron-withdrawing than sp2.
18.718.7Salts of Carboxylic AcidsSalts of Carboxylic Acids
Carboxylic Acids are Deprotonated by Strong Bases
Equilibrium lies far to the right; K is ca. 1011.For low molecular weight acids, sodium and potassium carboxylate salts are soluble in water.
strongeracid
weakeracid
RCOH + HO– RCO– + H2O
OO
Unbranched carboxylic acids with 12-18 carbonsgive carboxylate salts that form micelles in water.
Micelles O
ONasodium stearate
(sodium octadecanoate)
CH3(CH2)16CO
O
Na+–
O
ONa
polarnonpolar
Micelles
O
ONa
polarnonpolar
Sodium stearate has a polar "head" (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.
Micelles
Figure 18.5: A micelle
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.
Micelles
18.818.8Dicarboxylic AcidsDicarboxylic Acids
Dicarboxylic Acids
One carboxyl group acts as an electron-withdrawing group toward the other; effect decreases with increasing separation.
Oxalic acid
Malonic acid
Heptanedioic acid
1.2
2.8
4.3
COH
O
HOC
O pKa
HOCCH2COH
OO
HOC(CH2)5COH
O O
18.918.9Carbonic AcidCarbonic Acid
Carbonic Acid
HOCOH
O
CO2 + H2O HOCO–
O
H+ +
99.7% 0.3%
overall K for these two steps = 4.3 x 10-7
CO2 is the major species present in the equilibria above of "carbonic acid" in acidic media.
Carbonic Acid
HOCO–
O
–OCO–
O
H+ +
Ka = 5.6 x 10-11Second ionization constant:
18.1018.10Sources of Carboxylic AcidsSources of Carboxylic Acids
1. Side-chain oxidation of alkylbenzenes (Section 11.12)
2. Oxidation of primary alcohols (Section 15.9)
3. Oxidation of aldehydes (Section 17.15)
Synthesis of Carboxylic Acids: Review
18.1118.11Synthesis of Carboxylic Acids bySynthesis of Carboxylic Acids by
the Carboxylation of Grignard Reagents the Carboxylation of Grignard Reagents
Carboxylation of Grignard Reagents
RXMg
diethylether
RMgXCO2
H3O+
RCOMgX
O
RCOH
OConverts an alkyl (or aryl) halide to a carboxylic acid having one more carbon atom than the starting halide
R
MgX
C
O••
••
–diethylether
O ••••
MgX+
–
R C
O••
•• ••
O ••••
H3O+
••
R C
O H••
••O ••
Carboxylation of Grignard Reagents
Example: Alkyl Halide
CH3CHCH2CH3
(76-86%)
1. Mg, diethyl ether
2. CO2
3. H3O+
CH3CHCH2CH3
Cl CO2H
Example: Aryl Halide
(82%)
1. Mg, diethyl ether
2. CO2
3. H3O+
CH3
CO2HBr
CH3
18.1218.12Synthesis of Carboxylic Acids bySynthesis of Carboxylic Acids by
the Preparation and Hydrolysis of Nitrilesthe Preparation and Hydrolysis of Nitriles
Preparation and Hydrolysis of Nitriles
RX RCOH
O
The reactions convert an alkyl halide to a carboxylic acid having one more carbon atom than the starting halideA limitation is that the halide must be reactive toward substitution by SN2 mechanism.
– ••••C NRC ••N
SN2
H2O,H3O+
heat
+ NH4+
Example
NaCN
DMSO
(92%)
CH2Cl
CH2CN
(77%)
H2O
H2SO4
heatCH2COH
O
Example: Dicarboxylic Acid
BrCH2CH2CH2Br
NaCN H2O
(77-86%)NCCH2CH2CH2CN
H2O, HCl heat
(83-85%)HOCCH2CH2CH2COH
OO
via Cyanohydrin
1. NaCN
2. H+
CH3CCH2CH2CH3
O
CH3CCH2CH2CH3
OH
CN
(60% from 2-pentanone)
H2O
HCl, heat
CH3CCH2CH2CH3
OH
CO2H
18.1318.13Reactions of Carboxylic Acids:Reactions of Carboxylic Acids:
A Review and a PreviewA Review and a Preview
1. Acidity (Sections 18.4-18.6)2. Reduction with LiAlH4 (Section 15.3)
3. Esterification (Section 15.8)
4. Formation of acyl chlorides (Section 12.7)
Reactions already discussed:
Reactions of Carboxylic Acids
New reactions in this chapter:
1. Decarboxylation
2. First, revisit acid-catalyzed esterificationto examine the mechanism.
18.1418.14Mechanism of Acid-CatalyzedMechanism of Acid-Catalyzed
EsterificationEsterification
Acid-catalyzed Esterification
+ CH3OH
COH
OH+
+ H2O
COCH3
O
Important fact: the oxygen of the alcohol isincorporated into the ester as shown.
(also called the Fischer esterification)
The mechanism involves two stages:
1) formation of a tetrahedral intermediate from the C=O. (3 steps)
2) loss of the tetrahedral intermediate and regeneration of the C=O. (3 steps)
Mechanism of Fischer Esterification
C
OH
OH
OCH3
tetrahedral intermediate in esterification of benzoic acid with methanol.
First stage: formation of tetrahedral intermediate
C
OH
OH
OCH3
+ CH3OH
COH
O
H+
Methanol adds to the carbonyl group of the carboxylic acid.The tetrahedral intermediate is analogous to formation of a hemiacetal structure.
Second stage: conversion of tetrahedralintermediate to ester
+ H2O
H+
This stage corresponds to an acid-catalyzed dehydration.
COCH3
O
C
OH
OH
OCH3
Step 1
C
O
O H
•• ••
••••
H
••+
CH3
OH
••
C
O
O H
••
••
+ H
H
••O ••
CH3
+
Steps in the Mechanism of formation of the tetrahedral intermediate
protonation
Step 1, cont.
••
C
O
O H
••
••
+ H
Protonation of the carbonyl oxygen produces a cation that is stabilized by resonance (electron delocalization).
+
C
O
O H
•••• H
•• resonancestabilized
Step 2
CH3
••O ••
H
••
H
C
OH
OH
••••
••••
O+
CH3
••
C
O
O H
••
••
+ H
Attack by methanol
Step 3 CH3
O ••
H
H+
••
C
OH
OH
••••
••
O ••
CH3
•• +
tetrahedral intermediate
••
H
C
OH
OH
••••
••••
O+
CH3
CH3
••O ••
Hdeprotonation
Step 4
••
C
OH
O
••••
OCH3••
••
H H+
Steps from theTetrahedral intermediate to the Ester stage
+
+O ••
CH3
H
H
OCH3••
••
C
OH
O
••••
••
••
H
CH3
••O ••
H
protonation of OH
Step 5 O••H H
••+
••
C
OH••••
OCH3••
+
••
••
C
OH••••
OCH3
+OCH3••
C
OH••
••
+ ••
C
OH
O
••••
OCH3••
••
H H+
loss of water
resonancestabilized
Step 6
••
C
O••
OCH3••
+ H
••O••
H CH3 ••
+OH CH3
H
••
••
C
O••
OCH3
••
deprotonation
ester
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.
Key Features of Mechanism
18.1518.15Intramolecular Ester Formation:Intramolecular Ester Formation:
LactonesLactones
Lactones are cyclic esters.
They are formed by intramolecular esterification in a compound that contains a both a hydroxyl group and a carboxylic acid function.
Lactones
Examples
IUPAC nomenclature: replace the -oic acid ending of the carboxylic acid by –olide.Identify the oxygenated carbon by number.
HOCH2CH2CH2COH
O O
O+ H2O
4-hydroxybutanoic acid 4-butanolide
Examples
HOCH2CH2CH2COH
O O
O+ H2O
4-hydroxybutanoic acid 4-butanolide
HOCH2CH2CH2CH2COH
O O
O
+ H2O
5-hydroxypentanoic acid 5-pentanolide
Common names O
O
O
O
-butyrolactone -valerolactone
Ring size is designated by Greek letter corresponding to oxygenated carbonA lactone has a five-membered ring.A lactone has a six-membered ring.
Reactions designed to give hydroxy acids often yield the corresponding lactone, especially if the resulting ring is 5- or 6-membered.
In the following reaction, aδ-hydroxy acid was desired but aδ-lactone formed.
Lactones
Example
5-hexanolide (78%)
O
H3C
O
CH3CCH2CH2CH2COH
OO
1. NaBH4
2. H2O, H+
via:via:
CHCH33CHCHCHCH22CHCH22CHCH22COHCOH
OOOHOH
18.1618.16Decarboxylation of Malonic AcidDecarboxylation of Malonic Acid
and Related Compoundsand Related Compounds
Decarboxylation of Carboxylic Acids
Simple carboxylic acids do not decarboxylatereadily.
RH + CO2RCOH
O
But malonic acid does (requires a β C=O).
150o CCH3COH
O
+ CO2HOCCH2COH
O O
O
HO O
O
H H
H
Mechanism of Decarboxylation of Malonic Acid
The enol form of acetic acid.
O O
OHHO
H H
H
H
OH
HO+ C
O
O
One carboxyl group assists the loss of the other.
HOCCH3
O
R
Mechanism of Decarboxylation of Malonic Acid
Substituted malonic acids do the same.
HOCCHR'
O R
R’
OH
HO+
O
HO O
O
R R’
H O O
OHHO
R R’
C
O
O
185°C
Decarboxylation is a general reactionfor 1,3-dicarboxylic acids
160°C
CO2H
CO2H
CO2H
H
(74%) CH(CO2H)2
(96-99%)
CH2CO2H
Decarboxylation of Other β C=O Compounds O O
OHR"
R R'
R
β-keto acids also decarboxylate.
Need not be a 1,3-diacid, just needs β-C=O.
R"CCHR'
O
R
O
O
O
R R'
H R"
R'
OH
+R"
C
O
O
Mechanism of Decarboxylation of -keto acids O O
OHR"
R R'
This kind of compoundis called a -keto acid.
R"CCHR'
O
R
Decarboxylation of a -keto acid gives a ketone.
Decarboxylation of a -Keto Acid
25°C
CO2
CCH3C
O
CH3
CH3
H
+
CCH3C
O
CH3
CH3
CO2H
18.1718.17Spectroscopic Analysis of Spectroscopic Analysis of
Carboxylic AcidsCarboxylic Acids
A carboxylic acid is characterized by peaks due to OH and C=O groups in its infrared spectrum.
C=O stretching gives an intense absorptionnear 1700 cm-1.
OH peak is broad and overlaps with C—H absorptions.
Infrared Spectroscopy
Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2030 The McGraw-Hill Companies, Inc. All rights reserved.
Figure 18.8 Infrared Spectrum of 4-Phenylbutanoic acid
The proton on the OH group of a carboxylic acid is normally the least shielded of all of the protons in a 1H NMR spectrum:
( 10-12 ppm; broad and off-scale on a normal scan.)
1H NMR
Chemical shift (, ppm)
13C NMR
The carbonyl carbon is at very low field ( 160-185 ppm), but is not as deshielded as the carbonyl carbon of an aldehyde or ketone ( 190-215 ppm).
UV-VIS
Carboxylic acids absorb near 210 nm, butUV-VIS spectroscopy is not very useful for structure determination of carboxylic acids.
Aliphatic carboxylic acids undergo a varietyof fragmentations.Aromatic carboxylic acids first form acylium ions, which then loses CO giving m/z = 77.
Mass Spectrometry
ArCOH
••O •
•
ArCOH
•+O •
•
ArC O ••
+Ar
+
End of Chapter 18 End of Chapter 18 Carboxylic AcidsCarboxylic Acids