chapter 6 acid anhydride

Chapter 7

ACID ANHYDRIDE

Norfazrin Mohd Hanif

Faculty of Applied Science

UiTM Negeri Sembilan

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.

R C

O

OH HO C

O

R R C

O

O C

O

R H2O

two molecules of acid acid anhydride

General structure:

R C

O

O C

O

R

acid anhydride

or (RCO)2O

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

H3C C

O

O C

O

CH3

IUPAC : ethanoic anhydridecommon: acetic anhydride

CH3CH2 C

O

O C

O

CH2CH3

IUPAC : propanoic anhydridecommon: propionic anhydride

Anhydrides composed of two different acids are called mixed anhydrides and are named by using the names of the individual acids.

H3C C

O

O C

O

H

IUPAC : ethanoic methanoic anhydridecommon: acetic formic anhydride

NOMENCLATURE OF ACID

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.

Acid chloride and carboxylic acid

Acid chloride and carboxylate salt

Heating carboxylic acids with ZnO

Heating dicarboxylic acids

PREPARATION OF ACID

ANHYDRIDES

FROM ACYL CHLORIDES

ether

EXAMPLE

R C

O

Cl R' C

O

O R C

O

O C

O

R'

ether

CH3CH2 C

O

Cl CH3 C

O

O CH3CH2 C

O

O C

O

CH3

carboxylate salts

1) Acyl chlorides react with carboxylate salts to form acid anhydrides.

Can be used to prepare both symmetrical and unsymmetrical

anhydrides

2) Acyl chlorides also reacts with carboxylic acid to

give acid anhydride.

EXAMPLE

R C

O

Cl R' C

O

OH R C

O

O C

O

R'

CH3 C

O

Cl CH3 C

O

OH CH3 C

O

O C

O

CH3

carboxylic acid

heatHCl

HClheat

HEATING CARBOXYLIC ACIDS

WITH ZnO

Acid anhydride can be prepared from heating simple

carboxylic acids with zinc oxide.

EXAMPLE

R C

O

OH R C

O

O C

O

R

CH3 C

O

OH CH3 C

O

O C

O

CH3

heat

ZnO

2

heat

ZnO

2

HEATING DICARBOXYLIC ACIDS

Certain cyclic anhydride can be prepared by heating

dicarboxylic acid such as succinic and phthalic

anhydride.

HO C CH2

O

CH2 C

O

OH H2O300OC

OH

O

OH

O

230OC

O

O

O

H2O

O

O

O

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.

REACTIONS OF ACID

ANHYDRIDES

Hydrolysis

Alcoholysis

Ammonolysis

Friedel-Crafts acylation of benzene

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.

R C

O

O C

O

R H OH R C

O

OH R C

O

OH

EXAMPLE

H+

R C

O

O C

O

R H OH R C

O

O R C

O

OOH

H3C C

O

O C

O

CH3 H OH H3C C

O

OH H3C C

O

OHH+

ALCOHOLYSIS

R C

O

O C

O

R R' OH R C

O

OR' R C

O

OH

EXAMPLE

H3C C

O

O C

O

CH3 CH3CH2 OH H3C C

O

OCH2CH3 H3C C

O

OH

H3C C

O

O C

O

CH3

OH

H3C C

O

O H3C C

O

OH

ester carboxylic acid

ethanoic anhydride ethanol ethyl ethanoate ethanoic acid

phenol phenyl ethanoate ethanoic acidethanoic anhydride

Acid anhydrides react with alcohol to produce esters and carboxylic acids.

Does not required catalyst, but still requires heating.

AMMONOLYSIS

Amide can be prepared through ammonolysis of acid

anhydrides with ammonia, primary and secondary amines.

R C

O

O 2NH3 R C

O

NH2

2RNH2 R C

O

NHR

2R2NH R C

O

NR2

C

O

R R C

O

O NH4+

R C

O

O C

O

R

R C

O

O C

O

R

R C

O

O RNH3+

R C

O

O R2NH2+

H3C C

O

O 2NH3 H3C C

O

NH2

2CH3NH2 H3C C

O

NHCH3

C

O

CH3 H3C C

O

O NH4+

H3C C

O

O C

O

CH3 H3C C

O

O CH3NH3+

EXAMPLES:

FRIEDEL-CRAFTS ACYLATION OF

BENZENE

Acid anhydride reacts with benzene to produce ketones.

Carboxylic acids is the by-product.

R C

O

O C

O

R HO C

O

R

HR

O

ketone carboxylic acid

AlCl3

EXAMPLE

H3C C

O

O C

O

CH3HO C

O

CH3

HCH3

O

AlCl3

Thank you..