👉 Thionines are a class of organic compounds that are formed when two or more hydroxyl groups are attached to a nitrogen atom in a benzene ring. These compounds are known for their ability to undergo a variety of chemical reactions, including nucleophilic substitution reactions and esterification reactions. The formation of thionines is typically mediated by the action of a metal catalyst, such as a metal ion or a metal complex. The reaction proceeds through the following steps: 1) Formation of a