Question

How to make anhydride from carboxylic acid?

Answer 1

Hint: When a water molecule is removed from any acid molecule, the product that we get is known as an anhydride. In organic acid anhydride, there are two acyl groups (R-C=O) bonded to the same oxygen atom. Its generic structure is

Complete step-by-step answer: The anhydride obtained from carboxylic acid is one of the most common types of organic acid anhydride. It is also known as carboxylic anhydride. The molecular formula of carboxylic anhydride is ${(RC(O))}_{2}O$.
Formation of acid anhydrides from carboxylic acid occurs when the carboxylic acid reacts with an acid chloride in the presence of a base.

Mechanism of synthesis of carboxylic anhydride from acid chloride:

- In the carboxylic acid synthesis, addition-elimination mechanism is followed.
- Chloride anion $(C{l}^{-})$ acts as a leaving group.
- First, a proton is extracted by the base from carboxylic acid and forms carboxylate anion.
- The negatively charged oxygen of the carboxylate anion attacks the carbonyl carbon of acyl chloride, which is electrophilic in nature, and forms a tetrahedral intermediate.
- Lastly, the tetrahedral intermediate eliminates the chloride anion to form acid anhydride.
Hence by adding acid anhydride to a carboxylic acid, in the presence of a strong base, acid anhydride can be obtained.

Note: While naming the acid anhydrides, it is important to note that in an unsymmetrical acid anhydride, both of the carboxylic acids reacted must be named before adding the word anhydride. Whereas in symmetrical acid anhydride, simply renaming the word "acid" to "anhydride" of the parent carboxylic acid would suffice.