2. STRUCTURE OF ACID ANHYDRIDE
• The word ‘anhydride’ means without water.
• Contains two molecules of an acid, with loss of a molecule of water.
• Addition of water to an anhydride regenerates two molecules of the
carboxylic acid.
O
R C
OH
HO C
R
two molecules of acid
General structure:
O
O
R
C
O
C
acid anhydride
R
O
O
O
or (RCO)2O
R
C
O
C
acid anhydride
R
H2O
3. NOMENCLATURE OF ACID
ANHYDRIDE
• The word ‘acid’ is changed to ‘anhydride’ in both common
name and the IUPAC name.
• Examples:
- ethanoic acid → ethanoic anhydride
- propanoic acid → propanoic anhydride
O
H3 C
C
O
O
O
C
CH3
IUPAC : ethanoic anhydride
common: acetic anhydride
CH3CH2
C
O
O
C
CH2CH3
IUPAC : propanoic anhydride
common: propionic anhydride
4. NOMENCLATURE OF ACID
ANHYDRIDE
Anhydrides composed of two different acids are called mixed
anhydrides and are named by using the names of the individual acids.
O
H3C
C
O
O
C
H
IUPAC : ethanoic methanoic anhydride
common: acetic formic anhydride
• Symmetrical anhydrides : change the word acid of the carboxylic acid
to the word anhydride.
• Mixed anhydrides : alphabetizing the names for both acids and
replacing the word acid with the word anhydride.
6. FROM ACYL CHLORIDES
1)
Acyl chlorides react with carboxylate salts to form acid anhydrides.
Can be used to prepare both symmetrical and unsymmetrical
anhydrides
O
O
R C Cl
R' C O
ether
O
O
R C O C R'
carboxylate salts
EXAMPLE
O
CH3CH2 C Cl
O
CH3 C O
ether
O
O
CH3CH2 C O C CH3
7. 2)
Acyl chlorides also reacts with carboxylic acid to
give acid anhydride.
O
O
R C Cl
R' C OH
O
heat
O
HCl
R C O C R'
carboxylic acid
EXAMPLE
O
CH3 C Cl
O
CH3 C OH
heat
O
O
CH3 C O C CH3
HCl
8. HEATING CARBOXYLIC ACIDS
WITH ZnO
Acid anhydride can be prepared from heating simple
carboxylic acids with zinc oxide.
O
ZnO
2 R C OH
heat
O
O
R C O C R
EXAMPLE
O
2 CH3 C OH
ZnO
heat
O
O
CH3 C O C CH3
9. HEATING DICARBOXYLIC ACIDS
Certain cyclic anhydride can be prepared by heating
dicarboxylic acid such as succinic and phthalic
anhydride.
O
O
O
O
HO C CH2 CH2
C
OH
300 C
O
O
O
O
OH
OH
O
O
230 C
O
O
H2O
H2O
10. REACTIVITY OF ACID
ANHYDRIDES
Comparing acid anhydride with acyl chloride.
- ethanoic anhydride has a bulky ethanoate, CH3COO-
group attached to the carbonyl carbon.
- ethanoyl chloride has a simple Cl atom attached to the
carbonyl carbon.
- therefore ethanoic anhydride is less reactive than ethanoyl
chloride.
12. HYDROLYSIS
Acid anhydrides undergoes hydrolysis to produce carboxylic acids.
Can be carried out in acid or base.
Carboxylate salts are formed if hydrolysis is done in basic solution.
O
R
C O C
O
H+
R
R
H OH
O
O
R
O
R
R
O
OH
C O C
C OH
O
R
H OH
C OH
O
C O
R
C O
EXAMPLE
O
H3C
O
C O C
O
+
H
CH3
H OH
H3C
C OH
O
H3C
C OH
13. ALCOHOLYSIS
Acid anhydrides react with alcohol to produce esters and carboxylic acids.
Does not required catalyst, but still requires heating.
O
O
R
C O C
O
R
R
R' OH
O
C OR'
ester
R
C OH
carboxylic acid
EXAMPLE
O
H3C
O
C O C
O
CH3
ethanoic anhydride
O
H3C
O
C O C
CH3CH2 OH
ethanol
H3C
C OCH2CH3
ethyl ethanoate
ethanoic anhydride
H3C
phenol
H3C
C O
phenyl ethanoate
C OH
ethanoic acid
O
O
OH
CH3
O
H3C
C OH
ethanoic acid
14. AMMONOLYSIS
Amide can be prepared through ammonolysis of acid
anhydrides with ammonia, primary and secondary amines.
O
R C
O
O
R C
C
R
2NH3
R
O
C
R
2RNH2
R
C
NH2
C
R
2R2NH
R
C
R C
+
O NH4
O
NHR
O
O
O
C
O
O
O
O
R C
O
O
R C
+
O RNH3
O
NR2
R C
O R2NH2
+
16. FRIEDEL-CRAFTS ACYLATION OF
BENZENE
Acid anhydride reacts with benzene to produce ketones.
Carboxylic acids is the by-product.
O
R C
O
O
C
R
H AlCl
3
O
O
R
HO
C
R
carboxylic acid
ketone
EXAMPLE
O
H3C C
O
O
C
H AlCl
3
CH3
O
O
CH3
HO
C
CH3
