Carboxylic Acids

Report
Carboxylic Acids
The functional group of a carboxylic acid is a carboxyl
group, which can be represented in any one of three ways.
Nomenclature
IUPAC names:
• For an acyclic carboxylic acid, take the longest carbon
chain that contains the carboxyl group as the parent
alkane.
• Drop the final -e from the name of the parent alkane
and replace it by -oic acid.
• Number the chain beginning with the carbon of the
carboxyl group.
• Because the carboxyl carbon is understood to be
carbon 1, there is no need to give it a number.
Nomenclature
• In these examples, the common name is given in
parentheses.
• An -OH substituent is indicated by the prefix hydroxy-;
an -NH2 substituent by the prefix amino-.
Nomenclature
To name a dicarboxylic acid, add the suffix -dioic acid
Nomenclature
Nomenclature
For common names, use, the Greek letters alpha (a),
beta (b), gamma (g), and so forth to locate substituents.
Physical Properties
The carboxyl group contains three polar covalent bonds;
C=O, C-O, and O-H.
The polarity of these bonds determines the major
physical properties of carboxylic acids.
Physical Properties
The higher boiling points are a result of their polarity and
the fact that hydrogen bonding between two carboxyl
groups creates a dimer that behaves as a highermolecular-weight compound.
Physical Properties
Carboxylic acids are more soluble in water than are
alcohols, ethers, aldehydes, and ketones of comparable
molecular weight.
Fatty Acids
Fatty acids: Long chain carboxylic acids derived from
animal fats, vegetable oils, or phospholipids of biological
membranes.
• More than 500 have been isolated from various cells
and tissues.
• Most have between 12 and 20 carbons in an
unbranched chain.
• In most unsaturated fatty acids, the cis isomer
predominates; trans isomers are rare.
Fatty Acids
The Most Abundant Fatty Acids in Animal Fats, Vegetable
Oils, and Biological Membranes.
Fatty Acids
Saturated fatty acids are solids at room temperature.
• They pack close together to maximize interactions (by
London dispersion forces) between their chains.
Fatty Acids
All unsaturated fatty acids are liquids at room temperature
because the cis double bonds interrupt the regular
packing of their hydrocarbon chains.
Fatty Acids
Unsaturated fatty acids can have more than one double
bond which is almost always cis
Soaps
• Soaps are sodium salts or potassium salts of fatty acids.
• They are prepared from triglycerides.
• Triglycerides are triesters of glycerol.
• The reaction that takes place is called saponification
(Latin: saponem, “soap”).
Soaps
In water, soap molecules cluster into micelles.
The hydrophobic tails are shielded from the aqueous
environment, and their hydrophilic heads are in contact
with the aqueous environment.
Soaps
When soap and dirt are mixed in water, the nonpolar
hydrocarbon tails “dissolve” the nonpolar substances
such as grease and oil.
Soaps
• Soaps form water-insoluble salts in hard water.
• Hard water contains Ca2+, Mg2+, and Fe3+ ions.
Detergents
The problem of formation of precipitates in hard water
was overcome by using a molecule containing a sulfonate
(-SO3- ) group in the place of a carboxylate (-CO2-) group.
• Calcium, magnesium and iron salts of sulfonic acids,
RSO3H, are more soluble in water than are their salts of
fatty acids.
Acidity of Carboxylic Acids
• Carboxylic acids are weak acids.
• Values of Ka for most unsubstituted aliphatic and
aromatic carboxylic acids fall within the range 10-4 to
10-5 (pKa 4.0 - 5.0).
Acidity of Carboxylic Acids
When a carboxylic acid is dissolved in aqueous solution,
the form of the carboxylic acid present depends on the pH
of the solution in which it is dissolved.
Reaction with Bases
Carboxylic acids react with NaOH, KOH, and other strong
bases to form water-soluble salts.
They also form water-soluble salts with ammonia and
amines.
Fischer Esterification
In Fischer esterification, a carboxylic acid is reacted with
an alcohol in the presence of an acid catalyst, most
commonly concentrated sulfuric acid.
Fischer esterification is reversible.
It is possible to drive it in either direction by the choice of
experimental conditions (Le Chatelier’s principle).
Fischer Esterification
• In Fischer esterification, the alcohol adds to the
carbonyl group of the carboxylic acid to form a
tetrahedral carbonyl addition intermediate.
• The intermediate then loses H2O to give an ester.
Decarboxylation
• Decarboxylation: The loss of CO2 from a carboxyl group.
• Many carboxylic acids, when heated to a very high
temperature, will undergo thermal decarboxylation.
• Most carboxylic acids, however, are resistant to moderate
heat and melt and even boil without undergoing
decarboxylation.
• An exception is any carboxylic acid that has a carbonyl
group on the carbon b to the COOH group.
Decarboxylation
• Decarboxylation of a b-ketoacid.
• The mechanism forms a enol tautomer.
Decarboxylation
• An important example of decarboxylation of a b-ketoacid
in biochemistry occurs in the Krebs cycle.
• Oxalosuccinic acid, one of the intermediates in this cycle,
has a ketone b to one of the three carboxyl groups.
Structure Summary
• Polar Groups
• Monoacids form dimers through hydrogen bonding
• Diacids have two carboxyl groups
• Long-chain acids are called fatty acids
• Fatty acids are saturated or unsaturated
• Double bonds of fatty acids are cis
• Fatty Acid salts are called soaps
Reactions Summary
• Acid Base Reactions
• Reduction with LAH
• Fischer Esterification
• Decarboxylation
• Esters of Sulphonic Acids and Phosphoric Acid
• Anhydrides of Carboxylic Acids and Phosphoric Acid
Reactions Summary
Acid Base Reactions
• Reaction with bases give carboxylates (salts)
O
H 3C
O
ONa
O
NaO H
H 3C
OH
KOH
H 3C
• Reaction with ammonium give ammonium salts
O
O
H
N
H
3
O
O
N
H
4
OK
Reactions Summary
• Reduction with LAH
O
OH
LAH
H 3C
OH
H 3O
+
H 3C
OH
Reactions Summary
• Esterification
Reactions Summary
• Decarboxylation
Related Chemistry of Acids
• Sulfonic Acid and Phosphoric Acid also form Esters
O
SO
H
O
O
C
H
O
H
3
+
H
O
3
O
H
O
PO
H
O
C
H
O
H
3
+
H
O
3
SO
C
H
O
3 +H
2
O
O
PO
C
H
O
H
O
3 +H
2
O
Related Chemistry of Acids
• Carboxylic Acids Condense to form Anhydrides
O
O
O
O
+
+
H
3
C
O H
H O
C H
3
H
3
C
C H
O
H
2
O
3
• Phosphoric Acid also forms Anhydrides
O
O
H
O
PO
H+
H
O
PO
H
O
O
OO
O
POPO
H
H
O
O
O
P
h
o
s
p
h
o
r
ic
A
c
id
A
n
h
y
d
r
id
e
a
ls
o
c
a
lle
d
a
d
ip
h
o
s
p
h
a
t
e
O
P P
O
H
H
OO
+
PH
H
OO
OO
O
O
O OO
P P
O
P
O
H
H
OO
O OO
t
r
i
p
h
o
s
p
h
a
t
e

similar documents