Systematic IUPAC names replace "-e" ending of alkane with "oic acid". Systematic Name
O HCOH
Common names are based on natural origin rather than structure. Systematic Name Common Name
O
methanoic acid
HCOH
O
methanoic acid
formic acid
ethanoic acid
acetic acid
octadecanoic acid
stearic acid
O
CH3COH
ethanoic acid
CH3COH
O
O octadecanoic acid
CH3(CH2)16COH
CH3(CH2)16COH
Table 19.1 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
19.2 Structure and Bonding
Electron Delocalization
Formic Acid is Planar
Stabilizes carbonyl group O C
120 pm
R
H
H
O
••
C •• O ••
134 pm
O ••
R
+ C •• O ••
H
•• – O ••
R
••
C + O ••
H
•• – O •• ••
H
Boiling Points
19.3 Physical Properties
O
OH
80°C
99°C
O OH
bp (1 atm): 31°C
141°C
Intermolecular forces, especially hydrogen bonding, are stronger in carboxylic acids than in other compounds of similar shape and molecular weight.
Hydrogen-bonded Dimers
O
H
Hydrogen-bonded Dimers
O CCH3
H3CC O
H
O
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.
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.
Solubility in Water Carboxylic acids are similar to alcohols in respect to their solubility in water. Form hydrogen bonds to water. H O
H
19.4 Acidity of Carboxylic Acids Most carboxylic acids have a pKa close to 5.
O
H3CC
H O
H
O H
Free Energies of Ionization Carboxylic Acids are Weak Acids But carboxylic acids are far more acidic than alcohols.
CH3CH2O– + H+
O CH3COH
CH3CH2OH
pKa = 4.7
pKa = 16
ΔG°= 64 kJ/mol ΔG°= 91 kJ/mol
O CH3CO– + H+
ΔG°= 27 kJ/mol
O CH3CH2OH
Greater Acidity of Carboxylic Acids is Attributed Stabilization of Carboxylate Ion by
CH3COH
Figure 19.3b: Electrostatic Potential Maps of Acetic Acid and Acetate Ion
Inductive effect of carbonyl group O – RC O δ+ Resonance stabilization of carboxylate ion ••
O •• RC
•• – ••
O ••
•• – •• O ••
Acetic acid
RC O •• ••
Acetate ion
Carboxylic Acids are Neutralized by Strong Bases O 19.5 Salts of Carboxylic Acids
RCOH +
O RCO– +
HO–
stronger acid
H2O weaker acid
Equilibrium lies far to the right; K is ca. 1011. As long as the molecular weight of the acid is not too high, sodium and potassium carboxylate salts are soluble in water.
Micelles
Micelles O
Unbranched carboxylic acids with 12-18 carbons give carboxylate salts that form micelles in water.
ONa polar
nonpolar
O ONa sodium stearate (sodium octadecanoate) O – CH3(CH2)16CO Na+
Micelles
Figure 19.5: A micelle O ONa
nonpolar
polar
Sodium stearate has a polar end (the carboxylate end) and a nonpolar "tail“. The polar end is "water-loving" or hydrophilic. The nonpolar tail is "water-hating" or hydrophobic. In water, many stearate ions cluster together to form spherical aggregates; carboxylate ions on the outside and nonpolar tails on the inside.
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.
Substituent Effects on Acidity
19.6 Substituents and Acid Strength
Substituent Effects on Acidity O
standard of comparison is acetic acid (X = H) X
O X
CH2COH
Electronegative groups increase acidity
Alkyl groups have negligible effect
pKa = 4.7
Substituent Effects on Acidity
CH2COH
X
pKa
X
pKa
H
4.7
H
4.7
CH3
4.9
F
2.6
CH3(CH2)5
4.9
Cl
2.9
Effect of electronegative substituent decreases as number of bonds between it and carboxyl group increases. pKa
O
CH3CH2CHCO2H
CH2COH
Cl
electronegative substituents withdraw electrons from carboxyl group; increase K for loss of H+
CH3CHCH2CO2H
X
2.8
4.1
Cl ClCH2CH2CH2CO2H
4.5
Hybridization Effect
19.7 Ionization of Substituted Benzoic Acids
H2C HC
O
pKa
COH O
4.2
CH
COH O
4.3
C
COH
1.8
sp2-hybridized
carbon is more electronwithdrawing than sp3, and sp is more electronwithdrawing than sp2
Table 19.3 Ionization of Substituted Benzoic Acids O
X
COH
Substituent H CH3 F Cl CH3O NO2
ortho 4.2 3.9 3.3 2.9 4.1 2.2
effect is small unless X is electronegative; effect is largest for ortho substituent
pKa meta 4.2 4.3 3.9 3.8 4.1 3.5
19.8 Dicarboxylic Acids
para 4.2 4.4 4.1 4.0 4.5 3.4
Dicarboxylic Acids O HOC O
COH
Oxalic acid
1.2
O
HOCCH2COH O
pKa
O
Malonic acid
2.8
Heptanedioic acid
4.3
O
HOC(CH2)5COH
one carboxyl group acts as an electronwithdrawing group toward the other; effect decreases with increasing separation
19.9 Carbonic Acid
Carbonic Acid
Carbonic Acid O
O
HOCO–
HOCO–
O CO2 + H2O 99.7%
HOCOH
H+ +
0.3%
Second ionization constant:
Ka = 5.6 x 10-11
overall K for these two steps = 4.3 x 10-7
O
CO2 is major species present in a solution of "carbonic acid" in acidic media
+
H+
–OCO–
Synthesis of Carboxylic Acids: Review side-chain oxidation of alkylbenzenes (Section 11.13)
19.10 Sources of Carboxylic Acids
oxidation of primary alcohols (Section 15.10) oxidation of aldehydes (Section 17.15)
Carboxylation of Grignard Reagents 19.11 Synthesis of Carboxylic Acids by the Carboxylation of Grignard Reagents
O RX
Mg diethyl ether
RMgX
converts an alkyl (or aryl) halide to a carboxylic acid having one more carbon atom than the starting halide
CO2
RCOMgX H3O+ O RCOH
Carboxylation of Grignard Reagents ••
••
O •• δ–
R
Example: Alkyl Halide
O ••
diethyl ether
C
R
C •• O •• + •• – MgX
MgX O •• •• H3
2. CO2 3. H3O+
Cl
••
O+ R
CH3CHCH2CH3
1. Mg, diethyl ether
O ••
CH3CHCH2CH3 CO2H (76-86%)
C •• OH ••
Example: Aryl Halide 1. Mg, diethyl ether 2. CO2 CH3 3. H O+ 3
CH3
Br
CO2H
19.12 Synthesis of Carboxylic Acids by the Preparation and Hydrolysis of Nitriles
(82%)
Preparation and Hydrolysis of Nitriles
RX
– •C • S N2
O N ••
RC
N ••
H3
O+
Example NaCN CH2Cl
DMSO
RCOH
heat
+ NH4+ converts an alkyl halide to a carboxylic acid having one more carbon atom than the starting halide limitation is that the halide must be reactive toward substitution by SN2 mechanism
O CH2COH (77%)
CH2CN (92%) H2O H2SO4 heat
Example: Dicarboxylic Acid
O
BrCH2CH2CH2Br NaCN
via Cyanohydrin 1. NaCN
CH3CCH2CH2CH3
H2O
2. H+
OH CH3CCH2CH2CH3 CN
NCCH2CH2CH2CN H2O, HCl O
(77-86%)
H2O HCl, heat
OH
heat
CH3CCH2CH2CH3
O
HOCCH2CH2CH2COH
(83-85%)
CO2H
(60% from 2-pentanone)
Reactions of Carboxylic Acids 19.13 Reactions of Carboxylic Acids: A Review and a Preview
Reactions already discussed Acidity (Sections 19.4-19.9) Reduction with LiAlH4 (Section 15.3) Esterification (Section 15.8) Reaction with Thionyl Chloride (Section 12.7)
Reactions of Carboxylic Acids New reactions in this chapter α−Halogenation Decarboxylation But first we revisit acid-catalyzed esterification to examine its mechanism.
19.14 Mechanism of AcidCatalyzed Esterification
Acid-catalyzed Esterification (also called Fischer esterification) O
H+
COH + CH3OH O
Mechanism of Fischer Esterification The mechanism involves two stages: 1) formation of tetrahedral intermediate (3 steps) 2) dissociation of tetrahedral intermediate (3 steps)
COCH3 + H2O Important fact: the oxygen of the alcohol is incorporated into the ester as shown.
Mechanism of Fischer Esterification First stage: formation of tetrahedral intermediate The mechanism involves two stages: 1) formation of tetrahedral intermediate (3 steps) 2) dissociation of tetrahedral intermediate (3 steps) OH C
OCH3
OH
O COH + CH3OH H+ OH C
tetrahedral intermediate in esterification of benzoic acid with methanol
OCH3
methanol adds to the carbonyl group of the carboxylic acid the tetrahedral intermediate is analogous to a hemiacetal
OH
Second stage: conversion of tetrahedral intermediate to ester O
Mechanism of formation of tetrahedral intermediate
COCH3 + H2O H+
this stage corresponds to an acid-catalyzed dehydration
OH C OH
OCH3
Step 1
CH3
••
O ••
O• + •
H
C •• O ••
Step 1 •• •• O
H
C
H
+O ••
•• +O
CH3 H
•• +O
•O • • •
H
C •• O ••
carbonyl oxygen is protonated because cation produced is stabilized by electron delocalization (resonance)
H
H
C •• O ••
H
Step 2
H
H
Step 3 •• •• OH
+ O ••
C •• OH ••
•• +O
C H
C
H
CH3 CH3 •• O • •
H
H
•• • OH •
CH3 •• O • •
H
+ O ••
•• OH ••
H
C •• O ••
•• •• OH
CH3
CH3 O •• ••
H
• OH • ••
+ O ••
CH3 H
Step 4 •• • OH •
Tetrahedral intermediate to ester stage
C H
+ O ••
••
CH3
OCH3 ••
•• O • •
H
H
•• • OH •
C H
O •• ••
••
OCH3
CH3
••
H
O •• +
H
Step 5
Step 5 ••
• OH • ••
C H
OCH3
+ O ••
••
H
•• • OH •
•• • OH •
+ C + •• OCH3
••
H
O
C + •• OCH3
H
••
••
••
+ OH C
••
Step 6
••
OCH3 ••
Key Features of Mechanism H ••
O •• C
••
O+
CH3 Activation of carbonyl group by protonation of carbonyl oxygen
H H
••
••
OCH3 ••
••
O
••
+O C
H
CH3
Nucleophilic addition of alcohol to carbonyl group forms tetrahedral intermediate Elimination of water from tetrahedral intermediate restores carbonyl group
Lactones are cyclic esters Formed by intramolecular esterification in a compound that contains a hydroxyl group and a carboxylic acid function
Examples
Examples
O
O
HOCH2CH2CH2COH
O
4-hydroxybutanoic acid
O +
H2O
4-butanolide
4-hydroxybutanoic acid
IUPAC nomenclature: replace the -oic acid ending of the carboxylic acid by -olide identify the oxygenated carbon by number
β O
γ-butyrolactone
O 5-pentanolide
Lactones Reactions designed to give hydroxy acids often yield the corresponding lactone, especially if the resulting ring is 5- or 6-membered.
O δ
O + H2O
α
γ
O
γ
H2O
4-butanolide
HOCH2CH2CH2CH2COH
Common names α
+
O
O
5-hydroxypentanoic acid
β
O
HOCH2CH2CH2COH
O
δ-valerolactone
Ring size is designated by Greek letter corresponding to oxygenated carbon A γ lactone has a five-membered ring A δ lactone has a six-membered ring
Example O
O
CH3CCH2CH2CH2COH 1. NaBH4 2. H2O, H+
via: OH
O
CH3CHCH2CH2CH2COH O O
H3C 5-hexanolide (78%)
19.16 α-Halogenation of Carboxylic Acids: The Hell-Volhard-Zelinsky Reaction
α-Halogenation of Carboxylic Acids O
O
R2CCOH + X2
O
R2CCOH
H
But...
+ HX
X
R2CCOH + X2
P or PX3
O + HX
R2CCOH
analogous to α-halogenation of aldehydes and ketones
H X reaction works well if a small amount of phosphorus or a phosphorus trihalide is added to the reaction mixture
key question: Is enol content of carboxylic acids high enough to permit reaction to occur at reasonable rate? (Answer is NO)
this combination is called the Hell-VolhardZelinsky reaction
Example
Example O
O
CH2COH + Br2
CH3CH2CH2COH
Br2 P
O CH3CH2CHCOH Br (77%)
PCl3 benzene 80°C
Value: α-Halogen can be replaced by nucleophilic substitution
O CHCOH
(60-62%)
Br
Synthesis of α-Amino Acids
Value O CH3CH2CH2COH
Br2 P
O CH3CH2CHCOH
O (CH3)2CHCH2COH
Br2 PCl3
O (CH3)2CHCHCOH
Br
Br
(77%) O CH3CH2CHCOH
K2CO3 H2O heat
O (CH3)2CHCHCOH
OH
NH2
(69%)
(48%)
NH3 H2O
(88%)
Decarboxylation of Carboxylic Acids 19.17 Decarboxylation of Malonic Acid and Related Compounds
Simple carboxylic acids do not decarboxylate readily. O RH + CO2
RCOH But malonic acid does. O
O
150°C
HOCCH2COH
O CH3COH +
CO2
Mechanism of Decarboxylation of Malonic Acid Mechanism of Decarboxylation of Malonic Acid One carboxyl group assists the loss of the other. O
O HO
OH H
One carboxyl group assists the loss of the other.
O H O HO
H
O H
H
O H O
O
O HO
OH H
HO
H
O H
H
These hydrogens play no role. OH This compound is the enol form of HO acetic acid.
H
+
O
O
C
HOCCH3
+
H
HO
O
H
O
OH
C O
H
Mechanism of Decarboxylation of Malonic Acid Decarboxylation is a general reaction for 1,3-dicarboxylic acids One carboxyl group assists the loss of the other. O O O H O
HO
OH R
HO
R'
O R
R'
CO2H
185°C
CO2H
CO2H H (74%)
Groups other than H may be present. O
O
OH
HOCCHR' R
R'
HO R
+
C O
CH(CO2H)2
160°C
CH2CO2H (96-99%)
Mechanism of Decarboxylation of Malonic Acid Mechanism of Decarboxylation of Malonic Acid One carboxyl group assists the loss of the other. O O O H O HO
OH R
HO
R'
O R
R'
R'
+
R"
β
R
R
OH
O R"CCHR'
R' O
OH
O
R
This kind of compound is called a β-keto acid.
R'
O R"CCHR'
O α
O R
C
Mechanism of Decarboxylation of Malonic Acid O
R"
R'
Groups other than OH may be present.
HO
R
OH
O
OH
HOCCHR'
R" R
This OH group plays no role. O
One carboxyl group assists the loss of the other. O O O H O
+
R'
R"
C O
R
Decarboxylation of a β-Keto Acid O CH3C
O
CH3 C
CO2H
25°C
CH3
CH3C
C
CH3
H
CH3 + CO2
Decarboxylation of a β-keto acid gives a ketone.
R
Infrared Spectroscopy
Section 19.18 Spectroscopic Analysis of Carboxylic 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 absorption near 1700 cm-1. OH peak is broad and overlaps with C—H absorptions.
Figure 19.8 Infrared Spectrum of 4-Phenylbutanoic acid
1H
NMR
C6H5CH2CH2CH2CO2H
Proton of OH group of a carboxylic acid is normally the least shielded of all of the protons in a 1H NMR spectrum: (δ 10-12 ppm; broad).
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reaction of sodium hydroxide (NaOH) and hydrochloric acid (HCl) shown in the equation below. When the acid loses a proton and the base provides a hydroxide ion that accepts a proton, the product is water. The sodium (Na+) and the chloride (Cl-) ions
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