Download Carboxylic acid

Carboxylic acid
Introduction
• Carboxylic acids are compounds containing
O
the carboxyl group, RCOOH
C
OH
• A carboxyl group contains a carbonyl group,
C=O, present in aldehydes and ketones. And a
hydroxyl group, -OH present in alcohols.
However the two groups influence and change
the properties of each other.
Nomenclature of carboxylic acids
• Carboxylic acids are named by replacing the –
e of the coressponding alkane by –oic
followed by the acid. For example,
O
O
H
C
OH
methanoic acid
H3C
C
OH
ethanoic acid
H3C
CH3
CH3
O
C
H
C
H
C
OH
2,3-dimethylbutanoic acid
Name the following
(a)
(c)
O
H2
C
H3C
(b)
H2C
Cl
C
CH3
O
C
H
C
OH
H3C
(d)
OH
H3C
OH
O
C
H
C
CH3
O
C
H
C
OH
OH
Physical Properties of Carboxylic Acids
• Carboxylic acids are very polar due to both the carbonyl
group and the hydroxyl group
• Carboxylic acids can H-bond with each other, and in fact
exist primarily of dimers (two molecules held together by
H-bonding)
• Because of the above properties, carboxylic acids have
high boiling points (higher than corresponding alcohols)
• Those with less than 5 carbons are soluble in water
- those with more than 5 C’s can be soluble when ionized
O
C
+

-
H
O
O
H +

O
H
O
O
O
O
Na
carboxylic acids are more soluble in water than are alcohols, ethers, aldehydes, and
ketones of comparable molecular weight
Preparation of COOH
1. Oxidation of primary alcohol
• When primary alcohol is heated under reflux
with a solution of potassium manganate (VII)
or sodium dichromate (VI), the carboxylic
acid is formed.
O
R
H2
C
OH
KMnO4
reflux
R
C
OH
+ H2O
O
H3C
H2
C
OH
KMnO4
reflux
H3C
C
OH
+ H2O
2. Oxidation of aldehyde
• The reagents and conditions are the same as
those for the oxidation of primary alcohols
given above.
O
R
C
O
H
KMnO4
reflux
R
O
H3C
C
C
OH
O
H
KMnO4
reflux
H3C
C
OH
3. Hydrolysis of nitriles
• When a nitrile is heated under reflux with
dilute H2SO4, it is hydrolysed to the carboxylic
acid.
O
R
C
N
+ 2 H2O + H+
dilute H2SO4
R
C
OH
+ NH4+
O
H3C
C
N
ethanenitrile
+ 2 H2O + H+
dilute H2SO4
H3C
C
OH
ethanoic acid
+ NH4+
4. Hydrolysis of ester
• When an ester is heated with an aqueos alkali,
sodium hydroxide, the alcohol is liberated and
the acid is left as the sodium salt.
O
O
H3C
C
O
+ NaOH
R'
heat
H3C
C
O- Na+
+ R'
OH
• The acid can be obtained by distilling the
sodium salt with an acid.
O
O
dilute H2SO4
+
H3C
C
-
+
O Na
sodium ethanoate
H+
heat
H3C
C
ethanoic acid
OH
+ Na+
5. Oxidation of alkyl benzene
• When an alkyl benzene is heated under reflux
with a solution of potassium manganate (VII)
acidified with dilute H2SO4 , benzoic acid is
formed.
CH3
KMnO4
heat
COOH
Exercise
1. Write equation for the following reactions.
(a) Ethanol is heated with acidified KMnO4
(b) Ethanal is heated with acidied KMnO4
(c) Propylbenzene is refluxed with acidified
KMnO4
(d) Ethanenitrile is heated with dilute HCl
Chemical reactions
1. React with bases or alkalis to form salts
R
COOH
+ NaOH
R
COO- Na+
+ H2O
2. React with PCl5 or SOCl2 at room temperature
to form acyl chlorides
R
COOH
+ PCl5
R
COCl
+ POCl3 + HCl
R
COOH
+ SOCl2
R
COCl
+ SO2 + HCl
Acid Chlorides from RCOOH
Reaction with Thionyl Chloride
Reaction with Phosphorus Trichloride
Reaction with Phosphorus Pentachloride
RCOOH
3 RCOOH
RCOOH
SOCl 2
PCl 3
PCl 5
R
C
Cl + SO 2 + HCl
O
3R
C
Cl + H 3PO 3
O
R
C
O
Cl + POCl 3 + HCl
3. React with alcohols in the presence of
concentrated H2SO4 to form esters.
COOH + R'
R
OH
R
COOR'
+ H2O
4. Reduction – React with LiAlH4 to form primary
alcohols
R
COOH
+ 4 [H]
LiAlH4
RCH2OH
+ H2O
5. React with electropositive metals to form salts
and H2 gas.
O
R
COOH
+ K
R
C
O- K+ + 1/2 H2
6. Ammonium salts of carboxylic acids undergo
dehydration to form amides.
O
O
R
C
O- NH4+
R
C
amides
NH2
+ H2O
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