Anhydride has important applications in multiple fields

Anhydride is the remaining part of a certain oxygen-containing acid after removing one or several molecules of water. Specifically, general inorganic acids are anhydrides formed by the direct loss of one molecule of water from the acid, and the valence of the elements that determine acidity in the anhydrides remains unchanged. Organic acids are formed by dehydration reactions between two or more molecules of the acid. Only oxygen-containing acids have anhydrides, while anaerobic acids do not have anhydrides.
Anhydride can generally be regarded as an oxide formed by acid dehydration (but the anhydride of organic acids does not belong to oxides). Many can react with water again to form the original acid. According to the properties of acids, acid anhydrides can be divided into inorganic acid anhydrides and organic acid anhydrides. The anhydrides of inorganic acids are formed by the dehydration and condensation of one or two acid molecules, such as carbon dioxide (acid anhydride) and dinitrogen pentoxide (nitrate anhydride). The anhydride of an organic acid is a compound formed by the dehydration and condensation of two monobasic acid molecules or one binary acid molecule. Although it is not an oxide, it is also called an anhydride, such as acetic anhydride and phthalic anhydride.
Anhydride has important applications in multiple fields, such as organic synthesis, dyes, pharmaceutical industry, and the manufacturing of acetyl compounds. It is also used as a solvent and dehydrating agent, and is an important acetylation reagent and polymer initiator. In addition, there are various methods for synthesizing organic laboratory anhydrides, including acid catalysis, esterification, and oxidation.