Download Carboxylic Acids General formula R C O OH C O OH carboxyl group

Carboxylic Acids
O
O
General formula
R
C
C
OH
carboxyl group
OH
Nomenclature
IUPAC Drop "e" from alkane with the same number of carbons and add "oic acid". The carbon of the
carboxylic acid group is always numbered 1, and this functional group takes precedence over all other
functional groups. Carboxylic acids also have trite names,
O
H C
H 3C C
OH
OH
ethanoic acid
acetic acid
acetum is Latin
for vinegar
methanoic acid
formic acid
formica is Latin
for ant
H3 CH 2CH 2C C
H 3CH 2C C
OH
propanoic acid
propionic acid
H3 CH2 CH2 CH 2CH 2C C
OH
pentanoic acid
valeric acid
very bad smell
rancid butter
goat
OH
butanoic acid
butyric acid
butyrum is Latin
for butter
H 3C
O
O
H3 CH2 CH 2CH 2C C
O
O
O
O
H3 CH 2CH 2C C
OH
γ β α
OH
3-methylbutanoic acid
β-methylbutyric acid
isovaleric acid
very unpleasant smell
like that of dirty socks
hexanoic acid
caproic acid
limburger cheese
goat
Properties
Hydrogen bonding present, stronger and more efficient than in alcohols; c.f. the ir spectrum. Acids form
very stable dimers by hydrogen bonding:
O
H 3C
C
H O
O H
C
CH 3
O
Carboxylic acids are sufficiently strong that they ionize to a small extent in aqueous solution. The
presence of H3O + gives rise to the sour taste of vinegar.
The carboxylic acids react with NaOH and with NaHCO 3 to give water soluble sodium salts:
O
O
+
+
OH
+ H2 O
Na
R C
R C
O - Na+
OH
R
C
O
+
Na+ HCO 3-
O
R
C
-
+
O Na
OH
The salts are soluble in water and insoluble in organic solvents.
+ H2 O + CO 2
Although they both contain the -OH group, carboxylic acids are stronger acids than alchols. This is due to
the resonance stabilization of the anion formed from a carboxylic acid by loss of a proton:
R
C
O
R
O
C
O
O
This type of stabilization of the anion formed by loss of a proton is not available for alcohols.
Electon withdrawing groups increase the acidity of the acid by further stabilizing the anion.
A measure of the acidity is given by K a where K a is defined as the equilibrium constant of the reaction of
an acid to form an anion.
e.g. for acetic acid:
H3C C
O
+ H2 O
OH
H 3C C
Ka =
H3C C
O
O
+
H3O +
H 3O+
O
H3C C
= 1.75 x 10 -5
O
O
OH
at 25 oC
Some values of K a:
Compound
Ka
C2H5OH
C2H5COOH
ClCH2COOH
Cl2 CHCOOH
Cl3 CCOOH
F3CCOOH
~ 1 x 10 -18
1.34 x 10 -5
1.4 x 10 -3
3.32 x 10-2
2.0 x 10 -1
completely ionized in aqueous solution, like HCl
Preparation of Carboxylic Acids
1. Oxidation of aromatic hydrocarbons
COOH
C H
KMnO4, H3O+
Oxidation of Primary Alcohols
K2Cr2O7 , H2 SO4
RCH 2OH
RCOOH
or KMnO4, H 2SO 4
Reaction of Grignard Reagent with CO 2
XMgO
RMgX
+
O=C=O
HO
H 3O +
C O
C O
R
R
e.g.
COOH
MgBr
Br
Br2/FeBr 3
1) CO2
Mg/ether
2) H 3O+
C(CH 3)3
C(CH 3)3
CH 3
H 3C CH CH 2OH
PBr 3
CH3
H3C CH CH 2COOH
1) Mg/ether
CH 3
H 3C CH CH2Br 2) CO2
3) H 3O+
CCl4 , heat
CH 3
H3C C CHCH3
C(CH3)3
C(CH3)3
CH3
1) Mg/ether
H3C C CH 2CH 3 2) CO
2
Cl
3) H 3O+
HCl
CH3
H3C C CH 2CH 3
COOH
Hydrolysis of Nitriles with acid or base
R
e.g.
C
N
H3O+/heat
1) OH2) H 3O +
KCN
CH3CH2CH2Br
O
HI, 100 OC
R
H 3O +
I
C
O
OH
CH 3CH 2CH 2CN
CH 2CH 2CH 2CH 2 I
H 3O +
heat
KCN
H 3O +
tetrahydrofuran, THF
CH3CH2CH2COOH
Br2
peroxides
KCN
+
CH2COOH
CH2CN
CH2Br
NH4OH
CN(CH2 )4CN
1) OH2) H 3O +
COOH(CH 2)4COOH
CH 3
+
H3O+
H 3O +
phenylacetic acid
2NH4OH