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William H. Brown Thomas Poon
www.wiley.com/college/brown
Chapter FourteenCarboxylic Acids
14-2
Structure• The functional group of a carboxylic acid is a
carboxyl group.
• The general formula of an aliphatic carboxylic acid is RCOOH.
• That of an aromatic carboxylic acid is ArCOOH.
14-3
Nomenclature• IUPAC names: drop the -ee from the parent
alkane and add the suffix -oic acidoic acid.• If the compound contains a carbon-carbon double
bond, change the infix -anan- to -enen-.COOH C6H5
COOH
3-Methylbutanoic acid(Isovaleric acid)
trans-3-Phenylpropenoic acid(Cinnamic acid)
14-4
Nomenclature• The carboxyl group takes precedence over
most other functional groups.
5-Oxohexanoic acid 4-Aminobutanoic acid
(R)-5-Hydroxyhexanoic acidCOOH
OH
H2N COOHCOOHO
Don’t worry about R or S…we didn’t learn about that.
14-5
Nomenclature Dicarboxylic acids: add -dioic aciddioic acid to the name of
the parent alkane containing both carboxyl groups.
• There is no need to use numbers to locate the carboxyl groups; they can only be on the ends of the chain.
HO OH
O
Propanedioic acid(Malonic acid)
Ethanedioic acid(Oxalic acid)
OHO OH
O
O
Hexanedioic acid(Adipic acid)
Pentanedioic acid(Glutaric acid)
Butanedioic acid(Succinic acid)
OOH
OHOOH
O
HO
OHO OH
O
O
14-6
Nomenclature• If the carboxyl group is bonded to a ring, name the
ring compound and add the suffix -carboxylic acidcarboxylic acid.
32
1
2-Cyclohexene-carboxylic acid
trans-1,3-Cyclopentane-dicarboxylic acid
COOH COOHHOOC
Don’t worry about ringswith multiple –COOHs!
14-7
Nomenclature• Benzoic acid is the simplest aromatic carboxylic
acid.• Use numbers to show the location of substituents.
Benzoicacid
2-Hydroxybenzoic acid
(Salicylic acid)
COOH COOHOH
1,2-Benzene-dicarboxylic acid(Phthalic acid)
1,4-Benzene-dicarboxylic acid
(Terephthalic acid)
COOHCOOH
COOH
COOH
Don’t worry about benzene ringswith multiple –COOHs!
14-8
Nomenclature• When common names are used, the letters
etc. are often used to locate substituents.
• In common nomenclature, ketoketo indicates the presence of a ketone, and CH3CO- is named an aceto groupaceto group.
alanine)(-Aminopropionic acid;(-Aminobutyric acid;
GABA)
2-Aminopropanoic acid4-Aminobutanoic acid
4 3 25 H2N OH
O
OH
O
NH2
OH
O
1
OH
OO OCH3C-
3-oxobutanoic acid-Ketobutyric acid;
acetoacetic acid)
Acetyl group(aceto group)
14-9
Physical Properties• In the liquid and solid states, carboxylic acids
are associated by hydrogen bonding into dimeric structures.
14-10
Physical Properties• Carboxylic acids have significantly higher boiling points
than other types of organic compounds of comparable molecular weight.• They are polar compounds and form very strong
intermolecular hydrogen bonds.• Carboxylic acids are more soluble in water than alcohols,
ethers, aldehydes, and ketones of comparable molecular weight.• They form hydrogen bonds with water molecules
through their C=O and OH groups.
14-11
Physical Properties• Water solubility decreases as the relative size of
the hydrophobic portion of the molecule increases.
14-12
Acidity• Carboxylic acids are weak acids
• Values of pKa for most aliphatic and aromatic carboxylic acids fall within the range 4 to 5.
• The greater acidity of carboxylic acids relative to alcohols, both of which contain an OH group, is due to resonance stabilization of the carboxylate anion.
CH3COOH H2O CH3COO- H3O+
[CH3COO-][H3O+][CH3COOH]
+
= 1.74 x 10-5Ka =
pKa = 4.76
+
14-13
Acidity• Electron-withdrawing substituents near the
carboxyl group increase acidity through their inductive effect.
14-14
Acidity• The acid-strengthening effect of a halogen
substituent falls off rapidly with increasing distance from the carboxyl group.
14-15
Reaction with Bases• Carboxylic acids, whether soluble or insoluble in water,
react with NaOH, KOH, and other strong bases to give water-soluble salts.
• They also form water-soluble salts with ammonia and amines.
+ +
Benzoic acid(slightly soluble
in water)
Sodium benzoate(60 g/100 mL water)
COOH COO-Na+NaOH H2 O H2 O
+
Ammonium benzoate(20 g/100 mL water)
Benzoic acid(slightly soluble
in water)
COOH COO- NH4+NH3 H2 O
14-16
Reaction with Bases• Carboxylic acids react with sodium bicarbonate
and sodium carbonate to form water-soluble salts and carbonic acid.• Carbonic acid (H2CO3), in turn, breaks down to
carbon dioxide and water.
CH3COOH Na+HCO3- H2O
H2CO3
CH3COOH Na+HCO3-
CO2
CH3COO-Na+
CH3COO-Na+
H2O
CO2
H2CO3
H2O
+ +
+
+ ++
14-17
Reaction with Bases• Figure 14.1
Flowchart for separation of benzoic acid from benzyl alcohol.
14-18
Reduction• The carboxyl group is very resistant to
reduction.• It is not affected by catalytic hydrogenation under
conditions that easily reduce aldehydes and ketones to alcohols, and reduce alkenes and alkynes to alkanes.
• It is not reduced by NaBH4.
14-19
Reduction• Lithium aluminum hydride reduces a carboxyl
group to a 1° alcohol.• Reduction is carried out in diethyl ether, THF, or
other nonreactive, aprotic solvents.• (Notice that the alkene double bond is
unaffected!)
1. LiAlH4, ether2. H2O
LiOH Al(OH)3+ +H
OOH1. LiAlH4, ether
2. H2OLiOH Al(OH)3+ +H
OOHOH
14-20
Selective Reduction• Catalytic hydrogenation does not reduce a
COOH group!• we can use H2/Metal to reduce an alkene in the
presence of a COOH group.
• we can use NaBH4 to reduce an aldehyde or ketone in the presence of a COOH group.
OH
O
H2+ Pt OH
O
Hexanoic acid5-Hexenoic acid 25°C, 2 atm
C6H5 OH
OO1. NaBH42. H2O
C6H5 OH
OOH
5-Hydroxy-5-phenylpentanoic acid5-Oxo-5-phenylpentanoic acid
14-21
Fischer Esterification• Esters can be prepared by treating a carboxylic
acid with an alcohol in the presence of an acid catalyst, commonly H2SO4 or gaseous HCl.
CH3COHO
CH3CH2OHH2SO4
CH3COCH2CH3O
H2OEthanoic acid(Acetic acid)
++Ethyl ethanoate(Ethyl acetate)
Ethanol(Ethyl alcohol)
14-22
Fischer Esterification• Fischer esterification is an equilibrium
reaction.• By careful control of experimental conditions, it is
possible to prepare esters in high yield.• If the alcohol is inexpensive relative to the
carboxylic acid, it can be used in excess to drive the equilibrium to the right.
• A key intermediate in Fischer esterification is the tetrahedral carbonyl addition intermediate formed by addition of ROH to the C=O group.
C OHRO
HOCH3H+
C OCH3
OH
OR
H
H+
C OCH3RO
HOH+ +
14-23
Acid Chlorides• The functional group of an acid halide is a
carbonyl group bonded to a halogen atom.• Among the acid halides, acid chlorides are by far
the most common and the most widely used.
Functional group of an acid halide
Acetylchloride
Benzoylchloride
-C-X CH3CClO O
C-ClO
14-24
Acid Chlorides• Acid chlorides are most often prepared by
treating a carboxylic acid with thionyl chloride.
+ + +Butanoic
acidThionylchloride
Butanoylchloride
SOCl2 SO2 HClOH
O
Cl
O
14-25
Decarboxylation• Decarboxylation:Decarboxylation: loss of CO2 from a carboxyl
group.• Most carboxylic acids, if heated to a very high
temperature, undergo thermal decarboxylation.
• Most, however, are quite resistant to moderate heat and melt or even boil without decarboxylation.
RCOHO
RH CO2decarboxylation +
heat
14-26
Decarboxylation• Exceptions are carboxylic acids that have a
carbonyl group beta to the carboxyl group.• This type of carboxylic acid undergoes
decarboxylation on mild heating.
OH
OO OCO2
Acetone3-Oxobutanoic acid(Acetoacetic acid)
+warm
14-27
Decarboxylation• Thermal decarboxylation of a -ketoacid involves
rearrangement of six electrons in a cyclic six-membered transition state.
O+
enol ofa ketone
(A cyclic six-membered transition state)
CO2(1) (2)O OH
O
OCOH
O
14-28
Decarboxylation• Thermal decarboxylation of malonic acids also
involves rearrangement of six electrons in a cyclic six-membered transition state.
O OH
HO O
O H
HO CO
O HO
OCO2+
Enol of acarboxylic acid
A cyclic six-membered transition state
(1) (2)
14-29
Decarboxylation• Problem:Problem: Draw the product of decarboxylation.
• Problem:Problem: Draw the -ketoacid that undergoes decarboxylation to give this ketone.
OCOOH heat
O
CO2-ketoacid heat +
14-30
Carboxylic Acids
End Chapter 14End Chapter 14