CARBOXYLIC ACIDS
Carboxyl group:
O
C OH
Nomenclature —
Common Name
formic acid,
acetic acid,
propionic acid,
butyric acid,
valeric acid,
caproic acid,
Structure
H-COOH
CH3-COOH
CH3CH2-COOH
CH3(CH2)2-COOH
CH3(CH2)3-COOH
CH3(CH2)4-COOH
IUPAC
methanoic acid
ethanoic acid
propanoic acid
butanoic acid
pentanoic acid
hexanoic acid
Positions along the chain are indicated by Greek letters
(common names) or numbers (IUPAC).
Br
O
CH3 CHCH2CH2 C OH
5 4 3 2
1
δ γ β α
γ-bromovaleric
acid
4-bromopentanoic acid
Dicarboxylic acids —
Structure
HOOC-COOH
Common Name
oxalic acid,
IUPAC
ethanedioic acid
HOOC-CH2-COOH
malonic acid,
propanedioic acid
HOOC-(CH2)2-COOH succinic acid,
butanedioic acid
HOOC-(CH2)3-COOH glutaric acid,
pentanedioic acid
HOOC-(CH2)4-COOH adipic acid,
hexanedioic acid
Aromatic acids —
O
C
O
OH
benzoic acid
C
C
O
C
OH
O
C
OH
OH
O
O
C
OH
O
isophthalic acid
phthalic acid
OH
C
OH
terephthalic acid
Salts —
Name cation followed by
acid name with -ic
changed to -ate, eg
Na+ -O
C
O
2
sodium benzo ate
Physical Properties —
Lower molecular weight acids are soluble in H2O:
hydrogen bonding. Usually soluble in organic solvents.
Sodium and potassium
salts of lower molecular
weight acids are soluble in
H2O and not soluble in
organic solvents of low
polarity. The salts of longchain carboxylic acids are
soaps. Sodium and
potassium soaps form
micelles in water.
A Micelle
Carboxylic acids form dimers (pronounced die'-mers) in
the liquid phase --O
H O
R C
C R
These dimers also
O H
O
exit to some extent
in the gas phase,
consequently, carboxylic acids have high boiling points
for a given molecular weight.
3
Acidity —
O
O
+
R C
+ H 2O
R C
+ H3O
O-
O H
O
O
R C
OO
Keq =
H3O
+
R
OO
Ka =
H2O
R C
+
C
R
H3O
C
O H
O H
pKa = -log K a
Compare acidity of carboxylic acids with alcohols:
CH3CH2OH
CH3CH2O- + H+, Ka ~ 10-16
CH3COOH
CH3COO- + H+, Ka ~ 10-5
4
Reason —
Carboxylate anion is more stable compared to
carboxylic acid than alkoxide ion is compared to alcohol.
Neither alcohol nor alkoxide ion are stabilized by
resonance, but the
situation is different
for a carboxylic acid
O
O
and its anion;
R C
R C
in this case the
O H
O H
anion is stabilized
more important
less important
more than the acid:
O
O
R
R
C
C
O
O
equally important
Substituent groups —
Those which stabilize the anion more than its
conjugate acid (electron withdrawing) increase acid
strength. Those which destabilize anion more than acid
(electron donating) decrease acid strength.
5
Synthesis of Carboxylic Acids —
Oxidation of Primary Alcohols or Alkylbenzenes –
KMnO 4
RCH2OH heat
Ar-R
[RCHO]
KMnO4
heat
RCOOH
Ar-COOH, where R is 1o or 2o
Grignard Synthesis --O
O
C + R
C R
O
MgX
O
HX
O MgX
C R
+ MgX 2
O H
eg
+
MgBr
H3O
+ CO 2
COOH
This method results in a product which has one carbon
more than the reactant.
6
Nitrile Hydrolysis —
May be carried out under acidic or basic conditions –
conditions used may depend on other functional groups
present.
R
C N
H2O, H2SO4
heat
RCOOH + (NH4)2SO4
H2O, NaOH
heat
RCOO- Na + +
NH3
Mechanism —
Acidic Hydrolysis:
+
R
H 3O
C N
R
C N H
H2O
H
R C
N H
R C
HO
N H
HO H
H
N H
C
HO
+
C N H
H2O
H2O H
R C N H
HO
H
HO
R
R
H2O
HO
HO H
R C
R C N H
N H
H
7
O
R
C
HO
H
+
H N H
H
Basic Hydrolysis:
OH
R
C
OH
OH
N
R
H 2O
C N
R
C
O
N H
R
C
N H
H
OH
O
R
C
O
N H
OH H
R
C
O
+
N H
OH
R
H
C
O
+ H N H
H
Aliphatic Nitrile Synthesis —
RX
+
K+CN-
DMSO
RC N
+
K+X -
Best if R is primary, a disaster if it is tertiary.
eg
DMSO
CH3CH2CH2CH 2Br + CN CH3CH2CH2CH 2CN
SN2
but
CH3
CH3
DMSO
H3C C Br + CN+ HCN
H2C C
E2
CH3
CH3
8
Reactions of Carboxylic Acids —
Reduction to Primary Alcohols —
R-COOH
(or Ar)
1. LiAlH 4
2. H 3O+
R-CH 2OH
In addition to reducing the carboxyl group, LiAlH4 will
reduce nitro, nitrile, and various carbonyl groups, among
others. If any of these groups are present, they must be
protected or another method must be used to reduce the
carboxylic acid.
9