It is applied to cap or acetylate reactive functional groups particularly hydroxyl and amine groups that remain unreacted after a preceding synthetic step.

The main purpose of Capping A is to terminate residual reactive sites and prevent them from participating in subsequent reactions or causing unwanted side products.

Capping A consists of 9% acetic anhydride dissolved in tetrahydrofuran (THF) where acetic anhydride acts as the acetylatingcapping agent and THF serves as the solvent.