Acid Anhydrides

Acid Anhydrides

What is an Acid Anhydride?

Acid anhydrides are the molecules that are capable of forming acidic solutions in water. Before going deep into it. Let’s remember some basic concepts.

Acids – These are the substances that are ready to donate hydrogen ions in water.

Bases – these are the substances that hydroxide ions in water.

What is anhydride? Anhydride literally means ‘without water’. It can be defined as the chemical compound formed by eliminating water from another compound. Anhydrides react with water to produces either base or an acid.

Acid Anhydride

Carboxylic Acid Anhydride Structure

Acid anhydrides: Definition & Meaning

An Acid anhydride can be defined as a non-metal oxide which forms an acidic solution when reacted with water. In organic chemistry, it is a functional group consisting of 2 acyl groups combined together by an Oxygen atom. The non-metals which are capable of reacting with water are only called as Acid anhydrides and non-metals that do not react with water are not acids anhydrides. So we can conclude that all acid anhydrides are non-metals and all non-metals are not acid anhydride. For example, carbon monoxide is not an acid anhydride, though it is an oxide of carbon because it does not react with water.

Let’s try to understand better with the following reactions.

Acid anhydrides- Reactions

  • Formation of carbonic acid:

    When carbon dioxide reacts with water, it forms sulphuric acid. The chemical equation will be like this.

    CO2(g) + H2O → H2CO3(aq)

    As discussed earlier, Carbon Dioxide being a non-metal reacted with water to form H2CO3 (Carbonic acid) which is an acid as it has hydrogen to donate. This reaction is responsible for acid rain. It also plays a key role in changing the pH levels of streams, rivers and oceans. It can be observed in the above equation that if there is more CO2 in the air, more carbonic acid is produced which is harmful to life. This is the major problem we are facing today.

  • Formation of Sulphuric acid:

    When Sulphur trioxide reacts with water it forms Sulphuric acid. It can be chemically explained like this.

    SO3(g) + H2O → H2SO4(aq)

    Later, Sulphur trioxide gas reacts with water and resulted in the formation of Sulphuric acid. Sulphur dioxide in the air reacts with oxygen and forms sulphur trioxide. When it reacts with water during rain, that is how acid rain occurs. These have very harmful effects on the environment.

Uses of Acids anhydrides

Acids anhydrides have wide uses in organic chemistry.

  • They are used in the manufacture of many things like pharmaceuticals, industrial chemicals, explosives and perfumes.
  • Synthesis of esters by acetylation of alcohols
  • Synthesis of aspirin (Acetyl salicylic acid)
  • Synthesis of heroin by diacetylation of morphine
  • Use as a protecting group

Practise This Question

In each of the following question, an Assertion (A) is followed by a corresponding Reason (R). Use the following keys to choose the appropriate answer: 

Assertion (A): There is a natural asymmetry between converting work to heat and converting heat to work. 
Reason (R): No process is possible in which the sole result is the absorption of heat from a reservoir and its complete conversion into work.