Transcript Chapter-16A

Chapter 14: Carboxylic Acids
Sections 14.1-14.7
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14-1
Sections
Chapter 14: Carboxylic Acids
1.
2.
3.
4.
5.
6.
7.
8.
9.
Introduction
Structure
Nomenclature
Physical properties
Acidity
Reduction
Fisher esterification
Conversion to acid halides
Decaroxylation
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Structure
The functional group of a carboxylic acid is a
carboxyl group
:
O:
:
C
:O
COOH
CO 2 H
H
• the general formula of an aliphatic carboxylic acid is
RCOOH
• that of an aromatic carboxylic acid is ArCOOH
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Nomenclature
IUPAC names: drop the -e from the parent alkane
and add the suffix -oic acid
• if the compound contains a carbon-carbon double
bond, change the infix -an- to -enCOOH
3-Methylbutanoic acid
(Isovaleric acid )
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C6 H5
COOH
t rans -3-Phenylprop enoic acid
(Cinn amic acid)
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Nomenclature
The carboxyl group takes precedence over most
other functional groups
OH
COOH
(R)-5-Hydroxyhexanoic acid
O
COOH
5-Oxohexanoic acid
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H 2N
COOH
4-Aminobutanoic acid
14-5
Nomenclature
• dicarboxylic acids: add -dioic 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
O
HO
O
OH
O
Ethanedioic acid
(Oxalic acid)
O
HO
O
OH
O
Butanedioic acid
(Succinic acid)
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HO
O
HO
OH
Propanedioic acid
(Malonic acid)
O
O
OH
Pentanedioic acid
(Glutaric acid)
HO
OH
O
Hexanedioic acid
(Adipic acid)
14-6
Nomenclature
• if the carboxyl group is bonded to a ring, name the ring
compound and add the suffix -carboxylic acid
2
3
1
COOH
2-Cyclohexenecarboxylic acid
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H OOC
COOH
trans- 1,3-Cyclopentanedicarboxylic acid
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Nomenclature
• benzoic acid is the simplest aromatic carboxylic acid
• use numbers to show the location of substituents
COOH
COOH
COOH
OH
COOH
COOH
COOH
Benzoic
acid
2-Hydroxybenzoic
acid
(Salicylic acid)
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1,2-Benzenedicarboxylic acid
(Phthalic acid)
1,4-Benzenedicarboxylic acid
(Terephthalic acid)
14-8
Nomenclature
• when common names are used, the letters etc.
are often used to locate substituents
O
  

5
1
4 3 2
O
H2 N 
OH
O

OH
4-A min ob utanoic acid
(-Aminobutyric acid;
GABA )
OH
NH2
2-Aminopropanoic acid
(-Aminop ropionic acid;
alanin e)
• in common nomenclature, keto indicates the presence
of a ketone, and CH3CO- is named an aceto group
O
O
OH
3-oxob utanoic acid
-Ketobutyric acid ;
acetoacetic acid)
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O
CH3 CAcetyl group
(aceto group)
14-9
Physical Properties
In the liquid and solid states, carboxylic acids are
associated by hydrogen bonding into dimeric
structures
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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
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Physical Properties
• water solubility decreases as the relative size of the
hydrophobic portion of the molecule increases
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Acidity
Carboxylic acids are weak acids
• values of pKa for most aliphatic and aromatic
carboxylic acids fall within the range 4 to 5
CH3 COOH + H2 O
CH3 COO- + H3 O+
+
[CH
COO
]
[H
O
]
3
3
Ka =
[CH3 COOH]
= 1.74 x 10-5
pK a = 4.76
• 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
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Acidity
electron-withdrawing substituents near the
carboxyl group increase acidity through their
inductive effect
CH3 COOH
Acetic
acid
pKa : 4.76
ClCH2 COOH
Cl2 CHCOOH
Cl3 CCOOH
Chloroacetic
acid
2.86
D ich loroacetic
acid
1.48
Trich loroacetic
acid
0.70
Increasin g acid strength
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Acidity
• the acid-strengthening effect of a halogen substituent
falls off rapidly with increasing distance from the
carboxyl group
Cl
COOH
COOH
Cl
2-Chlorob utanoic 3-Chlorobutan oic
acid
acid
(pK a 2.83)
(p Ka 3.98)
Cl
COOH
4-Ch lorobutanoic
acid
(pK a 4.52)
COOH
Butan oic
acid
(pK a 4.82)
Decreas ing acid strength
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Reaction with Bases
Carboxylic acids, whether soluble or insoluble in
water, react with NaOH, KOH, and other strong
bases to give water-soluble salts
COOH
+
N aOH
-
H2 O
Benzoic acid
(slightly soluble
in water)
COO Na
+
+
H2 O
Sodium benzoate
(60 g/100 mL water)
They also form water-soluble salts with ammonia
and amines
COOH + N H3
Benzoic acid
(slightly soluble
in water)
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-
H2 O
COO NH4
+
A mmonium benzoate
(20 g/100 mL water)
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Reaction with Bases
Carboxylic acids react with sodium bicarbonate and
sodium carbonate to form water-soluble salts and
carbonic acid
• carbonic acid, in turn, breaks down to carbon dioxide
and water
+
CH3 COOH + Na HCO3
-
H2 CO3
+
CH3 COOH + Na HCO3
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H2 O
+
CH3 COO Na + H2 CO3
CO2 + H2 O
+
CH3 COO Na + CO2 + H2 O
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Reaction with Bases
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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
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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
O
H 1 . LiAlH4 , eth er
2 . H2 O
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OH + LiOH + Al(OH)
3
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Selective Reduction
Catalytic hydrogenation does not reduce a COOH
group
• we can use H2/M to reduce an alkene in the presence of
a COOH group
O
O
OH + H2
5-Hexen oic acid
Pt
25°C, 2 atm
OH
Hexanoic acid
• we can use NaBH4 to reduce an aldehyde or ketone in
the presence of a COOH group
O
C6 H5
OH
O
OH
5-Oxo-5-p henylpentan oic acid
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1 . NaBH4
2 . H2 O
C6 H5
O
OH
5-Hyd roxy-5-phen ylpen tanoic acid
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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
O
H2 SO4
CH3 COH + CH3 CH2 OH
Ethanoic acid
Eth anol
(A cetic acid ) (Ethyl alcoh ol)
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O
CH3 COCH2 CH 3 + H2 O
Ethyl ethanoate
(Ethyl acetate)
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
H
O
R C OH + HOCH3
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H+
O
R C OCH3
O
H
H+
O
R C OCH3 + HOH
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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
O
- C- X
Functional group
of an acid halide
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O
CH3 CCl
A cetyl
chloride
O
C-Cl
Benzoyl
chloride
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Acid Chlorides
Acid chlorides are most often prepared by treating
a carboxylic acid with thionyl chloride
O
OH + SOCl 2
Butanoic
Thionyl
acid
chloride
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O
Cl + SO 2 + HCl
Butanoyl
chloride
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Decarboxylation
Decarboxylation: loss of CO2 from a carboxyl group
• most carboxylic acids, if heated to a very high
temperature, undergo thermal decarboxylation
O
RCOH
decarb oxylation
heat
RH + CO2
• most carboxylic acids, however, are quite resistant to
• moderate heat and melt or even boil without
decarboxylation
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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
O
O
 
OH
3-Oxobu tan oic acid
(Acetoacetic acid)
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O
warm
+
CO2
Acetone
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Decarboxylation
• thermal decarboxylation of a -ketoacid involves
rearrangement of six electrons in a cyclic sixmembered transition state
enol of
a ketone
O
H
O
(1)
O
O
H
O
C
(2)
O
O
+
CO 2
(A cyclic six-membered
trans ition s tate)
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Decarboxylation
• thermal decarboxylation of malonic acids also involves
rearrangement of six electrons in a cyclic sixmembered transition state
O
H
HO
O
O
A cyclic six-membered
transition state
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O
(1)
HO
H
O
C
O
(2)
O
+
HO
CO 2
Enol of a
carboxylic acid
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Decarboxylation
• Problem: draw the product of decarboxylation
O
COOH
heat
• Problem: draw the -ketoacid that undergoes
decarboxylation to give this ketone
O
-ketoacid
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heat
+ CO2
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