Acid Anhydrides: Overview, Questions, Preparation

An acid anhydride is formed by removing the molecules of water from any given acid. It can be represented in the functional group R(CO)O(CO)R'. When an acid anhydride is formed, one equivalent of water is removed from two equivalent acids in a chemical reaction. For example, the dehydration between benzoic acid and propanoic acid, C6H5COOH + CH3CH2COOH → C6H5C(O)OC(O)CH2CH3 + H2O, sulfur trioxide, SO3. They are non-metal in nature. This compound does not react with water.

Some of the formation compounds are mentioned below:

Formation of Carbonic acid

When carbon dioxide reacts with water, it forms sulphuric acid. The chemical equation will look thus:

CO2(g) + H2O → H2CO3(aq)

Formation of sulphuric acid:

When sulphur trioxide reacts with water it forms sulphuric acid. It can be chemically explained thus.

SO3(g) + H2O → H2SO4(aq)

Nitrogen oxides

Two oxides of nitrogen are acid anhydrides; that is, they react with water to form nitrogen-containing oxyacids. Dinitrogen trioxide is the anhydride of nitrous acid, HNO2, and dinitrogen pentoxide is the anhydride of nitric acid, HNO3.

N2O3 + H2O → 2HNO2

N2O5 + H2O → 2HNO3

• It is used in manufacturing several pharmaceutical chemicals, perfumes, and explosives.
• This compound can be used as a protecting group.
• Used in the synthesis of esters by acetylation of alcohols, heroin by deacetylation of morphine.
• Properties of acid anhydride:

They are colourless and have a pungent smell.

• They are soluble in organic solvents like ether and alcohol.
• The boiling point of acid anhydride is higher than that of its parent chemical from which it is prepared.
• The boiling point of this compound is 140 degrees Celsius.
• They are comparatively less reactive than acid chlorides.
• Used in the manufacturing of cellulose acetate.

The acid anhydride functional group results when two carboxylic acids combine and lose water (anhydride = without water). Symmetrical acid anhydrides are named like carboxylic acids except the ending -acid is replaced with -anhydride. Calcium oxide that is produced by a base upon the loss of water are designated basic anhydrides.

It is an important topic as a 5-mark question can be expected from this topic in CBSE exams.

A. Anhydrides are less stable because the electron donation to one carbonyl group competes with the electron donation to the second carbonyl group. Thus, compared with esters, where only one carbonyl group has to balance the oxygen atom, anhydrides are more reactive than esters.  

A. Acid anhydrides are derivatives of carboxylic acids. They may be symmetric (where the two groups of R are identical) or asymmetric.

A. Acid anhydrides are formed from the dehydration reaction of two classes of carboxylic acids, as their name suggests. Anhydrides are highly reactive to nucleophiles and are capable of acylating a variety of proteins and other macromolecules that are important functional groups.

A. A compound that has two acyl groups (R-C=O) bound to the same oxygen atom is an acid anhydride. Anhydrides are typically formed in the presence of a base when carboxylic acid reacts with an acid chloride.  

A. Acid anhydrides are formed from the dehydration reaction of two classes of carboxylic acids, as their name suggests.  

Q. Why are acid anhydrides so reactive?

Q. Is the nomenclature of acid anhydrides always symmetrical?

Q. Are acid anhydrides reactive?

Q. How are acid anhydrides formed?

Q. Describe an acid anhydride.