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Chemistry 20
Chapter 10
Carboxylic acids
Carboxylic acid: the functional group of a carboxylic acid is a carboxyl group (-COOH),
which is a carbonyl group (C=O) attached to a hydroxyl group (-OH). Carboxylic group can
be represented by three ways: -COOH, -CO2H, or the following method:
O
CH3-C-OH
or
CH3COOH
Naming of carboxylic acids: IUPAC names for carboxylic acids are similar to alcohols
except the final “-e” of the parent alkane is dropped and replaced by “-oic acid”. The parent
chain should contain the carboxyl group (-COOH). 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. We consider the hodroxyl group (-OH) and the amino group (NH2) as a substituent (list them in alphabetical order).
O
O
OH
O
H2 N
COOH
OH
OH
3-Methylbu tanoic acid 5-Hydroxyhexan oic acid
4-Amin ob enzoic acid
(Isovaleric
acid)
O
O O
1
3
1
HOwe
2 add the suffix “-dioic
Naming of dicarboxylic acids:
acid”
OH
HO
OH to the name of the parent
alkane that contains both carboxyl
O groups; thus, “-ane” becomes “-anedioic acid”. The
numbers of the carboxyl carbons
areedioic
not indicated
because
they acid
can be only at the ends of the
aned ioic
Ethan
acid Prop
(Malonic
acid
)
parent chain.
(Oxalic acid )
6
1
3
1
5
1
OH
Hexanoic acid
(Caproic acid )
O
HO
4
O
O
5
1
OH
O
1
HO
HO
OH
1
6
OH
O
O
Butaned ioic acid
Pen tanedioic acid
Hexan edioic acid
(Succinic acid)
(Glutaric acid)
(Ad ipic acid)
Physical properties of carboxylic acids: 1. They are so polar because they contain three
polar covalent bonds (C=O, C-O, and O-H). 2. They have higher boiling points than other
types of organic compounds of comparable molecular weight (because their polarity and the
presence of the hydrogen bonding between two carboxyl groups (it creates a dimer)).
hydrogen bondin g
betw een tw o
molecules
H3 C
O
+
H
O
C
C
O
H
+
CH3
O
-
3. They are more soluble in water than other types of organic compounds (because of
hydrogen bonding). 4. The liquid carboxylic acids have sharp, often disagreeable odors. 5.
They have sour taste (exist in pickles, limes and lemons).
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Fatty acids: long, unbranched chain carboxylic acids are found in animal fats, vegetable oils,
or phospholipids of biological membranes. Nearly all fatty acids have an even number of
carbon atoms, most between 12 and 20, in an unbranched chain.
COOH S tearic acid (18:0
(mp 70°C)
Note: In most unsaturated fatty acids (with double bond C=C), theCOOH
cis isomer
is usually
exists
Oleic acid (18;1)
S tearic
acidCOOH
(18:0)
(mp 16°C)
and the trans isomer is rare.
(mp 70°C)
COOH Oleic acid (18;1)
(mp 16°C)
CisCOOH Linoleic acid (18:
(mp-5°C)
COOH Linoleic acid (18:2)
Note: the saturated fatty acids are solid at room temperature and they have (mp-5°C)
higherCOOH
melting
Linolenic acid (18
points than the unsaturated fatty acids (they are liquid at room temperature). The reason is: the (mp -11°C)
saturated fatty acids are linear and they can be packed together in
close parallel alignment.
COOH Linolenic acid (18:3)
The attractive interactions between adjacent hydrocarbon chains (London dispersion
forces)
(mp -11°C)
are maximized. In unsaturated fatty acids, the cis double bonds interrupt the regular packing
of the chains and the London dispersion forces is smaller.
COOH
COOH
COOH
COOH
COOH
Esters: in esters the hydrogen atom in the carboxyl group is replaced with an alkyl group.
O
Ester group
CH3-C-O-CH3
Saponification: Natural soap are sodium or potassium salts of fatty acids. They are prepared
from a blend of tallow and coconut oils (triglyceride). Triglycerides are triesters of glycerol.
The solid fats are melted with steam, and the water insoluble triglyceride layer that forms on
the top is removed. the preparation of soaps begins by boiling these triglycerides with sodium
hydroxide (NaOH). Boiling with potassium hydroxide (KOH), gives a potassium soap.
O
O CH2 OCR
saponification
+ 3 N aOH
RCOCH
O
CH2 OH
CHOH
+
O
+
3 RCO N a
CH2 OH
1,2,3-Propanetriol S odium soaps
(Glycerol; glycerin)
CH2 OCR
A triglyceride
( a triester of glycerol)
How soap cleans: soap consists of two parts: 1. Hydrophobic part, which is nonpolar and it is
made of fatty acid (long hydrocarbon chain). 2. Hydrophilic part, which is polar and it is made
of carboxylate group (-COO-) with a negative charge and a metal ion (Na+ or K+). In water,
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nonpolar hydrocarbon chains tend to cluster in such a way as to minimize contact of their
hydrocarbon chains with surrounding water molecules. The polar part stays in contact with
the surrounding water molecules. Soap molecules cluster into micelles (a spherical
arrangement of molecules). Many of the things we commonly think of as dirt, are nonpolar
and insoluble in water. When the dirt and soap are mixed together, the nonpolar hydrocarbon
inner parts of the soap can dissolve the dirt and they keep the dirt inside of the micelles. In
this way, the dirt is washed away in the polar wash water.
Chemical reaction of carboxylic acids:
1. They are weak acids (pKa = 4-5). Substituents of high electronegativity (especially -OH, Cl, and -NH3+), near the carboxyl group increase the acidity of carboxylic acids.
Formula: CH3 COOH
N ame:
pK a:
Acetic
acid
4.76
ClCH2 COOH
Cl2 CHCOOH
Cl3 CCOOH
Chloroacetic D ichloroacetic Trich loroacetic
acid
acid
acid
2.86
1.48
0.70
In creasing acid strength
2. As they are weak acids, they react with strong bases like NaOH, KOH, and NH3 to form
water-soluble salts.
COOH + NaOH
H2 O
Ben zoic acid
(slightly soluble in w ater)
COOH + NH3
+
COO Na + H2 O
Sodiu m b enzoate
(60 g/100 mL w ater)
H2 O
Benzoic acid
(s ligh tly solub le in w ater)
-
COO NH4
+
Ammoniu m b enzoate
(20 g/100 mL water)
3. Reduction: the carboxyl group is one of the organic functional groups that is most resistant
to reduction (it cannot be reduced by using the transition metals and NaBH4). The most
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common reagent for the reduction of a carboxylic acid to a 1 aclcohol is the very powerful
reducing agent lithium aluminum hydride, LiAlH4.
O
LiAlH4, ether
COH
CH2OH
H2O
4-Hydroxymethylcyclopentene
3-cyclopentenecarboxylic acid
4. Fischer esterification: reaction of a carboxylic acid with an alcohol in presence of an acid
catalyst (most commonly, concentrated sulfuric acid) gives an ester.
O
H2 SO4
+
CH3 C-OH
H-OCH2 CH3
Eth anoic acid
Eth anol
(Acetic acid ) (Ethyl alcoh ol)
O
CH3 COCH2 CH3 + H2 O
Ethyl ethanoate
(Ethyl acetate)
5. Decarboxylation: is the loss of CO2 from a carboxyl group. Almost any carboxylic acid,
when heated to a very high temperature, undergoes thermal decarboxylation:
O
RCOH
decarboxylation
RH +
CO 2
Note: Most carboxylic acids, however, are quite resistant to moderate heat and melt or even
boil without decarboxylation. Exceptions are carboxylic acids that have a carbonyl group  to
the carboxyl group.
O
O
O
 
OH
3-Oxobutanoic acid
(Acetoacetic acid)
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+
CO 2
Acetone
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